Seasonal cycle of surface ocean pCO2 and pH in the northern Indian Ocean and their controlling factors

Abstract This paper examines the seasonal variability of surface ocean pCO2 and pH in the northern Indian Ocean. It aims to identify their controlling factors using a high-resolution, regional ocean-ecosystem model simulation and available surface ocean carbon observations. The seasonal variability of pCO2 and pH of the northern Indian Ocean is attributed to the changes in surface temperature (T), dissolved inorganic carbon (DIC), total alkalinity (ALK), and salinity (S). The western Arabian Sea has an enormous seasonal variance of pCO2 due to coastal upwelling dynamics. In contrast, the seasonal variance of pCO2 in the Bay of Bengal is governed by the upper ocean mixed layer dynamics, albeit with smaller amplitudes. The contribution of T (DIC) in the seasonal variance of pCO2 and pH at the western Arabian Sea is ±90 (∓100) μatm and ±0.18 (∓0.20) pH units, respectively. The contribution of ALK and S is complementary to each other in inducing seasonal variances of pCO2 and pH in the southeastern Arabian Sea with a magnitude of ±5∼10 μatm and ±0.02 pH units, respectively. In the northern Bay of Bengal, salinity plays a significant role in controlling seasonal variability of pCO2 and pH with amplitudes of roughly ± 20 µatm and ±0.18 pH units, respectively, along the pathways of freshwater spreading. The maxima of seasonal amplitudes of pCO2 and pH occur during April-May and August-September for both basins. This study highlights the hotspots of pCO2 and pH in the northern Indian Ocean, where observational efforts to monitor pCO2 and pH become an essential requirement.