Dominant physical-biogeochemical drivers for the seasonal variations in the surface chlorophyll-a and subsurface chlorophyll-a maximum in the Bay of Bengal

Abstract The seasonal variations and driving mechanisms of the surface and subsurface chlorophyll-a concentrations in the Bay of Bengal are far from resolved, as only a few local, short-term studies have been performed. Hence, this study investigates a comprehensive basin-wide framework of the seasonal variations in the chlorophyll-a concentration, its dominant external forcing, and the internal dynamics of the Bay of Bengal. Multivariate empirical orthogonal function decomposition and heterogeneous correlation analyses are applied to numerous observational, reanalysis, and satellite datasets, including chlorophyll-a, nutrients, temperature, salinity, turbidity, and wind stress curl datasets collected from various sources, including the Copernicus Marine Environment Monitoring Service, World Ocean Atlas, and ERA-Interim. This study suggests that the chlorophyll-a concentrations at both the surface and the subsurface chlorophyll-a maximum (SCM) are higher during summer and early autumn than during the other seasons, especially along the coastal regions and western part of the Bay of Bengal. During summer and early autumn, riverine nutrient inputs, the intrusion of nutritious water from the Arabian Sea, and coastal upwelling are the three dominant drivers controlling the chlorophyll-a concentrations at both the surface and the SCM. The positive wind stress curl-induced uplift of the thermocline increases the nutrient supply and thus significantly enhances the chlorophyll-a concentration at the SCM along the entire western side of the bay during the second half of the year. During spring, the deep euphotic depth plays a vital role in controlling the concentration and depth of the SCM. The depth of the 26 °C isotherm can be used as a proxy of the depth of the SCM. This study provides an improved understanding of the high chlorophyll-a concentrations and their drivers in five potential zones within the Bay of Bengal, which will help to identify the rich marine ecosystems therein.