Monthly variability in surface pCO₂ and net air-sea CO₂ flux in the Arabian Sea

Abstract

Recent studies on biogeochemical cycling of carbon in the Arabian Sea, by Joint Global Ocean Flux Study (JGOFS), revealed that the Arabian Sea is a perennial source of carbon dioxide to the atmosphere. The surface pCO₂ is governed by physical processes such as water mass transports, upwelling and winter convective mixing and associated biological processes. Hence surface pCO₂ distribution showed significant seasonal and spatial variability. As a result, it is difficult to extrapolate observed data to the entire basin to assess basin-wide fluxes of CO₂ to the atmosphere. On the basis of the data collected during Indian, U.S. JGOFS and Indian Land-Ocean Interactions in the Coastal Zone process study programs in the Arabian Sea, multiple regression equations were developed on a seasonal basis to compute Dissolved Inorganic Carbon (DIC) from surface temperature, salinity and chlorophyll as constraints. Total Alkalinity (TA) was computed from surface salinity. On the basis of satellite derived chlorophyll (Coastal Zone Color Scanner) and Levitus surface temperature and salinity climatology, surface TA and DIC were constructed at 1° × 1° grids. Using carbonate dissociation constants, pCO₂ levels were computed. The results show the southwest coast of India occupied with very low-pCO₂ waters because of inflow from Bay of Bengal low-saline water mass during NE monsoon (December–February). On the other hand, this area is occupied with pCO₂ -rich (>500 μatm) waters during SW monsoon (June–August) because of coastal upwelling. The highest pCO₂ (>700 μatm) were found along the west coast of the Arabian Sea driven by strong upwelling. In the open ocean, higher pCO₂ levels were observed in the north (north of 15°N) compared to the southern Arabian Sea round the year, with values higher than those in the atmosphere even in the south. Though no strong physical forcing occurs during spring intermonsoon (March–May), surface pCO₂ levels were higher than in the atmosphere. The air-sea flux of CO₂ was found to be high during SW monsoon throughout the Arabian Sea. The west coast of the Arabian Sea releases higher quantities of CO₂ to the atmosphere per unit area followed by the northern and eastern Arabian Sea. As a whole, the Arabian Sea (north of 10°N) releases about 90 TgC yr⁻¹ to the atmosphere.