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

Abstract

Situated in similar latitudes and subjected to similar atmospheric forcing, the tropical basins of the Arabian Sea looses fresh water due to excess evaporation over precipitation while Bay of Bengal receives freshwater via excess rain and river run off. The hydrological imbalance thus created on an annual scale will have to be balanced by the inter-basin exchange. In winter this happens through the intrusion of Bay of Bengal waters into the Arabian Sea, when the southward flowing East India Coastal Current carrying low salinity waters from the northern Bay feeds into the West India Coastal Current flowing north along the shelf in the Arabian Sea. Advection of nutrients by this intrusion triggers enhanced levels of chlorophyll near the southern part of the western shelf of India and may play a role in altering the biogeochemistry of this intense hypoxic region. 1. Introduction - The Arabian Sea and the Bay of Bengal are both tropical basins located in similar latitudes but separated by the landmass of Indian peninsula. Both the basins are forced by seasonally reversing monsoon winds- southwesterly during June to September and northeasterly during November till February. In response to this forcing, the surface circulation also undergoes seasonal reversal. Annually, the Arabian Sea looses fresh water through intense evaporation (~1 m yr-1) while the Bay of Bengal receives immense quantities of fresh water by way of heavy precipitation [~ 2 m yr-1, Prasad, 1997] as well as by run off from peninsular rivers [1.625 x 10-12 m3 yr-1, Subramanian, 1993]. This creates a hydrological imbalance and the seasonally reversing circulation plays an important role in exchanging the water masses between the Arabian Sea and the Bay of Bengal and maintaining the salt-balance of this region. During summer (June-September) the eastward flowing Indian Monsoon Current (IMC) carries saltier Arabian Sea high salinity water mass [Prasanna Kumar and Prasad, 1999] into the Bay of Bengal [Vinayachandran, 1999]. In contrast, during winter (November-February) both the westward flowing north equatorial current (NEC) between equator and 10oN and the southward flowing East India Coastal Current (EICC) brings warm and low-salinity waters into the Arabian Sea from the Bay of Bengal. Though there exist some documentation of the inflow of Bay of Bengal waters into the Arabian Sea [Wyrtki, 1973; Hareesh Kumar and Mathew, 1997], there is no information available on the effect of such exchange of water mass between the basins in the context of chemical and biological changes of the region. This has special significance in the biogeochemistry of the west coast of India which experiences severe hypoxia and its potential link to global change phenomena [Naqvi et al., 2000]. In this paper we explore the implication of intrusion of Bay of Bengal waters into the Arabian Sea along the western shelf of Indian peninsula in terms of chemical and biological changes based on the supporting evidences from the observation (in situ as well as satellite).