High-resolution numerical modelling of seasonal volume, heat, and freshwater transport along the Indian coast

The Indian subcontinent divides the north Indian Ocean (IO) into Arabian Sea (AS) and the Bay of Bengal (BoB), with different thermohaline properties. Seasonal reversal of winds and equatorial remote forcing due to proximity to equator, influences circulation of these basins. In this study, we numerically modelled the physical characteristics of AS and BoB, using MITgcm with a high spatial resolution of 1/20° (~ 5 km) and 49 vertical levels in a z-coordinate system, on a climatological scale. Temperature, salinity and flow fields were validated with satellite and gridded ARGO datasets. Statistically we established that the model setup simulates the upper ocean features and subsurface circulation in these two basins well.

Then, we computed the alongshore volume, heat, and freshwater transport along the coastline of India and eastern Sri Lanka. We observed that the alongshore transport along the eastern coast is stronger with high seasonal variability due to the poleward flowing Western Boundary Current (WBC) and equatorward flowing East Indian Coastal Current (EICC). The west coast transport is influenced by intraseasonal oscillations. We computed the freshwater transport to be an order less than the volume transport. Seasonality of alongshore volume and freshwater transport contradicts each other on the western coast whereas they are in phase on the eastern coast. Then we computed the contribution of freshwater transport in total flow as a percentage of the total volume transport. In the BoB, this is maximum during JJAS season which is limited to the northeastern coast of India, followed by October-November season as it covers the entire east coast. We also observed a weak and narrow freshwater export pathway flowing across the Palk Strait into the Gulf of Mannar. The seasonality of transport and upper ocean salinity highly correlate to each other in this region.

Meridional heat transport (MHT) was also computed over AS and BoB. Analysis shows that MHT over AS is stronger than BoB. Both basins act as heat source during summer monsoon and heat sink during winter monsoon. Zonal transport correlates positively with zonal wind whereas meridional transport correlates negatively with meridional wind. Various factors such as wind reversal, Ekman transport and vertical thermal wind shear play a role. Net heat flux positively correlates with total heat transport along the eastern coast of India and southeastern AS. It can be attributed to coastal currents and equatorial forcing which help in advection of heat and thermal ventilation.

Understanding the effects of these exchanges on nutrient and carbon transport along the coastal waters via nearshore processes would be aided by further research into these interactions. The high-resolution climatological set-up lays the groundwork for additional research into the physical and biological processes occurring in the Indian coastal seas.