Short-term variability of phytoplankton blooms associated with a cold eddy in the northwestern Arabian Sea

The northern Arabian Sea is a semienclosed sea with high primary productivity and a complicated flow pattern consisting of several eddies. This paper reports on phytoplankton blooms, which were associated with a cold eddy in the northern Arabian Sea during November 1996, inferred from Ocean Color and Temperature Scanner (OCTS) and Sea-view Wide Field-of-view Sensor (SeaWiFS)-derived chlorophyll a (Chl-a hereafter), AVHRR sea surface temperature (SST), and other available oceanography data. The blooms emerged at 100 km from both coasts in the Gulf of Oman (60.5°E, 24.5°N) where the depth is about 3000 m. The Chl-a concentrations patch first appeared on November 2 and decayed after about 4 weeks (December 3, 1996). The high Chl-a concentrations patch was about 100 km in diameter and it was located at 60.3–61.3°E, 23.5–24.5°N. The bloom, having a mean Chl-a concentration of 6.8 mg m−3 on November 6, was located in a cold SST eddy, which was accompanied by another feature, an anticyclone eddy (of 100 km in diameter) with high SST and low Chl-a concentrations to the southwest (61.5°E, 22.5°N). An SST drop occurred around November 14, which coincides with a peak of the vertical pumping velocity derived from NSCAT-derived wind stress. Two SeaWiFS-derived Chl-a images obtained in November 1998 and 1999 show good agreement in terms of the locations and features with those described above through the OCTS observations. The possible mechanism for this newly identified Chl-a patch is discussed.