The Leeuwin Current off Western Australia, 1986–1987

Robert L. Smith College of Oceanography, Oregon State University, Corvallis, Oregon

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Adriana Huyer College of Oceanography, Oregon State University, Corvallis, Oregon

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J. Stuart Godfrey CSIRO Division of Oceanography, Hobart, Tasmania

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John A. Church CSIRO Division of Oceanography, Hobart, Tasmania

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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 current over the upper slope exceeded 50 cm s−1 poleward at times and had a 325-day mean of 30 cm s−1; an equatorward undercurrent existed below about 300 m and had a 325-day mean of 10 cm s−1 at 450 m. The strong, narrow Leeuwin Current depends on the large-scale alongshore gradient of geopotential anomaly at the sea surface, with a value greater than 2 × 106 m s−2, which is anomalously large compared to other eastern boundary regions. The onshore geostrophic transport exceeded the offshore Ekman transport induced by the equatorward wind stress, and was presumably balanced over the upper slope and outer shelf by the offshore Ekman transport near the bottom under the Leeuwin Current. The seasonal variation in the strength of the Leeuwin Current seemed to be the result of variations in the wind stress and not in the alongshore pressure gradient, which had little seasonal dependence.

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 current over the upper slope exceeded 50 cm s−1 poleward at times and had a 325-day mean of 30 cm s−1; an equatorward undercurrent existed below about 300 m and had a 325-day mean of 10 cm s−1 at 450 m. The strong, narrow Leeuwin Current depends on the large-scale alongshore gradient of geopotential anomaly at the sea surface, with a value greater than 2 × 106 m s−2, which is anomalously large compared to other eastern boundary regions. The onshore geostrophic transport exceeded the offshore Ekman transport induced by the equatorward wind stress, and was presumably balanced over the upper slope and outer shelf by the offshore Ekman transport near the bottom under the Leeuwin Current. The seasonal variation in the strength of the Leeuwin Current seemed to be the result of variations in the wind stress and not in the alongshore pressure gradient, which had little seasonal dependence.

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