Decoupling of cyclone-induced surface blooms and water-column chlorophyll responses across 44 cyclones in the Bay of Bengal

Abstract

Tropical cyclones often produce satellite-observed surface chlorophyll-a blooms, which are commonly interpreted as increased phytoplankton biomass. Using 44 cyclones in the Bay of Bengal, we found that surface chlorophyll-a increased rapidly during tropical cyclone passage but declined within a few days, while the basin-scale composite showed no significant increase in chlorophyll-a integrated over the upper 100 m. The surface chlorophyll-a response was weaker in the central deep basin than along the northwestern shelf and off Sri Lanka. These regional differences were associated with freshwater stratification, barrier-layer structure, coastal transport, and upwelling. The combination of surface enhancement, deep chlorophyll maximum shoaling, and no significant basin-scale increase in water-column chlorophyll-a was consistent with vertical redistribution of subsurface chlorophyll-a.