Contrasting Chl-a responses to the tropical cyclones Thane and Phailin in the Bay of Bengal

Satellite-derived ocean colour data was used to identify the phytoplankton bloom associated with tropical cyclones Thane and Phailin in the Bay of Bengal (BoB). Cyclone Thane was category 2 cyclone (25-31 December 2011), and Phailin was category 5 cyclone (8-14 October 2013), and both occurred during the post-monsoon season. The present study examined the effect of cyclone intensity difference on the chlorophyll a (Chl-a) production in the BoB. Two and seven times Chl-a enhancement was observed after the passage of Phailin and Thane respectively. Possible causes were examined using the satellite-derived sea surface temperature, sea surface height anomalies and sea surface winds. In both cases, cyclone intensified the strength of the pre-existing cyclonic eddies after its passage. Our study shows that the strong stratification along the Phailin track suppresses the Chl-a enhancement after its passage. However, comparatively weaker stratification along the Thane track promotes entrainment of more nutrients and enhances the Chl-a production even though with lesser intensity. In this study, the extent of Chl-a production associated with cyclones was attributed to upper ocean physical processes than the intensity.