Monsoons, plumes, and blooms: intraseasonal variability of subsurface primary productivity in the Bay of Bengal

Abstract

During the southwest monsoon, seasonal storms bring torrential rainfall to the South Asian subcontinent and the northern Indian Ocean. Dense cloud cover limits the amount of sunlight that reaches the ocean surface, and sediment-laden river runoff limits the depths to which light can penetrate. Changing light availability should affect phytoplankton primary productivity and its dependent biogeochemical processes. Yet little is known about how subtropical weather is linked to ecosystem processes below the ocean’s surface. Here, using novel physical and bio-optical measurements from an array of free-drifting, autonomous systems, we show that the onset of cloudy, rainy conditions associated with stormy periods led to >50% reduction in subsurface gross primary productivity (GPP) relative to clear, sunny break periods. Simultaneous bioacoustic measurements collected onboard the autonomous platforms suggested that this intraseasonal variability in GPP generated a response in higher trophic levels. Long-term measurements from biogeochemical (BGC) Argo floats indicated that the magnitude of the days-to-weeks variability reported here was similar to that of the regional annual cycle in the region. Our findings demonstrate that intraseasonal subtropical air-sea variability modulates important regional biogeochemical ocean processes in the Northern Indian Ocean, and may help improve our ability to forecast the changing global climate system.