Why Is the Monsoon Coastal Upwelling Signal Subdued in the Bay of Bengal?

Abstract

The Indian summer monsoon, which brings heavy precipitation to the densely populated Indian subcontinent, plays an important role in the development of a coastal upwelling circulation that brings colder, nutrient-rich water to the surface. Although the western shores of the Arabian Sea (AS) and Bay of Bengal (BoB) both experience upwelling-favorable winds during June-August, only the AS coastline exhibits significant surface cooling. In contrast, the BoB remains warm and its upwelling is characterized by a transient, weak sea surface temperature (SST) response confined to the east coast of India. A weaker mean alongshore wind stress and coastal circulation do not sufficiently explain the lack of SST response in the BoB. Here, we examine other reasons for the differing behavior of these two coastal margins. Firstly, we show that while winds are persistently upwelling-favorable in the western AS, intraseasonal wind variability in the BoB induces intermittent upwelling. Secondly, the vertical density stratification is controlled by salinity in the BoB, and upwelled waters are saltier, but only marginally cooler than surface waters. By contrast, the density in the AS is temperature-controlled, and upwelled waters are substantially colder than the surface. Additionally, satellite-based SST in the BoB does not adequately resolve the upwelling signal. Using a numerical model, we find that salinity stratification has a greater influence on the mean SST, while wind frequency alters near-shore SST and its temporal variability. This work has implications for the sensitivity of upwelling regions and their response to wind stress and stratification in a warming climate.