Biophysical processes in the Indian Ocean

International audienceBasic physical processes that impact biological activity in the Indian Ocean (IO), namely, near-surface processes (upwelling, entrainment, detrainment, and advection) and subsurface circulations (shallow overturning cells and subthermocline currents), are reviewed. In the Arabian Sea, there are upwelling blooms during the southwest monsoon (SWM) along Somalia, Oman, and the west coast of India. In the central Arabian Sea, the overall SWM (northeast monsoon; NEM) blooms appear to be a series of entrainment (detrainment) blooms forced by intraseasonal winds. In the western Bay of Bengal, a prominent NEM bloom results from the entrainment of a preexisting deep chlorophyll maximum (DCM). South of Sri Lanka, the SWM bloom is caused by coastal upwelling and Ekman suction, and is swept into the Bay of Bengal by the Southwest Monsoon Current. In the tropical, South IO (5-20°S), there is a weak, surface bloom during boreal summer when new production is enhanced by nutrient entrainment; the surface bloom is even weaker (or absent) during boreal winter because the mixed layer is thinner, the thermocline is deeper, and hence, nutrient entrainment weaker. At intraseasonal timescales, blooms are associated with wind events and Rossby waves/eddies, and they can be generated by both new production and entrainment of a preexisting DCM. During the 1997/1998 El Niño-Southern Oscillation/IO zonal dipole event, there was an upwelling bloom near Sumatra/Java in fall 1997, a much deeper DCM and weaker surface bloom along 5-10°S in spring 1998, and a weaker bloom in the Arabian Sea during the SWM of 1998