Abstract

Abstract. The super cyclonic storm Amphan originated in the central Bay of Bengal (BoB) in May 2020, and a phytoplankton bloom occurred in the upper ocean that was devoid of background nutrients. The dynamic mechanism of the chlorophyll a (Chl a) bloom was researched based on reanalysis data, remote sensing and Argo float data. During the passage of Amphan, an inertial oscillation with a 2 d period appeared in the thermocline and lasted for approximately 2 weeks. After the passage of Amphan, a cyclonic eddy with a maximum vorticity of approximately 0.36 s−1 formed in the study area (Box A). Additionally, horizontal transport of Chl a also occurred when the maximum inlet fluxes through the western and northern sides of Box A were 0.304 and −0.199 mg m−2 s−1, respectively. With the weakened thermocline and thinner barrier layer thickness (BLT), nitrate and Chl a were uplifted to the upper ocean by upwelling. Then, with the high photosynthetically available radiation (PAR) in the upper ocean, a phytoplankton bloom occurred. This study provides new insights into the biological responses in the BoB during the passage of tropical cyclones (TCs).

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