Observational evidence on the coastal upwelling along the northwest coast of India during summer monsoon

The lesser known coastal upwelling in the North Eastern Arabian Sea (NEAS) during summer monsoon, its associated dynamics and forcing mechanisms is elucidated for the first time using basin scale monthly time-series in-situ and satellite data. The presence of cool upwelled waters along northwest coast of India from July to early October with an associated increase in productivity was evident in both data. The low level Findlater jet blows towards west coast of India with high wind magnitude (10-12 m/s) during summer monsoon generates strong Ekman transport (1416 kg/m/s) at offshore and Ekman pumping velocity (1.349 m/s) at coastal region initiates upwelling. It was identified that the currents and remote forcing also regulate upwelling along the region. Although upwelling seems to exist along the northwest coast, it was weaker (25.5 degrees C) compared to the southwest coast where the SST dropped to 24 degrees C. The upwelling was observed in the south during June as a surface process, while it was observed along the northwest coast of India by the end of August. Even though the onset of upwelling in the NEAS and South Eastern Arabian Sea (SEAS) had a lag of two months, the recession of upwelling happened during late and early September respectively. The cause for the lag in the onset and cessation of upwelling between SEAS and NEAS is attributed to the propagation of Kelvin waves and southwest monsoon winds. The study also reveals that temperature and chlorophyll profiles show bi-modal peaks of high and low associated with winter cooling (winter) and upwelling (summer).