Inter-annual Variability of Chlorophyll-a in the Arabian Sea and its Gulfs

Abstract

The analysis of chlorophyll-a for a 7 year period from the ocean color images of SeaWiFS in the Arabian Sea and its marginal seas showed considerable inter-annual and geographical variability. The variability in chlorophyll-a was essentially centered on the Southwest and the Northeast monsoon seasons. The mean seasonal chlorophyll-a of the Northeast Monsoon in the western Arabian Sea showed an increasing trend from 1998 to 2004. The period of elevated chlorophyll-a levels in the Gulf of Mannar is apparently extended due to advection of chlorophyll-a rich water from the Bay of Bengal along the east coast of India. The mean distribution of chlorophyll-a from the SeaWiFS images shows a coastally trapped chlorophyll-a rich front from the east coast of India that enriches the Gulf of Mannar and continues northward along the west coast of India. As this front moves northward along the coast, the southern coastal waters turn oligotrophic. The chlorophyll-a concentration in the region is also influenced by the large-scale climatic events such as El Nino/La Nina during 1998-1999. The chlorophyll-a data from selected locations were subjected to spectral analysis to extract periodic signals. The annual signal was found to be dominant in the areas affected by only one of the two monsoons such as the Gulf of Mannar. The semiannual signal was predominant in areas affected by both the monsoons such as western and northern Arabian Sea. We built an empirical model incorporating the annual and semiannual modes of chlorophyll-a from the ocean color data. The ratio of the standard deviations of SeaWiFS-observed to the modeled chlorophyll-a, taken as a measure of model suitability, varied between 0.71 and 0.86. Thus the chlorophyll-a variability in this region can reasonably be predicted using the empirical model developed in this study.