Formation of the southern Bay of Bengal cold pool

A pool of relatively cooler water, called here as the southern Bay of Bengal cold pool, exists around Sri Lanka and southern tip of India during the summer monsoon. This cold pool is enveloped by the larger Indian Ocean warm pool and is believed to affect the intraseasonal variations of summer monsoon rainfall. In this study, we have investigated the mechanisms responsible for the formation of the cold pool using a combination of both satellite data sets and a general circulation model of the Indian Ocean. Sea surface temperature (SST) within the cold pool, after the steady increase during the February-April period, decreases first during a pre-monsoon spell in April and then with the monsoon onset during May. The onset cooling is stronger (similar to 1.8 ) than the pre-monsoon cooling (similar to 0.8 ) and culminates in the formation of the cold pool. Analysis of the model temperature equation shows that SST decrease during both events is primarily due to a decrease in incoming solar radiation and an increase in latent heat loss. These changes in the net heat flux are brought about by the arrival of cloud bands above the cold pool during both periods. During the pre-monsoon period, a cloud band originates in the western equatorial Indian Ocean and subsequently arrives above the cold pool. Similarly, during the monsoon onset, a band of clouds originating in the eastern equatorial Indian Ocean comes over the cold pool region. A lead-lag correlation calculation between daily SST and rainfall anomalies suggest that cooling in SST occurs in response to rainfall events with a lag of 5 days. These sequence of events occur every year with certain amount of interannual variability

Influence of Rainfall Over Eastern Arabian Sea on Its Salinity

The west coast of India and the adjoining eastern Arabian Sea (EAS) is one of the high rainfall zones of Indian summer monsoon. The summer monsoon rainfall in this region is about 1,036km(3), which is comparable to the annual runoff of the Ganga-Brahmaputra river system. We have investigated the impact of EAS rainfall and Bay of Bengal (BoB) low-salinity water on the Arabian Sea salinity with a suite of experiments using an ocean general circulation model. The sea surface salinity (SSS) of EAS decreases progressively from June to September by 0.5 to 1psu. A numerical experiment that isolates the effect of EAS rainfall suggests that this SSS decrease is largely due to local rainfall over the EAS. The spatial pattern of SSS decrease, however, is influenced by the prevailing West India Coastal Current. The role of low-salinity water originating in the BoB on reducing the EAS salinity has also been examined. In the South Eastern Arabian Sea, during winter, the SSS decreases by about 1.5psu. This freshening is caused by rainfall during the early winter in the southwestern BoB between 6 degrees N and 15 degrees N. Neither rainfall to the north of 15 degrees N nor river runoff into the BoB contributes much to the South Eastern Arabian Sea freshening during winter

Dynamics of summer monsoon current around Sri Lanka

From June-September, the summer monsoon current (SMC) flows eastward south of Sri Lanka and bends northeastward to form a swift jet that enters the Bay of Bengal (BoB). This part of SMC serves as the major component of the water exchange between the Arabian Sea and the BoB, maintaining the salt and freshwater of the North Indian Ocean. The processes that determine the evolution, intensification, and meandering of the SMC involve both local and remote forcing by winds. Interactions of the SMC with westward-propagating Rossby waves and eddies are only partly understood. In this study, we investigate these processes using an Indian Ocean general circulation model (MOM4p1) that is capable of simulating the SMC realistically. Because eddies and meanders are smoothed out in the climatology, our analyses focus on a single year of 2009, a period when a strong anticyclonic bend in the SMC was observed. An eddy-kinetic-energy budget analysis shows the region to be a zone of significant eddy activity, where both barotropic and baroclinic instabilities are active. Based on the analysis, we classify the evolution of SMC into stages of onset, intensification, anticyclonic bend, anticyclonic vortex formation, meandering, and termination. In addition, analysis of eddy-potential-vorticity flux and eddy-enstrophy decay reveal when, where, and how the eddies tend to drive the mean flow. Rossby waves and westward-propagating eddies arriving from the east energize the SMC in June and accelerate the mean flow through an up-gradient eddy-potential-vorticity flux. At the same time, local winds also strengthen the flow, by increasing its mean near-surface kinetic energy and raising isopycnals, the latter building up available potential energy (APE). The baroclinic instability that takes place in late July and early August releases APE, thereby generating the SMC meanders