Eddies and currents in the Bay of Bengal during summer monsoons

Abstract

The Southwest Monsoon Current (SMC) in the Bay of Bengal (BoB) and prominent eddies surrounding it, in particular, an anticyclonic eddy (AE) to the southeast and a cyclonic eddy (Sri Lanka Dome, SLD) to the east of Sri Lanka, were investigated using field observations, numerical simulations, satellite imagery, laboratory experiments and theoretical studies. The field campaigns were conducted in 2015 and 2018, respectively, in conjunction with the Air-Sea Interactions in the Northern Indian Ocean (ASIRI) and Monsoon Intra-Seasonal Oscillations in Bay of Bengal (MISO-BoB) research initiatives. Ship observations confirmed the presence of a annually recurring AE about 500 km southeast of Sri Lanka, with surface velocities up to ~1 m s−1, size in the meridional direction ~200 km, and penetration approximately to the depth of the thermocline ~150 m. Satellite observations and COAMPS® model simulations show that the AE and the SLD are formed following the appearance of the SMC in early summer, and both evolve during July/August and disappear in September. This result is at odds with the prevailing notion that the AE is a result of the interaction between the SMC and Rossby waves arriving from the southeast of the BoB, carrying energy of a blocked Wyrtki jet at the Sumatra coast; such waves do not arrive in the southwestern BoB until August/September. Simultaneous appearance of AE and SLD is inconsistent with a recently proposed mechanism in which SLD is formed due to separation of the SMC from the Sri Lanka coast. A new hypothesis is proposed wherein the AE and the SLD are generated by topographically trapped Rossby wave response of the SMC to perturbations by the Sri Lankan coast. Observations of the size, location and origins of the AE were broadly consistent with this hypothesis, so were the results of a laboratory experiment designed to mimic natural flow in the BoB by creating an eastward jet (SMC) on a simulated β plane. Sea surface temperature (SST) observations show that the SMC carries colder upwelled water from the southern Sri Lankan and Indian coasts and distributes it within the surface waters of the AE and beyond. Thus, the surface waters of the AE are colder than that of the SLD, notwithstanding perceived local upwelling in the SLD, suggesting that SMC may play a controlling role in the air-sea interactions of the southern BoB.