Effect of Stratification and Wind on Upwelling Source Depth and SST: A Case Study Comparing the Arabian Sea and Bay of Bengal During the Summer Monsoon

Summertime coastal upwelling in the Arabian Sea and Bay of Bengal is an important process that sets sea surface temperature (SST), which affects air-sea exchanges of moisture and influences monsoon precipitation. Despite receiving similar seasonal wind forcing and insolation, the Arabian Sea and Bay of Bengal differ drastically in their upwelling responses to Southwest monsoon winds. During the summer, SST in the Arabian Sea cools by 4-5 ºC compared to the spring, whereas SST in the Bay of Bengal cools by only 1-2 ºC in the same time frame. This study seeks to better understand what drives these different coastal upwelling responses. We focus on the effects of wind—which drives coastal upwelling—and stratification, because the Bay of Bengal is significantly more stratified than the Arabian Sea due to large influxes of freshwater. We quantified the effects of stratification and alongshore wind stress on upwelling source depth (the depth that upwelled water originates from) and volume transport through analyzing satellite and Argo data, as well as running numerical process-study experiments. As predicted by theory, the upwelling volume transport is a linear function of the alongshore wind stress and is insensitive to stratification. However, we find that the source depth increases with weaker stratification and stronger wind stress, and we present a scaling for this dependence. These results show that the contrast in summertime SST in the Arabian Sea and Bay of Bengal could be partially due to a difference in upwelling source depth. Moreover, this study augments our understanding of what sets the source depth of upwelled water, and helps us better predict how coastal upwelling dynamics may change in the future.