JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. C7, PAGES 12,667-12,683, JULY 15, 1991 The Seasonal Circulation of the Upper Ocean in the Bay of Bengal JAMES T. POTEMRA,1 MARK E. LUTHER, 2 AND JAMES J. O'BRIEN Mesoscale Air-Sea Interaction Group, Florida State University, Tallahassee Analysis of the results of a multilayer, adiabatic, numericalmodel of the upper Indian Ocean, driven by climatological monthly mean winds, shows that the simulated currents in the northeastern Indian Ocean are in generalagreement with available observations and interpretations.The main features of the ocean currents include large anticyclonic flow in the Bay of Bengal surface waters during the northern hemisphere winter. This gyre decaysinto eddiesin springand then transitionsinto a weaker, cyclonic gyre by late summer. The western recirculationregion of this flow is an intensifiedwestern boundary current which changes direction twice during the year. In the Andaman Sea, east of the Bay of Bengal, the oceanicflow changes direction twice duringthe year; it is cyclonic during the springand early summer and anticyclonic the rest of the year. Flow in the equatorial region shows the North Equatorial Current (NEC) flowing west during winter. Further south is the eastward flowing Equatorial Counter Current (ECC) and the westward flowing South Equatorial Current. In summer, the NEC switchesdirection,joins the ECC, and forms the Indian Monsoon Current. Investigation of the secondlayer of the model (the upper 450 m of the ocean) shows that flow during much of the year is baroclinic (strongvertical shear). Model layer thickness reveals coastal Kelvin waves propagating along the coast, traveling the entire perimeter of the Andaman Sea and the Bay of Bengal. This wave excites westward propagatingRossby waves into the interior of the bay. Time series analysis of transport calculationsyield significant peaks in the 20- to 30-day range and 50- to 60-day range which are not likely directly forced by the applied wind stress. 1. INTRODUCTION The Indian Ocean is a particularly interesting,dynamically rich area because of the changingwind patterns associated with the Indian Monsoon. In spite of this, it is a relatively poorly studied area. In addition, data coverage for the area is sparse. While some data sets do exist, they are predomi- nantly on large spaceand time scales. Recent efforts, suchas the monsoon experiment (MONEX) of the Global Atmo- spheric Research Program (GARP), the First GARP Global Experiment (FGGE), and the Indian Ocean Experiment (INDEX), have substantially improved the availability of measureddata in this region. Individual studieshave also contributed in recent years to the understanding of the dynamics in the Indian Ocean. An early effort by Wyrtki [1961] synthesized the available data sets (at the time) of the properties of the Southeast Asian waters. More recently, Legeckis [1987] demonstrated the appearance of a western boundary current in the Bay of Bengal using sea surfacetemperature data obtainedfrom the advancedvery high resolution radiometer (AVHRR) carded on board the NOAA 9 satellite. Data coverage from this satellite encompasses the entire region of the Bay of Bengal [McClain et al., 1985]. Rao et al. [1989] have generated mean monthly mixed layer depth, sea surface temperature, and surface current climatologies for the tropical Indian Ocean. Molinari et al. [1960] analyzed three different setsof satellite-tracked drifting buoys and compiled a monthly climatology of surface currents in the tropical Indian Ocean. Finally, oceanic data for the Indian Ocean is compiled in a 1Now at STX, NASA Goddard Space Flight Center, Greenbelt, Maryland. 2Nowat Department of Marine Science, University of South Florida, St. Petersburg,. Copyright 1991 by the American Geophysical Union. Paper number 91JC01045. 0148-0227/91/91JC-01045505.00 few excellent atlases [see Diiing, 1970; Wyrtki, 1971; Has- tenrath and Lamb, 1979; Cutler and Swallow, 1984]. Al- though these sources contribute significantly to the under- standingof dynamics in the tropical Indian Ocean, data on smaller scales, particularly in the Bay of Bengal, are some- what limited. The focus of this study therefore is to use a realistic, wind-driven model to simulate ocean currents in the region of the Bay of Bengal, from 75øE to 111øE and from 7øS to 23øN. Similar studies have been done for other areas of the Indian Ocean [Jensen, 1990] and using different models [Woodberry et al., 1989]. A review of modeling efforts in the Indian Ocean is given by Luther [1987]. The results of the model are compared with measured data in an attempt to understandthe dynamics of the area. The model used for this study, developed at the Florida State University [Jensen, 19901, is a reduced gravity, three and one-half layer model. Since the model has multiple layers, the subsurfacecur- rents can be studied as well as the surface. The lowest layer is at rest, however, and only the results from the first (surface) and second (subsurface) layers will be examined here. Figure 1 shows the typical large-scale circulation pattern associated with the winter and summer monsoons as com- piled in Monatskarten fiir den Indischen Ozean (Deutsches Hydrographisches Institut [1960], as depicted by Diiing [1970]). The general surface circulation of the region in northern winter includes large anticyclonic flow in the Bay of Bengal, a westward flowing North Equatorial Current (NEC), and eastward flowing Equatorial Counter Current (ECC) and a westward flowing South Equatorial Current (SEC). The northern summer pattern is characterized by counterrotating eddies in the Bay of Bengal [Diiing, 19701, an eastward flowing Indian Monsoon Current (IMC) and a westward flowing SEC. The model reproduces these flows, in general agreement with the atlases previously discussed.In this presentation the model is first described. Next, a description of the 12,667