Influence of coastal upwelling on the surface current in the Bay of Bengal using HF radar and satellite observations

Coastal Upwelling is a phenomenon in which cold and nutrient-enriched water from the Ekman layers reaches the surface enhancing the biological productivity of the upwelling region. In this work, an attempt is made to understand the influence of coastal upwelling on surface current variations during May 2018 to August 2018, when HF radar current observation (source: NIOT, India) is available. The wind-based Upwelling Index(UI_wind) showed coastal upwelling throughout the study period. But the SST based upwelling index (UI_sst) showed upwelling occurred only from May to the first week of June. Cross-shore components of HF radar-derived ocean surface current (CSSC)  showed strong similarity with UI_sst. The first phase of upwelling from UI_sst is observed to start on 5^th May and lasts till 14^th May with a maximum peak on around 10^th May and having a horizontal extension of ~40 km. Then, there is a break period for about three days and after that, the second phase of upwelling starts on 17^th May and lasts till 25^th May with a maximum peak on around 20^th May, but this time the horizontal extension is ~100 km which is much larger than during the first phase. A strong positive (from coast to offshore) CSSC is observed to start on around 5^th May and lasts till 13^th May with a maximum peak on around 10^th May and having a horizontal extension of ~40 km, as observed from UIsst. A reversal of CSSC (towards coast) is noted on 14^th May when the break of coastal upwelling is evident from UI_sst. The CSSC then again started intensifying 15^th May onwards and continued for ten days till 25^th May, similar to UI_sst.  The horizontal extension of the upwelling signature in the second phase of upwelling is ~70 km. Therefore, a 7-10 days of the coastal upwelling and its horizontal extension are identified in this study. This study suggests the use of high resolution (~6 km) HF radar current observation on the monitoring of coastal upwelling processes.