An analysis of the characteristics of chlorophyll in the Sulu Sea

The seasonal and spatial variations of chlorophyll concentrations, Sea Surface Temperature (SST), wind fields and wind-induced Ekman pumping in the Sulu Sea are investigated using a set of new remote sensing measurements from October 1997 to December 2004. The results show the seasonality of chlorophyll, wind fields and SST and reveal the phytoplankton blooming events in the Sulu Sea basin during the northeast monsoon season. In summer, chlorophyll concentrations were relatively low (< 0.2 mg/m3) and distributed uniformly throughout the basin with a narrow belt of high chlorophyll concentrations along the coastal waters, particularly the coasts of Borneo and of the Sulu Archipelago. In winter, chlorophyll concentrations increased (> 0.2 mg/m3) throughout the entire basin, and phytoplankton bloomed southward to the central basin, while chlorophyll concentrations reached high levels (1 mg/m3) in the center of the blooms. One peak was observed during the northeast monsoon season each year. SSTs have significant negative correlations with chlorophyll concentrations; i.e., high and uniformly distributed in summer but lower with an obvious tongue of cold waters southward to the central basin in winter. The seasonal variation of chlorophyll concentrations and SST distribution were associated with the seasonally reversing monsoon. The winter phytoplankton blooming and the tongue of the cold waters were correlated to the vertical upwelling cold and nutrient-rich waters drawn by the northeast wind, with the center of the blooms and the location of cold tongues coinciding with the maximum of the wind speeds and the Ekman pumping velocities.