Effect of thermal and non-thermal processes on the variability of ocean surface pCO2 and buffering capacity in the north Indian Ocean

The oceans have absorbed nearly 30% of the anthropogenic CO₂ that alters the ocean carbon chemistry. The oceanic processes are highly complex, which mandate approaches that couple its physical, chemical and biological states. Here, we use a coupled atmosphere–ocean-biogeochemistry model, incorporating spatially and temporally varying atmospheric CO₂ to simulate the north Indian Ocean (NIO) carbon dynamics for the period 2013–2020. We assess the seasonal variability of Dissolved Inorganic Carbon (DIC), total Alkalinity (ALK), ocean surface pCO₂ and buffering capacity. To assess the mechanisms that control carbon dynamics in the region, we segregate the ocean surface pCO₂ into temperature-driven (thermal) and bio-physical processes induced (non-thermal) pCO₂. We find that the thermally driven pCO₂ is dominant in summer (June, July, August and September; JJAS), but the non-thermal component in winter (December, January and February; DJF) in the northern Arabian Sea (AS). The northern AS is characterised by a deep mixed layer and convection-induced vertical mixing during winter. DIC from the subsurface layer is uplifted to the surface, which results in high ocean surface pCO₂ in winter. Off the Oman coast, the non-thermal processes control the surface pCO₂ in summer. In the northern bay, the thermal component of pCO₂ is dominant in summer and non-thermal component in winter as in northern AS, but their magnitudes are lower due to large riverine flux. The budget analysis reveals strong influence of diffusion, CO₂ flux and biological processes in controlling DIC variability in NIO. Low buffering capacity in upwelling regions indicates that pCO₂ changes are more sensitive to changes in DIC, primarily due to the upwelled DIC-rich surface waters. Therefore, it results in a reduced ability to absorb CO₂. This warrants the need to address recent changes in carbon dynamics in response to the increased levels of atmospheric CO₂.