Surface circulation and upwelling patterns around Sri Lanka

A De Vos,C B Pattiaratchi,E M S Wijeratne

doi:10.5194/bg-11-5909-2014

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, and experiences bi-annually reversing monsoon winds. Aggregations of blue whale (Balaenoptera musculus) have been observed along the southern coast of Sri Lanka during the northeast (NE) monsoon, when satellite imagery indicates lower productivity in the surface waters. This study explored elements of the dynamics of the surface circulation and coastal upwelling in the waters around Sri Lanka using satellite imagery and numerical simulations using the Regional Ocean Modelling System (ROMS). The model was run for 3 years to examine the seasonal and shorter-term (~10 days) variability. The results reproduced correctly the reversing current system, between the Equator and Sri Lanka, in response to the changing wind field: the eastward flowing Southwest Monsoon Current (SMC) during the southwest (SW) monsoon transporting 11.5 Sv (mean over 2010–2012) and the westward flowing Northeast Monsoon Current (NMC) transporting 9.6 Sv during the 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 southern coast. During the SW monsoon, the island deflects the eastward flowing SMC southward, whilst along the eastern coast, the southward flow results from the Sri Lanka Dome recirculation. The major upwelling region, during both monsoon periods, is located along the southern coast, resulting from southward flow converging along the southern coast and subsequent divergence associated with the 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 eastern and western coasts: during the SW (NE) monsoon, the flow along the western (eastern) coast was stronger, migrating the upwelling centre to the east (west).

Highlights

Sri Lanka is situated 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 (Fig. 1)
During the SW monsoon, the Southwest Monsoon Current (SMC) flows from west to east, transporting higher salinity water from the Arabian Sea, whilst during the NE monsoon, the currents reverse in direction, with the Northeast Monsoon Current (NMC) transporting lower salinity water originating from the Bay of Bengal from east to west (Schott and McCreary, 2001)
The data include wind speed and direction data from a coastal meteorological station located at Hambantota (Fig. 1), meteorological information from ECMWF ERA interim data which were used for model forcing, and MODIS satellite imagery accessed from the ocean colour website (Feldman and McClain, 2013)

Summary

Introduction

Sri Lanka is situated 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 (Fig. 1). The location of Sri Lanka is unique, with its offshore waters transporting water with different properties through reversing ocean currents driven by monsoon winds. The northern Indian Ocean is characterised by bi-annually reversing monsoon winds resulting from the seasonal differential heating and cooling of the continental land mass and the ocean. During the SW monsoon, the Southwest Monsoon Current (SMC) flows from west to east, transporting higher salinity water from the Arabian Sea, whilst during the NE monsoon, the currents reverse in direction, with the Northeast Monsoon Current (NMC) transporting lower salinity water originating from the Bay of Bengal from east to west (Schott and McCreary, 2001).

Objectives

Methods

Results

Conclusion