A model study of the seasonal cycle of the Arabian Sea surface temperature

The Annual variation of the SST along a zonal strip from the coast of Somalia to the southwest coast of India was simulated using available data (monthly-mean heat and momentum fluxes across the air-sea interface, surface advective field, etc.) as input to a Kraus-Turner mixed-layer model. Three cases were examined. In the first, influence of surface fluxes alone was considered. The second included the effects of surface fluxes and vertical advection. Then, effect of horizontal advection was added. The model forced with the surface heat and momentum fluxes alone simulated reasonably well the SST variability throughout the year except during the May-August (southwest monsoon) cooling phase. The model was found to be inadequate to handle the coastal areas during this phase. Over the open-sea regime the performance of the model was better; and, it improved when the influence of advection was included. The important contribution of the horizontal advection during June-August was to remove most of the heat gained at the surface during the course of a year. Though downwelling in the open-sea had little influence on the SST, it had noticeable impact on the vertical heat transport. The numerical experiments suggest that the Kraus-Turner thermodynamics alone dominate the Arabian SST variability throughout the year except during the southwest monsoon, when dynamics too playa significant role

Thermohaline forcing of eastern boundary currents: with application to the circulation off the west coast of Australia

The linear, viscid, continuously stratified model of McCreary (1981) is extended to allow for thermohaline forcing by a specified, longitudinally independent, surface density field ρs. When the ocean is unbounded and ρs is steady, the density field is altered throughout the water column by vertical diffusion. If ρs increases poleward, the resulting pressure field slopes downward toward the pole in the upper water column, and there is an associated eastward geostrophic current. This interior current forces downwelling at an eastern ocean boundary, and generates a poleward surface coastal current and an equatorward undercurrent. For realistic choices of model parameters the coastal circulation is as strong as, and opposite in direction to, that caused by a typical equatorward wind stress τy. When ρs oscillates at the annual cycle, the unbounded flowis confined to a surface boundary layer. The coastal circulation is qualitatively quasi-steady, but also has characteristics of a vertically propagating Kelvin wave. One solution is forced by an idealized representation of the observed ρs and τy fields off the west coast of Australia. This solution compares well with observations of the oceanic circulation there. In particular, there is a surface coastal jet (the model Leeuwin Current) that flows against the wind. This successful comparison suggests that the mean circulation in the region is significantly forced by ρs, whereas the annual variability is strongly forced by τy

Monthly mean wind stress along the coast of the north Indian Ocean

Monthly-mean wind stress and its longshore and offshore components have been computed using the bulk aerodynamic method for each of a string of 36 two-°-latitude by two-°-longitude squares along the coast of the north Indian Ocean. The data source for the computation is the sixty-year mean resultant winds of Hastenrath and Lamb. The main features exhibited by the components, taking the longshore components as positive (negative) when the Ekman transport is away from (towards) the coast, are: (1) Along the coasts of Somalia and Arabia, the magnitude of the wind stress is among the highest in the north Indian Ocean, and its direction is generally parallel to the coastline. This results in a longshore component which is large (as high as 2·5 dyne/cm2) and positive during the southwest monsoon, and weaker (less than 0·5 6 dyne/cm2) and negative during the northeast monsoon. (2) Though weak (less than 0·5 2 dyne/cm2) during the northeast monsoon, the monthly-mean longshore component along the west coast of India remains positive throughout the year. The magnitude of the offshore component during the southwest monsoon is much larger than that of the longshore component. (3) The behaviour of the wind stress components along the east coast of India is similar to that along the Somalia-Arabia coast, but the magnitudes are much smaller

West India coastal current and Lakshadweep high/low

The West India Coastal Current (WICC) flows northward during November-February and southward during April-September. At the time of formation of the northward (southward) phase of the current, a high (low) in sea level-the Lakshadweep High (Low), forms off southwestern India, and migrates westward across the Arabian Sea. The annual cycle of the WICC and that of the Lakshadweep High/Low arise from a set of circumstances that are special to the North Indian Ocean. This relatively small tropical basin is driven by seasonal monsoon winds. As a result, its wind-driven near-surface circulation consists primarily of annual and semi-annual long, equatorially-trapped Kelvin and Rossby waves, and coastally-trapped Kelvin waves. In terms of these waves, the West India Coastal Current is a superposition of annual and semiannual coastally-trapped Kelvin waves. The Lakshadweep High/Low forms when the Kelvin waves, on turning around Sri Lanka, and propagating northward along the west coast of India, radiate Rossby waves

