Around Sri Lanka and to the south of India sea surface temperatures (SST) are cooler compared to the surrounding region during summer monsoon. This region where SSTs are relatively cooler is known as the cold pool. Owing to its possible impact on monsoon variability, some studies have been carried out to understand the evolution of cold pool SST during this period. These studies suggest, coastal upwelling along southern coast of Sri Lanka and eastward advection of cooler water contributes to the decrease in SST during summer monsoon. However, the processes leading to the formation of cold pool, still, remain unknown. In this study, we have investigated the mechanism responsible for the formation and maintenance of southern Bay of Bengal (BOB) cold pool using high resolution satellite data, model simulations and in-situ observations for the year 2009. Our study reveals formation of cold pool is dominated by atmospheric processes, whereas oceanic processes dominate its maintenance. Cooling of SSTs during premonsoon and onset phase acts as a prerequisites for the formation of cold pool, which are linked to the reduction in Net Heat flux (NHFX) during theses periods. The changes in NHFX during premonsoon and onset phase are dominated by reduction in Short-wave (SW) radiation associated with strong convective activity over cold pool. Convective activity over the cold pool are associated with the northward movement of Maximum Cloud Zone (MCZ) that forms over Equatorial Indian Ocean (EIO) during these periods. SST within the cold pool after the steady increase during February-April months, cools first during premonsoon rain event and then during monsoon onset. Analysis of high resolution satellite data for the period 2003-2009 suggest that, these sequence of events occurs with minor amount of inter-annual variability. Lead-lag correlation also made it clear that SST response in 5 days to the corresponding variation in atmospheric processes. SST within the cold pool shows several intraseasonal cooling events during the summer mon-soon. Considering that rainfall above the cold pool is very low during the summer monsoon, these cooling events occurring within the summer should be necessary for maintaining the cold pool. The seasonal evolution of SST shows that it continues to decrease till the end of the summer monsoon. In-situ data collected during CTCZ field program in 2009, at two time series locations (TSL) and model simulations were used to determine the processes responsible for such cooling events. To estimate the contribution from advection to the observed SST tendency at fixed location, a measurement stratergy called ‘opertaion advection’ was used in this study. This stratergy involves measurement of oceanographic parameters along four edges from TSL directod along North, South, East and West for estimation of horizontal temperature gradients. Our results from SST cooling events captured by CTD at two fixed locations suggests that horizontal advection and entrainment dominate the SST evolution. Model temperature equation evaluated near the TSLs are convinient with the observations and suggest that atmospheric forcing is not responsible for intraseasonal cooling events.

Ce travail avait pour but de reconstituer l'évolution de la température et du δ¹⁸O des eaux de surface et des eaux de la thermocline dans la Warmpool indo-pacifique (IPWP) en combinant la thermométrie Mg / Ca et la mesure des isotopes stables de l'oxygène sur des foraminifères planctoniques de surface et de sub-surface prélevés dans des carottes de sédiments situées dans l'océan Indien tropical oriental. Ce travail a permis de ré-évaluer les effets des différentes méthodes de nettoyage et de la dissolution in situ sur la thermométrie Mg/Ca des foraminifères planctoniques, mettant en évidence la nécessité de corrections différentes suivant les espèces. L’évolution de l’IPWP au cours des 270 000 dernières années a été reconstituée. Les résultats indiquent que le δ¹⁸O des eaux de surface reflètent principalement l'advection latérale plutôt que l'historique des précipitations régionales, et suggèrent que l'hydrologie de surface IPWP est contrôlée par la migration latitudinale de la zone de convergence intertropicale aux échelles de temps orbitales mais aussi en réponse aux événements climatiques abrupts de l'hémisphère nord (eg. événements de Heinrich). Les variations de salinité de surface sont étroitement corrélées aux changements d’export vers l’Atlantique au niveau du Courant des Aiguilles (Sud de l’Afrique). Puis, les changements dans le transport des eaux de la thermocline issues de l’ITF vers l'océan Indien ont été étudiés. Les résultats montrent que le transport était plus faible pendant les glaciations (ie. MIS 6 et 4-2) que pendant les périodes interglaciaires (ie. MIS 7, MIS 5 et Holocène) et exerçaient une influence significative sur les changements de la température de la thermocline dans l'Océan Indien. / This work aimed at reconstructing the late Quaternary evolution of surface and thermocline temperature and ocean surface water δ¹⁸O in the Indo-Pacific Warm Pool by combining Mg/Ca-thermometry and stable oxygen isotope analyses on surface and thermocline-dwelling planktonic foraminifers retrieved from sediment cores in the eastern tropical Indian Ocean. This study allowed to re-evaluate the effects of different cleaning methods and in-situ dissolution on the Mg-thermometry of planktonic foraminifers, evidencing the need for species-dependent corrections. Then, the IPWP evolution over the last 270,000 years has been explored. Results indicate that surface water δ¹⁸O chiefly reflects lateral advection rather than local precipitation history, and suggest that surface IPWP hydrology is controlled by the latitudinal migration of the Intertropical Convergence Zone at orbital timescale as well as during abrupt northern hemisphere climatic events (i.e. Heinrich events). Ocean surface salinity in the IPWP and Agulhas leakage region varied synchronously, implying their teleconnection through oceanic and atmospheric circulation. Moreover, changes in the transport of thermocline water to the Indian Ocean by the Indonesian Throughflow (ITF) have been reconstructed. Results show that thermocline water transport was weaker during glacials (i.e. MIS 6 and 4-2) than during interglacials (MIS 7, MIS 5 and Holocene), and exerted significant influence on Indian Ocean TWT change.