SeaWiFS observations of the Arabian Sea southwest monsoon bloom for the year 2000

Abstract

During the southwest monsoon (SWM) in the northern Arabian Sea, the phytoplankton bloom associated with upwelling has remained incompletely characterized by satellite ocean-color observations. This is due to the prevalence of dust storms in the region from May to September that prevents good satellite-derived chlorophyll retrievals. To ameliorate this problem, SeaWiFS data from May to September 2000 were processed with the spectral-matching algorithm (SMA) using a suite of Saharan dust models. The use of SMA significantly increased chlorophyll data recovery in areas otherwise completely masked by standard SeaWiFS processing, and permitted a description of the bloom over the SWM period. Maps of the frequency and spatial distribution of TOMS-absorbing aerosol index indicate that Africa and Arabia are the most probable dust sources, and that the nearshore area parallel to the Arabian coast was the most affected by dust events during the SWM, which is also where chlorophyll retrievals are also poorest. Weekly averages of the SeaWiFS images produced using the SMA show that at the onset of upwelling in June, the bloom along the Omani coast has limited offshore extent except for a few small thin filaments. From July to September, the high-chlorophyll features evolve into more complex structures that extend offshore into the Arabian Sea interior. In October, when the SWM has ended, the filaments appear to merge, forming a high-chlorophyll band off the Omani coast. Quikscat wind data suggest that the widespread filament formation occurs only when the SWM winds begin to weaken, which results in greater spatial variability in the zone of maximum wind stress (the Findlater Jet). Although the bloom is generally restricted to the cold-water upwelling area (as indicated by TMI sea-surface temperatures) that lies northwest of the Findlater Jet axis, the high-chlorophyll structures do not occur solely in association with coldest upwelling features. This suggests that thermal satellite imagery cannot be used to infer chlorophyll distributions for periods in which no ocean-color data are available, e.g., the earlier JGOFS cruises.