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The Mechanism of the Freshwater Outflow Through the Ganges‐Brahmaputra‐Meghna Delta

The Ganges‐Brahmaputra‐Meghna (GBM) delta is a major source of freshwater for the Bay of Bengal. The flow through this megadelta is complex because of its large size and numerous river networks, making the region a challenging area for river‐routing models. This study investigates the dynamics of the riverine outflow across the GBM delta and its interaction with the ocean on monthly to seasonal time scales, using a two‐layer model that represents the riverine water and the oceanic water. The model simulates a seasonal increase and decrease of discharge and sea surface height (SSH) at both the main stem and the distributaries. Although discharge through the main stem is driven by that propagating from the upstream, model experiments show that the distributaries are an active region of river‐ocean interactions as a result of a river plume established on the oceanic side of the river mouth. This river plume is induced by river discharge in the presence of the Coriolis force and is associated with an increase in SSH along the coast. Backwater is induced at the river mouth, resulting in convergence and higher SSH in distributaries. The narrow, meandering, and shallow paths of the distributaries enhance the role of friction on the flow from the upstream, making their river mouths more sensitive to oceanic variability. The results of our study demonstrate that river plumes may play a central role in the dynamics of megadelta flow by connecting the dynamics at the main stem and the distributaries with ocean dynamics.