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

Abstract

Although situated on the same geographical latitude, the two ocean basins (the Bay of Bengal (BoB) and the Arabian Sea (AS)) of the northern Indian Ocean (NIO) experience diverse met-ocean conditions. These ocean basins are the warmest world oceans exhibiting high sea surface temperatures (SST) year-round. Conducive SST, its gradient and coupled interactions of vertical wind shear, absolute vorticity, relative humidity, and potential intensity lead to NIO being host to 7%-10% of all the tropical cyclones (TCs) in the world (Gray, 1979(Gray, , 1998. However, compared to the AS, the BoB witnesses more cyclones, a majority of which propagate in the west-northwest/north-northwest direction (Evan & Camargo 2011;Mohanty 1994). The National Cyclone Risk Mitigation Project (NCRMP) report of India states that during 1891-2000 nearly 308 TCs (out of which 103 were severe) affected the east coast of India. In contrast, only 48 TCs affected the west coast (of which 24 were severe) within the same period. Cyclones in BoB are known to be stronger and more destructive than those in AS (Singh Abstract 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) μatm whereas dissolved inorganic carbon (DIC) driven changes are 23.6 (19.5) and 27.8 (78.0) μatm. 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 CO 2 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 pr...