A Large Winter Chlorophyll‐a Bloom in the Southeastern Bay of Bengal Associated With the Extreme Indian Ocean Dipole Event in 2019

Abstract

AbstractA large Chlorophyll‐a (Chla) bloom developed in the southeastern Bay of Bengal (BoB) (4°N–10°N, 88°E−95°E) during winter 2019 and was observed from Moderate Resolution Imaging Spectroradiometer‐Aqua Chla data. By analyzing various observational and reanalysis datasets, we found that remote Equatorial Indian Ocean (EIO) upwelling waves and local Ekman pumping associated with the extreme Indian Ocean Dipole (IOD) event in 2019 were responsible for the Chla bloom. The extreme IOD 2019 favored strong upwelling Kelvin waves propagating from the eastern EIO into the BoB along the eastern boundary. These Kelvin waves were reflected into the central BoB as upwelling Rossby waves manifested by eddylike sea surface height propagating westward approximately along 5°N. In addition, local wind stress curl generated strong Ekman pumping in the southeastern BoB. As a result, upwelling processes associated with westward‐propagating Rossby waves and local Ekman pumping together uplifted the thermocline, breaking the stratification so that subsurface nutrient‐rich water was transported to the surface to cause a large Chla bloom in the southeastern BoB during winter 2019. The Chla bloom had a ring structure and its spatial shape was modulated by westward upwelling Rossby waves and cyclonic eddies. This study highlights the important role of IOD events in the formation of Chla bloom in the southeastern BoB during winter.