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Paper No1Publication ID : 648   &   Year : 2016  
TitleLatent heat flux sensitivity to sea surface temperature-regional perspectives
Authors BP Kumar, MF Cronin, S Joseph, M Ravichandran, N Sureshkumar
Source Journal of Climate,
AbstractA global analysis of latent heat flux (LHF) sensitivity to sea surface temperature (SST) is performed, with focus on the tropics and the north Indian Ocean (NIO). Sensitivity of LHF state variables (surface wind speed Ws and vertical humidity gradients οq) to SST give rise to mutually interacting dynamical (Ws driven) and thermodynamical (οq driven) coupled feedbacks. Generally, LHF sensitivity to SST is pronounced over tropics where SST increase causes Ws (οq) changes, resulting in a maximum decrease (increase) of LHF by ~15 W m⿿2 (°C)⿿1. But the Bay of Bengal (BoB) and north Arabian Sea (NAS) remain an exception that is opposite to the global feedback relationship. This uniqueness is attributed to strong seasonality in monsoon Ws and οq variations, which brings in warm (cold) continental air mass into the BoB and NAS during summer (winter), producing a large seasonal cycle in air⿿sea temperature difference οT (and hence in οq). In other tropical oceans, surface air is mostly of marine origin and blows from colder to warmer waters, resulting in a constant οT ~ 1°C throughout the year, and hence a constant οq. Thus, unlike other basins, when the BoB and NAS are warming, air temperature warms faster than SST. The resultant decrease in οT and οq contributes to decrease the LHF with increased SST, contrary to other basins. This analysis suggests that, in the NIO, LHF variability is largely controlled by thermodynamic processes, which peak during the monsoon period. These observed LHF sensitivities are then used to speculate how the surface energetics and coupled feedbacks may change in a warmer world.

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Paper No2Publication ID : 651   &   Year : 2016  
TitleImpact of an upgraded model in the NCEP Global Ocean Data Assimilation System: The tropical Indian Ocean
Authors Rahaman, H., D. W. Behringer, S. G. Penny, and M. Ravichandran
Source J. Geophys. Res. Oceans, 121, 8039⿿8062, doi:10.1002/2016JC012056.
AbstractThe National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produces global ocean analysis based on the Global Ocean Data Assimilation System (GODAS). This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS. Three experiments were performed with same atmospheric forcing fields: (i) using MOM4p0d (GODAS_p0), (ii) using MOM4p1 (GODAS_p1), both using observed temperature and synthetic salinity, and (iii) using MOM4p1 (GODAS_p1S) assimilating both observed temperature and observed salinity. Validation with independent observations shows significant improvement of subsurface temperature and salinity in the new analysis using MOM4p1 versus MOM4p0d. There is also improvement in the upper ocean current of the equatorial Indian Ocean. The impact of observed salinity on the upper ocean surface current is marginal, but there is significant improvement in the subsurface current. The seasonal and inter-annual variability of the Wyrtki jet and the equatorial undercurrent is improved in GODAS_p1 versus GODAS_p0. All analyses reproduced the Indian Ocean dipole, with the GODAS_p1S simulated sea surface temperature (SST) the most accurate. The temperature inversion over the north Bay of Bengal (BoB) is reproduced only in GODAS_p1S. The mean sea level over BoB and equatorial Indian Ocean improved in GODAS_p1S as compared with AVISO observation. The combined model upgrade and assimilation of observed salinity led to reduced root-mean-square deviation and higher correlation coefficient values in the sea level anomaly (SLA) when compared with satellite observations.

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Paper No3Publication ID : 650   &   Year : 2016  
TitleOcean atmosphere thermal decoupling in the eastern equatorial Indian ocean
Authors Sudheer Joseph, M Ravichandran, B. Praveen Kumar, Raju V. Jampana, Weiqing Han
Source Climate Dynamics, DOI: 10.1007/s00382-016-3359-1
AbstractEastern equatorial Indian ocean (EEIO) is one of the most climatically sensitive regions in the global ocean, which plays a vital role in modulating Indian ocean dipole (IOD) and El Niño southern oscillation (ENSO). Here we present evidences for a paradoxical and perpetual lower co-variability between sea-surface temperature (SST) and air-temperature (Tair) indicating instantaneous thermal decoupling in the same region, where signals of the strongly coupled variability of SST anomalies and zonal winds associated with IOD originate at inter-annual time scale. The correlation minimum between anomalies of Tair and SST occurs in the eastern equatorial Indian ocean warm pool region (⿿70°E⿿100°E, 5°S⿿5°N), associated with lower wind speeds and lower sensible heat fluxes. At sub-monthly and Madden⿿Julian oscillation time scales, correlation of both variables becomes very low. In above frequencies, precipitation positively contributes to the low correlation by dropping Tair considerably while leaving SST without any substantial instant impact. Precipitation is led by positive build up of SST and post-facto drop in it. The strong semi-annual response of SST to mixed layer variability and equatorial waves, with the absence of the same in the Tair, contributes further to the weak correlation at the sub-annual scale. The limited correlation found in the EEIO is mainly related to the annual warming of the region and ENSO which is hard to segregate from the impacts of IOD.

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Paper No4Publication ID : 647   &   Year : 2016  
TitleTrends and relationship between chlorophyll-a and sea surface temperature in the central equatorial Indian Ocean
Authors G S Kumar, S Prakash, M Ravichandran, AC Narayana
Source Remote Sensing Letters 7 (11), 1093-1101
AbstractThis work presents the trend analysis and relationship between chlorophyll-a (chl-a) concentration and sea surface temperature (SST) in the central equatorial Indian Ocean (CEIO) using Aqua MODIS chl-a Level-3 Standard Mapped Image (SMI) data for a period of 10 years (2002⿿2012). In order to understand the monsoonal variability of chl-a concentration and SST and to evaluate their relationships over the CEIO, trend analysis of chl-a values was carried out. The area average chl-aconcentration in the region shows a weak annual cycle with high concentration during winter (October⿿December) and low in summer (June). High chl-aconcentration (~0.22 mg m⿿3) is observed during early winter in the region. Chl-aconcentration starts decreasing from March onwards until the onset of summer monsoon. The data reveal low chl-a concentrations during summer period, i.e., from June to September, which is in accordance with several observations, and higher concentrations during October to December. The other reason is that satellite sensor may not capture chl-a variability more accurately because of cloud cover during summer monsoon time. A reasonably significant coefficient of determination (R2 = 0.51; significant at p < 0.05 level) between SST and chl-a concentration is recorded. This study clearly suggests that the SST acts as a proxy for variables which cause high chl-a concentration in the CEIO.

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Paper No5Publication ID : 646   &   Year : 2016  
TitleIntraseasonal Variability of SST and Precipitation in the Arabian Sea during Indian Summer Monsoon: Impact of Ocean Mixed Layer Depth
Authors Yuanlong Li, Weiqing Han, Wanqiu Wang, M Ravichandran
Source Journal of Climate, DOI:
AbstractThis study investigates sea surface temperature (SST) and precipitation variations in the eastern Arabian Sea (EAS) induced by the northward-propagating Indian summer monsoon (ISM) intraseasonal oscillations (MISOs) through analyzing satellite observations and the Climate Forecast System Reanalysis (CFSR) and performing ocean general circulation model (OGCM) experiments. MISOs in the EAS achieve the largest intensity in the developing stage (May⿿June) of the ISM. The MISOs induce intraseasonal SST variability primarily through surface heat flux forcing, contributed by both shortwave radiation and turbulent heat flux, and secondarily through mixed layer entrainment. The shallow mixed layer depth (MLD < 40 m) in the developing stage and decaying stage (September⿿October) of the ISM significantly amplifies the heat flux forcing effect on SST and causes large intraseasonal SST variability. Meanwhile, the high SST (>29°C) in the developing stage leads to enhanced response of MISO convection to SST anomaly. It means that the ocean state of the EAS region during the developing stage favors active two-way air⿿sea interaction and the formation of the strong first-pulse MISO event. These results provide compelling evidence for the vital role played by the ocean in the MISO mechanisms and have implications for understanding and forecasting the ISM onset. Compared to satellite observation, MISOs in CFSR data have weaker SST variability by ~50% and biased SST⿿precipitation relation. Reducing these biases in CFSR, which provides initial conditions of the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2), may help improve the ISM rainfall forecast.

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Paper No6Publication ID : 645   &   Year : 2016  
TitlePrediction of sea surface temperature by combining numerical and neural techniques
Authors Kalpesh Patil, MC Deo, M Ravichandran
Source Journal of Atmospheric and Ocean Technology, DOI:
AbstractThe prediction of sea surface temperature (SST) in real-time or online mode has applications in planning marine operations and forecasting climate. This paper demonstrates how SST measurements can be combined with numerical estimations with the help of neural networks and how reliable site-specific forecasts can be made accordingly. Additionally, this work demonstrates the skill of a special wavelet neural network in this task. The study was conducted at six different locations in the Indian Ocean and over three time scales (daily, weekly, and monthly). At every time step, the difference between the numerical estimation and the SST measurement was evaluated, an error time series was formed, and errors over future time steps were forecasted. The time series forecasting was affected through neural networks. The predicted errors were added to the numerical estimation, and SST predictions were made over five time steps in the future. The performance of this procedure was assessed through various error statistics, which showed a highly satisfactory functioning of this scheme. The wavelet neural network based on the particular basic or mother wavelet called the ⿿Meyer wavelet with discrete approximation⿝ worked more satisfactorily than other wavelets.

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Paper No7Publication ID : 644   &   Year : 2016  
TitleIntroduction to the special issue on the Bay of Bengal: From monsoons to mixing.
Authors Mahadevan, A., T. Paluszkiewicz, M. Ravichandran, D. Sengupta, and A. Tandon.
Source Oceanography29(2):14⿿17,
AbstractThe Bay of Bengal has a surprisingly large influence on the world. It nurtures the South Asian summer monsoon, a tremendous ocean-atmosphere-land phenomenon that delivers freshwater to more than a third of the human population on this planet. During summer, southwesterly winds gather moisture from the ocean and carry it deep inland over the Indian subcontinent, bringing welcome rains to a parched land. During winter, the winds reverse to northeasterly, and the ocean circulation responds by dispersing the terrestrial freshwater runoff concentrated in the northern part of the bay. This freshwater impacts the ocean⿿s structure, circulation, and biogeochemistry in numerous ways and, through modification of sea surface temperature, feeds back to influence air-sea fluxes. Because the atmosphere obtains its moisture and heat for convection from the ocean, the interplay between ocean and atmosphere is crucial for the development and sustenance of the monsoon. 

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Paper No8Publication ID : 643   &   Year : 2016  
TitleA tale of two spicy seas.
Authors MacKinnon, J.A., J.D. Nash, M.H. Alford, A.J. Lucas, J.B. Mickett, E.L. Shroyer, A.F. Waterhouse, A. Tandon, D. Sengupta, A. Mahadevan, M. Ravichandran, R. Pinkel, D.L. Rudnick, C.B. Whalen, M.S. Alberty, J. Sree Lekha, E.C. Fine, D. Chaudhuri, and G.L. Wagner.
Source Oceanography 29(2):50⿿61,
AbstractUpper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean⿿s horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

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Paper No9Publication ID : 642   &   Year : 2016  
TitleMonsoon mixing cycles in the Bay of Bengal: A year-long subsurface mixing record.
Authors Warner, S.J., J. Becherer, K. Pujiana, E.L. Shroyer, M. Ravichandran, V.P. Thangaprakash, and J.N. Moum
Source Oceanography 29(2):158⿿169,
AbstractBased on the first year-long record of mixing collected in the eastern central Bay of Bengal, we quantify the role that subsurface turbulent heat fluxes play in upper-ocean cooling brought on by southwest (SW) and northeast (NE) monsoons. During the NE (dry, or winter) monsoon, atmospheric and subsurface turbulent heat fluxes each contribute about 50% of the net sea surface cooling. During the SW (wet, or summer) monsoon, the atmospheric heat flux varied widely due to ⿿active⿝ and ⿿break⿝ cycles of the monsoon intraseasonal oscillations, but had a net positive seasonal average. The subsurface turbulent heat flux during the SW monsoon led to surface cooling at rates more than three times greater than those measured during the NE monsoon. Since the seasonally averaged atmospheric heat flux was positive, subsurface mixing accounted for nearly all of the cooling during the SW monsoon. During the transition between the NE and SW monsoons, subsurface heat flux was near zero, and atmospheric heating rapidly warmed the sea surface. Following the SW monsoon, passage of Tropical Storm Hudhud led to O(1) m2 s⿿1 rates of turbulence diffusivity and strong subsurface heat flux, accounting for roughly half of the 1.4°C surface cooling that occurred over a 60-hour period. 

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Paper No10Publication ID : 641   &   Year : 2016  
TitleAdrift upon a salinity-stratified sea: A view of upper-ocean processes in the Bay of Bengal during the southwest monsoon. 
Authors Lucas, A.J., J.D. Nash, R. Pinkel, J.A. MacKinnon, A. Tandon, A. Mahadevan, M.M. Omand, M. Freilich, D. Sengupta, M. Ravichandran, and A. Le Boyer
Source Oceanography 29(2):134⿿145,
AbstractThe structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument⿿s roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular ⿿barrier-layer⿝ stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above. 

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Paper No11Publication ID : 640   &   Year : 2016  
TitleWhat controls seasonal evolution of sea surface temperature in the Bay of Bengal? Mixed layer heat budget analysis using moored buoy observations along 90°E.
Authors Thangaprakash, V.P., M.S. Girishkumar, K. Suprit, N. Suresh Kumar, D. Chaudhuri, K. Dinesh, A. Kumar, S. Shivaprasad, M. Ravichandran, J.T. Farrar, R. Sundar, and R.A. Weller.
Source Oceanography 29(2):202⿿213,
AbstractContinuous time-series measurements of near surface meteorological and ocean variables obtained from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) moorings at 15°N, 90°E; 12°N, 90°E; and 8°N, 90°E and an Ocean Moored buoy Network for Northern Indian Ocean (OMNI) mooring at 18°N, 90°E are used to improve understanding of air-sea interaction processes and mixed layer (ML) temperature variability in the Bay of Bengal (BoB) at seasonal time scales. Consistent with earlier studies, this analysis reveals that net surface heat flux primarily controls the ML heat balance. The penetrative component of shortwave radiation plays a crucial role in the ML heat budget in the BoB, especially during the spring warming phase when the ML is thin. During winter and summer, vertical processes contribute significantly to the ML heat budget. During winter, the presence of a strong barrier layer and a temperature inversion (warmer water below the ML) leads to warming of the ML by entrainment of warm subsurface water into the ML. During summer, the barrier layer is relatively weak, and the ML is warmer than the underlying water (i.e., no temperature inversion); hence, the entrainment cools the mixed layer. The contribution of horizontal advection to the ML heat budget is greatest during winter when it serves to warm the upper ocean. In general, the residual term in the ML heat budget equation is quite large during the ML cooling phase compared to the warming phase when the contribution from vertical heat flux is small. 

