Phytoplankton bloom triggered by an anticyclonic eddy: The combined effect of eddy‐Ekman pumping and winter mixing

Abstract

AbstractRecent composite based researches reveal that anticyclonic eddies facilitate the growth of phytoplankton in the subtropical gyres. Two dynamical mechanisms, eddy‐Ekman pumping and winter mixing, have been examined individually, but their relative and combined effects remain unclear. Using satellite observations and model simulations, this study investigated the process of a distinct phytoplankton bloom generated in an anticyclonic eddy in the nutrient‐depleted southeastern Indian Ocean. The bloom propagated westward along with the eddy for more than 600 km from late April to August in 2010. The peak of surface chlorophyll concentration in the eddy is 2.2 times larger than the mean value of the ambient. The development of the bloom is dominated by the winter deepening of mixed layer, whose velocity in vertical nutrient flux is on average 3 times larger than that of eddy‐Ekman pumping. The results of a 1‐D physical‐biogeochemical model demonstrate that the role of eddy‐Ekman pumping is also indispensable, because it not only transports extra nutrients into the mixed layer, but also results in significant chlorophyll enrichment in subsurface water. The superposition of eddy‐Ekman pumping on winter mixing triples the chlorophyll both at the surface and in the upper layer, and the entrainment of subsurface phytoplankton into the mixed layer contributes significantly to the surface bloom, especially in its initial stage. Both the satellite observations and model simulation show that eddy‐Ekman pumping can lead to an early occurrence of the bloom for more than 2 weeks.