Merging gauge data and models with satellite data from multiple sources to aid the understanding of long-term trends in chlorophyll-a concentrations

Several studies have analysed the long-term trends of chlorophyll-a (chl-a) concentration and related its intensity to sea surface temperature (SST) and wind forcing. In this article, we present trends of chl-a concentrations for the period 1997-2016 derived from multi-merged satellite data and analysed the role of SST, wind mixing (WM) and surface incoming short wave flux (SISF) and river discharges in the distribution of chl-a in the coastal waters of southwest India. The results show decreasing trend of anomalies for chl-a. Chl-a shows negative trend, though not significant, with all other variables, i. e., WM, SISF and river discharge over the last two decades in the study area. Whereas, an increasing trend of SST has recorded for the region during the same period. Our results suggest that the warming of sea surface as observed from SST, decreased river input, weakening of winds and reduced solar radiation are among the most prominent reasons for decline in chl-a concentration for the study region. Further, the positive relation between chl-a with WM and SISF and negative correlation with SSTs suggest that these trends are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers.