No abstract

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] 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.

A linear, continuously stratified model is used to investigate the dynamics of the East India Coastal Current (EICC). Solutions are found numerically in a basin that resembles the Indian Ocean basin north of 29øS, and they are forced by Hellerman and Rosenstein [1983] winds. Effects due to the following four forcing mechanisms are isolated: local alongshore winds adjacent to the east coasts of India and Sri Lanka, remote alongshore winds adjacent to the northern and eastern boundaries of the Bay, remotely forced signals propagating from the equator, and interior Ekman pumping. Each process contributes significantly to the EICC surface flow at some locations and at some times during the year. Along the

Abstract. Hydrography and altimetry show that a "high" in surface topography forms off southwest India in January (midnortheast monsoon) and that a "low" forms during the southwest monsoon; the high and low, called Lakshadweep high and low after the island chain in the vicinity of where they form, propagate westward, extending across the southern Arabian Sea a few months after genesis. We investigate the dynamics of the high and low with an analytic model and with numerical simulations using a dynamical reduced-gravity model for the north Indian Ocean. We conclude that the high and low do not owe their existence to nonlinearity. They are a consequence of westward propagating Rossby waves radiated by Kelvin waves propagating poleward along the western margin of the Indian subcontinent. Most important for the annual cycle of the high and low are the annual and semiannual Kelvin waves off southwest India; these appear to be forced primarily by alongshore winds in the Bay of Bengal and by winds in the equatorial Indian Ocean. Other Kelvin waves, provided their period is greater than about 40 days, can contribute to the high and low; these Kelvin waves are expected to introduce intra-annual and interannual variability in the annual cycle. Below the critical period of 40 days, energy is trapped at the coast. The analysis provides a mechanism for the early onset of upwelling off southwest India, which is known to begin in February, well before the onset of the upwelling-favorable southwest monsoon winds.