Impact of seasonal river input on the Bay of Bengal simulation

• We simulated the Bay of Bengal (BoB) with and without river input.
• We obtained seasonal salinity cycles at river-ocean meeting points for ten rivers.
• River input is distributed unevenly along the coast based on variable discharges.
• Two large-scale plumes (Kelvin Number≫1) for GBM and ISS are generated.
• River input is critical for barrier layer formation and near-surface stratification.

This study seeks to better understand the impact of seasonal river input on the Bay of Bengal (BoB) using the Regional Ocean Modeling System (ROMS). To quantify the effects, two parallel climatological simulations (with and without rivers) were carried out for fifteen years. While the temperatures at river mouths along the model’s coastal boundary were relaxed to the monthly climatology, observation-based reconstructed seasonal cycles were provided for salinity with monthly discharges for ten selected rivers. The result shows significant improvement (50–70%) in the model-simulated near-surface salinity, freshwater plume, stratification, mixed-layer depth and barrier-layer thickness (BLT) with river input. The river input reduced the domain-averaged annual surface salinity and surface density by 0.99 psu and 0.73 kg/m3, respectively, when compared to the no-river simulation.One of the major impacts of river inclusion in the simulation is the formation, dispersion and demise of the freshwater plume system in the Bay. Two separate large-scale (Kelvin Number≫1) freshwater plumes form by surface-advection during April–May, one near the Ganges–Brahmaputra–Meghna (GBM) river system mouths at the head of the Bay, and another further south near the mouths of the Irrawaddy–Sittang–Salween (ISS). They merge and evolve as one plume system to cover almost half of the domain in October and to occupy the largest volume in November. The formation and dispersion of the plume system correspond well with freshening in the northern Bay, affecting seasonal stratification with the formation of barrier layer, and subsequent advection of the freshwater by eddies and fronts along the East India Coastal Current (EICC). The simulated domain-averaged BLT was minimum during March–May and maximum during October–February. Simulations indicate that the river input is necessary in addition to precipitation-minus-evaporation in setting up the near-surface stratification and for the formation of barrier layer in the BoB.