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Paper No12Publication ID : 639   &   Year : 2016  
TitlePenetrative radiative flux in the Bay of Bengal. 
Authors Lotliker, A.A., M.M. Omand, A.J. Lucas, S.R. Laney, A. Mahadevan, and M. Ravichandran.
Source Oceanography 29(2):214⿿221,
AbstractThe Bay of Bengal (BoB), a semi-enclosed basin in the northern Indian Ocean, is a complex region with large freshwater inputs and strong vertical stratification that result in a shallow, spatially variable mixed layer. With the exception of shortwave insolation, the air-sea heat exchange occurs at the sea surface and is vertically redistributed by mixing and advection. Strongly stratified, shallow mixed layers inhibit vertical mixing, and the penetration of solar radiation through the base of the mixed layer can lead to redistribution of upper-ocean heat. This paper compiles observations of hyperspectral downwelling irradiance (Ed) from 67 profiles collected during six research cruises in the BoB that span a broad range of regions and seasons between 2009 and 2014. We report attenuation length scales computed using double and single exponential models and quantify the penetration of radiative flux below the mixed layer depth (Qpen). We then evaluate estimates of Qpen obtained from published chlorophyll-based models and compare them to our observations. We find that the largest penetrative heat flux (up to 40% of the incident Ed) occurs near 16°N where the mixed layers are shallow and the water is optically clear.

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Paper No13Publication ID : 638   &   Year : 2016  
TitleTechnological advancements in observing the upper ocean in the Bay of Bengal: Education and capacity building
Authors Tandon, A., E.A. D⿿Asaro, K.M. Stafford, D. Sengupta, M. Ravichandran, M. Baumgartner, R. Venkatesan, and T. Paluszkiewicz
Source Oceanography 29(2):242⿿253,
AbstractBecause the monsoon strongly affects India, there is a clear need for indigenous expertise in advancing the science that underlies monsoon prediction. The safety of marine transport in the tropics relies on accurate atmospheric and ocean environment predictions on weekly and longer time scales in the Indian Ocean. This need to better forecast the monsoon motivates the United States to advance basic research and support training of early career US scientists in tropical oceanography. Earlier Indian field campaigns and modeling studies indicated that an improved understanding of the interactions between the upper ocean and the atmosphere in the Bay of Bengal at finer spatial and temporal scales could lead to improved intraseasonal monsoon forecasts. The joint US Air-Sea Interactions Regional Initiative (ASIRI) and the Indian Ocean Mixing and Monsoon (OMM) program studied these interactions, resulting in scientific advances described by articles in this special issue of Oceanography. In addition to these scientific advances, and while also developing long-lasting collaborations and building indigenous Indian capability, a key component of these programs is training early career scientists from India and the United States. Training has been focusing on fine-scale and mixing studies of the upper ocean, air-sea interactions, and marine mammal research. Advanced methods in instrumentation, autonomous robotic platforms, experimental design, data analysis, and modeling have been emphasized. Students and scientists from India and the United States at all levels have been participating in joint cruises on Indian and US research vessels and in training participants in modern tools and methods at summer schools, at focused research workshops, and during research visits. Such activities are building new indigenous capability in India, training a new cadre of US scientists well versed in monsoon air-sea interaction, and forging strong links between Indian and US oceanographic institutions. 

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Paper No14Publication ID : 596   &   Year : 2016  
TitleASIRI: An Ocean-Atmosphere Initiative for Bay of Bengal
Authors Hemantha W Wijesekera, Emily Shroyer, Amit Tandon, M Ravichandran, Debasis Sengupta, SUP Jinadasa, Harindra JS Fernando, Neeraj Agarwal, K Arulananthan, GS Bhat, Mark Baumgartner, Jared Buckley, Luca Centurioni, Patrick Conry, J Thomas Farrar, Arnold L Gordon, Verena Hormann, Ewa Jarosz, Tommy G Jensen, Shaun Johnston, Matthias Lankhorst, Craig M Lee, Laura S Leo, Iossif Lozovatsky, Andrew J Lucas, Jennifer Mackinnon, Amala Mahadevan, Jonathan Nash, Melissa M Omand, Hieu Pham, Robert Pinkel, Luc Rainville, Sanjiv Ramachandran, Daniel L Rudnick, Sutanu Sarkar, Uwe Send, Rashmi Sharma, Harper Simmons, Kathleen M Stafford, Louis St Laurent, Karan Venayagamoorthy, Ramasamy Venkatesan, William J Teague, David W Wang, Amy F Waterhouse, Robert Weller, Caitlin B Whalen
Source Bulletin of the American Meteorological Society in Press
Abstract An observation and modeling campaign in the Bay of Bengal is aimed at studying upper ocean and lower atmosphere processes and interactions in relation to Indian Ocean Monsoons. Air-Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013-2017) aimed at understanding and quantifying coupled atmosphere-ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than twenty research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB⿿s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB; energetic mesoscale and submesosacle features including an intrathermocline eddy in the central BoB; and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (~ 300 km wide) that carries high-salinity water into BoB during northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden Julian Oscillation (MJO) permit the study of multi-scale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air-sea interactions control the ABL and upper ocean processes.

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Paper No15Publication ID : 598   &   Year : 2016  
TitleNear surface salinity and stratification in the north Bay of Bengal from moored observations
Authors Debasis Sengupta, GN Bharath Raj, M Ravichandran, J Sree Lekha, Fabrice Papa
Source Geophysical Research Letters, (in Press)
AbstractA thin layer of fresh water from summer monsoon rain and river runoff in the Bay of Bengal (BoB) has profound influence on air-sea interaction across the south Asian region, but the mechanisms that sustain the low-salinity layer are as yet unknown. Using the first long time series of high-frequency observations from a mooring in the north BoB and satellite salinity data, we show that fresh water from major rivers is transported by large-scale flow and eddies, and shallow salinity stratification persists from summer through winter. The moored observations show frequent 0.2-1.2 psu salinity jumps with time scales of 10⿿minutes to days, due to O(1-10) km sub-mesoscale salinity fronts moving past the mooring. In winter, satellite sea surface temperature shows 10⿿km-wide filaments of cool water, in line with moored data. Rapid salinity and temperature changes at the mooring are highly coherent, suggesting slumping of salinity-dominated fronts. Based on these observations, we propose that sub-mesoscale fronts may be one of the important drivers for the persistent fresh layer in the north BoB.

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Paper No16Publication ID : 597   &   Year : 2016  
TitleFifteen years of ocean observations with the global Argo array
Authors Stephen C Riser, Howard J Freeland, Dean Roemmich, Susan Wijffels, Ariel Troisi, Mathieu Belbéoch, Denis Gilbert, Jianping Xu, Sylvie Pouliquen, Ann Thresher, Pierre-Yves Le Traon, Guillaume Maze, Birgit Klein, M Ravichandran, Fiona Grant, Pierre-Marie Poulain, Toshio Suga, Byunghwan Lim, Andreas Sterl, Philip Sutton, Kjell-Arne Mork, Pedro Joaquín Vélez-Belchí, Isabelle Ansorge, Brian King, Jon Turton, Molly Baringer, Steven R Jayne
Source Nature Climate Change Volume 6 Issue 2 Pages 145-153
AbstractMore than 90% of the heat energy accumulation in the climate system between 1971 and the present has been in the ocean. Thus, the ocean plays a crucial role in determining the climate of the planet. Observing the oceans is problematic even under the most favourable of conditions. Historically, shipboard ocean sampling has left vast expanses, particularly in the Southern Ocean, unobserved for long periods of time. Within the past 15 years, with the advent of the global Argo array of profiling floats, it has become possible to sample the upper 2,000 m of the ocean globally and uniformly in space and time. The primary goal of Argo is to create a systematic global network of profiling floats that can be integrated with other elements of the Global Ocean Observing System. The network provides freely available temperature and salinity data from the upper 2,000 m of the ocean with global coverage. The data are available within 24 hours of collection for use in a broad range of applications that focus on examining climate-relevant variability on seasonal to decadal timescales, multidecadal climate change, improved initialization of coupled ocean⿿atmosphere climate models and constraining ocean analysis and forecasting systems.

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Paper No17Publication ID : 649   &   Year : 2016  
TitleIndian moored observatory in the Arctic for long-term in situ data collection
Authors R Venkatesan, KP Krishnan, M Arul Muthiah, B Kesavakumar, David T Divya, MA Atmanand, S Rajan, M Ravichandran
Source The International Journal of Ocean and Climate Systems 7 (2), 55-61. DOI: 10.1177/1759313116642898
AbstractThis article describes the first Indian arctic multi-sensor mooring (IndARC) observatory in Kongsfjorden of Arctic waters. The Ministry of Earth Sciences evolved a unique moored observatory ⿿IndARC⿿ with suite of sensors for physico-chemical and oceanographic in situ long-term data collection. The Arctic Ocean plays an important role in governing the earth⿿s climate and also faithfully records its past climatic history and represents a significant gap in ocean observations. This indigenously designed and installed observatory IndARC, for the first time, collected various parameters from July 2014 to July 2015. The uniqueness of this system, the challenges faced and results from data collected are presented in this article.

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Paper No18Publication ID : 544   &   Year : 2015  
TitleRelation between the upper ocean heat content in the equatorial Atlantic during boreal spring and the Indian monsoon rainfall during June⿿September
Authors Vijay Pottapinjara, M. S. Girishkumar, S. Sivareddy, M. Ravichandran and R. Murtugudde
Source Int. J. Climatol., Published online in Wiley Online Library, DOI: 10.1002/joc.4506
AbstractEarlier studies have identified a teleconnection between the Atlantic zonal mode (AZM) and Indian summer monsoon rainfall (ISMR), both of which are active during the boreal summer (AZM: June⿿August; ISMR: June⿿September). It is known that El Niño-Southern Oscillation (ENSO)-like coupled dynamics are operational in the tropical Atlantic during the AZM events. Our goal here is to extend this process understanding to seek a predictive relation between the tropical Atlantic and the ISMR based on these known teleconnections. Monthly composite analysis of the zonal surface winds, heat content, and sea surface temperature (SST) in the equatorial Atlantic tells us that signatures of a warm or cold AZM event begin to emerge as early as January of that year. We found significant correlations between the ISMR and the low level zonal winds in the western equatorial Atlantic and heat content in the eastern equatorial Atlantic in the boreal spring season. Tracking coherent changes in these winds and the evolution of the heat content in the deep tropical Atlantic in the boreal spring may offer the potential for skillful predictions of the ensuing summer monsoon anomalies, especially during non-ENSO years when the predictability of ISMR tends to be low.

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Paper No19Publication ID : 543   &   Year : 2015  
TitleObserved interannual variability of near-surface salinity in the Bay of Bengal
Authors Pant, V., M. S. Girishkumar, T. V. S. Udaya Bhaskar, M. Ravichandran, F. Papa, and V. P. Thangaprakash
Source J. Geophys. Res. Oceans, 120, doi:10.1002/2014JC010340
AbstractAn in situ gridded data of salinity, comprising Argo and CTD profiles, has been used to study the interannual variability of near-surface salinity (within 30 m from sea surface) in the Bay of Bengal (BoB) during the years 2005⿿2013. In addition to the broad agreement with earlier studies on the north-to-south gradient of surface salinity and general features of seasonal variability of salinity, the data also revealed few episodes of enhanced freshening in the BoB. The observations showed distinct anomalous low salinity (< 2 psu) waters in the northern BoB during June⿿February of the years 2006⿿2007 (Y67), 2011⿿2012 (Y12), and 2012⿿2013 (Y23). The anomalous freshening during these years showed similar life cycle, such as, it starts in the northern BoB during July⿿September of current summer and extends up to February⿿March of next winter with a southward propagation. Analysis showed that the oceanic and atmospheric conditions associated with positive Indian Ocean Dipole (pIOD) lead to these freshening events, and IOD rather than El Niño/Southern Oscillation (ENSO) controls the interannual variability of salinity in the BoB. The mixed layer salt budget analysis indicated the dominant role of local fresh water flux (horizontal advection) on the observed salinity tendency during summer (winter) monsoon season. Enhanced precipitation associated with pIOD lead to enhanced freshening in northern BoB during June⿿September, which remained to this region with prevailing summer monsoon circulation. The weakening or absence of southward east India coastal current (EICC) during October⿿December of these freshening years trapped anomalous freshwater in the northern BoB.

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Paper No20Publication ID : 542   &   Year : 2015  
TitleAssessing the impact of various wind forcing on INCOIS-GODAS simulated ocean currents in the equatorial Indian Ocean
Authors S. Sivareddy M. Ravichandran, M. S. Girishkumar, , K V S R Prasad
Source Ocean Dynamics, September 2015, Volume 65, Issue 9, pp 1235-1247
AbstractThe Global Ocean Data Assimilation System configured at Indian National Centre for Ocean Information Services (INCOIS-GODAS) has been forced with satellite-based QuikSCAT gridded winds (QSCAT) to obtain accurate operational ocean analysis, particularly ocean currents, as compared to the default National Centers for Environmental Prediction-Reanalysis 2 (NCEP-R2) wind forcing in the tropical Indian Ocean (TIO). However, after termination of QuikSCAT mission in November 2009, an alternate wind forcing was required for providing operational ocean analysis. The present study examines the suitability of an Advanced Scatterometer (ASCAT)-based daily gridded wind product (DASCAT) for the INCOIS-GODAS. Experiments were performed by forcing INCOIS-GODAS with three different momentum fluxes derived from QSCAT, DASCAT, and NCEP-R2 wind products. Simulated ocean currents from these experiments are validated with respect to in situ current measurements from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. Results suggested that the quality of simulated ocean currents from the daily DASCAT forcing is on par with the QSCAT forcing in the TIO, except for the equatorial Indian Ocean (EIO). Although QSCAT-forced current simulations are slightly better than DASCAT-forced simulations, both QSCAT and DASCAT provide a much better result than NCEP-R2. Our analysis shows that the better simulations of currents over the EIO, with the QSCAT forcing compared to DASCAT forcing, can be attributed to the smoothening of the wind field in the DASCAT compared to QSCAT. The impact of the error in the DASCAT on ocean current analysis is, however, limited to local scales and upper 100 m of water column only. Thus, our study demonstrated that, in the absence of QSCAT, DASCAT is a better alternative for INCOIS-GODAS ocean analysis than the NCEP-R2.

