Characteristics of a cyclonic eddy and its influence on mesozooplankton community in the northern Bay of Bengal during early winter monsoon.

Characteristics of a cold-core eddy and its influence on the mesozooplankton community were studied along the central (87° E) Bay of Bengal during winter monsoon (November 2008) based on in situ data. The thermo-haline structure and the satellite-derived sea level anomaly maps showed the presence of a cyclonic eddy between 16° N and 20° N. The nutrient enhancement due to the eddy pumping in the euphotic column (∼ 50 m) had resulted in high chlorophyll a concentration, a factor of 8 times higher than that outside the eddy, which led to higher mesozooplankton biovolume (0.35 ± 0.36 ml m(-3)) and abundance (276 ± 184 ind m(-3)). The northern cyclonic eddy (NCE) seems to exist for approximately 6 months between July and January. During summer, the NCE is forced by local wind stress curl and the resultant Ekman pumping, whereas during fall and early phase of the winter, it is sustained by westward propagating semi-annual Rossby waves. The longer existence of NCE in the study region, which originated 6 months prior to the present observation, provides a favourable environment for the mesozooplankton community to grow and reproduce, resulting in noticeable increase in the biovolume. Hence, the persistent and longer existence of NCE significantly influences the biological production of the generally oligotrophic BoB, making it locally biologically 'active'.