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Paper No1Publication ID : 68   &   Year : null  
TitleDroughts of the Indian summer monsoon: Role of clouds over the Indian Ocean
Authors Sulochana Gadgil, P. N. Vinayachandran and P. A. Francis
SourceCURRENT SCIENCE, VOL. 85, NO. 12, 25 DECEMBER 2003
AbstractComparison of the evolution of the normal summer monsoon of 2003 with the unanticipated drought of 2002 led to this investigation of the role of the deep convection in the atmosphere (deep cloud systems) over the equatorial Indian Ocean. We find that the link of the Indian monsoon to events over the equatorial Indian Ocean is as important as the well-known link to the dramatic events over the Pacific (El Niño Southern Oscillation; ENSO). Over the equatorial Indian Ocean, enhancement of deep convection in the atmosphere over the western part is generally associated with suppression over the eastern part and vice versa. We call the oscillation between these two states, which is reflected in the pressure gradients and the wind along the equator, the Equatorial Indian Ocean Oscillation (EQUINOO). We show that every season with excess rainfall/drought during 1979–2002 can be ‘explained’ in terms of favourable/unfavourable phase of either this oscillation or the ENSO or both. In particular, in the monsoon season of 2002 while the ENSO phase was unfavourable but weak, the phase of the EQUINOO was highly unfavourable and a large deficit occurred in the monsoon rainfall.

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Paper No2Publication ID : 67   &   Year : null  
TitleExtremes of the Indian summer monsoon rainfall, ENSO and equatorial Indian Ocean oscillation
Authors Sulochana Gadgil, P. N. Vinayachandran, P. A. Francis and Siddhartha Gadgil
SourceGeophys. Res. Lett., 31, L12213, doi:10.1029/2004GL019733.
Abstract It is well known that anomalies of the Indian Summer Monsoon Rainfall (ISMR) are linked to El Niño and Southern Oscillation (ENSO). We show that large anomalies of the ISMR are also linked to the Equatorial Indian Ocean Oscillation (EQUINOO) between states with enhancement/suppression of atmospheric convection over the western part of the equatorial Indian Ocean with suppression/enhancement over the eastern part and associated changes in the anomaly of the zonal wind along the equator. EQUINOO is the atmospheric component of the coupled Indian Ocean Dipole mode. There is a strong relation between the large anomalies of ISMR and a composite index which is a linear combination of the indices for ENSO and EQUINOO with all seasons with large deficits (excess) characterized by small (large) values of the index. However, the variation of ISMR within one standard deviation is more complex and does not appear to be related to the composite index.

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Paper No3Publication ID : 66   &   Year : null  
TitleIntense rainfall events over the west coast of India
Authors Francis, PA and Gadgil, Sulochana
SourceMeteorology and Atmospheric Physics, 94 (1-4). pp. 27-42.
AbstractThe west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm $day^{-1}$ at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment,as carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between $14^\circ N-16^\circ N$ and near $19^\circ N$. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.

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Paper No4Publication ID : 65   &   Year : null  
TitleTriggering of the positive Indian Ocean dipole events by severe cyclones over the Bay of Bengal
SourceTellus A, Volume 59 Issue 4, Pages 461 - 475
AbstractIn this paper, we suggest that positive Indian Ocean dipole events could be triggered by the occurrence of severe cyclones over the Bay of Bengal during April–May. All positive Indian Ocean dipole events during 1958–2003 are preceded by at least one such severe cyclone. Severe cyclones over the Bay of Bengal strengthen the meridional pressure gradient across the eastern equatorial Indian Ocean (EEIO) and hence lead to the intensification of the upwelling favourable southeasterlies along the Sumatra coast. Severe cyclones can also lead to a decrease in the integrated water vapour content and suppress convection over the EEIO. We suggest that the suppression of convection over the EEIO in turn, leads to the enhancement of convection over the western equatorial Indian Ocean (WEIO) and hence to the weakening of the westerlies along the central equatorial Indian Ocean (CEIO). This can lead to a positive feedback between suppression of convection over the EEIO and enhancement of the convergence and convection over the WEIO. If this positive feedback continues until the winds over the CEIO become easterlies, the convection over the EEIO remains suppressed for a period much longer than the synoptic scale. The strong upwelling caused by the easterlies along the equator and southeasterlies along the Sumatra coast decreases the sea surface temperature of the EEIO very rapidly and a positive dipole event gets triggered

