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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C9, PAGES 19,825–19,843, 2001

Effects of river inputs into the Bay of Bengal

Stephan D. Howden

Department of Marine Science, University of Southern Mississippi, Stennis Space Center, Mississippi

Raghu Murtugudde

Earth Systems Science Interdisiplinary Center, University of Maryland at College Park, College, Park, Maryland

Abstract

The effect of river runoff in the Bay of Bengal is examined using a reduced gravity primitive equation ocean model coupled to an atmospheric boundary layer model. Model simulations are carried out by including river discharges as surface freshwater forcing at the mouths of the rivers. To assess the effect of river inputs on the dynamics and thermodynamics of the tropical Indian Ocean, parallel simulations are carried out by neglecting the river inputs. Additionally, another set of parallel runs without penetrative radiation loss through the mixed layer is carried out. The freshwater flux due to rivers results in lower salinities and shallower mixed layers, as expected. However, the influence of this additional freshwater flux into the bay is rather counterintuitive. With the inclusion of river discharges more heat is absorbed by the ocean, but sea surface temperatures are slightly cooler in the bay because of enhanced entrainment cooling of the shallower mixed layer, enhanced penetrative radiation, and an enhanced effect of latent heat loss on the temperature tendency. This is despite the greater latent heat loss when river input is neglected. Conversley, neglect of penetrative radiation results in a shallower but slightly warmer mixed layer with river input. River input and penetrative radiation each affect the mixed layer depths, the salinity and temperature structure, and currents in the Bay of Bengal, but they have a small effect on SST. Annual SST, averaged over the Bay of Bengal, is only 0.1°C colder with river input. Neglecting penetrative radiation in the river run results in an increase of only 0.2°C for the annual SST. The lack of persistence of a barrier layer in the bay helps regulate SST even in the presence of enhanced buoyancy forcing due to river input. Averaged over the bay, a barrier layer forms as mixed layer detrainment occurs, and the thermocline deepens just after the southwest monsoon and the northeast monsoon. The barrier layer is short-lived in each case it is eroded by mixing. The effect of riverine input in the bay is not confined to the surface waters. A pool of cold anomaly (−1°C) and fresher waters is centered near 100 m depth in the bay with riverine input. This cold pool beneath the mixed layer allows entrainment cooling of the mixed layer to be more effective, even though mass entrainment is lower relative to the case neglecting river input. The more diffuse thermocline in the bay is consistent with enhanced vertical mixing despite the large positive buoyancy forcing.

Received 25 September 2000; accepted 22 March 2001.

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Citation: Howden, S. D., and R. Murtugudde (2001), Effects of river inputs into the Bay of Bengal, J. Geophys. Res., 106(C9), 19,825–19,843.

Copyright 2001 by the American Geophysical Union.