Impact of initial and lateral open boundary conditions in a Regional Indian Ocean Model on Bay of Bengal circulation

This study uses three simulations (Exp-1, Exp-2, and Exp-3) from a nested basin-scale, regional, eddy-permitting Indian Ocean model forced with CORE-II inter-annual forcing, to study the impact of initial and lateral open boundary conditions on Bay of Bengal (BoB) circulation. It also evaluates the state of art reanalysis products from GODAS, SODA3 and ORAS5. Sensitivity studies using more realistic initial and lateral boundary conditions suggest that accurate initial conditions are essential for realistic simulation of the mean and the variability of temperature and salinity in the upper ocean. We also show that accurate lateral boundary conditions are also essential for realistic basin-scale simulations for the upper ocean circulation in the BoB. The model captured well the observed seasonal and inter-annual variability of salinity in the BoB. The comparison of surface currents from the regional simulations and reanalysis products with OSCAR analyses shows Exp-3 (with improved initial and lateral boundary​ conditions) to be the closest among all three. The summer monsoon current is also most realistic in Exp-3, while the reanalysis products are unable to reproduce the BoB Coastal Current structure and magnitude. This study also reports the first evidence of a coastally trapped, narrow boundary current at thermocline depth along the eastern boundary of BoB. The presence of this boundary current is noticed in all three regional solutions while most prominent in Exp-3 and nearly absent in the reanalysis products. The salt transport across 8°N shows large seasonal and inter-annual variations during 2003–2009 in the best simulation (Exp-3), with standard deviation ranging between 77–150 psu-m3/s up to 1000 m depth. Apart from summer and winter monsoon periods, which show the intrusion of salt into BoB from the equatorial Indian Ocean in the upper layers, we found significant salt transport at deeper depths (300–1500 m) as well. The integrated inter-annual variations of the seasonal salt flux at different depths across 8 °N show presence of strong seasonal cycle in the upper layer (0–200 m). However, the seasonality is most prominent in the upper 50 m. The intra-seasonal variation in the deeper layers are more prominent as compared to the upper layer.