Hydrography and circulation in the western Bay of Bengal during the northeast monsoon

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...

Intrusion of the Bay of Bengal water into the Arabian Sea during winter monsoon and associated chemical and biological response

“…This could be understood in the context of prevailing circulation along the western boundary of the Bay of Bengal and eastern boundary of the Arabian Sea. During winter the East India Coastal Current (EICC) along the western boundary of the Bay of Bengal flow towards south [Shetye et al, 1996] (also see Figure 1, details on HYCOM, are given by Bleck [2002]). Though the freshwater influx in the Bay of Bengal is considerably reduced during winter, the waters in the eastern Bay remain fresher than the southern Bay (Figure 1).…”

Cyclone‐induced mixing does not cool SST in the post‐monsoon north Bay of Bengal

“…The sub-mixed layer, unaffected by evaporative and longwave cooling, warms rapidly. Solar penetration could also contribute to the formation of temperature inversions such as that at station 16 ( Figure S2; Shetye et al, 1996). It is possible that basin-scale dynamics, as well as freshwater from monsoon rain and river runoff, influence the intensity of BoB cyclones.…”

“…A deep warm layer underlying a fresh plume is associated with the equatorward post-monsoon East India Coastal Current (Shetye et al, 1996;Pankajakshan et al, 2007). Ocean models suggest that relaxation of monsoon winds, and downwelling waves from the equatorial Indian Ocean, deepen the thermocline off the east coast of India (Schott and McCreary, 2001).…”

Trace Metal Concentrations in Zooplankton from the Eastern Arabian Sea and Western Bay of Bengal

“…Port activities such as shipping, loading of Fe-Mn ore, and unloading of Zn-Pb ore concentrates and other bulk cargo also partly contribute to the enhanced levels of some of these trace metals in seawater and zooplankton of the coastal areas of both the Arabian Sea and Bay of Bengal. Moreover, a large quantity of sediments impregnated with heavy metals are supplied by major rivers along the east coast and were constantly moved by waves either towards north or south depending on the direction and angle of wave approach with respect to the coast (Rengasamy and Jing, 2005) Bioaccumulation 888,857 629,361 402,160 34,021 294,196 265,742 5663 factors (baf) stations off Nagapattinam transect (station position: B4), seems to be related to the southerly flowing East India coastal current from the head of the Bay of Bengal and the peculiar gyral circulation of the Bay of Bengal (Shetye et al, 1996). Under the influence of these gyres and as a consequence of the geometry of the east coast of India (U-shaped basin), the low-saline waters, probably of riverine origin, drift offshore by the simultaneous prevalence of anticlockwise surface currents, thereby favoring significant accumulation of trace metals in zooplankton.…”