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Paper No21Publication ID : 541   &   Year : 2015  
TitleVolume transports of the Wyrtki jets and their relationship to the Indian Ocean Dipole
Authors McPhaden, M. J., Y. Wang, and M. Ravichandran
Source J. Geophys. Res. Oceans, 120, 5302⿿5317, doi:10.1002/2015JC010901
AbstractThe equatorial Indian Ocean is characterized by strong eastward flows in the upper 80⿿100 m during boreal spring and fall referred to as the Wyrtki jets. These jets are driven by westerly winds during the transition seasons between the southwest and northeast monsoons and represent a major conduit for mass and heat transfer between the eastern and western sides of the basin. Since their discovery over 40 years ago, there have been very few estimates from direct observations of the volume transports associated with these currents. In this paper we describe seasonal-to-interannual time scale variations in volume transports based on 5 years of unique measurements from an array of acoustic Doppler current profilers in the central equatorial Indian Ocean. The array was centered at 0°, 80.5°E and spanned latitudes between 2.5°N and 4°S from August 2008 to December 2013. Analysis of these data indicates that the spring jet peaks in May at 14.9±2.9 Sv and the fall jet peaks in November at 19.7±2.4 Sv, around which there are year-to-year transport variations of 5⿿10 Sv. The relationship of the interannual transport variations to zonal wind stress forcing, sea surface temperature, sea surface height, and surface current variations associated with the Indian Ocean Dipole (IOD) are further highlighted. We also illustrate the role of wind-forced equatorial waves in affecting transport variations of the fall Wyrtki jet during the peak season of the IOD.

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Paper No22Publication ID : 583   &   Year : 2015  
TitleA reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean
Authors Roxy, M. K., A. Modi, R. Murtugudde, V. Valsala, S. Panickal, S. P. Kumar, M. Ravichandran, M. Vichi, and M. Lévy
Source Geophys. Res. Lett., 42, doi:10.1002/2015GL066979.
AbstractAmong the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming trend in sea surface temperatures in the tropics during the past century⿿though the contribution of such a large warming to productivity changes have remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.

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Paper No23Publication ID : 533   &   Year : 2014  
TitleInfluence of Pacific Decadal Oscillation on the relationship between ENSO and tropical cyclone activity in the Bay of Bengal during October⿿December
Authors M. S. Girishkumar, V. P. Thanga Prakash and M. Ravichandran
Source Climate Dynamics DOI 10.1007/s00382-014-2282-6
AbstractThe relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October⿿December under cold (1950⿿1974) and warm (1975⿿2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Niño and La Niña years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Niña years (2.62 TCs per season) than during El Niño years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Niña years (1.4 TCs per season) compared to El Niño years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Niña and El Niño years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Niña years compared to El Niño years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO.

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Paper No24Publication ID : 532   &   Year : 2014  
TitleThe role of ENSO and MJO on rapid intensification of tropical cyclones in the Bay of Bengal during October⿿December
Authors M. S. Girishkumar, K. Suprit, S. Vishnu , V. P. Thanga Prakash and M. Ravichandran
Source Theor Appl Climatol DOI 10.1007/s00704-014-1214-z
AbstractThe influence of El Niño/Southern Oscillation (ENSO) and Madden⿿Julian Oscillation (MJO) and their combined effect on the rapid intensification (RI) of tropical cyclones (TCs) in the Bay of Bengal (BoB) during the primary cyclone season (October⿿December) is investigated. An empirical index, called genesis potential index (GPI), is used to quantify the relative importance of four environmental parameters responsible for the modulation of TCs characteristics. The analysis shows that TC frequency and RI of TC⿿s is higher in La Niña than El Niño regime during the primary TC season in the BoB. The combined effect of enhancement (reduction) in mid-tropospheric humidity (primary factor) and relative vorticity (secondary factor) played a major role in the enhancement (reduction) of the TC activity under La Niña (El Niño) regime. In addition, when the MJO is active over the BoB (phases 3⿿4; characterized by enhanced convective activity in the BoB) under La Niña regime, environmental conditions were more conducive for enhancement of TC activity and RI of TCs compared to corresponding MJO phase under El Niño regime. Increase in mid-tropospheric humidity and reduction in vertical wind shear were identified as the primary and secondary factors enhancing the likelihood of RI of TCs in the BoB during phases 3⿿4 of MJO under La Niña regime. Further, the role of accumulated tropical cyclone heat potential (ATCHP) on the RI of TC during primary TC season is also investigated. Our analysis demonstrates that ATCHP is large for TCs which undergo RI compared to TCs not undergoing RI.

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Paper No25Publication ID : 531   &   Year : 2014  
TitleMixing to Monsoons: Air-Sea Interactions in the Bay of Bengal
Authors J. Lucas, E. L. Shroyer, H. W. Wijesekera, H. J. S. Fernando, E. E⿿Asaro, M. Ravichandran, S. U. P. Jinadasa, J. A. Mackinnon, J. D. Nash, R. Sharma, L. Centurioni, J. T. Farrar, R. Weller, R. Pinkel, A. Mahadevan, D. Sengupta, and A. Tandon
Source Eos, Vol. 95, No. 30, 29 July 2014
AbstractMixing to Monsoons: Air-Sea interactions in the Bay of Bengal

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Paper No26Publication ID : 534   &   Year : 2014  
TitleRevisiting the Wintertime Intraseasonal SST Variability in the Tropical South Indian Ocean: Impact of the Ocean Interannual Variation
Authors Li, Y., W. Han, T. Shinoda, C. Wang, M. Ravichandran, and J. Wang
Source J. Phys. Oceanogr. doi:10.1175/JPO-D-13-0238.1
AbstractIntraseasonal sea surface temperature (SST) variability over the Seychelles⿿Chagos Thermocline Ridge (SCTR; 55°-85°E, 12°-4°S) induced by boreal wintertime Madden-Julian oscillations (MJOs) is investigated with a series of OGCM experiments forced by the best available atmospheric data. The impact of the ocean interannual variation (OIV), e.g., the thermocline depth changes in the SCTR, is assessed. The results show that surface shortwave radiation (SWR), wind speed-controlled turbulent heat fluxes, 8 and wind stress-driven ocean processes are all important in causing the MJO-related intraseasonal SST variability. The effect of the OIV is significant in the eastern part of the SCTR (70°-85°E), where the intraseasonal SSTs are strengthened by about 20% during the 2001-2011 period. In the western part (55°-70°E), such effect is relatively small and not significant. The relative importance of the three dominant forcing factors is adjusted by the OIV, with increased (decreased) contribution from wind stress (wind speed and SWR). The OIV also tends to intensify the year-to-year variability of the intraseasonal SST amplitude. In general a stronger (weaker) SCTR favors larger (smaller) SST responses to the MJO forcing. Due to the nonlinearity of the upper-ocean thermal stratification, especially the mixed layer depth (MLD), the OIV imposes asymmetric impact on the intraseasonal SSTs between the strong and weak SCTR conditions. In the eastern SCTR, both the heat flux forcing and entrainment are greatly amplified under the strong-SCTR condition but only slightly suppressed under the weak-SCTR condition, leading to an overall strengthening effect by the OIV.

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Paper No27Publication ID : 537   &   Year : 2014  
TitleSalinity measurements collected by Fishermen reveal a ⿝River in the Sea⿝ flowing along the eastern coast of India.
Authors A. V. S. Chaitanya, M. Lengaigne, J. Vialard, V. V. Gopala krishna, F. Durand, C. Kranthikumar, S. Amritash, V. Suneel, F. Papa, and M. Ravichandran
Source Bulletin of the American Meteorological Society, 95, pp 1897-1908, 2014
AbstractBeing the only tropical ocean bounded by a continent to the north, the Indian Ocean is home to the most powerful monsoon system on Earth. Monsoonal rains and winds induce huge river discharges and strong coastal currents in the northern Bay of Bengal. To date, the paucity of salinity data has prevented a thorough description of the spreading of this freshwater into the bay. The potential impact of the salinity on cyclones and regional climate in the Bay of Bengal is, however, a strong incentive for a better description of the water cycle in this region. Since May 2005, the National Institute of Oceanography conducts a program in which fishermen collect seawater samples in knee-deep water at eight stations along the Indian coastline every 5 days. Comparison with open-ocean samples shows that this cost-effective sampling strategy is representative of offshore salinity evolution. This new dataset reveals a salinity drop exceeding 10 g kg⿿1 in the northern part of the bay at the end of the summer monsoon. This freshening signal propagates southward in a narrow (~100 km wide) strip along the eastern coast of India, and reaches its southern tip after 2.5 months. Satellite-derived alongshore-current data shows that the southward propagation of this ⿿river in the sea⿝ is consistent with transport by seasonal coastal currents, while other processes are responsible for the ensuing erosion of this coastal freshening. This simple procedure of coastal seawater samples collection could further be used to monitor phytoplankton concentration, bacterial content, and isotopic composition of seawater along the Indian coastline.

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Paper No28Publication ID : 536   &   Year : 2014  
TitleInfluence of the Atlantic zonal mode on monsoon depressions in the Bay of Bengal during boreal summer
Authors Pottapinjara, V., M. S. Girishkumar, M. Ravichandran, and R. Murtugudde
Source J. Geophys. Res. Atmos., 119, doi:10.1002/ 2014JD021494.
AbstractThe influence of the Atlantic Zonal Mode (AZM) or the Atlantic Niño on monsoon depressions in the Bay of Bengal during the boreal summer (June⿿August) is studied. Our analysis shows that there is a statistically significant difference in the number of monsoon depressions in the Bay of Bengal between the warm and cold phases of the AZM; more (fewer) monsoon depressions form during the cold (warm) phase of AZM. It also shows that there are differences in spatial pattern of trajectories of monsoon depressions; during the cold phase of AZM, the tracks are relatively long and seem to cluster along the axis of core monsoon region compared to the warm phase of AZM. The analysis indicates an increase (a reduction) in low-level cyclonic vorticity and mid tropospheric humidity but a reduction (an increase) in vertical wind shear due to anomalous circulation pattern. All of these changes are favorable for the enhancement (suppression) of monsoon depressions during the cold (warm) phase of the AZM. Our analysis further shows a teleconnection pathway by which the AZM can influence the remote Indian Ocean. This could have implications for enhancing monsoon prediction skill, especially during non-El Niño⿿Southern Oscillation years.

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Paper No29Publication ID : 535   &   Year : 2014  
TitleInterannual variability of the Arabian Sea Warm Pool: observations and governing mechanisms
Authors R. R. Rao, V. Jitendra, M. S. GirishKumar , M. Ravichandran and S. S. V. S. Ramakrishna
Source Climate Dynamics DOI 10.1007/s00382-014-2243-0
AbstractThe near-surface layers in the Arabian Sea progressively warm up from February to early May resulting in the formation of pool of warm waters popularly known as the Arabian Sea Warm Pool (ASWP). The availability of high quality TMI sea surface temperature (SST) data for the years 1998⿿2010 is exploited to describe the evolution of the ASWP on seasonal and interannual time scales and to explain the associated mechanisms. The multi-year (1998⿿2010) averaged TMI SSTs during April⿿May show peak values of the ASWP in excess of 30 °C with its core >30.5 °C extending offshore as a well-marked southwestward tongue stretching from the southwest coast of India. The ASWP shows both seasonal and interannual variability in the evolution of spatio-temporal characteristics such as amplitude, phase and spatial extent. Among these 13 years, the ASWP was most (least) pronounced during 1998, 2003 and 2010 (1999, 2000, 2001 and 2008). The mechanisms that govern the observed interannual variability of the ASWP are examined addressing the most relevant issues such as⿿(1) dynamic pre-conditioning: background pycnocline topography influenced by the westward propagating Rossby waves during October⿿May, (2) thermal pre-conditioning: background SST/heat content signal during October⿿January influenced by the strength of the preceding year⿿s summer monsoon and the post-monsoon cyclones during October⿿December, (3) haline pre-conditioning: near-surface vertical salinity stratification during November⿿February influenced by the advection of low saline waters from the Bay of Bengal, (4) influence of surface net heat flux forcing during February⿿May, and (5) influence of El Nino/La Nina.

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Paper No30Publication ID : 490   &   Year : 2013  
TitleTemperature inversions and their influence on the mixed layer heat budget during the winters of 2006-2007 and 2007-2008 in the Bay of Bengal
Authors M. S. Girishkumar, M. Ravichandran, and M. J. McPhaden
Source JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 1-12, doi:10.1002/jgrc.20192, 2013
AbstractTime series measurements of temperature, salinity and surface meteorological parameters recorded at 8_N, 90_E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African-Asian-Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006�2007 (W67) and 2007�2008 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year-to-year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer.

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Paper No31Publication ID : 491   &   Year : 2013  
TitleObserved intraseasonal thermocline variability in the Bay of Bengal
Authors Girishkumar, M. S., M. Ravichandran and W. Han
Source Journal of Geophysical Research: Oceans In Press DOI 10.1002/jgrc.20245
AbstractThe time series of temperature data obtained from moored buoys deployed at 8N,12N and 15N along 90E in the Bay of Bengal (BoB) shows a persistent intraseasonal variability on 30-120 day time scale in three distinct periods 30-70 day, near 90-day and near 120-day in the thermocline region. The standard deviation of moored buoy temperature data shows that half of the variability in the thermocline region is contributed from the 30-120 day variability. The relative contribution of local Ekman pumping velocity and remote wind forcing from equatorial Indian Ocean (EIO) to the intraseasonal thermocline variability in the BoB is examined using satellite derived Sea surface height anomaly (SSHA), wind and depths of 23 isotherm (D23, proxy for thermocline depth) derived from moored buoys temperature data. The analysis shows that large amplitude intraseasonal oscillations of thermocline - particularly the near 90-day and 120-day variability - could not be explained by local Ekman pumping velocity alone. The SSHA, D23 and wind fields reveal that the first and second baroclinic mode Kelvin and Rossby waves, which are generated remotely by winds from the EIO and eastern BoB, can significantly influence the thermocline variability in the BoB. The near 90-day and 120-day thermocline variability is driven primarily by the variability of equatorial zonal wind stress. While the 30-70 day thermocline variability is affected most by interior Ekman pumping over the Bay, it also appears to be influenced by zonal wind stress in the EIO and alongshore wind stress in the eastern BoB.

