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Wind forcing of the Ganga-Brahmaputra river plume

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Abstract

Response of Ganga-Brahmaputra river plume to wind forcing in the Bay of Bengal is studied using a numerical ocean circulation model. Four different wind forcing scenarios, namely, winds over the entire model domain, no winds anywhere over the model domain, winds over Equatorial Indian Ocean only and winds over Bay of Bengal only, are considered. Model simulations are carried out in an idealized setting where forcing from other rivers and precipitation is ignored. Despite the absence of this forcing, model captures observed phases of Ganga-Brahmaputra river plume, seasonal cycle of sea surface temperature and spatio-temporal structure of East India Coastal Current (EICC) reasonably well. Horizontal structure of the plume is investigated using surface salinity, surface currents and freshwater thickness obtained from the simulation that includes Ganga-Brahmaputra river discharge and winds over the entire model domain. The plume spreads upstream (eastward) but remains confined to the coast in northern bay during spring. During summer monsoon, plume spreads southward along the east coast of India and subsequently southeastward over the open bay and reaches the northern tip of Andaman islands by the end of October. During winter monsoon, the plume flows southward, assisted by EICC along the east coast of India and recedes northward in the central bay. In the absence of winds, the plume flows southward along the coast of India throughout the year. Equatorial winds force the plume farther (compared to no winds case) southward along the coast of India prominently in the winter monsoon. Local winds control the horizontal structure of the plume in the bay as they produce the seasonal structure that closely resembles that produced by the winds over the entire model domain. Momentum balance of the plume reveals that geostrophy controls the westward drift of the freshwater around the river mouth. Wind friction and associated Ekman flow are important in the upstream spreading of the plume during spring and in the eastward expansion over the open bay during summer monsoon.

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Acknowledgments

The authors acknowledge Indian National Center for Ocean Information Services (INCOIS) and Ministry of Earth Sciences (MoES), Government of India, for their support for this study. P.N. Vinayachandran acknowledges partial financial support from J C Bose Fellowship, SERB, Government of India. The authors also acknowledge support from The Bay of Bengal Boundary Layer Experiment (BoBBLE), a joint MoES, India - Natural Environment Research Council (NERC), U.K. program. The BoBBLE program was funded by the MoES, Government of India, under its Monsoon Mission program administered by the Indian Institute of Tropical Meteorology, Pune. ERA-Interim reanalysis surface forcing was provided by European Centre for Medium-Range Weather Forecasts (https://www.ecmwf.int). Numerical code employed for the simulations in this study was obtained from Regional Ocean Modeling System website (http://www.myroms.org). The authors are thankful towards the research personnel at Rutgers University and University of California, Los Angeles, numerous other contributors and ROMS community for developing and maintaining ROMS code. Simulations were carried out on Mandhan, a High Performance Computing (HPC) system at the Center for Atmospheric & Oceanic Sciences, Indian Institute of Science (IISc). Mandhan was funded by the Department of Science and Technology and Divecha Center for Climate Change. A portion of the simulations were also carried out on SAHASRAT, another HPC system at the Supercomputer Education and Research Centre, IISc. We are thankful to the two anonymous reviewers for their comments and suggestions which improved the presentation of the paper.

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  1. Center for Atmospheric & Oceanic Sciences, Indian Institute of Science, Bengaluru, Karnataka, India

    Shrikant M. Pargaonkar & P. N. Vinayachandran

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Pargaonkar, S.M., Vinayachandran, P.N. Wind forcing of the Ganga-Brahmaputra river plume. Ocean Dynamics 71, 125–156 (2021). https://doi.org/10.1007/s10236-020-01429-6

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