Variability of SeaWiFS-derived surface chlorophyll

Abstract

Ocean colour data obtained from SeaWiFS through NASA's Goddard DAAC Center are processed and analyzed. Raw data of 1km resolution within the area of latitudes 2N-13.5N and longitudes between 76.5E-88.0E on relatively cloud free days from December 1998 to December 1999 are processed using OC4 algorithm of Seadas to produce chlorophyll maps. Processed data include about 95 single day maps and composite maps were made for each month and season. The seasons are defined approximately as follows: Northeast (NE) Monsoon (December-February), First Inter-monsoon (March-April), Southwest (SW) Monsoon (May-September) and Second Inter-Monsoon (October-November). During SW Monsoon waters off west and southwest coasts exhibit higher surface pigment concentrations (> 1 mg m). This region exhibits a shallow mixed layer (only about 30m deep) during SW monsoon, which supports possible upwelling in the region. During SW Monsoon currents originate from the Arabian Sea, where strong upwelling is observed, flow from west to east in south of Sri Lanka and bring nutrient rich upwelled waters towards south. This enhances surface pigment concentrations as well. During the SW Monsoon patches of high chlorophyll are visible to the east of Sri Lanka where colder Sri Lanka Dome is located. During NE Monsoon and First Inter-monsoon, waters around Sri Lanka show relatively low chlorophyll-a concentrations (< 0.5 mg m’ ) except at few areas off northeast coast. The effect of surface wind forcing on variability of surface chlorophyll is studied as well. During the month of March when the Northeast Monsoon tapers down and when winds are somewhat weaker, over 60% correlation is found between chlorophyll and meridional wind stress in the Northeast region.