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...

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.

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The tropical oceans have long been recognized as the most important region for large-scale oceanatmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Niño-related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large-and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east-west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current...