The Role of South Monsoon Current in the Air-Sea Interactions of Bay of Bengal

Harindra Joseph Fernando, University of Notre Dame, Department of Civil and Environmental Engineering and Earth Sciences, Notre Dame, United States, Annunziata Pirro, University of Notre Dame, Department of Civil Engineering, Environmental and Earth Sciences, Notre Dame, IN, United States, Hemantha W Wijesekera, Naval Research Laboratory, Ocean Sciences, Stennis Space Center, MS, United States, Tommy G Jensen, US Naval Research Laboratory, Ocean Sciences Division, Stennis Space Center, MS, United States and S.U.P. Jinadasa, National Aquatic Resources Research and Development Agenc, Colombo, Sri Lanka
The Southwest Monsoon Currents (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 Intraseasonal Oscillations in Bay of Bengal (MISO-BoB) research initiatives. Ship observations confirmed the presence of a 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 AE and SLD are formed following the appearance of SMC in early summer, and both evolve during July/August and disappear in September. This result is at odds with the current notion that AE is a result of the interaction between SMC and Rossby waves arriving from the southeast of the BoB with energy from a blocked Wyrtki jet at the Sumatra coast; such waves do not arrive in the southwestern BoB until August/September. Almost simultaneous appearance of AE and SLD is inconsistent with a recently proposed mechanism of SLD formation due to separation of SMC from the Sri Lanka coast. A new hypothesis is advanced wherein AE and SLD result in from topographically trapped Rossby wave response of SMC to perturbations of the Sri Lankan coast. Observations of the size, location and origins of AE were broadly consistent with this hypothesis, as did a laboratory experiment designed to mimic the natural flow in BoB by creating an eastward jet (SMC) on a simulated β plane in a rotating tank. Sea surface temperature observations show that SMC carries colder upwelled water from the southern Sri Lankan and Indian coasts and distributes within the surface waters of AE and beyond. Thus, surface waters of AE are colder than that of SLD, notwithstanding possible local upwelling in the latter, highlighting possible roles of SMC in swaying air-sea interactions in BoB.