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Adriana Huyer

Abstract

Repeated CTD observations were made along a single section spanning the continental shelf off northern California at 38°40′N during 1981. The section consisted of nine standard stations between 1 and 46 km from the shore in water depths between 40 and 1700 m. The shelf break was at a depth of 150 m about 25 km from shore. The section was occupied 17 times between 13 April and 3 August and twice in December, and a similar section had been occupied in February.

During the April–August period, winds were strong and persistently favorable for upwelling. Isotherms, isohalines and isopycnals in the upper 200 in sloped persistently upward toward the coast, and coldest, saltiest and densest surface waters almost always occurred at the most inshore nation. Variations in wind strength caused changes in the surface layer over the entire shelf: stronger winds were associated with lower temperatures and higher salinities. Subsurface temperatures increased gradually between April and August. We were unable to account for most of the salinity variance during this period.

Shelf waters were considerably warmer and less saline in winter, when the wind direction and speed were highly variable. Isotherms, isohalines and isopycnals were nearly level in winter. Dynamic height and coastal sea level were high in winter and low in summer, there was good agreement between them.

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Adriana Huyer

Abstract

Between April 1976 and May 1977, more than 35 repeated hydrographic sections were made just south of Cabo Nazca, Peru at 15°S, and sea level was measured continuously by a tide gage at San Juan. The sea level data were filtered to remove diurnal and shorter variations, and adjusted for the inverted barometer effect. For each hydrographic section, the dynamic height of the sea surface was computed relative to 500 and 600 db. Changes in the dynamic height at the station nearest the coast agree well with changes in the coastal sea level. The dynamic height data are therefore a good representation of the height of the sea surface along the section, and are used to examine the offshore structure of sea level fluctuations. The amplitude of these fluctuations decreases rapidly with distance from shore, with an offshore length scale of 30–60 km. The variations in sea level are associated with changes in the depth of subthermocline isotherms at depths of ∼300–500 m, rather than with a change in the depth of the thermocline which remains between 50 and 100 m. The variations in sea level are also associated with fluctuations in the alongshore geostrophic velocity. Near the continental slope the amplitude of the geostrophic velocity fluctuations is constant down to ∼300 m and then decreases rapidly down to ∼500 m. These characteristics are consistent with the idea that the sea level variations are a manifestation of a poleward propagating internal Kelvin wave, with the interface at ∼400 m rather than at the thermocline. Mean geostrophic velocity profiles show a poleward undercurrent with maximum southeastward flow of about 15 cm s−1 at a depth of 100 m near the continental slope (35 km from shore); the mean undercurrent is barely discernible 60 km from shore, and not observed 100 km from shore.

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Adriana Huyer
and
Robert L. Smith

Abstract

Observations of the hydrographic regime over the continental shelf off Oregon from the R/V Yaquina during the summer of 1972 showed the presence of an alongshore, subsurface ribbon of relatively cool water. Its properties and its evolution during the 1972 season are described. Examination of earlier observations showed that evidence of the ribbon was found during the upwelling season in almost every year between 1961 and 1971. It is usually observed when the upwelling index (the monthly mean Ekman transport directed offshore) is high. The ribbon can be accounted for by southward advection of sub-arctic water due to the coastal jet associated with upwelling. A warm temperature anomaly, occurring at somewhat higher salinity, is frequently observed inshore of the ribbon.

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Pijusi K. Kundu
,
Robert C. Beardsley
, and
Adriana Huyer

Abstract

Datasets taken near the coasts of Peru and California have been analyzed to explore the evidence of a correlation between the high frequency (period < 10.8 hour) and low frequency (period > 4 day) motions. A large part of the high frequency current is consistent with internal wave dynamics. They have a near 180° phase change across the water column; there is also evidence of a clockwise veering with depth, presumably due to frictional effects. No correlation of the vertical Reynolds stress vw and vertical shear Vz is found. The horizontal stress uv is found to be negatively correlated with the horizontal shear V x only when averaged over time and space scales larger than those of the mean velocity field. The implied eddy viscosity is of order ν H ∼105 cm2 5−1. It is suggested that the relaxation time for the wave field to lose asymmetry is larger than the time for the wave to propagate across a typical scale of the mean shear field. Due to this the fluctuating field may locally behave like a negative eddy viscosity at certain places.

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Robert L. Smith
,
Adriana Huyer
,
J. Stuart Godfrey
, and
John A. Church

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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