Leeuwin Current Off Research Articles

Thermohaline forcing of the Indian Ocean by the Pacific Ocean

An idealized box model of the Indian Ocean forced by steady winds and Haney-type surface heat fluxes is used to examine the importance of the warm, fresh throughflow from the equatorial Pacific on the climate of the Indian Ocean. In particular, the hypothesis proposed by Godfrey and Weaver (1991, Progress in Oceanography, 27, 225–272), that the anomalous poleward buoyancy-forced Leeuwin current off the west coast of Australia is a manifestation of a basinwide thermohaline circulation driven by the Indonesian throughflow, is examined. The stronger Sverdrup circulation dominates the thermohaline circulation in most of the model ocean except near the eastern boundary. The effects of the throughflow, however, can be determined by comparing two runs forced by a Pacific Ocean with either the warm, fresh profile of the western equatorial Pacific or a cooler, more saline profile more typical of the eastern equatorial Pacific (where a more usual equatorward wind-driven boundary current is found). It is found that heat imported from the Pacific is transported zonally across the Indian Ocean to the western boundary by the South Equatorial Current. The enhanced meridional steric height gradient south of the SEC drives an eastward return flow back to the eastern boundary, where it turns south to form the poleward Leeuwin Current. The reverse path is traced out by the waters immediately below the thermocline. None of these features are observed when the model Pacific has the cooler profile typical of the eastern boundaries of other oceans. Indonesian throughflow significantly affects the surface heat fluxes and the meridional heat transport in the Indian Ocean. The importance of the throughflow in maintaining the very warm climate of the Indian Ocean (a net exporter of heat) is noted. In the model, the poleward western boundary current along the coast of Africa south of about 27°S appears to play only a very minor role in the basinwide thermohaline circulation described above. This differs from the “warm water route” proposed by Gordon (1986, Journal of Geophysical Research, 91, 5037–5046) where heat is returned to the South Atlantic past the Agulhas Retroflection. Large-scale, fairly long period (>100 days) barotropic eddies are found in the western portion of the basin for some solutions. The generation mechanism for these eddies appears to be barotropic instability in the model South Equatorial Current.

The coastal current off western India during the northeast monsoon

Hydrographic data collected during December 1987 and January 1988 are used to describe the poleward coastal current along the west coast of India that flows against weak winds during the northeast monsoon. Near the southern end of the coast, at about 10°N, the current was approximately 400 km wide, 200 m deep and carried the low salinity Equatorial Surface Water. The isopycnals tilted down on approaching the coast. Near the northern end of the coast, at about 22°N, the flow was restricted mainly to the vicinity of the continental slope; the current was a narrow (100 km), 400 m deep jet with a transport of about 7 × 10 6 m 3 s −1. Along most of the coastline, a southward moving undercurrent was inferred from the distribution of salinity, temperature and dynamic topography. To understand the driving mechanism of the current, the annual cycle of the contribution of the longshore pressure gradient and that of the winds to the near-surface momentum balance was examined using available climatologies. It is seen that the longshore pressure gradient overwhelms the winds during the northeast monsoon, whereas during the southwest monsoon the winds dominate. In the Leeuwin Current off western Australia, the only other known eastern boundary current that flows against the winds, the pressure gradient dominates the winds throughout the year. The overall structure of the northeast monsoon coastal current is consistent with that predicted in the analytic model proposed by McCreary et al. (1986, Journal of Marine Research, 44, 71–92) to explain the Leeuwin Current. In the model a poleward baroclinic pressure gradient is generated by a density gradient along the coast. The latter is known to exist along the west coast of India. However, it seems likely that the current is also influenced by a barotropic pressure gradient.

The Leeuwin Current off Western Australia, 1986–1987

Abstract The Leeuwin Current in the Indian Ocean off Western Australia differs from the other major eastern boundary currents, e.g., California Current, since it flows rapidly poleward against the prevailing equatorward wind. The first large-scale study of the Leeuwin Current was conducted between North West Cape (22°S) and the south-western corner of Australia (35°S) from September 1986 to August 1987. As part of this Leeuwin Current Interdisciplinary Experiment (LUCIE), current meters were deployed along the shelf-edge (from 22° to 35°S) and across the shelf and upper slope (at 29.5° and 34°S), and CTD surveys were made out from the shelf at several latitudes. Except for about one month (January) the flow between the surface and about 250 m was strongly poleward within 100 km of the shelf-edge, with a poleward transport of about 5 Sv (Sv ≡ 6 m3 s−1). The 325-day mean currents at the shelf-edge were poleward at about 10 cm s−1, opposing a mean equatorward wind stress of 0.3 dyn cm−2. The monthly mean cur...

An analytic model for the Leeuwin Current off western Australia

A linear, steady-state, analytic model for the Leeuwin Current is developed by coupling a continental shelf region to a two layer deep ocean. The model is forced by both the observed annual mean alongshore density gradient in the Indian Ocean and the seasonal winds. It is shown that even during the period between September and February, when the winds are strongest, the forcing due to the alongshore density gradient in the Indian Ocean is sufficient to overcome the opposing wind effects, with the net result that a strong southward flow exists near the shelf break. As the current flows southwards it accelerates into the opposing winds. It is shown that average seasonal southerly wind speeds of the order of 17 m s −1 are required to arrest the southward transport. These speeds, while attainable during the strongest storms, are unrealistic for an average over a seasonal timescale. When the same analytic model is applied to the Peru Current off the western coast of South America the flow is everywhere northward. A discussion of the differences between these two regions is presented. It is deduced that the anomalous poleward-flowing Leeuwin Current is driven by a deep alongshore density gradient which is set up in the top 200 m of the Indian Ocean by convective surface cooling.

On the Dynamics of the Throughflow from the Pacific into the Indian Ocean

Abstract The circulation forced by an inflow of water through an eastern ocean boundary is investigated using two linear, viscid, and continuously stratified models. One of the models has a flat bottom, and solutions are obtained analytically; the other has a continental shelf, and solutions are found numerically. Without vertical mixing all the inflow continues across the ocean. With vertical mixing, however, part of it bends poleward to generate a coastal circulation. The presence of a shelf displaces the coastal currents offshore, but otherwise changes their structure and magnitude very little. Solutions suggest that the southward bending of the throughflow from the Pacific into the Indian Ocean may contribute to the Leeuwin Current off western Australia, but that it is not the dominant mechanism for driving the circulation there.

Observations of the Leeuwin Current off Western Australia

Abstract Shipboard observations made in May 1982 showed a definite poleward surface flow (the Leeuwin Current) over the West Australian shelf from 22°S to 28°S. The surface current was relatively fresh, warm, low in dissolved oxygen concentration, and high in nutrients. The current flowed against a strong wind. Only a small portion of its flux of 4 × 106 m3 s−1 came from the Northwest Shelf. There was a subsurface equatorward current at a few hundred meters depth which was salty, high in oxygen concentration and low in nutrients. Observations from previous cruises show a surface geopotential gradient that could drive the surface current. It is suggested that winter deepening of the mixed layer may allow the geopotential gradient to overcome the wind stress.

Observations of the Leeuwin Current off Western Australia : Thompson, Rory, 1984. J. phys. Oceanogr., 14(3):623–628

Observations of the Leeuwin Current off Western Australia : Thompson, Rory, 1984. J. phys. Oceanogr., 14(3):623–628