Dissolved Inorganic Carbon Dynamics of the Eastern Arabian Sea

With the increasing uptake of CO₂ by the oceans, it is important to understand the fate of dissolved inorganic carbon (DIC) in the oceans. The Arabian Sea is one of the productive regions of the world oceans and experiences strong seasonality due to semi-annually reversing monsoon winds and currents. In order to understand the influence of seasonally varying physical processes on DIC dynamics, repeated basin-scale studies (10 times) with seven to ten coast-offshore transects were studied along the eastern Arabian Sea (EAS) during December 2017 – January 2019. In general, the surface DIC concentrations were dependent on salinity. The mean DIC concentrations during the summer monsoon (June-September) was lower (1972 ± 96 µmol L^-1) compared to high saline periods (2066 ± 50 µmol L^-1) of winter (December-February) and transition months (March and October). EAS showed two characteristic contrasting processes during winter monsoon; the intrusion of low saline Bay of Bengal waters in the south near Cape lowered DIC (2018 ± 11 µmol L^-1) whereas the injection of convective mixing driven deeper saline waters in the north off Okha increased surface DIC (2200 ± 17 µmol L^-1). With the advent of the summer monsoon, this high saline DIC pool advected southerly and occupied the offshore regions. This offshore subsurface waters (100-200 m) with DIC maxima (~2200 µmol L^-1) was uplifted to shallow depths (<50 m) and formed an upwelling source onto the EAS shelf. Despite this, the surface DIC concentrations over the shelf off Mumbai to Cape were lower than the offshore. This is due to the influx of freshwater from monsoonal rivers and the resulting stratification which restricted upwelling of high DIC waters to sub-pycnocline depths. However, lack of river runoff and weak stratification facilitated strong upwelling of DIC rich waters in the north (off Okha) compared to off Mumbai. The present study showed a robust dependence of DIC on salinity and was more prominent in the south than the northeastern regions due to seasonality in precipitation/river runoff in the Arabian Sea.