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Paper No5Publication ID : 64   &   Year : null  
TitleMonsoon variability : links to major oscillations over the equatorial Pacific and Indian Ocean
Authors S. Gadgil, M. Rajeevan and P.A.Francis
SourceCurrent Science, 2007, 93, 182-194.
AbstractIn this article, we first discuss our perception of the factors which are critical for inter-annual variation of the Indian summer monsoon rainfall and the major milestones leading to this understanding. The nature of the two critical modes for monsoon variability, viz. El Nino Southern Oscillation and equatorial Indian Ocean Oscillation is considered and their links to the monsoon elucidated. We suggest possible reasons for the rather poor skill of simulation of the interannual variation of the Indian summer monsoon rainfall by atmospheric general circulation models, run with the observed sea surface temperature as boundary condition. We discuss implications of what we have learned for the monsoon of 2006, and possible use of information on the two important modes for prediction of the rainfall in all or part of the summer monsoon season. We conclude with our view of what the focus of research and development should be for achieving a substantial improvement in the skill of simulation and prediction of the Indian summer monsoon rainfall in the near future.

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Paper No6Publication ID : 63   &   Year : null  
TitleIndian ocean dipole: Processes and impacts
SourceIndian Academy of Sciences, Platinum Jubilee, P1, pp. 1–21
AbstractEquatorial Indian Ocean is warmer in the east, has a deeper thermocline and mixed layer, and supports a more convective atmosphere than in the west. During certain years, the eastern Indian Ocean becomes unusually cold, anomalous winds blow from east to west along the equator and southeastward off the coast of Sumatra, thermocline and mixed layer lift up and the atmospheric convection gets suppressed. At the same time, western Indian Ocean becomes warmer and enhances atmospheric convection. This coupled ocean-atmospheric phenomenon in which convection, winds, sea surface temperature (SST) and thermocline take part actively is known as the Indian Ocean dipole (IOD). Propagation of baroclinic Kelvin and Rossby waves, excited by anomalous winds, play an important role in the development of SST anomalies associated with the IOD. Since mean thermocline in the Indian Ocean is deep compared to the Pacific, it was believed for a long time that the Indian Ocean is passive and merely responds to the atmospheric forcing. Discovery of the IOD and studies that followed demonstrate that the Indian Ocean can sustain its own intrinsic coupled ocean-atmosphere processes. About 50% percent of the IOD events in the past 100 years have co-occurred with El Nino Southern Oscillation (ENSO) and the other half independently. Coupled models have been able to reproduce IOD events and process experiments by such models – switching ENSO on and off – support the hypothesis based on observations that IOD events develop either in the presence or absence of ENSO. There is a general consensus among different coupled models as well as analysis of data that IOD events co-occurring during the ENSO are forced by a zonal shift in the descending branch over to the eastern Indian Ocean. Processes that initiate the IOD in the absence of ENSO are not clear, although several studies suggest that anomalies of Hadley circulation are the most probable forcing function. Impact of the IOD is felt in the vicinity of Indian Ocean as well as in remote regions. During IOD events, biological productivity of the eastern Indian Ocean increases and this in turn leads to death of corals over a large area. Moreover, the IOD affects rainfall over the maritime continent, Indian subcontinent, Australia and eastern Africa. The maritime continent and Australia suffer from deficit rainfall whereas India and east Africa receive excess. Despite the successful hindcast of the 2006 IOD by a coupled model, forecasting IOD events and their implications to rainfall variability remains a major challenge, as is, understanding reasons behind an increase in frequency of IOD events in recent decades.

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Paper No7Publication ID : 61   &   Year : null  
TitleThe aberrant behaviour of Indian monsoon in June 2009
Authors Francis P. A. and Sulochana Gadgil
SourceCurrent Science
AbstractIn this note, we present in brief, the results of a study aimed at identifying the factors which could have led to the unusual behaviour of the monsoon in June 2009. We consider first the special features of the monsoon of June 2009 in the light of what we know about the nature of variability of the Indian summer monsoon.

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Paper No8Publication ID : 62   &   Year : 2010  
TitleTowards understanding the unusual Indian monsoon in 2009
Authors Francis P. A. and Sulochana Gadgil
SourceJ. Earth System Science
AbstractIn press

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