The mechanism of the Freshwater outflow through the Ganges-Brahmaputra-Meghna Delta

Abstract:

We investigated the dynamics of the riverine outflow across the Ganges–Brahmaputra–Meghna (GBM) delta using a two-layer model with a focus on its interaction with the ocean on monthly to seasonal timescales. The GBM delta is a major source of freshwater for the Bay of Bengal and is known to affect the local Monsoonal climate. Because the GBM delta is large and consists of numerous and complex river networks, it makes the region a challenging area for river-routing models. By using a two-layer model, our goal is to understand the basic underlying dynamics of the river-ocean interaction.

We find the model successfully simulating a seasonal increase and decrease of discharge and sea surface height (SSH) at both the mainstem and the distributaries. Model experiments show the distributaries an active region of river–ocean interactions. While discharge through the mainstem is driven by the SSH signal propagating from the upstream, the distributaries are strongly affected by the ocean. This is because of the river plume established on the oceanic side of the river mouth. A river plume induced by the mainstem in the presence of the Coriolis force increases the SSH along the coast and the river mouth of the distributaries, thus causing backwater and convergence and raising the SSH in distributaries. Further idealized experiments suggest the narrow, meandering, and shallow paths of the distributaries are enhancing the role of friction on the flow from the upstream, which makes the river mouths of the distributaries more sensitive to oceanic variability. Our study demonstrates that river plumes may play a central role in the dynamics of mega-delta flow by connecting the dynamics at the mainstem and the distributaries through ocean dynamics.