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Paper No32Publication ID : 492   &   Year : 2013  
TitleEvaluation of the Global Ocean Data Assimilation System at INCOIS: The Tropical Indian Ocean
Authors M. Ravichandran, D. Behringer, S. Sivareddy, M.S. Girishkumar, Neethu Chacko, R. Harikumar
Source Ocean Modelling. (2013), In Press
AbstractA new version of NCEPs Global Ocean Data Assimilation System (GODAS), which is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular ocean Model version 4.0 (MOM4.0) and a three-dimensional variational (3D-VAR) data assimilation scheme, was configured and operationalized at Indian National Centre for Ocean Information Services (INCOIS). The primary objective of the GODAS at INCOIS (INCOIS-GODAS) is to provide an accurate estimate of the ocean state, which will be used to initialize a coupled model for the seasonal monsoon forecast and also to understand the variability of the ocean at different time scales. In this paper, we assess the quality of ocean analyses in the Tropical Indian Ocean (TIO) obtained from the operational INCOIS-GODAS. In addition to this, we examined the sensitivity of INCOIS-GODAS to different momentum forcing and to the assimilation of temperature and synthetic salinity based on the experiments carried out with different wind products: NCEP2 and QuikSCAT and a free run respectively. The present study reveals that the model with assimilation simulates most of the observed features of temperature, SSHA and currents with reasonably good accuracy in the TIO at both intra-seasonal and inter-annual time scales. The analysis further shows that there was a considerable improvement in the ocean current field, when the model was forced with QuikSCAT winds

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Paper No33Publication ID : 489   &   Year : 2013  
TitleEvaluation of ASCAT based daily gridded 1 winds in the tropical Indian Ocean
Authors S. Siva Reddy, M. Ravichandran, and M. S. Girishkumar
Source J. Atmos. Oceanic Technol. doi:10.1175/JTECH-D-12-00227.1, in press.
AbstractThe quality of daily gridded ASCAT (DASCAT) 25 blended winds is examined in the Tropical Indian Ocean using 3-day running mean gridded QuikSCAT (QSCAT) winds, and in-situ daily winds from the Research Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA). The primary objective of this study is to examine whether DASCAT is a reliable replacement for the widely-used QSCAT wind products. Spatial distributions of DASCAT and QSCAT winds show good agreement in speed and direction, except over a few localized regions. We find a significant spatial coherence between rainfall and the regions of discrepancy between DASCAT and QSCAT. Comparison of DASCAT and QSCAT wind products with RAMA moorings indicates that DASCAT better captures the overall wind variability compared to QSCAT, especially during rainy and low-wind (< 5 ms-134 ) conditions. The root mean square of the RAMA-DASCAT (RAMA-QSCAT) difference during rain fall in the zonal and meridional wind is 1.4 and 1.6 ms-1 (2.7 and 2.0 ms-136 ) respectively. The present study indicates that the DASCAT blended wind product is a reliable alternative to QSCAT in the tropical Indian Ocean

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Paper No34Publication ID : 530   &   Year : 2013  
TitleCan oxycline depth be estimated using sea level anomaly (SLA) in the northern Indian Ocean?
Authors Satya Prakash, Prince Prakash and M. Ravichandran
Source Remote Sensing Letters, 2013 Vol. 4, No. 11, 1097⿿1106,
AbstractInformation on depth of oxycline is critical not only for understanding magnitude and extent of the hypoxic zone but also for specifying potential fishing zones on operational basis.We analysed Argo-oxygen data from the northern Indian Ocean, along with sea level anomaly (SLA) data from altimeter, to demonstrate the correlation between depths of oxycline, thermocline and SLA. Our analysis suggests that observed variability in oxycline depth is mainly governed by physical processes such as vertical movement in the thermocline depth in the northern Indian Ocean basin. There exists strong positive correlations between depths of thermocline, oxycline and SLA. Oxycline depth and SLA are highly correlated in the Arabian Sea, but the correlation between the two is weaker in the Bay of Bengal and equatorial Indian Ocean. We propose a regression equation between SLA and oxycline depth, which may be used to estimate the depth at which water is oxygen deficient (through oxycline) in the northern Indian Ocean.

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Paper No35Publication ID : 487   &   Year : 2013  
TitlePredicting Sea Surface Temperatures in the North Indian Ocean with Nonlinear Autoregressive Neural Networks
Authors Kalpesh Patil, M. C. Deo, Subimal Ghosh, and M. Ravichandran
Source International Journal of Oceanography Volume 2013, Article ID 302479, 11 pages
AbstractPrediction of monthly mean sea surface temperature (SST) values has many applications ranging from climate predictions to planning of coastal activities. Past studies have shown usefulness of neural networks (NNs) for this purpose and also pointed to a need to do more experimentation to improve accuracy and reliability of the results. The present work is directed along these lines. It shows usefulness of the nonlinear autoregressive type of neural network vis-`a-vis the traditional feed forward back propagation type. Neural networks were developed to predict monthly SST values based on 61-year data at six different locations around India over 1 to 12months in advance.The nonlinear autoregressive (NAR) neural network was found to yield satisfactory predictions over all time horizons and at all selected locations.The results of the present study were more attractive in terms of prediction accuracy than those of an earlier work in the same region.The annual neural networks generally performed better than the seasonal ones, probably due to their relatively high fitting flexibility

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Paper No36Publication ID : 488   &   Year : 2013  
TitleEvaluation of near-surface air temperature and specific humidity from hybrid global products and their impact on latent heat flux in the North Indian Ocean
Authors Rahaman, H., and M. Ravichandran
Source JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS, VOL. 118, 10341047, doi:10.1002/jgrc.20085, 2013
AbstractDaily near-surface air temperature (Ta) and specific humidity (Qa) from three hybrid flux products, namely, Coordinated Ocean-Ice Reference Experiments version II (COREII), Objectively Analyzed Air-Sea Fluxes (OAFlux), and Air-Sea Fluxes for the Global Tropical Oceans (TropFlux), are evaluated using in situ data over the North Indian Ocean. The analysis shows that the root-mean-square error (RMSE) value of Ta is ~0.5_C for all products. TropFlux captures the daily variability of Ta very well, but it has a systematic deviation in Ta. The large drop in Ta observed during intense rainfall events is very well captured by TropFlux. All products overestimate Qa by 0.31.5 g/kg; OAFlux has the smallest systematic deviation, whereas TropFlux has the highest correlation with buoy data. The overestimation of Qa by the products is mainly caused by high values of Qa, in the range of 1822 g/kg. The RMSE of Qa ranges from 0.92 to 1.79 g/kg, with OAFlux having the lowest values. Latent heat flux (LHF) computed from a bulk algorithm is underestimated by all products, which can be primarily attributed to the positive bias in Qa. In the southern Bay of Bengal, LHF decreases with increasing Qa during winter and summer monsoons. In this region, a change in 1 g/kg Qa can cause about 1115 W/m2 errors in LHF. The air-sea humidity difference is linearly related to sea surface temperature for values greater than 28_C, similar to findings for the western Pacific Ocean

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Paper No37Publication ID : 436   &   Year : 2012  
TitleWeakening of spring Wyrtki jets in the Indian Ocean during 2006-2011
Authors Sudheer Joseph, Alan J. Wallcraft, Tommy G. Jensen, M. Ravichandran,S. S. C. Shenoi, and Shailesh Nayak
Source JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C04012, doi:10.1029/2011JC007581,
AbstractBeginning in 2006, the Indian Ocean experienced climatologically anomalous conditions due to large-scale coupled air-sea interactions that influenced the surface circulation of the equatorial Indian Ocean. Here we present evidence from observations as well as a general circulation model to demonstrate that spring Wyrtki jets (WJ) were weak during the past 6 years and were even reversed to westward flow during 2008. We note that this weakening coincided with uniformly high sea level as well as positive east to west gradient anomalies along the equatorial Indian Ocean during the month of May each year, starting in 2006. The weakened jets occur in conjunction with the latitude of zero zonal wind (LUZ) being close to the equator during these years, resulting in weaker than normal zonal winds along the equator from 2006 and onward. We find that starting in 2006, the normal tendency of westward propagation of the annual harmonic mode switches to eastward propagation, coherent with the wind forcing. In comparison to the annual harmonic component of the zonal current, the weak WJs are mainly associated with the semiannual harmonic WJs, as evident from an amplitude reduction of that mode by at least 0.3 m s1 during the post-2005 period. Our analysis demonstrates that the variance explained by the semiannual harmonic is reduced to half (30�40%) at the core of the WJ in 2006 and later years in comparison with earlier years when it was 70�80%.

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Paper No38Publication ID : 437   &   Year : 2012  
TitleObserved variability of chlorophyll-a using Argo profiling floats in the South Eastern Arabian Sea
Authors M. Ravichandran, M. S. Girish Kumar and Stephen Riser
Source Deep Sea Research Part I: Oceanographic Research Papers (March 2012), doi:10.1016/j.dsr.2012.03.003
AbstractThe time series of temperature, salinity, chlorophyll-a, particle scattering at 700 nm, and dissolved oxygen obtained from Argo floats deployed in the southeastern ArabianSea (AS), were used to investigate the variability of these quantities and the mechanisms that modulate them between March 2010 and March 2011. The observations show a persistent occurrence of a subsurface chlorophyll-a maximum (0.751 mg m−3) near depths of 40100 m throughout the study period, just above the top of permanent thermocline and euphotic depth. The analysis shows that upward and downward movement of thermocline, which is primarily due to westward movement of low-mode baroclinic Rossby waves, significantly influences the depth of the subsurface chlorophyll maxima and its intensity. Further, the vertical movement of the thermocline significantly influences the depth of the oxycline in the AS. The mixed layer deepening associated with wind induced vertical mixing and convective overturning lead to near surface blooms during the summer and winter monsoons. The analysis clearly shows that, in addition to entrainment of nutrients from rich subsurface water in the near surface layer, vertical fluxes from the subsurface chlorophyll maximum also contribute significantly to mixed layer blooms. The availability of light also plays an active role in the mixed layer bloom, particularly during the summer monsoon season

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Paper No39Publication ID : 465   &   Year : 2012  
TitleDetermination of Sonic Layer Depth from XBT Profiles and Climatological Salinities in the Arabian Sea
Authors Udaya Bhaskar, TVS and Swain, Debadatta and Ravichandran, M
Source International Journal of Earth Sciences and Engineering, 5 (1). pp. 36-44. ISSN 0974-5904
AbstractSonic layer depth (SLD) plays an important role in antisubmarine warfare in terms of identifying the shadow zones for submarine safe parking. Sound velocity profiles (SVP) are obtained from temperature and salinity (T/S) profiles which are inturn used for deriving SLD. Salinity data over oceanic region is sparse, compared to temperature alone profiles. Given the limited availability of salinity data in comparison to temperature, alternative methods need to be sought for generating SVPs. In the present work, a new method is proposed for utilizing all temperature measurements in conjunction with World Ocean Atlas climatological salinities to compute SVPs and then extract SLD. This approach is demonstrated by utilizing all T/S data obtained from Argo floats in the Arabian Sea (40 80 E and 0 30 N). SLD is estimated from SVP obtained from Argo T/S profiles first and again by replacing the Argo salinity with climatological salinity. Statistics revealed that more than 90% of cases, SLD obtained from Argo T/S and Argo temperature and climatological salinity matched exactly, with the root mean square deviation ranging from 3 12 m with an average of 7 m.

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Paper No40Publication ID : 471   &   Year : 2012  
TitleUpper ocean variability in the Bay of Bengal during the tropical cyclones Nargis and Laila
Authors K. Maneesha V.S.N. Murty, M. Ravichandran, T. Lee, Weidong Yu M.J. McPhaden
Source Prog. Oceanogr., 106, 49-61
AbstractUpper ocean variability at different stages in the evolution of the tropical cyclones Nargis and Laila is evaluated over the Bay of Bengal (BoB) during May 2008 and May 2010 respectively. Nargis initially developed on 24 April 2008; intensified twice on 2728 April and 1 May, and eventually made landfall at Myanmar on 2 May 2008. Laila developed over the western BoB in May 2010 and moved westward towards the east coast of India. Data from the Argo Profiling floats, the Research Moored Array for AfricanAsianAustralian Monsoon Analysis and prediction (RAMA), and various satellite products are analyzed to evaluate upper ocean variability due to Nargis and Laila. The analysis reveals pre-conditioning of the central BoB prior to Nargis with warm (>30˚C) Sea Surface Temperature (SST), low (<33 psu) Sea Surface Salinity (SSS) and shallow (<30 m) mixed layer depths during MarchApril 2008. Enhanced ocean response to the right of the storm track due to Nargis includes a large SST drop by ~1.76˚C, SSS increase up to 0.74 psu, mixed layer deepening of 32 m, shoaling of the 26 _C isotherm by 36 m and high net heat loss at the sea surface. During Nargis, strong inertial currents (up to 0.9 ms_1) were generated to the right of storm track as measured at a RAMA buoy located at 15˚ N, 90˚ E, producing strong turbulent mixing that lead to the deepening of mixed layer. This mixing facilitated entrainment of cold waters from as deep as 75 m and, together with net heat loss at sea surface and cyclone-induced subsurface upwelling, contributed to the observed SST cooling in the wake of the storm. A similar upper ocean response occurs during Laila, though it was a significantly weaker storm than Nargis.

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Paper No41Publication ID : 472   &   Year : 2012  
TitleDoes sea level pressure modulate the dynamic and thermodynamic forcing in the tropical Indian Ocean?
Authors Nisha, P.G., Muraleedharan, P.M. , Keerthi, M.G., Sathe, P.V., Ravichandran, M.
Source International Journal of Remote Sensing, Volume 33, Issue 7, April 2012, Pages 1991-2002
AbstractThree areas, the north Indian Ocean (NIO), the equatorial Indian Ocean (EIO) and the south Indian Ocean (SIO), were chosen over the tropical Indian Ocean to investigate the dependency of sea surface temperature (SST), wind speed (WS) and sea level pressure (SLP) on latent heat flux (LHF). The dynamic and thermodynamic behaviour of the tropical Indian Ocean was studied from the trends of the scatter represented by the mean and standard deviation of LHF, WS and humidity gradient, q s - q a, D q, binned in 1C SST interval plotted against SST. The direct linear relationship of LHF with SST reverses at some point to display an inverse relationship when the atmosphere is coupled with the ocean. The point at which the reversal takes place marks the threshold SST which appears to have an inherent relationship with the SLP, especially when the ocean-atmosphere system is coupled. North of 5 S, the LHF peaks at the threshold SST of 27.5C and decreases gradually on either side. The resemblance of the SST-LHF curve of SIO and EIO to that of the equatorial Pacific Ocean can be attributed to the fact that both regimes fell under the same pressure band that covers the equatorial Pacific. Shifting of LHF maxima to a lower SST regime as SLP increases is noticed at southern and northern latitudes, while such a regime shift is not noticed at the equator. This phenomenon can be attributed to relatively weaker air-sea coupling and subsequent lower LHF production at the EIO.

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Paper No42Publication ID : 474   &   Year : 2012  
TitleIs the trend in chlorophyll-a in the Arabian Sea decreasing?
Authors Prince Prakask, Satya Prakash, H. Rahaman, M. Ravichandran, and Shailesh Nayak
Source GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L23605, doi:10.1029/2012GL054187, 2012
AbstractRecent studies of satellite-derived Chlorophyll concentrations (Chl-a) in the westernArabian Sea (AS) have suggested an increasing temporal trend, but the length of the records used have typically been too short to resolve longer-term trends, if any. Our analysis of a long term satellite ocean color data shows a change of trend in the summer chlorophyll for the western AS before and after 2003; Chl-a concentration was indeed increasing till 2003, but appears to be declining since then, indicating a secular multi-year trend in Chl-a variability. However, this trend is not uniform over the entire region. Analysis of wind, sea surface temperature (SST), Sea Level Anomaly (SLA) and thermocline depth, suggests that the declining summer monsoon chlorophyll-a (Chl-a) concentration may be due to increasing SLA in this region. The earlier observed biological changes in the western AS could be an artifact of the change in local winds and ocean dynamics, which may be a part of the natural long-term variability.

