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This study investigates the seasonal transport variability of the North Indian Ocean (NIO) boundary currents, in particular, the East India Coastal Current (EICC) in the Bay of Bengal (BoB), West India Coastal Current (WICC) in the Arabian Sea (AS) and Somali Current (SC) in the Equatorial Indian Ocean (EIO) using a high-resolution Regional Ocean Modeling System (ROMS) with climatological river input. The simulation result indicates that northward EICC peaks in March and its mean transport is 2.65 Sv during February to April. The southward EICC extends down to 200 m depth and transports 0.67 Sv during October–December. Northward WICC which extends down to 100 m depth transports 0.31 Sv along the west coast of India during the winter monsoon. Southwestward SC flows between 6.5°N to 3°N, and northeastward cross-equatorial SC goes up to 11°N along the Somali coast. Northeastward SC which is the strongest coastal current in the NIO transports 13.05 Sv during the summer monsoon. The average transport of southwestward SC is 2.57 Sv only. The study suggests that wind stress and wind stress curl (WSC) plays a vital role in the semiannual reversal of transport and remote forcing modulated by equatorial wind anomalies dominates the southward EICC transport. [ABSTRACT FROM AUTHOR]

The western boundary current in the Bay of Bengal, also known as East India Coastal Current, is northward (southward) and continuous during pre‐ (post‐) Indian Summer Monsoon (ISM) but discontinuous during ISM (June–September). This study investigates the features of this discontinuity and role of eddies, local winds, and southern open boundary lateral forcing using AVISO TOPEX Poseidon, ERS, and Jason1 combined sea surface height anomaly, Ocean Surface Current Analysis Real‐time (OSCAR) surface currents and high‐resolution Regional Ocean Modelling System simulations. The study shows that the discontinuity is governed by westward and southwestward propagating eddies between two opposite facing flows along the boundary, northward of 10°N and southward of 21°N. This northward flow of 10°N is wind driven with lateral influence during September. The southward flow of 21°N is influenced by the Summer Monsoon Current, but the local wind influence starts from July onward. During ISM, about 79% cyclonic and 77% anticyclonic eddies move westward, while 68% cyclonic and 69% anticyclonic eddies move southward. Three highly active eddy genesis areas, having more than 50% activities during ISM, are identified: northern and southwestern bay for cyclonic and off Visakhapatnam (17.68°N, 83.21°E) coast for anticyclonic eddies. The pre‐ISM (post‐ISM) is dominated by anticyclonic (cyclonic) eddies, which are found across the western Bay of Bengal and away from northward (southward) East India Coastal Current. The ISM is dominated by westward and southwestward moving both cyclonic and anticyclonic eddies, which are confined to the boundary, where discontinuity is generally observed. Plain Language Summary: The western boundary current in Bay of Bengal, which is also known as East India Coastal Current, flows northward during February–May and southward during October–December. During Indian Summer Monsoon period (June–September), the boundary current is not consistent and various cyclonic and anticyclonic eddies are generated in the western Bay of Bengal. This study investigates the features of this disorganized boundary currents and the role of eddies, local winds, and southern boundary in making the currents disorganized using measurements and high‐resolution numerical model simulations. The study is important to understand the changes in upper ocean dynamics associated with the inconsistent flow and to assess the contribution of the above said forcings to trigger such inconsistency. The study can further be used to understand the discontinuity induced ocean mixing, upwelling, sea level variability, and energy distribution along the western boundary of Bay of Bengal. Key Points: The discontinuity features are more pronounced along the western boundaryThe discontinuity is governed by westward and southwestward propagating eddies along the boundary between two opposite flows of 10°N and 21°NLocal winds and southern boundary lateral forcing have significant roles in the boundary current discontinuity [ABSTRACT FROM AUTHOR]

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/m 3 , 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. [ABSTRACT FROM AUTHOR]