Observed physical and biogeochemical variability due to tropical cyclone Mocha using glider observations in the Bay of Bengal

Deep-sea glider measurements of temperature, salinity, chlorophyll-a, and dissolved oxygen (DO) collected during Tropical Cyclone (TC) Mocha (09-15 May 2023) provided a unique opportunity to examine the response of physical and biogeochemical parameters in the Bay of Bengal during the secondary TC peak season. Glider measurements show a significant reduction in SST (~2.5°C) and a mild increase in SSS (~0.9 psu) in response to the TC. Similarly,biogeochemical observations show a strong response, with increase in near-surface chlorophyll concentration (1.6 mg m-3) and DO (10 μM) driven by TC-induced vertical mixing and upwelling. The second near-surface chlorophyll peak (1.7 mg m-3), observed eight days after the passage of TC, was substantially stronger than the first peak (0.8 mg m-3) that occurred immediately after the TC passed. This difference may be attributed to extensive cloud cover during the first chlorophyll peak, which likely limited light availability at the surface. Both chlorophyll and dissolved oxygen reached their maximum concentrations approximately 8–9 days after the TC passed the study location. Statistical analysis indicates enhanced near-surface diurnal variability in the DO during the post-TC bloom period, compared to the non-bloom period. Further, the model shows a weaker response to the TC than that observed in the glider measurements.