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Paper No43Publication ID : 391   &   Year : 2012  
TitleThe Influences of ENSO on tropical cyclone activity in the Bay of Bengal during October-December
Authors Girishkumar, M. S., and M. Ravichandran
Source Journal of Geophysical Research (Oceans) - 117, C02033, doi:10.1029/2011JC007417.
AbstractThe El Nio/Southern Oscillation (ENSO) influence on Tropical Cyclone (TC) activity (frequency, genesis location and intensity) in the Bay of Bengal (BoB) during the primary TC peak season (October-December) are studied for the period of 1993-2010. The study shows that during primary TC peak season, Accumulated Cyclone Energy (ACE) in the BoB is negatively correlated with Nio3.4 sea surface temperature anomaly (SSTA). Under La Nia regime number of extreme TC cases (wind speed > 64 kt) increases significantly in the BoB during the primary TC peak season. The analysis further shows that negative Indian Ocean Dipole (IOD) year is also favorable for extreme TC activity in the BoB during the primary TC peak season. The existence of low level cyclonic (anticyclonic) vorticity, enhanced (suppressed) convection, and high (low) tropical cyclonic heat potential (TCHP) in the BoB provides favorable (unfavorable) conditions for the TC activity under La Nia (El Nio) regimes together with weak vertical wind shear and high SST. The genesis location of TC shifts to the east (west) of 87E in the BoB during La Nia (El Nio) regime due to the variability in convective activity. The probable reason for the intense TC during a La Nia regime is likely explained in terms of longer track for TCs over warm SST and high TCHP due to eastward shifting of genesis location together with other favorable conditions. The variability of Madden-Julian Oscillation (MJO) and its influence on TC activity in the BoB during La Nia and El Nio regime are also examined.

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Paper No44Publication ID : 390   &   Year : 2012  
TitleAn anomalous cooling event observed in the Bay of Bengal during June 2009
Authors Neethu Chacko, M. Ravichandran, R. R. Rao , S. S. C. Shenoi
Source Ocean dynamics, DOI 10.1007/s10236-012-0525-9, 62 (5), pp. 671-681.
AbstractSea surface temperature (SST) variability over the Bay of Bengal (BoB) has the potential to trigger deep moist convection thereby affecting the active-break cycle of the monsoons. Normally, during the summer monsoon season, SST over the BoB is observed to be greater than 28C which is a pre-requisite for convection. During June 2009, satellite observations revealed an anomalous basin wide cooling and the month is noted for reduced rainfall over the Indian subcontinent. In this study we analyze the likely mechanisms of this cooling event using both satellite and moored buoy observations. Observations showed deepened mixed layer, stronger surface currents and enhanced heat loss at the surface in the BoB. Mixed layer heat balance analysis is carried out to resolve the relative importance of various processes involved. We show that the cooling event is primarily induced by the heat losses at the surface resulting from the strong wind anomalies, and advection and vertical entrainment playing secondary roles.

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Paper No45Publication ID : 385   &   Year : 2012  
TitleObserved cholorphyll-a bloom in the southern Bay of Bengal during winter 2006-07
Authors M. S. Girishkumar, M. Ravichandran and Vimlesh Pant
Source Available on-online, International Journal of Remote Sensing, doi: 10.1080/01431161.2011. 563251. Vol. 33, No. 4, 20 February 2012, 12641275
AbstractThe analysis of 6-year chlorophyll-a data provided by the Moderate Resolution Imaging Spectral (MODIS) radiometer revealed anomalous chlorophyll-a bloom in the southern Bay of Bengal during the winter 20062007. The plausible causative mechanisms for such a large chlorophyll-a during the winter 20062007 are analysed through surface wind field, surface net heat flux and sea surface height anomaly (SSHA) data. The chlorophyll-a bloom developed inNovember 2006 near the northern tip of Sumatra. It was intensified and propagated slowly westward during December 2006 to January 2007 and then weakened by February 2007. The combined effect of shallowing of thermocline and weakening of barrier layer due to anomalous westward propagating upwelling Rossby waves associated with the Indian Ocean Dipole (IOD) event and relatively strong wind field causing entrainment of subsurface nutrient-rich water to euphotic zone leads to the bloom in the southern Bay of Bengal.

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Paper No46Publication ID : 392   &   Year : 2012  
TitleA New Atlas of Temperature and Salinity for the North Indian Ocean
Authors A. Chatterjee, D. Shankar, S. S. C. Shenoi, G. V. Reddy, G. S. Michael, M. Ravichandran, V. V. Gopalkrishna, E. P. Rama Rao, T. V. S. Udaya Bhaskar, and V. N. Sanjeevan
Source Journal of Earth System Science, 121 (3), pp 559-595.
AbstractThe most used temperature and salinity climatology for the world ocean, including the Indian Ocean, is the World Ocean Atlas (WOA) (Locarnini et al., 2006; Antonov et al., 2006; Locarnini et al., 2010; Antonov et al., 2010) because of the vast amount of data used in its preparation. The WOA climatology does not, however, include all the available hydrographic data from the Indian Exclusive Economic Zone (EEZ), leading to the potential for improvement if the data from this region are included to prepare a new climatology. We use all the data that went into the preparation of the WOA (Locarnini et al., 2010; Antonov et al., 2010), but add considerable data from Indian sources, to prepare new annual, seasonal, and monthly climatologies of temperature and salinity for the Indian Ocean. The addition of data improves the climatology considerably in the Indian EEZ, the differences between the new North Indian Ocean Atlas (NIOA) and WOA being most significant in the Bay of Bengal, where the patchiness seen in WOA, an artifact of the sparsity of 22 data, was eliminated in NIOA. The significance of the new climatology is that it presents a more stable climatological value for the temperature and salinity fields in the Indian EEZ.

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Paper No47Publication ID : 393   &   Year : 2011  
TitleIn-situ Ocean Observing system
Authors M. Ravichandran
Source Book chapter in "Operational Oceanography in the 21st Century", A. Schiller, G. B. Brassington (eds.), DOI 10.1007/978-94-007-0332-2_3, pp 55-90, Springer publications, 2011.
AbstractOcean Observing system consists of in-situ and satellite systems to detect, track, and predict changes in physical, chemical, geological and biological systems. In-situ observing systems consist of both Eulerian (based on fixed locations) and Legrangian (whose location varies with time) systems. The elements of in-situ observing system in terms of its principle, capability to observe the ocean, technology and some of the applications pertain to each system are described. A brief status on Indian Ocean Observing system (IndOOS) is also described. The strengths and weakness of each platform and the need for integrating different observational platforms/sensors are highlighted.

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Paper No48Publication ID : 473   &   Year : 2011  
TitleInvestigation of XBT and XCTD biases in the Arabian Sea and the Bay of Bengal with implications for climate studies
Authors Boyer, T. , Gopalakrishna, V.V., Reseghetti, F., Naik, A., Suneel, V., Ravichandran, M., Mohammed Ali, N.P., Mohammed Rafeeq, M.M., Anthony Chico, R.
Source Volume 28, Issue 2, February 2011, Pages 266-286, Journal of Atmospheric and Oceanic Technology
AbstractLong time series of XBT data in the Bay of Bengal and the Arabian Sea are valuable datasets for exploring and understanding climate variability. However, such studies of interannual and longer-scale variability of temperature require an understanding, and, if possible, a correction of errors introduced by biases in the XBT and expendable conductivity-temperature-depth (XCTD) data. Two cruises in each basin were undertaken in 2008/09 on which series of tests of XBTs and XCTDs dropped simultaneously with CTD casts were performed. The XBT and XCTD depths were corrected by comparison with CTD data using a modification of an existing algorithm. Significant probe-to-probe fall-rate equation (FRE) velocity and deceleration coefficient variability was found within a cruise, as well as cruise-to-cruise variability. A small (~0.01C) temperature bias was also identified for XBTs on each cruise. No new FRE can be proposed for either the Bay of Bengal or the Arabian Sea for XBTs. For the more consistent XCTD, basin-specific FREs are possible for the Bay of Bengal, but not for the Arabian Sea. The XCTD FRE velocity coefficients are significantly higher than the XCTD manufacturers' FRE coefficient or those from previous tests, possibly resulting from the influence of temperature on XCTD FRE. Mean temperature anomalies versus a long-term mean climatology for XBT data using the present default FRE have a bias (which is positive for three cruises and negative for one cruise) compared to the mean temperature anomalies for CTD data. Some improvement is found when applying newly calculated cruise-specific FREs. This temperature error must be accounted for in any study of temperature change in the regions.

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Paper No49Publication ID : 380   &   Year : 2011  
TitleDiurnal Cycle Induced Amplification of Sea Surface Temperature Intraseasonal Oscillations Over the Bay of Bengal in Summer Monsoon Season
Authors Mujumdar, M.; Salunke, K.;Rao, S. A.; Ravichandran, M.; Goswami, B. N.
Source IEEE Geoscience and Remote Sensing Society, 99, 206-210,
AbstractIn spite of strong mean summer monsoon winds, the magnitudes of diurnal and intraseasonal oscillations (ISO) of the sea surface temperature (SST) in the Bay of Bengal (BoB) are as strong as the respective magnitudes in the western Pacific. Using continuous observations during the peak summer monsoon of 1998 at BoB buoy (DS4) located at (89◦ E, 19◦ N), we show that the strong near-surface diurnal variation in the BoB during warming phases of the ISO leads to almost double the magnitude of the diurnal SST over the BoB as compared to that during the cooling phases. The simulation experiments with and without the diurnal cycle of surface fluxes indicate that more than one-third of the observed SST ISO amplitude could arise from the rectification of the diurnal cycle through the influence of late night and early daytime upper-ocean mixing processes during the warming phases. The rapid shoaling of the upper-ocean mixed layer occurs during afternoon while it deepens slowly during late night and early daytime which tends to retain the warm SSTs at the end of the nighttime cooling. The insight derived from these experiments on the influence of the diurnal cycle on ISOs of the SST underlines the need for a proper simulation of the diurnal cycle of the SST in climate models.

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Paper No50Publication ID : 381   &   Year : 2011  
Title Is there any observational evidence for southward flow of cooler and low salinity waters through the Indo-Sri Lanka Channel during winter?- In press,
Authors R.R. Rao, M. S. Girishkumar, M. Ravichandran, P.Thandahil, V V Gopalakrishna
Source International Journal of Remote Sensing, doi: 10.1080/01431161.2010.523728. Vol. 32, No. 22, 20 November 2011, 73837398
AbstractDuring winter, along the east coast of India, the near-surface flow is characterized by the southward-flowing East India Coastal Current (EICC) which bends around Sri Lanka and enters into the south-eastern Arabian Sea (AS). This current carries cooler, low-salinity waters from the head Bay of Bengal (BoB) into the south-easternAS. But due to a lack of any direct in situ measurements, it is not clear whether any part of this current that flows through the Indo-Sri Lanka Channel (ISLC) is significant. An attempt is made in this study to look for any observational evidence for the southward flow of cooler, low salinity waters through the ISLC during winter. In the absence of direct in situ measurements on the observed currents in the non-navigable shallow ISLC, the observed high resolution, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST), and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll-a and historic sea surface salinity (SSS) data are utilized as tracers to track any southward water flow through the Pamban Pass and Adams Bridge in the ISLC. The analysis suggests that both the non-navigable shallow Pamban Pass and the Adams Bridge in the ISLC act as barriers and limit the southward flow of cooler, low salinity waters into the Gulf of Mannar in the south during winter.

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Paper No51Publication ID : 382   &   Year : 2011  
TitleIntraseasonal variability in barrier layer thickness in the south central Bay of Bengal,
Authors M. S. Girishkumar, M. Ravichandran, McPhadan and R. R. Rao
Source Journal of Geophysical Research-Ocean, 116, C03009, doi: L10.1029/2010JC006657.
AbstractTime series measurements of temperature and salinity recorded at 8N, 90E in the south central Bay of Bengal from a Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction buoy, along with satellite altimetry and scatterometer data, are utilized to describe the seasonal and intraseasonal variability of barrier layer thickness (BLT) during November 2006 to April 2009. The BLT shows strong seasonality with climatological minima during both MarchMay and AugustSeptember and maxima during DecemberFebruary. Large‐amplitude, intraseasonal fluctuations in BLT are observed during September 2007 to May 2008 and during September 2008 to April 2009. The observed intraseasonal variability in BLT is mainly controlled by the vertical movement of isothermal layer depth (ILD) in the presence of a shallow mixed layer. Further, the analysis shows that both ILD and BLT are modulated by vertical stretching of the upper water column associated with westward propagating intraseasonal Rossby waves in the southern bay. These waves are remotely forced by intraseasonal surface winds in the equatorial Indian Ocean.

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Paper No52Publication ID : 202   &   Year : 2010  
TitleInterannual variability of Kelvin wave propagation in the waveguides of the equatorial Indian Ocean, the coastal Bay of Bengal and the southeastern Arabian Sea during 1993-2006 (2009)
Authors R. R. Rao, M. S. GirishKumar, M. Ravichandran, A. R. Rao , V. V. Gopalakrishna, P.Thadathil
Source Volume 57, Issue 1, January 2010, Pages 1-13, Deep-Sea Research Part I: Oceanographic Research Papers, doi:10.1016/j.dsr.2009.10.008
AbstractThe observed variability of the Kelvin waves and their propagation in the equatorial wave guide of the Indian Ocean and in the coastal wave guides of the Bay of Bengal (BoB) and the southeastern Arabian Sea (AS) on seasonal to interannual time scales during the years 1993�2006 is examined utilizing all the available satellite and in-situ measurements. The Kelvin wave regime inferred from the satellite-derived sea surface height anomalies (SSHA) shows a distinct annual cycle composed of two pairs of alternate upwelling (first one occurring during January-March and the second one occurring during August-September) and downwelling (first one occurring during April-June and the second one occurring during October-December) Kelvin waves that propagate eastward along the equator and hit the Sumatra coast and bifurcate. The northern branches propagate counterclockwise over varied distances along the coastal waveguide of the BoB. The potential mechanisms that contribute to the mid-way termination of the first upwelling and the first downwelling Kelvin waves in the wave guide of the BoB are hypothesized. The second downwelling Kelvin wave alone reaches the southeastern AS, and it shows large interannual variability caused primarily by similar variability in the equatorial westerly winds during boreal fall. The westward propagating downwelling Rossby waves triggered by the second downwelling Kelvin wave off the eastern rim of the BoB also shows large interannual variability in the near surface thermal structure derived from SODA analysis. The strength of the equatorial westerlies driven by the east-west gradient of the heat sources in the troposphere appears to be a critical factor in determining the observed interannual variability of the second downwelling Kelvin wave in the wave guides of the equatorial Indian Ocean, the coastal BoB, and the southeastern AS.