Tides in the Gulf of Kutch, India

In the Gulf of Kutch on the northwestern shelf of India the semi-diurnal constituents M<SUB>2</SUB> and S<SUB>2</SUB> get amplified approximately threefold. In contrast, amplification of the diurnal constituents K<SUB>1</SUB> and O<SUB>1</SUB> is small. From analytical and numerical solutions of linear, viscous, cross-sectionally averaged equations for tidal motion in a channel, it is seen that the observed amplification results from a combination of quarter-wavelength resonance, geometric effect due to decrease in width of the channel, and friction

Propagation of bottom-trapped waves over variable topography

The behavior of bottom-trapped waves on a topographic slope, as they propagate towards a deeper region of constant depth, is examined using a quasi-geostrophic model. It is seen that there can be no free transmitted wave to the region of constant depth. The bottom-trapped energy is thus segregated in the region of the slope and its vicinity. The details of how this occurs are worked out for a slope shoaling towards the west. For a given frequency and meridional wavenumber, two free bottom-trapped waves can exist on the slope, with the shorter (longer) of the two having an eastward (westward) group velocity. When the short wave propagates to the region of constant depth, a reflected wave is generated. There is no transmitted wave, but a 'fringe' which decays away from the interface between the slope and the region of flat topography is produced. Over the flat topography the fringe consists of baroclinic and barotropic motions which lead to bottom-intensification in the immediate vicinity of the slope and to increasingly barotropic currents farther away

The movement and implications of the Ganges-Brahmaputra runoff on entering the Bay of Bengal

The Ganges-Brahmaputra rivers discharge annually approximately 10 sup(2) m sup(3) of freshwater into the Bay of Bengal at its northern end. We propose that the spread of this water, accompaniEd. by mixing with the ambient waters, occurs in three phases. The Ganges-Brahmaputra discharge appears to lead to cooler sea surface temperatures in the northern Bay and to a stronger coastal current along the West Coast of India, both during the northeast monsoon

The effect of varying bathymetry on steady, zonal equivalent-barotropic jets

The influence of sudden changes in bathymetry on zonal, equivalent-barotropic, steady oceanic jets on a β plane is considered. Solutions to the perturbations induced by a step-like change in bathymetry yield the well-known results that an eastward-flowing jet will oscillate about the mean axis downstream of the step; upstream the perturbation results in a shear re-adjustment. However, the amplitude of an individual streamlined oscillation depends upon its lateral position in the incident jet. For westward-flowing jets, the perturbation results in a northward or southward migration of streamlines, accompanied by a shear adjustment. Both eastward and westward jets intensity on entering a deeper region and broaden on entering a shallower region. This change in width arises from the opposite sense of the shear vorticities on the two flanks of the jet. The hitherto unexplained broadening of the Kuroshio over the Izu Ridge has been shown to be quantitatively determined by the simple model presented

Biological control of surface temperature in the Arabian Sea

By far the dominant variable parameter controlling the absorption cross-section for short-wavelength solar radiation incident on the ocean surface is the concentration of photosynthetic pigment contained in phytoplankton cells. The abundance of phytoplankton depends on the intensity of incident radiation and on the supply of essential nutrients (nitrogen in particular). A higher abundance increases absorption of radiation and thus enhances the rate of heating at the ocean surface. In the Arabian Sea, the southwest monsoon promotes seasonal upwelling of deep water, which supplies nutrients to the surface layer and leads to a marked increase in phytoplankton growth. Using remotely sensed data on ocean colour, we show here that the resulting distribution of phytoplankton exerts a controlling influence on the seasonal evolution of sea surface temperature. This results in a corresponding modification of ocean-atmosphere heat exchange on regional and seasonal scales. Thus we show that this biological mechanism may provide an important regulating influence on ocean-atmosphere interactions