Wind-driven coastal upwelling along the western boundary of the Bay of Bengal during the southwest monsoon

Abstract

A hydrographic survey during the southwest monsoon (July-August 1989) showed that along most of the western boundary of the Bay of Bengal, in an approximately 40 km wide band, isopycnals from depths up to about 70 m surfaced due to upwelling forced by local winds, in a fashion similar to that observed along eastern boundaries. Below the upwelling band there were often signatures of downwelling, suggestive of an undercurrent. There were no indications of a large-scale remotely forced western boundary current. Geostrophic velocity in the upwelling band was in the direction of the winds. The dynamic topography outside the upwelling band had cellular structures possibly indicating the presence of shelf waves with longshore wavelength of 400-500 km. The near-surface stratification was dominated by salinity, a consequence of high freshwater input to the Bay. The upwelling led to a coastward increase in salinity, except near the northern end where the freshwater influx from the Ganges and the Brahmaputra rivers overwhelmed other processes and gave rise to a freshwater plume offshore of the upwelling band. This plume moved equatorward against local winds.