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Paper No53Publication ID : 386   &   Year : 2010  
TitleARGO - a decade of progress
Authors Freeland Howard J., Roemmich Dean, Garzoli Silvia L., Le Traon Pierre-Yves, Ravichandran Muthalagu, Riser Stephen, Thierry Virginie, Wijffels Susan, Belboch Mathieu, Gould John, Grant Fiona, Ignazewski Mark, King Brian, Klein Birgit, Mork Kjell Arne, Owens Breck, Pouliquen Sylvie, Sterl Andreas, Suga Toshio, Suk Moon-Sik, Sutton Philip, Troisi Ariel, Vlez-Belchi Pedro Joaquin, Xu Jianping
Source In Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society" Conference (Vol. 2), Venice, Italy, 2125 September 2009, Hall, J., D.E. Harrison, and D. Stammer, Eds., ESA Publication WPP-306
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Paper No54Publication ID : 387   &   Year : 2010  
TitleObserving systems in the Indian Ocean
Authors Masumoto, Y., W. Yu, G. Meyers, N. D'Adamo, L. Beal, W.P.M. de Ruijter, M. Dyoulgerov, J. Hermes, T. Lee, J.R.E. Lutjeharms, J.P. McCreary, Jr., M.J. McPhaden, V.S.N. Murty, D. Obura, C.B. Pattiaratchi, M. Ravichandran, C. Reason, F. Syamsudin, G. Vecchi, J. Vialard, and L. Yu
Source In Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society" Conference (Vol. 2), Venice, Italy, 2125 September 2009, Hall, J., D.E. Harrison, and D. Stammer, Eds., ESA Publication WPP-306, doi:10.5270/OceanObs09.cwp.60.
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Paper No55Publication ID : 388   &   Year : 2010  
TitleThe global tropical moored buoy array
Authors McPhaden, M.J., K. Ando, B. Bourls, H.P. Freitag, R. Lumpkin, Y. Masumoto, V.S.N. Murty, P. Nobre, M. Ravichandran, J. Vialard, D. Vousden, and W. Yu
Source In Proceedings of the "OceanObs'09: Sustained Ocean Observations and Information for Society" Conference (Vol. 2), Venice, Italy, 2125 September 2009, Hall, J., D.E. Harrison, and D. Stammer, Eds., ESA Publication WPP-306.
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Paper No56Publication ID : 389   &   Year : 2010  
TitleThe Role of Altimetry in Coastal Observing Systems
Authors P. Cipollini, J. Benveniste, J. Bouffard, W. Emery, C. Gommenginger, D. Griffin, J. Hyer, K. Madsen, F. Mercier, L. Miller, A. Pascual, M. Ravichandran, F. Shillington, H. Snaith, T. Strub, D. Vandemark, S. Vignudelli, J. Wilkin, P. Woodworth, J. Zavala-Garay
Source in Proceedings of OceanObs09: Sustained Ocean Observations and Information for Society (Vol. 2), Venice, Italy, 21-25 September 2009, Hall, J., Harrison, D.E. & Stammer, D., Eds., ESA Publication WPP-306
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Paper No57Publication ID : 377   &   Year : 2010  
TitleA new approach for deriving temperature and salinity fields in the Indian Ocean using artificial neural networks
Authors Prasad Kumar Bhaskaran, Ravindran Rajesh Kumar, Rahul Barman, Ravichandran Muthalagu
Source J Mar Sci Technol DOI 10.1007/s00773-009-0081-2.
AbstractThis work reports a new methodology for deriving monthly averages of temperature (T) and salinity (S) fields for the Indian Ocean based on the use of an artificial neural network (ANN). Investigation and analysis were performed for this region with two distinct datasets: (1) monthly climatological data for T and S fields (in 1x1grid boxes) at standard depth levels of the World Ocean Atlas 1994 (WOA94), and; (2) heterogeneous randomly distributed in situ ARGO, ocean station data (OSD) and profiling (PFL) floats. A further numerical experiment was conducted with these two distinct datasets to train the neural network model. Nonlinear regression mapping utilizing a multilayer perceptron (MLP) is employed to tackle nonlinearity in the data. This study reveals that a feed-forward type of network with a resilient backpropagation algorithm is best suited for deriving T and S fields; this is demonstrated by independently using WOA94 and in situ data, which thus tests the robustness of the ANN model. The suppleness of the T and S fields derived from the ANN model provides the freedom to generate a new grid at any desired level with a high degree of accuracy. Comprehensive training, testing and validation exercises were performed to demonstrate the robustness of the model and the consistency of the derived fields. The study points out that the parameters derived from the ANN model using scattered, inhomogeneous in situ data show very good agreement with state-of-the-art WOA climatological data. Using this approach, improvements in ocean climatology can be expected to occur in a synergistic manner with in situ observations. Our investigation of the Indian Ocean reveals that this approach can be extended to model global oceans

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Paper No58Publication ID : 378   &   Year : 2010  
TitleObserved intra-seasonal to interannual variability of the upper ocean thermal structure in the southeastern Arabian Sea during 2002-2008
Authors Gopalakrishna, V.V., F. Durand , K.Nisha, M.Lengaigne, T.P.Boyer , J.Costa , R.R.Rao , M. Ravichandran, S.Amrithash, L.John, K.Girish, C.Ravichandran, V.Suneel
Source Deep-Sea Research I, doi:10.1016/j.dsr.2010.03.010.
AbstractThe southeastern Arabian Sea (SEAS), located in the Indian Ocean warm pool, is a key-region of the regional climate system. It is suspected to play an important role in the dynamics of the Asian summer monsoon system. The present study reports the salient features derived from a newly harvested observational dataset consisting of repeated fortnightly XBT transects in the SEAS over the period 2002�2008. The fortnightly resolution of such a multi-year record duration is unprecedented in this part of the world ocean and provides a unique opportunity to examine the observed variability of the near-surface thermal structure over a wide spectrum, from intra-seasonal to interannual timescales. We find that most of the variability is trapped in the thermocline, taking the form of upwelling and downwelling motions of the thermal stratification. The seasonal variations are consistent with past studies and confirm the role of the monsoonal wind forcing through linear baroclinic waves (coastally-trapped Kelvin and planetary Rossby waves). Sub-seasonal variability takes the form of anomalous events lasting a few weeks to a few months and occurs at two preferred timescales: in the 30�110 day band, within the frequency domain of the Madden�Julian oscillation and in the 120�180 day band. While this sub-seasonal variability appears fairly barotropic in the offshore region, the sign of the anomaly in the upper thermocline is opposite to that in its lower part on many occasions along the coast. Our dataset also reveals relatively large interannual temperature variations of about 1 �C from 50 to 200 m depth that reflect a considerable year-to-year variability of the magnitude of both upwelling and downwelling events. This study clearly demonstrates the necessity for sustained long-term temperature measurements in the SEAS.

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Paper No59Publication ID : 379   &   Year : 2010  
TitleIntraseasonal response of mixed layer temperature and salinity in the Bay of Bengal to heat and freshwater flux,
Authors Sindu Raj Parampil, Anitha Gera, M. Ravichandran, and Debasis Sengupta
Source J. Geophys. Res, 115, C05002, doi:10.1029/2009JC005790.
AbstractBuoy and satellite data show pronounced subseasonal oscillations of sea surface temperature (SST) in the summertime Bay of Bengal. The SST oscillations are forced mainly by surface heat flux associated with the active break cycle of the south Asian summer monsoon. The input of freshwater (FW) from summer rain and rivers to the bay is large, but not much is known about subseasonal salinity variability. We use 20022007 observations from three Argo floats with 5 day repeat cycle to study the subseasonal response of temperature and salinity to surface heat and freshwater flux in the central Bay of Bengal. About 95% of Argo profiles show a shallow halocline, with substantial variability of mixed layer salinity. Estimates of surface heat and freshwater flux are based on daily satellite data sampled along the float trajectory. We find that intraseasonal variability of mixed layer temperature is mainly a response to net surface heat flux minus penetrative radiation during the summer monsoon season. In winter and spring, however, temperature variability appears to be mainly due to lateral advection rather than local heat flux. Variability of mixed layer freshwater content is generally independent of local surface flux (precipitation minus evaporation) in all seasons. There are occasions when intense monsoon rainfall leads to local freshening, but these are rare. Large fluctuations in FW appear to be due to advection, suggesting that freshwater from rivers and rain moves in eddies or filaments.

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Paper No60Publication ID : 383   &   Year : 2010  
TitleResponse of subsurface waters in the eastern Arabian Sea to tropical cyclones
Authors Rao A D, Madhu Joshi, Indu Jain and M Ravichandran, (2010)
Source Estuarine, Coastal and Shelf Science, 89, 267-276
AbstractThermister chain data at different depths for June 1998 cyclone in the Arabian Sea at a location (69.2 E,15.5 N) which is about 60 km to the left of the cyclone track indicates subsurface warming below 60 m and inertial oscillations of temperature with a periodicity of about 2 days. The oscillations continued for not, vert, similar15 days even after the cyclone crossed the coast. The analysis of the buoy, DS1 located at the same position also suggests a stabilized southward flow after about two weeks of the cyclone crossed the coast. Analysis of the buoy data for May 1999 cyclone in the same region also indicates similar pattern. In order to investigate the effect of cycloneocean interaction and primarily to understand the process for the subsurface warming, 3-dimensional Princeton Ocean Model is configured for the eastern part of the Arabian Sea. The model uses high horizontal resolution of about 6 km near the coast and a terrain following sigma coordinate in the vertical with 26 levels. The study focuses on surface cooling and temperature rise in the underlying waters and explains its mechanism through upwelling and downwelling respectively. The simulations in concurrence with the observations suggest that the occurrence of subsurface warming precedes the surface cooling with a lag of a day as the cyclone advances DS1. The simulations also demonstrate local temperature stratification plays an important role for cooling of the upper ocean and warming of the subsurface waters and extent of warming is directly related to the depth of the thermocline.

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Paper No61Publication ID : 384   &   Year : 2010  
TitleSonic layer depth variability in the Arabian Sea
Authors Bhaskar, T.V.S.U., Swain, D., Ravichandran, M.
Source International Journal of Oceans and Oceanography, 4 (1) pp. 17-28.
AbstractSpatial and temporal distribution of sonic layer depth (SLD) in the Arabian Sea (AS) was studied using temperature and salinity (T/S) profiles from Argo floats during the years 2003 2004 and World Ocean Atlas 2001 (WOA01) climatology. SLD was obtained from sound velocity profiles computed from T/S data. SLD variability as obtained from Argo matched well with those obtained from the WOA01 in certain locations and showed remarkable difference in some other. SLD variability in the AS is mainly related to seasonal variations in T/S owing to influence of seasonal phenomena as well as the geography of the region. Deeper SLDs were observed during summer monsoon (> 90 m) and winter monsoon (> 80 m) respectively. Up-welling and down-welling (Ekman dynamics) associated with the Findlater Jet controlled SLD during the summer monsoon. While in winter monsoon, cooling and convective mixing regulated SLD in the study region. Weak winds, high insolation and positive net heat flux lead to the formation of thin, warm and stratified sonic layer during pre and post summer monsoon periods, respectively. Examination of SLD in selected areas further revealed clear seasonal changes reflecting strong monsoon signals in the AS.

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Paper No62Publication ID : 209   &   Year : 2009  
TitleNumerical simulation of Bay of Bengal circulation features from Ocean General Circulation model (2009)
Authors Saheb Paul, Arun Chakraborty, P. C. Pandey, Sujit Basu, S. K. Satsangi, and M. Ravichandran, (2009)
Source Marine Geodesy, Vol.32, Issue.1, Pages 1-18.
AbstractThe Bay of Bengal circulation features have been analyzed using a three-dimensional Ocean General Circulation Model (OGCM). The northward propagation of eastern boundary coastal Kelvin wave and its movement throughout the entire Bay is evident from the study. The trapping of Kelvin waves in the central Bay for many months causes warming of the central basin, which acts as a potential breeding ground of cyclones. The seasonal variations of circulation, temperature, and salinity have been reproduced by the model. These variations compare favorably with observations. The existence of a salt finger towards north and its eastward movement are two important results of the study. The cyclonic gyres in the Bay of Bengal during different months agree reasonably well with the SeaWiFS chlorophyll observations and indicate the upwelling zones over the basin. The presence of a subsurface Bay-of-Bengal dipole in summer monsoon months is another important result of the study.

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Paper No63Publication ID : 211   &   Year : 2009  
TitleOcean- Atmosphere Interactions During Cyclone Nargis(2009),
Authors M. J. McPhaden, G. R. Foltz, T. Lee, V. S. N. Murty, M. Ravichandran, G.A. Vecchi, J. Vialard, J. D Wiggert, and L.Yu,
Source EOS, Vol. 90, No. 7.
AbstractOcean- Atmosphere Interactions During Cyclone Nargis(2009)

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Paper No64Publication ID : 210   &   Year : 2009  
TitleObserved low-salinity plume off Gulf of Khambhat, India, during post-monsoon period,(2009)
Authors Rao, A. D., M. Joshi, and M. Ravichandran,
Source Geophys. Res. Lett., 36, L03605, 2009 doi:10.1029/2008GL036091
AbstractFreshwater outflow from river systems, joining the northeastern Arabian Sea suggests that the discharge is maximum during summer monsoon and least during other seasons. However, analysis of climatological monthly salinity and ARGO data indicate, low-saline plumes off Gulf of Khambhat during post-monsoon, despite the fact that there is no river discharge during this period. It is puzzling to understand the existence of relatively low-saline waters in the region that warrants further investigation. Simulations from three-dimensional Princeton Ocean Model (POM) suggest that the opposing winds and the associated onshore coastal currents during July seem to trap the freshwater in the gulf region. As the monsoon withdraws by September, the currents change their direction that would help in releasing the low saline waters to open ocean. Winter cooling of the freshwater also enhances the prospects of temperature inversions.

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Paper No65Publication ID : 376   &   Year : 2009  
TitleArgo - the challenge of continuing 10 years of progress
Authors 17. Roemmich, M. Belbeoch, P.J.V. Belchi, H. Freeland, W.J. Gould, F. Grant, M. Ignaszewski, B. King, B. Klein, K.A. Mork, W.B. Owens, S. Pouliquen, M. Ravichandran, S. Riser, A. Sterl, T. Suga, M.-S. Suk, P. Sutton, V. Thierry, P.-Y. LeTraon, S. Wijffels, J. Xu,
Source Oceanography Magazine, vol 22, 3, 46-55.
AbstractIn only 10 years, the Argo Program has grown from an idea into a functioning global observing system for the subsurface ocean. More than 3000 Argo floats now cover the world ocean. With these instruments operating on 10-day cycles, the array provides 9000 temperature/salinity/depth profiles every month that are quickly available via the Global Telecommunications System and the Internet. Argo is recognized as a major advance for oceanography, and a success for Argo�s parent programs, the Global Ocean Data Assimilation Experiment and Climate Variability and Predictability, and for the Global Earth Observation System of Systems. The value of Argo data in ocean data assimilation (ODA) and other applications is being demonstrated, and will grow as the data set is extended in time and as experience in using the data set leads to new applications. The spatial coverage and quality of the Argo data set are improving, with consideration being given to sampling under seasonal ice at higher latitudes, in additional marginal seas, and to greater depths. Argo data products of value in ODA modeling are under development, and Argo data are being tested to confirm their consistency with related satellite and in situ data. Maintenance of the Argo Program for the next decade and longer is needed for a broad range of climate and oceanographic research and for many operational applications in ocean state estimation and prediction.

