Surface circulation and upwelling patterns around Sri Lanka

Abstract

Abstract. Sri Lanka occupies a unique location within the equatorial belt in the northern Indian Ocean with the Arabian Sea on its western side and the Bay of Bengal on its eastern side. The region is characterised by bi-annually reversing monsoon winds resulting from seasonal differential heating and cooling of the continental land mass and the ocean. This study explored elements of the dynamics of the surface circulation and coastal upwelling in the waters around Sri Lanka using satellite imagery and the Regional Ocean Modelling System (ROMS) configured to the study region and forced with ECMWF interim data. The model was run for 2 yr to examine the seasonal and shorter term (∼10 days) variability. The results confirmed the presence of the reversing current system in response to the changing wind field: the eastward flowing Southwest Monsoon Current (SMC) during the Southwest (SW) monsoon transporting 11.5 Sv and the westward flowing Northeast Monsoon Current (NMC) transporting 9.5 Sv during the Northeast (NE) monsoon, respectively. A recirculation feature located to the east of Sri Lanka during the SW monsoon, the Sri Lanka Dome, is shown to result from the interaction between the SMC and the Island of Sri Lanka. Along the eastern and western coasts, during both monsoon periods, flow is southward converging along the south coast. During the SW monsoon the Island deflects the eastward flowing SMC southward whilst along the east coast the southward flow results from the Sri Lanka Dome recirculation. The major upwelling region, during both monsoon periods, is located along the south coast and is shown to be due to flow convergence and divergence associated with offshore transport of water. Higher surface chlorophyll concentrations were observed during the SW monsoon. The location of the flow convergence and hence the upwelling centre was dependent on the relative strengths of wind driven flow along the east and west coasts: during the SW (NE) monsoon the flow along the western (eastern) coast was stronger and hence the upwelling centre was shifted to the east (west). The presence of upwelling along the south coast during both monsoon periods may explain the blue whale (Balaenoptera musculus) aggregations in this region.