A case study of sea surface temperature-cooling dynamics near the Indian tip during May 1997

The sea surface temperature (SST) off the southwest coast of India is important to the Indian summer monsoon onset, which generally occurs in late May off this coast. It is known that both locally and remotely forced coastal processes contribute to the SST cooling. In this paper we focus on the latter by studying the SST-cooling dynamics near the Indian tip (IT) during 1-15 May, using advanced very high resolution radiometer (AVHRR) SST, NASA scatterometer (NSCAT) winds, National Centers for Environmental Prediction/National Center for Atmospheric Research surface meteorological variables, and the Princeton Ocean Model. High spatial resolution (9 km) daily mean AVHRR SST maps indicate that the SST cooling near the IT is a recurrent phenomenon, wherein the cold SST water occupies the southwest India shelf and extends southward of the IT. During the 15 day period the NSCAT-observed winds increased from near zero, the net surface heat flux into the ocean changed by ∼62 W/m², and the SST decreased by 1.8°C near the IT. Under these conditions the model integration is carried out as an initial value problem with realistic bottom topography, climatological density stratification, and observed NSCAT winds. The results show that the Ekman dynamics along the west India shelf promote upwelling and initiate SST cooling. Shallow and spatially variable shelf topography near the IT enhance wind mixing, while sharp gradients in the isobaths at the shelf break and stratification enhance upwelling/downwelling intensity through cross- isobath flow. It is inferred that the SST cooling near the IT is a locally forced process that is dictated by shelf topography.