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Paper No66Publication ID : 207   &   Year : 2009  
TitleReduced Near-Surface Thermal Inversions in 2005-06 in the Southeastern Arabian Sea (Lakshadweep Sea)(2009)
Authors Nisha, K., S. A. Rao, V. V. Gopalakrishna, R. R. Rao, M. S. Girishkumar, T. Pankajakshan, M. Ravichandran, S. Rajesh, K. Girish, Z. Johnson, M. Anuradha, S. S. M. Gavaskar, V. Suneel, and S. M. Krishna,
Source Journal of Physical Oceanography, 39, 1184-1199
AbstractRepeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 200206. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of nearsurface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification.

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Paper No67Publication ID : 206   &   Year : 2009  
TitleRAMA: The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction, (2009)
Authors M. J. McPhaden, G. Meyers, K. Ando,Y. Masumoto, V. S. N. Murty, M. Ravichandran, F. Syamsudin, J. Vialard, L. Yu, and W. Yu,
Source Bulletin of the American Meteorological Society, vol. 90, No.4, 479-459.
AbstractThe Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25N by the Asian land mass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean-atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the worlds population. The Indian Ocean also remotely influences the evolution of El Nio and the Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and Atlantic hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multi-nationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article describes the scientific rationale, design criteria, and progress towards implementing the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Potential future applications of the data for societal benefit are also described.

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Paper No68Publication ID : 205   &   Year : 2009  
TitleComparison of NCEP turbulent heat fluxes with in situ observations over the south-eastern Arabian Sea (2009)
Authors D. Swain S. H. Rahman and M. Ravichandran
Source Meteorol Atmos Phys, DOI 10.1007/s00703-009-0023-x.
AbstractTurbulent surface heat fluxes (latent and sensible heat) are the two most important parameters through which airsea interaction takes place at the oceanatmosphere interface. These fluxes over the global ocean are required to drive ocean models and to validate coupled oceanatmosphere global models. But because of inadequate in situ observations these are the least understood parameters over the tropical Indian Ocean. Surface heat fluxes also contribute to the oceanic heat budget and control the sea surface temperature in conjunction with upper ocean stratification and ocean currents. The most widely used flux products in diagnostic studies and forcing of ocean general circulation models are the ones provided by the National Centres for Environment Prediction (NCEP) reanalysis. In this study we have compared NCEP reanalysed marine meteorological parameters, which are used for turbulent heat fluxes, with the moored buoy observation in the south-eastern Arabian Sea. The NCEP latent heat flux (LHF) and sensible heat flux (SHF) derived from bulk aerodynamic formula are also compared with that of ship and buoy derived LHF and SHF. The analysis is being carried out during the pre-monsoon and monsoon season of 2005. The analysis shows that NCEP latent as well as sensible heat fluxes are largely underestimated during the monsoon season, however, it is reasonably comparable during the pre-monsoon period. This is largely due to the underestimation of NCEP reanalysis air temperature (AT), wind speed (WS) and relative humidity (RH) compared to buoy observations. The mean differences between buoy and NCEP parameters during the monsoon (pre-monsoon) period are ~21% (~14%) for WS, ~6% (~3%) for RH, and ~0.75% (0.9%) for AT, respectively. The sudden drop in AT during rain events could not be captured by the NCEP data and, hence, large underestimations in SHF. During the pre-monsoon period, major contribution to LHF variations comes from WS, however, both surface winds and relative humidity controls the LHF variations during the monsoon. LHF is mainly determined by WS and RH during the monsoon and, WS is the main contributor during the pre-monsoon.

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Paper No69Publication ID : 204   &   Year : 2009  
Title Comparison of AMSR-E and TMI sea surface temperature with Argo near-surface temperature over the Indian Ocean, (2009)
Authors Udaya Bhaskar, T. V. S., Rahman, S. H., Pavan, I. D., Ravichandran, M. and Nayak, S.(
Source International Journal of Remote Sensing, 30:10, 2669 2684.
AbstractSea surface temperature (SST) measurements from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) are compared with near-surface temperature (foundation SST) in situ measurements obtained from Argo floats over the Indian Ocean. Spatial variation was compared for 2002-2006 and 11 floats were used for temporal variation collocated observations. The results show that TMI and AMSR-E SST measurements are slightly overestimated during the pre-and post-monsoon seasons and underestimated during the monsoon season. Statistical analysis shows that the SST from the AMSR-E is better correlated with the Argo foundation SST compared to the TMI. The standard deviation (SD) and root mean square error (RMSE) for AMSR-E SST are 0.58C and 0.35C, respectively, over the Equatorial Indian Ocean (EIO). The corresponding values for the TMI are 0.66C and 0.47C. Over the Arabian Sea the SD values are slightly higher compared to the EIO values, whereas RMSE values are less for both TMI and AMSR-E SST. These retrieval accuracies are above the expected retrieval accuracy. The seasonal average spatial distribution of AMSR-E SST shows a better match with the Argo foundation SST compared to TMI SST distributions. The robustness of the good spatial match during the monsoon season may be attributed to strong winds.

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Paper No70Publication ID : 203   &   Year : 2009  
TitleVariability of Indian summer monsoon rainfall in daily data from gauge and satellite (2009)
Authors Rahman, S. H., D. Sengupta, and M. Ravichandran
Source J. Geophys. Res., 114, D17113, doi:10.1029/2008JD011694
AbstractIt has long been thought that tropical rainfall retrievals from satellites have large errors. Here we show, using a new daily 1 degree gridded rainfall data set based on about 1800 gauges from the India Meteorology Department (IMD), that modern satellite estimates are reasonably close to observed rainfall over the Indian monsoon region. Daily satellite rainfalls from the Global Precipitation Climatology Project (GPCP 1DD) and the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) are available since 1998. The high summer monsoon (JuneSeptember) rain over the Western Ghats and Himalayan foothills is captured in TMPA data. Away from hilly regions, the seasonal mean and intraseasonal variability of rainfall (averaged over regions of a few hundred kilometers linear dimension) from both satellite products are about 15% of observations. Satellite data generally underestimate both the mean and variability of rain, but the phase of intraseasonal variations is accurate. On synoptic timescales, TMPA gives reasonable depiction of the pattern and intensity of torrential rain from individual monsoon low-pressure systems and depressions. A pronounced biennial oscillation of seasonal total central India rain is seen in all three data sets, with GPCP 1DD being closest to IMD observations. The new satellite data are a promising resource for the study of tropical rainfall variability.

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Paper No71Publication ID : 208   &   Year : 2009  
TitleDevelopment of a New Comprehensive Ocean Atlas for Indian Ocean utilizing ARGO Data (2009)
Authors B. Prasad Kumar, Rahul Barman, S. K. Dube, P. C. Pandey, M. Ravichandran and Shailesh Nayak
Source Int. J. Climatology., 2009 doi: 10.1002/joc.188
AbstractThe World Ocean Atlas (WOA), also termed Levitus Climatology, is a global ocean climatology containing monthly, seasonal and annual means of temperature (T) and salinity (S) fields at standard ocean depths. The monthly climatology for T and S is available for standard depths up to 1000 m. The database used in the preparation of this climatology (WOA) are historical records of Conductivity, Temperature and Depth (CTD) casts and other available marine observations collected in the past. The methodology used in preparation of this WOA is objective analysis which is essentially non-synoptic and widely scattered in the space domain. We understand that ARGO data has so far not been blended with WOA, nor has its impact for improving WOA climatology been attempted. Presently, with the wealth of marine data from ARGO profilers in the Indian Ocean, we propose a new approach to reconstruct T and S fields optimally utilizing the ARGO data. Here we develop a new model using Delaunay Tessellation with QHull algorithm delivering three-dimensional T and S fields from a non-uniform scattered database up to a depth of 1000 m. For gaps in a data-sparse region, we use all available quality-checked Ocean Station Data (OSD) and Profiling Float Data (PFL) information on T and S, in addition to the existing ARGO data. The initiative here was to replace WOA data points with realistic information from ARGO and in situ data, thereby producing a new climatology atlas. We demonstrate the robustness of our approach, and the final climatology on T and S is better compared with the existing state-of-the-art WOA. The advantage of the proposed methodology is the scope of improving the ocean atlas with the addition of more ARGO data in the near future. The clustered approach in modelling enables ocean parameter retrieval in geometrically disconnected regions with an option for hot restart. We believe that the new climatology will benefit the research community immensely.

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Paper No72Publication ID : 212   &   Year : 2008  
TitleSeasonal variability of Sonic layer Depth in the Central Arabian Sea,(2008)
Authors Udaya Bhaskar, D. Swain and M. Ravichandran,
Source Ocean Science Journal, Vol. 43, No.3, 147-152.
AbstractThe seasonal variability of sonic layer depth (SLD) in the central Arabian Sea (CAS) (0 to 25N and 62-66E) was studied using the temperature and salinity (T/S) profiles from Argo floats for the years 2002-2006. The atmospheric forcing responsible for the observed changes was explored using the meteorological data from NCEP/NCAR and Quickscat winds. SLD was obtained from sound velocity profiles computed from T/S data. Net heat flux and wind forcing regulated SLD in the CAS. Up-welling and down-welling (Ekman dynamics) associated with the Findlater Jet controlled SLD during the summer monsoon. While in winter monsoon, cooling and convective mixing regulated SLD in the study region. Weak winds, high insolation and positive net heat flux lead to the formation of thin, warm and stratified sonic layer during pre and post summer monsoon periods, respectively.

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Paper No73Publication ID : 215   &   Year : 2008  
TitleA cold pool south of Indo-Sri Lanka Channel and its intrusion into the southeastern Arabian Sea during winter,(2008)
Authors R. R. Rao, M. S. Girish Kumar, M. Ravichandran, V.V.Gopalakrishna and P.Thadathil,
Source Deep Sea Research I, 55, 1009-1020
AbstractDuring winter, south of the Indo-Sri Lanka Channel (ISLC), the observed sea-surface temperature (SST) distribution shows a distinct mini-cold pool (MCP) with relatively cooler waters (SSTo28 1C). All the available satellite and in-situ measurements are utilized to characterize and explain the mechanisms that govern the evolution of the observed MCP. During DecemberJanuary, the northeasterly surface winds blow through the ISLC manifesting a patch of strong winds in the south with peak intensity of about 10m/s, enhance surface turbulent heat losses and drive near-surface vertical mixing resulting in the observed cooling. The vertical temperature profiles in this region also show cooling and deepening of the near-surface isothermal layer from November to January. This cooling occurs episodically on an intra-seasonal time scale with a typical periodicity of 815 days and is stronger when the surface winds intensify, surface net heat losses are larger and the near-surface circulation is more pronounced. The cooling episodes varied in number, intensity, duration and spatial extent in each winter during 19982006. The cooler surface waters from this MCP flow initially southwestward and are then topographically steered northwestward by the Maldives Island Chain. The resultant near-surface circulation also appears to strengthen the amplitude of the near-surface thermal inversions observed in the SouthEastern Arabian Sea (SEAS).

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Paper No74Publication ID : 216   &   Year : 2008  
TitleSurface buoyancy flux in Bay of Bengal and Arabian Sea (2008)
Authors G. Anitha, M. Ravichandran, and R. Sayanna ,
Source Ann. Geophys., 26, 395-400,
AbstractThe seasonal variation of thermal, haline, net surface buoyancy flux, the Monin-Obukhov length (M-O length, L) and stability parameter, i.e. the ratio of M-O length to mixed layer depth (h) were studied in the Bay of Bengal (BoB) and the Arabian Sea (AS) for the years 2003 and 2004 using Argo temperature and salinity profiles. The relative quantitative influence of winds to surface buoyancy and the applicability of scaling mixed layer using M-O length in BoB and AS was brought out. Rotation and light penetration modify the mixed layer depth from M-O length during shoaling in spring giving L/h<1.

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Paper No75Publication ID : 213   &   Year : 2008  
TitleOceanic upwelling and downwelling processes in waters off the west coast of India(2008)
Authors Rao, A.D., Joshi, M., Ravichandran, M.,
Source Ocean Dynamics, 58 (3-4), 213-226.
AbstractA three-dimensional model based on the Princeton Ocean Model (POM) has been implemented to study the circulation of the west coast of India. The model uses a curvilinear orthogonal horizontal grid with higher resolution near the coast (39 km) and a terrain following sigma coordinate in the vertical. The model is able to simulate Lakshadweep High and Lakshadweep Low (LL) during the winter and summer monsoons, respectively. During winter, the downwelling processes noticed along the coast help in the formation of temperature inversions. The inversions can be seen even up to the depths of ∼50 m, which agrees with the available ARGO data in the region. Model simulations show that coastal upwelling off Kerala is at its peak in July. The intensity of upwelling reduces along the coast towards north. During the existence of LL, there is a cyclonic eddy in the sub-surface waters over the South-Eastern Arabian Sea, with vertical extent up to the depths of 100150 m and it is strengthened due to the presence of northward counter current in the shelf region. The southerly coastal jet formed along the southern coast as a result of upwelling is noticed a westward shift along with LL. The location of the eddy off Kerala is tilted towards the open ocean with depth and our experiments suggest that this flow can be understood as a first baroclinic mode.

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Paper No76Publication ID : 214   &   Year : 2008  
TitleObserved anomalous upwelling in the Lakshadweep Sea during the summer monsoon season of 2005(2008)
Authors Gopalakrishna, V. V., R. R. Rao, K. Nisha, M. S. Girishkumar, T. Pankajakshan, M. Ravichandran, Z. Johnson, K. Girish, N. Aneeshkumar, M. Srinath, S. Rajesh, and C. K. Rajan ,
Source J. Geophys. Res., 113, C05001, doi:10.1029/2007JC004240
AbstractRepeat near-fortnightly expendable bathythermograph (XBT) transects made along Kochi-Kavaratti (KK) shipping lane in the Lakshadweep Sea (LS) during 20022006 are examined to describe the observed year-to-year variability of upwelling during summer monsoon season (SMS). Among all the years, the upwelling characterized by up-sloping of 25_C isotherm is relatively weaker and persisted until November during SMS of 2005 and is stronger during the SMS of 2002. As a result of prolonged upwelling, the sea surface temperature has shown cooling extending into the postmonsoon season. The estimated marine pelagic fish landings along the southwest coast of India (SWCI) have also shown increase until December. The governing mechanisms both in terms of local and remote forcings are examined to explain the observed anomalous upwelling during SMS of 2005. The equatorward alongshore wind stress (WS) along the KK XBT transect persisted in a transient manner beyond September only during SMS of 2005. The westerly wind bursts over the equator during the winter of 20042005 are both short-lived and relatively weaker triggering weaker upwelling Kelvin waves that propagated into LS in the following SMS of 2005. The observed distribution of negative sea surface height anomaly in the LS is relatively weaker during the SMS of 2005 and lasted longer. The correlation analysis suggests that the local alongshore WS off the SWCI and the remote forcing from the southern coast of Sri Lanka has greater influence on the observed interannual variability of upwelling in the LS when compared to the remote forcing from the equator.

