The Bay of Bengal, a semienclosed tropical basin that comes under the influence of monsoonal wind and freshwater influx, is distinguished by a strongly stratified surface layer and a seasonally reversing circulation. We discuss characteristics of these features in the western Bay during the northeast monsoon, when the East India Coastal Current (EICC) flows southward, using hydrographic data collected during December 1991. Vertical profiles show uniform temperature and salinity in a homogeneous surface layer, on average, 25 m deep but shallower northward and coastward. The halocline, immediately below, is approximately 50 m thick; salinity changes by approximately 3 parts per thousand. About two thirds of the profiles show temperature inversions in this layer. Salinity below the halocline hardly changes, and stratification is predominantly due to temperature variation. The halocline is noticeably better developed and the surface homogeneous layer is thinner in a low-salinity plume that hugs the coastline along the entire east coast of India. The plume is, on average, 50 km wide, with isohalines sloping down toward the coast. Most prominent in the geostrophic velocity field is the equatorward EICC. Its transport north of about 13 ø N, computed with 1000 dbar as the level of reference, varies between 2.6 and 7.1 x 106 m 3 s -1; just south of this latitude, a northwestward flow from offshore recurves and merß .. 6ges with the coastal current. At the southern end of the region surveyed, the transport is 7.7 x 10 m 3 s -1 . Recent model studies lead us to conclude that the EICC during the northeast monsoon is driven by winds along the east coast of India and Ekman pumping in the interior bay. In the south, Ekman pumping over the southwestern bay is responsible for the northwestward flow that merges with the EICC. IntroductionThe Bay of Bengal, a semienclosed basin in the North Indian Ocean (Figure 1), is forced by the seasonally reversing monsoon winds. The basin is essentially tropical, lying south of 22 ø N; its maximum zonal extent is about 1200 km, with the eastern boundary intersecting the equator. Proximity to the equator implies that the region can support rapidly propagating, tropical planetary waves; the presence of seasonal winds implies the existence of mechanisms for generating such low-frequency waves. Such considerations make the bay an interesting natural laboratory for the study of seasonal wind-driven tropical ocean circulation. The bay is also distinguished by strong near-surface stratification. The four major rivers flowing into the bay, Irrawaddy, Brahmaputra, Ganga, and Godavari, discharge annually approximately 1.5 x 1012 m 3 of fresh water into the bay [Martin et al., 1981]. In addition, annual rainfall over the bay varies between 1 m off the east coast of India to more than 3 m in the Andaman Sea and the coastal region north of it [Batongartner and Reichel, 1975]. This Paper number 95JC03307. 0148-0227/96/95JC-03307509.00 large influx of fleshwater leads to a strongly stratified near-surface la...

[1] An analysis of the heat budgets of the near-surface Arabian Sea and Bay of Bengal shows significant differences between them during the summer monsoon (JuneSeptember). In the Arabian Sea the winds associated with the summer monsoon are stronger and favor the transfer of heat to deeper layers owing to overturning and turbulent mixing. In contrast, the weaker winds over the bay force a relatively sluggish oceanic circulation that is unable to overturn, forcing a heat budget balance between the surface fluxes and diffusion and the rate of change of heat in the near-surface layer. The weak winds are also unable to overcome the strong near-surface stratification because of a lowsalinity surface layer. This leads to a shallow surface mixed layer that is stable and responds quickly to changes in the atmosphere. An implication is that sea surface temperature (SST) in the bay remains higher than 28°C, thereby supporting large-scale deep convection in the atmosphere during the summer monsoon. The atmospheric heating associated with the convection plays a critical role in sustaining the monsoon winds, and the rainfall associated with it, not only over the bay but also over the Indian subcontinent, maintains a low-salinity surface layer. In the Arabian Sea the strong overturning and mixing lead to lower SST and weak convective activity, which in turn, lead to low rainfall and runoff, resulting in weak stratification that can be overcome easily by the strong monsoon winds. Thus, in both basins, there is a cycle with positive feedback, but the cycles work in opposite directions. This locks monsoon convective activity primarily to the bay.

[1] A climatology of Sea-viewing Wide Field-of-View Sensor (SeaWiFS) chlorophyll data over the Indian Ocean is used to examine the bloom variability patterns, identifying spatio-temporal contrasts in bloom appearance and intensity and relating them to the variability of the physical environment. The near-surface ocean dynamics is assessed using an ocean general circulation model (OGCM). It is found that over a large part of the basin, the seasonal cycle of phytoplankton is characterized by two consecutive blooms, one during the summer monsoon, and the other during the winter monsoon. Each bloom is described by means of two parameters, the timing of the bloom onset and the cumulated increase in chlorophyll during the bloom. This yields a regional image of the influence of the two monsoons on phytoplankton, with distinct regions emerging in summer and in winter. By comparing the bloom patterns with dynamical features derived from the OGCM (horizontal and vertical velocities and mixed-layer depth), it is shown that the regional structure of the blooms is intimately linked with the horizontal and vertical circulations forced by the monsoons. Moreover, this comparison permits the assessment of some of the physical mechanisms that drive the bloom patterns, and points out the regions where these mechanisms need to be further investigated. A new outcome of this study is that in many distinct areas, time shifts of 1-2 months are witnessed in the timing of the bloom onsets in adjoining regions. These time shifts are rationalized in terms of horizontal advection and Rossby wave propagation.

According to recent estimates, the annual total continental runoff into the Bay of Bengal (BoB) is about 2950 km3, which is more than half that into the entire tropical Indian Ocean (IO). Here we use climatological observations to trace the seasonal pathways of near surface freshwater from BoB runoff and Indonesian Throughflow (ITF) by removing the net contribution from precipitation minus evaporation. North of 20°S, the amount of freshwater from BoB runoff and ITF changes with season in a manner consistent with surface currents from drifters. BoB runoff reaches remote regions of the Arabian Sea; it also crosses the equator in the east to join the ITF. This freshwater subsequently flows west across the southern tropical IO in the South Equatorial Current.

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