Beneath a Hole in the Monsoon: The Southern Bay of Bengal Cold Pool

Abstract:

The minimum in Indian Summer monsoon rainfall is located around Sri Lanka and the ocean below is characterized by a cold pool – having cooler water in comparison to the surrounding region. The meridional gridient in SST between this cold pool and the northern Bay of Bengal has been suggested to be critical for triggering deep atmospheric convection furhter north. As a part of the CTCZ (Continental Tropical Convergence Zone) project under the Indian Climate Research Programme, the first field experiment within the cold pool was conducted during the summer monsoon of 2009 to understand the oceanography of this region. Hydrographic (CTD), ADCP and surface meteorological observations at two time-series locations (TSL) within the core of the Summer Monsoon Current (SMC) during 27 July – 8 August constituted the major data set of this experiment. These observations captured interesting features in both salinity and temperature fields. CTD time series revealed the existence of a salt pump by which the saltier Arabian Sea water located at subsurface layers is pumped up to the surface of the Bay of Bengal. The pumping occurred intermittently during the summer monsoon, in several bursts, along the meandering path of the SMC and is hypothesized to be the mechanism for importing saltier water into the bay in order to maintain the salt balance of the bay. Simulation using an Indian Ocean model reproduces pupming events albeit with certain difference in its characteristics. The temperature data captured two cooling events during the observation period. The first cooling event was during 24 -27 July in which the SST decreased by 0.3°C and the second was during 30 July – 2 August in which the SST dropped by about 0.5°C. These observations were made within the region where cold water is advected eastward by the SMC that carries cooler upwelled water from the coasts of India and Sri Lanka. Estimates of horizontal advection using in situ data sets and simulations using an Indian Ocean model suggest that cooling events taking place within the summer monsoon are controlled by ocean dynamics dominated by the powerful presence of the SMC.