Seasonal variability and drivers of air-sea CO2 exchange in the Arabian Sea

Whereas the ocean as a whole serves as the major sink of anthropogenic carbon, the Arabian Sea (AS) acts as a net source of CO₂ to the atmosphere. The magnitude of this carbon flux, however, is strongly dependent upon seasonal forcing, most notably the summer and winter Monsoons. Historic under-sampling precludes a data-driven seasonal resolution of surface carbon chemistry in the AS, which necessitates modeling studies. Global ocean circulation models currently underestimate CO₂ outgassing in the AS, as well as the timing of seasonal maxima in relation to independent estimates (Sarma et al., 2013).

In order to diagnose the physical, chemical, and biological processes creating these discrepancies, we use a high-resolution (5-km) hydrodynamic model, the Regional Oceanic Modeling System (ROMS), coupled to a nitrogen-based nitrogen-phytoplankton-zooplankton-detritus (NPZD) ecosystem model with an implemented carbon chemistry module initialized and forced with climatological fields. With increased horizontal resolution, maximum CO₂ outgassing correctly occurs during the summer Monsoon, and the annual CO₂ flux is increased. Additionally, we elucidate the dominant roles of dissolved inorganic carbon (DIC) and temperature in determining CO₂ flux in the Omani upwelling region and offshore, respectively. Sensitivity experiments also highlight the relative role of circulation and biogeochemical processes in contributing to these factors.

Establishing a seasonal baseline of the drivers responsible for parameters such as pCO₂ and pH in the AS is necessary in its own right due to their impact upon marine ecosystems, but it is furthermore essential for projecting how future climate change impacts will modify the role of the AS as a source of CO₂ to the atmosphere.

Reference: Sarma et al., 2013 doi: 10.5194/bg-10-7035-2013