Madhu Joshi

A three-dimensional numerical model of coastal upwelling along the west coast of India

A three-dimensional baroclinic numerical model is developed and used to study the response of a coastal ocean to mean monthly pure wind-stress forcing representative of northeast monsoon and different phases of the southwest monsoon along the west coast of India. The model incorporates coastal ocean physics as realistically as possible by resolving surface and bottom Ekman layers. Conservation equations are applied for mass, momentum, thermal energy, salinity and turbulent energy. The governing equations together with their boundary conditions are solved by finite-difference techniques. Experiments are performed to investigate the thermal response and associated alongshore currents of the coastal waters in the southeast Arabian Sea using monthly mean wind-stress forcing. The model simulations suggest that the upwelling process is initiated during pre-monsoon month of May along the coastal waters off the extreme southwest peninsular region. The upwelling is more intense when the monsoon is at its peak in July compared to May and September and the associated reduction of SST increases towards south in the coastal waters as a result of pure wind-stress forcing. During northeast monsoon season, which is represented in the model by the January wind stress forcing, the model simulations indicate that the wind forcing does not favour upwelling. The relationship between coastal upwelling and longshore currents is also studied. The model predictions of coastal circulation, alongshore current and vertical temperature structure at selected stations are compared with the limited observations available.

Observed low‐salinity plume off Gulf of Khambhat, India, during post‐monsoon period

[2] Arabian Sea (AS) is an area which is strongly influenced by the seasonally reversing monsoon winds. During summer monsoon (June-September), an anticyclonic circulation evolves in the AS. The equatorward component at the eastern boundary of this anticyclonic circulation is known as the West India Coastal Current (WICC). During this period, the surface circulation of the shelf waters in the Arabian Sea has been studied by various workers [Stramma et al., 1996] using the hydrographic data. The seasonal variation of the freshwater flow into the coastal ocean shows that its dynamics is dominated by the estuarine processes. Presence of these low-saline plumes in the coastal water has a great practical relevance to physical and biological problems. The WICC is northward in winter (November-January) which favors the sinking processes along the eastern AS. The in-situ data collected along the west coast shows that the Temperature Inversion (TI) in this area is a stable seasonal feature [ThadathilandGosh,1992].ThedownwelledAShigh-saline sub-surfacewatersarewarmandleadtoTIinthemixedlayer. The dynamics of TI along the west coast of India is explained in detail by Rao et al. [2008]. [3] Even thoughthere are no major river systems along the west coast of India compared to the east coast, it is interesting to study the effect of freshwater discharge on the coastal circulation. The Indus, Mahi, Narmada and Tapti are the main rivers which discharge freshwater into the eastern AS. The freshwater outflow from the Indus River inhibits the upwelling tendency of the isotherms in the surface layer [Sanilkumar and Unny, 2005; Rao et al., 2005, 2008]. Shankar et al. [2005] studied the water-masses off the southeastern Arabian Sea during 2002 summer monsoon. Their study suggests that the freshwater discharge from various rivers located along the west coast of India and rainfall over the Western Ghats forms ephemeral layer of low-saline waters over the eastern AS. The present study is focused to the region of Gulf of Khambhat. The paradoxical nature of the observed salinity field in the waters of this region is pointed out here. The low-salinity plumes are observed in the post-monsoon season and not during the monsoon as may be expected. This has been explained by the local circulation of the region through model studies using the POM. Though the model has been implemented for the entire eastern part of the AS, the simulations are focused particularly for the northeastern region near the Gulf of Khambhat. The details of the model setting and numerical experiments are given by Rao et al. [2008] and hence not repeated here. However, it is worth mentioning that there are about 175 250 26 computational points in the model grid. Resolution in the zonal direction varies from 3–7km with finer resolution near the coast while, it is about 7–10km in the meridional direction. In the present study, the rivers namely Indus, Narmada, Tapi and Mahi are incorporated in the model by replacing rigid boundary condition by an openboundary at the coast where these rivers are joining the AS.