Satellite derived chlorophyll a distribution in the Bay of Bengal during northeast monsoon

Abstract

The Bay of Bengal is a semi-enclosed tropical ocean basin driven by the monsoon winds. The Bay receives a large quantity of freshwater from river discharges and rainfall, which leads to very low salinity values in the near-surface layers and a strongly stratified upper layer. The existing knowledge of the Chlorophyll a distribution in the Bay of Bengal is based on few shipboard measurements. We have studied the seasonal distribution of satellite-derived Chlorophyll a in the BoB using OCTS and SeaWiFS data.

Monthly composites of Chlorophyll a prepared from six years (1997–2003) of SeaWiFS data show that the Arabian Sea is richer in Chlorophyll a concentration when compared to the Bay of Bengal. The strong monsoon winds during summer and convection during winter favor high biological productivity in the Arabian Sea. The stratification is so strong in the Bay that it prevents the supply of nutrients from the nutricline to the euphotic zone. When the winds are at their maximum strength during the summer monsoon, high cloud cover inhibits higher Chlorophyll a in the Bay.

We have found that there is a phytoplankton bloom in the southwestern part of the Bay of Bengal during the November–January period. The Chlorophyll a concentration of the bloom can be as high as 2 mg m?³ compared to the near-zero value before the bloom. Nutrient injection into the euphotic zone due to open ocean upwelling driven by Ekman pumping causes the bloom. During the Northeast monsoon, the Bay also receives sufficient sunlight needed for the phytoplankton growth.

The bloom appears every year, but its intensity, duration, and spatial spread vary from year to year. However, during 1997 there was no bloom compared to other years. The Ekman pumping was weak during 1997 as a result of large-scale anomalies in both atmosphere and ocean associated with the Indian Ocean Dipole. The sea level anomaly suggests that the cyclonic gyre that usually forms in the southwestern Bay during the Northeast Monsoon was absent in 1997.

Several cyclones form over the Bay of Bengal during October–December. The internal mixing caused by the strong winds and upwelling due to the positive wind stress curl associated with the cyclone can enrich the upper layer with nutrients. Cyclones lead to localized intense blooms in the Bay of Bengal. In the southern Bay of Bengal, a cyclone during December 2000 cooled the sea surface temperature by 3.0°C and increased the Chlorophyll a concentration by 3.1 mg m?³. In the highly stratified waters of the northern Bay, a cyclone formed during November 1996 cooled the sea surface temperature by 2.6°C and increased the Chlorophyll a concentration by 3.6 mg m?³. The seasonal blooms are found to be weaker during years without cyclones.