Hydrography and circulation off the west coast of India during the Southwest Monsoon 1987

Abstract

During June-August 1987 thirteen hydrographic sections, each approximately perpendicular to the coast, covered the coastal region off western India. The southernmost section showed occurrence of upwelling, the nearshore surface temperature being about 2.5°C lower than farther offshore. There was a shallow (approximately 75 to 100 m deep) equatorward surface current, below which there were signatures of downwelling indicative of a poleward undercurrent hugging the continental slope. T-S characteristics showed that the undercurrent carried low salinity water found in the southwestern Bay of Bengal. Conditions similar to those found at this section existed up to around 15N but the intensity of upwelling, and the signatures of the surface current and the undercurrent grew weaker progressively from the south to the north, and ceased to be noticeable at about 20N. The width of the surface current was about 150 km, whereas the undercurrent was about 40 km wide. The transport in the equatorward surface current increased from less than 0.5 × 106 m3/s to about 4 × 106 m3/s from the north to the southern end of the coast. The winds during the observation varied between west-northwesterly near the southern end of the coast to west-southwesterly near the northern end. The longshore component of the wind stress was generally equatorward. Its magnitude was maximal (0.5 dyn/cm2) near the southern end. On the basis of these observations we propose that the circulation off the west coast of India during the southwest monsoon, though weak, is dynamically similar to the wind-driven eastern boundary currents found elsewhere in the oceans.