Phytoplankton bloom due to Cyclone Sidr in the central Bay of Bengal

Abstract

The evidence of phytoplankton bloom in response to the cyclone Sidr in the central Bay of Bengal (BoB) basin has been reported. Satellite observations of surface chlorophyll-a concentration (Chl-a) from MODIS-Aqua, sea surface temperature (SST) from MODIS-Aqua and TMI were studied for before, during and after cyclone periods. The mixed-layer depth (MLD) derived from the nearest Argo float that was closest to the cyclone track along with the float temperature and salinity (T/S) profiles near the cyclone track were studied to further complement the satellite observations. Nearly 900 km long phytoplankton bloom was observed along the cyclone path. Along the path of cyclone Sidr, the average increase in Chl-a due to Sidr during the cyclone and after cyclone period was about 285 per cent and 150 per cent respectively, from the before-cyclone period. The mean decrease in SST from before cyclone period was about 1.75°C and 1.27°C between during and after cyclone period respectively, with maximum cooling of 3.2°C observed during cyclone period. The Argo float observations indicate deepening of MLD by 15 m, during the cyclone and shoaling of MLD after the cyclone by about 35 m. Upward Ekman pumping due to cyclonic winds caused upwelling of nutrient rich subsurface waters, which led to the bloom in the BoB.