Physical and Biochemical Responses to Sequential Tropical Cyclones in the Arabian Sea

Abstract

The upper-ocean physical and biochemical responses to sequential tropical cyclones (TCs) Kyarr and Maha in the Arabian Sea (AS) were investigated using data from satellites and Bio-Argo floats. Corresponding to slow and strong sequential TCs, two cooling processes and two short chlorophyll a (chl-a) blooms occurred on the sea surface, separated by 6–7 days, and three cold eddies appeared near the TC paths, with sea surface temperatures dropping more than 6 °C. Phytoplankton blooms occurred near cold eddies e1, e2, and e3, with chl-a concentrations reaching 12.76, 23.09, and 16.51 mg/m3, respectively. The depth-integrated chl-a analysis confirmed that the first chl-a enhancement was related to the redistribution of chl-a associated with TC-induced Ekman pumping and vertical mixing at the base of the mixed layer post-TC Kyarr. The subsequent, more pronounced chl-a bloom occurred due to the net growth of phytoplankton, as nutrient-rich cold waters were brought into the euphotic layer through Ekman pumping, entrainment, and eddy pumping post-TC Maha. Upwelling (vertical mixing) was the dominant process allowing the resupply of nutrients near (on the right side of) the TC path. The results derived from a biogeochemistry model indicated that the chl-a evolution was consistent with the observations recorded on Bio-Argo floats. This study suggests that in sequential TC-induced phytoplankton blooms, the redistribution of chl-a is a major mechanism for the first bloom, when high chl-a concentrations occur in the subsurface layer, whereas the second bloom is fueled by nutrients supplied from the deep layer.