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Paper No77Publication ID : 217   &   Year : 2007  
TitleMixed layer variability in Northern Arabian Sea as detected by an Argo float (2007)
Authors Udaya Bhaskar, D. Swain and M. Ravichandran,
Source Ocean Science Journal, Vol. 42, No.4, 241-246.
AbstractSeasonal evolution of surface mixed layer in the Northern Arabian Sea (NAS) between 17 N20.5 N and 59 E69 E was observed by using Argo float daily data for about 9 months, from April 2002 through December 2002. Results showed that during April - May mixed layer shoaled due to light winds, clear sky and intense solar insolation. Sea surface temperature (SST) rose by 2.3 oC and ocean gained an average of 99.8 Wm-2. Mixed layer reached maximum depth of about 71 m during June - September owing to strong winds and cloudy skies. Ocean gained abnormally low ~18Wm-2 and SST dropped by 3.4 oC. During the inter monsoon period, October, mixed layer shoaled and maintained a depth of 20 to 30 m. November -December was accompanied by moderate winds, dropping of SST by 1.5 oC and ocean lost an average of 52.5 Wm-2. Mixed layer deepened gradually reaching a maximum of 62 m in December. Analysis of surface fluxes and winds suggested that winds and fluxes are the dominating factors causing deepening of mixed layer during summer and winter monsoon periods respectively. Relatively high correlation between MLD, net heat flux and wind speed revealed that short term variability of MLD coincided well with short term variability of surface forcing

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Paper No78Publication ID : 219   &   Year : 2006  
TitleInferring mixed-layer depth variability from Argo observations in the western Indian Ocean (2006)
Authors T. V. S Uday Bhaskar, D. Swain, M. Ravichandran,
Source Journal of Marine Research, 64, 393406.
AbstractThe seasonal and spatial variability of mixed layer depth (MLD) was examined in the Western Indian Ocean (WIO) (30E 80E and 10S 30N) for three consecutive years starting from June 2002 May 2005 using Argo temperature and salinity (T/S) profiles. These were compared with MLD estimates from World Ocean Atlas 2001 (WOA01) T/S data. Temporal and spatial variability of MLD estimated from Argo T/S profiles were found to correspond well with the MLD obtained from WOA01 T/S data. However, slight deviations in the form of months of occurrence of minima and maxima MLDs were observed. MLD from WOA01 climatology is underestimated compared to MLD from Argo for almost the entire three years of study. It is also observed that MLD variability features as brought out by both the data sets followed the dynamics that govern the mixed layer variability in this region.

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Paper No79Publication ID : 218   &   Year : 2006  
TitleIndian Moorings: Deep-sea current meter moorings in the Eastern Equatorial Indian Ocean(2006)
Authors Murty, V.S.N., M.S.S. Sarma, A. Suryanarayana, D. Sengupta , A. S. Unnikrishnan, Vijayan Fernando, Anselm Almeida, S. Khalap, Areef Sardar, K. Somasundar and M. Ravichandran,
AbstractIndian Moorings: Deep-sea current meter moorings in the Eastern Equatorial Indian Ocean(2006)

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Paper No80Publication ID : 221   &   Year : 2006  
TitleObserved mini-cold pool off the southern tip of India and its intrusion into the south central Bay of Bengal during summer monsoon season(2006)
Authors R. R. Rao, M. S. Girish Kumar, M. Ravichandran, B. K. Samala, and Nandkumar Sreedevi
Source Geophys. Res. Letters, Vol. 33, L06607.
AbstractThe observed mini-cold pool (MCP) off the southern tip of India (STI) and its intrusion into the south central Bay of Bengal (BoB) during the summer monsoon season is examined utilizing the available satellite and in situ measurements. The mechanisms that govern the occurrence of this MCP through surface wind field and its curl, net surface heat flux and divergence in the near-surface circulation (Ekman + geostrophic) are examined. The observed MCP primarily appears to be driven by the upwelling caused by the divergence in the near-surface circulation. The upwelling results in both shoaling of thermocline and enhanced blooming of chlorophyll-a. With the progress of the season the sea surface temperature (SST) cooling associated with MCP intrudes into the south central BoB by the Summer Monsoon Current (SMC).

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Paper No81Publication ID : 220   &   Year : 2006  
TitleObserved intraseasonal variability of mini-cold pool off the southern tip of India and its intrusion into the south central Bay of Bengal during summer monsoon season (2006)
Authors R. R. Rao, M. S. Girish Kumar, M. Ravichandran, B. K. Samala, and G. Anita,
Source Geophys. Res. Letters, Vol. 33, L06607
AbstractThe observed evolution of a mini-cold pool (MCP) off the southern tip of India (STI) and its intrusion into the south central Bay of Bengal (BoB) during the summer monsoon season shows pronounced cooling episodes on intraseasonal time scale with differences in their number, intensity, duration and spatial extent. This variability that changes from year to year appears to be primarily determined by the corresponding variability in the upwelling driven by the divergence in the near-surface (Ekman + geostrophic) circulation and wind induced mixing. The signature of this cooling carried by the Summer Monsoon Current (SMC) is seen with reduction in intensity in the south central BoB limited mostly only to south of about 10_N. In the background of slow cooling caused by SMC, the cooling episodes of different amplitudes occur in the south central BoB suggesting that spatially variable wind forcing is responsible for producing these episodes simultaneously on intraseasonal time scale

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Paper No82Publication ID : 222   &   Year : 2005  
TitleSignature of active and break phases of ISM in subsurface argo data(2005)
Authors Anita, G, M. Ravichandran and R. Sayanna, ,
Source Journal of Environmental Geochemistry, Vol.8, 237-243.
AbstractSignature of active and break phases of ISM in subsurface argo data

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Paper No83Publication ID : 224   &   Year : 2004  
TitleResults from the First Argo float deployed by India (2004)
Authors M. Ravichandran, P. N. Vinaya chandran, Sudheer Joseph and K. Radhakrishnan,
Source Current Science, Vol. 86, No. 5, 651-659.
AbstractArgo is a revolutionary concept in ocean observation system that envisages real-time sampling of the temperature and salinity profiles of the global oceans at an approximate spatial resolution of 300 km, once in ten days. Argo float is an autonomous drifting profiler that pops up and down in the ocean from the surface up to 2000 m, measuring two most important physical properties of the water column, viz. temperature and salinity as a function of pressure (depth). Moreover, it can provide an estimate of currents both at the surface and at the parking depth in near-real time. India is an active participant in the Argo programme and has already deployed 31 out of 150 proposed floats. In this article, we describe the data received from the first Argo float deployed by India in the southeastern Arabian Sea. The data demonstrate that the temperature and salinity profiles from Argo floats present possibilities for oceanographic studies and spatial and temporal scales that had been hitherto impossible.

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Paper No84Publication ID : 223   &   Year : 2004  
TitleA new technique to determine the lightning charge location from the electric field vector measurements(2004)
Authors M. Ravichandran and A. K. Kamra
Source Journal of Atmospheric and Solar Terrestrial Physics (JATP), 66, 349-362.
AbstractA new technique to find out the magnitude and location of the net charge center and the charge destroyed in a lightning flash in a thundercloud has been proposed. The technique is based on the measurements of the electric field vector on the ground surface during a lightning flash. The technique has the advantage of field measurements being made at only one station if simultaneous measurements for the distance of lightning are made with time-to-thunder technique. From our measurements made with a spherical field meter with Maxwells current density, typical cases of cloud-to-cloud and cloud-to-ground discharges are analyzed. The values of above parameters calculated from this technique are within the normal range of these variables in thunderclouds inferred from other techniques. However, the charge values show significant change when the electric field vector instead of only the vertical electric field measured by conventional field mill is used for the calculations.

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Paper No85Publication ID : 227   &   Year : 2001  
TitleResponse of the Bay of Bengal to Gopalpur and Paradip super cyclones during 15-31 October 1999 (2001)
Authors Chintalu G.R., Seetaramayya P., Ravichandran M. and Mahajan P.N
Source Current Science, 81, 283-291.
AbstractResponse of the Bay of Bengal to two tropical cyclones, i.e. Gopalpur and Paradip super cyclones, during 1531 October 1999, is studied using a stationary mooring buoy for marine meteorological observations. The National Institute of Ocean Technology (NIOT, Chennai) has deployed this buoy at 13N, 87E, by fixing various meteorological instruments and sensors to acquire sea surface temperature (SST), air temperature (Ta), wind speed (Ws), wind direction (Wd) and ocean currents (Cs) using remote sensing technique through INSAT-1D satellite at an interval of 3 h. The results of the analysis of the above parameters have shown clearly a response (SST difference between before and after formation) of about 0.7C for the Gopalpur cyclone and 0.9C for the Paradip cyclone. Ta has shown rapid variations following the rapid movement of cloud decks across the buoy during the cyclone period. The observed changes in the wind speed and direction are in concurrence with analysed mean sea level pressure oscillations. Finally, this study recommends more buoy based marine meteorological observations over this region and the neighbouring areas, where the tropical cyclones generally occur and subsequently hit the Coromandal coast.

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Paper No86Publication ID : 225   &   Year : 2001  
TitleBOBMEX - The Bay of Bengal Monsoon Experiment(2001)
Authors Bhat G. S., S. Gadgil, P. V. Hareesh Kumar, S. R. Kalsi, P. Madhusoodanan, V. S. N. Murty, C. V. K. Prasada Rao, V. Ramesh Babu, L. V. G. Rao, R. R. Rao, M. Ravichandran, K. G. Reddy, P. Sanjeeva Rao, D. Sengupta, D. R. Sikka, J. Swain and P. N. Vinayachandran.
Source Bull. Am. Met. Soc., 82, 10, 2217-2243
AbstractThe first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during JulyAugust 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented. During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CAPE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 23 kJ kg−1 following convection, and recovered in a time period of 12 days. The surface wind speed was generally higher than 8 m s−1.The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

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Paper No87Publication ID : 226   &   Year : 2001  
TitleOscillations of Bay of Bengal sea surface temperature during the 1998 summer monsoon(2001)
Authors Sengupta, D. and M. Ravichandran
Source Geophysical Research Letters, 28, 10,2033-2036
AbstractNew measurements from moored buoys in the Bay of Bengal, along with satellite cloud data, reveal strong monsoon intraseasonal oscillations (ISO) during the summer of 1998. The active phase of the monsoon is marked by high surface wind and deep atmospheric convection. The buoy data show that sea surface temperature (SST) in the Bay of Bengal warm pool rises and falls with periods of weeks. These intraseasonal oscillations of SST are not adequately captured in a satellite derived weekly SST analysis. They are a direct response to ISO of net surface heat ux into the ocean, which is negative in the active phase of the monsoon and positive in the quiescent phase. Fresh water from rivers and rain appears to control northern Bay of Bengal SST in late summer by allowing sunlight to escape below a shallow mixed layer.

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Paper No88Publication ID : 229   &   Year : 1999  
TitleA spherical field meter to measure the electric field vector ; Measurements in fair-weather and inside a dust devil (1999)
Authors Ravichandran, M. and A. K. Kamra
Source Review of Scientific Instruments, vol.70, Issue 4, 2140-2149.
AbstractA spherical field meter has been developed to measure the atmospheric electric field vector above the ground surface. Its calibrations in the radial and linear electric fields and the method to find out the distortion coefficients are described. Extensive measurements of the components of the atmospheric electric field and the resulting electric field vector in fair weather and during the passage of a small dust devil over the spherical field meter are reported. Observations made at Pune (18�32 N, 73�51 E, 559 m above msl), show that in fair weather the horizontal component of the electric field may be upto 5% of the vertical component. Variations in the horizontal components of the electric field when averaged over an interval of more than 4 s are small and are generally but not always identical to its vertical components. However, at a frequency of few Hertz, variations in the horizontal components may be comparatively larger and generally independent of the vertical components. Observations made during the passage of a small dust devil over the spherical field meter show the development of appreciable horizontal component of electric field above the ground surface. These observations demonstrate the capability and reliability of the spherical field meter to measure the magnitude and direction of the electric field vector. They also demonstrate that the movement of small space charge pockets close to the surface may cause the development of horizontal component of the electric field

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Paper No89Publication ID : 228   &   Year : 1999  
TitleFirst results from a new observational system over the Indian Seas (1999)
Authors Premkumar, K., M. Ravichandran, S.R. Kalsi, D. Sengupta, S. Gadgil.
Source Current Science, 78, 323-330.
AbstractThe Department of Ocean Development has recently installed several moored metocean buoys in the Bay of Bengal and Arabian Sea to measure sea surface temperature (SST) and other near-surface atmospheric and oceanic variables every three hours. We present here a preliminary analysis of the data from one buoy in the Arabian Sea and three buoys in the Bay of Bengal, during JuneAugust 1998, along with daily INSAT data on outgoing longwave radiation at the buoy locations. Data show that SST dropped abruptly in the eastern Arabian Sea during passage of the June 1998 cyclone. In the Bay of Bengal, SST shows prominent intraseasonal oscillations at all three buoy locations. The SST variation seems to be determined by variations in cloudiness and wind speed at the buoy location.

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Paper No90Publication ID : 230   &   Year : 1993  
TitleOn the Assumption of the Earth's Surface as a perfect Conductor in Atmospheric Electricity, (1993)
Authors Kamra, A. K and M. Ravichandran
Source Journal of Geophysical Research, Vol. 98, No.D12, 22,875-22,885
AbstractThe assumption of considering the Earths surface as a plane perfect conductor is examined. The electric field produced due to a point charge or a model thundercloud in the atmosphere and the field changes produced due to a leader in a lightning flash have been calculated if the Earths surface, instead of a perfect conductor, is considered as a dielectric of finite dielectric constant and zero conductivity. It is shown that when the dielectric constant of the surface is <10, the horizontal and vertical components of electrical field may be comparable to each other at comparatively large distances from the charge. The net electric field in such a case may be inclined from the vertical by as much as 45 or even more. At different distances, the field changes caused due to the leader in a lightning flash change not only its magnitude but direction as well. Certain igneous and metamorphic rocks with low moisture contents, fine-grained sand when it is dry or has a moisture content of <1% or is at low temperatures, and ice at very low temperatures of <−50 C or oriented such that its c axis is perpendicular to the direction of electric field have dielectric constant values <10 and conductivity <10−9 S/m. Therefore, for some regions , such as polar regions, high-latitude regions covered with sea ice or dry sand, hot and dry regions covered with sand in the midst of vast deserts, and regions covered with hot and dry igneous and metamorphic rocks, the average representative values of dielectric constant and conductivity of the surface may be such that the Earths surface at those places cannot be assumed as a perfect conductor. Consequently, the assumption of treating the Earths surface as an insulating surface generally taken for fast field changes during a stepped leader in a lightning flash, may be extended for much slower field changes occurring even in fair weather. It needs to be mentioned, however, that thunderstorm activity is generally minimal in most of the regions mentioned above. It is proposed that instead of measuring only the vertical component of electric field, as is normally done, a measurement of the electric field vector will give a full description of the intensity of the electric field at some places

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