Unusual phytoplankton blooms in the southwestern Bay of Bengal: a comparative study

Two unusual phytoplankton bloom events were identified in the southwestern Bay of Bengal from MODIS-derived chlorophyll-a concentration data collected between 2003 and 2015. The occurrence of the unusual phytoplankton bloom in December 2005 (called Bloom 1 in this study) has been reported in the literature to be triggered by multiple forcings, including strong cyclonic eddy, frequent typhoons, and strong wind-induced mixing. Interestingly, the other unusual phytoplankton bloom (called Bloom 2 in this study) was identified in almost the same location in December 2013. Further, it is the strongest bloom during our study period with large area of high Chl-a > 1.0 mg/m(3) and shared some similar features with Bloom 1, such as wide coverage and long duration. At the same time, there were also frequent typhoons and a cyclonic eddy. The possible causes of Bloom 2 were examined using time series of multi-satellite datasets, including sea surface height anomalies (SSHA), sea surface temperature (SST), together with Argo profile data. We found that the cyclonic eddy might be not yet the dominant factor for Bloom 2 as the eddy was much weaker than that of Bloom 1. Specially, SST in December 2013 was lowest among all the December from 2003 to 2015. That is, the stratification is weakest. Therefore, the weak stratification can be broken easily by mixing induced by typhoons and cyclonic eddies and finally result in the strong bloom. This comparative studies could provide us some insight in understanding the role of eddies and tropical cyclones in phytoplankton dynamics in the Bay of Bengal.