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

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 degrees C. Phytoplankton blooms occurred near cold eddies e1, e2, and e3, with chl-a concentrations reaching 12.76, 23.09, and 16.51 mg/m(3), 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.

Automated summary

This study investigated the physical and biochemical responses in the upper ocean of the Arabian Sea to sequential tropical cyclones. The results showed that the cyclones led to cooling processes, phytoplankton blooms, and the appearance of cold eddies. The redistribution of chlorophyll a was found to be a major mechanism for the first bloom, while the second bloom was fueled by nutrients from the deep layer. The findings were consistent with the observations made by Bio-Argo floats.