Response of Surface Ocean pCO2 to Tropical Cyclones in Two Contrasting Basins of the Northern Indian Ocean

This study examines the impact of two very severe tropical cyclonic events, Phailin (over the Bay of Bengal) and Ockhi (over the Arabian Sea), on surface ocean pCO(2) and the associated changes in the upper ocean structure using a coupled biogeochemical ROMS model. The primary productivity averaged over the mixed layer is increased from 6.9 to 12.0 (7.2-45.6) mgCm(-3)d(-1) in response to Phailin (Ockhi). A decomposition analysis reveals that the mean contribution of temperature-driven changes in inducing pCO(2) variability in response to pre- and post-cyclonic conditions, in case of Phailin (Ockhi), are 5.4 (-8.8) and -7.6 (-58.8) mu atm whereas dissolved inorganic carbon (DIC) driven changes are 23.6 (19.5) and 27.8 (78.0) mu atm. Although salinity and total alkalinity have a relatively lesser control in inducing pCO(2) variability, salinity's response to the post-Phailin conditions is significant owing to strong salinity stratification in the Bay of Bengal. The enhancement of DIC is more in the near-surface waters than its removal by net biological processes resulting in the dominance of cyclone-induced upwelling and associated vertical mixing driven changes over the enhanced biology-driven changes in controlling pCO(2) variability during both cyclones. Despite comparable magnitudes of environmental forcing during both cyclones, the oceanic response to Phailin is comparatively short-lived and subdued due to stronger stratification in the Bay of Bengal. A relatively large ratio of DIC to total alkalinity in the upper layers of Arabian Sea facilitates a higher pCO(2) response to Ockhi. This makes Ockhi a greater source of CO2 to the atmosphere. Plain Language Summary Tropical cyclones dissipate large amounts of energy into the upper ocean, enhancing vertical mixing under the influence of strong winds. The enrichment of nutrients and carbon due to upwelling of deeper waters enhance pCO(2) levels making a favorable environment for biological productivity eventually utilizing the CO2, and decreasing the net pCO(2) level. This study examines the impact of two very severe cyclones on surface pCO(2) variability in two contrasting basins of the northern Indian Ocean. In both the basins, physical processes like heat, water fluxes, air-sea exchanges of inorganic carbon and mixing are largely controlling pCO(2) variability during tropical cyclones. Biological processes like carbonate production/dissociation and organic matter cycling have a considerable control on pCO(2) variability but not comparable with pCO(2) variability driven by the physical processes. The strong upper ocean stratification in the Bay of Bengal and the vertical distribution of the inorganic carbon inventory inherent to both basins result in differential oceanic response to tropical cyclones. The Arabian Sea (Ockhi) acts as a greater source of CO2 to the atmosphere in comparison to the Bay of Bengal (Phailin) during tropical cyclones.

Automated summary

This study examines the impact of two severe tropical cyclonic events on surface ocean pCO(2) and upper ocean structure. The results show that these cyclones have significant effects on primary productivity and pCO(2) variability. The response to the cyclones is relatively short-lived and subdued in the Bay of Bengal, while the Arabian Sea acts as a greater source of CO2 to the atmosphere during the cyclones.