Wind and topographic effects on the Labrador current at Carson Canyon

Kinsella, Eric Douglas (1984) Wind and topographic effects on the Labrador current at Carson Canyon. Masters thesis, Memorial University of Newfoundland.

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Abstract

Carson Canyon is located on the eastern slope of the Grand Bank and is about 10 km wide, 80 km long, and 700 m deep where it intersects the shelf break (110 m). Data from June of 1980 and 1981 are interpreted in terms of steady and time-dependent (wind -driven) interactions of the Labrador Current with the canyon. The mean velocity field is estimated from direct measurements and the dynamic topography relative to 200 dbar. Up and downstream of the canyon, the flow was predominantly along-isobath with speeds of 0 (20cm s⁻¹) near the shelf break 10 m above bottom, and 48 cm s⁻¹ at the velocity axis over the 500 m isobath. A 20 m thick cold core of <-1°C water upstream of the canyon separated into inner and outer cold cores over the canyon axis, and coalesced to reform a single core downstream. The degree to which the canyon perturbed the mean flow changed with position both along and perpendicular to the canyon axis with along-isobath flow near the shelf break: cross-isobath flow on the upstream side and cross-canyon flow in the center: further off-shelf, the flow remained undisturbed by the canyon. Divergence of the mean flow in the vicinity of a secondary head on the upstream side is indicated by the measurements, implying steady upwelling. This is supported by a 0.9 cm s⁻¹ residual, near-bottom flow toward the secondary head. A single upwelling event lasting 5 d was observed following 18 - 23 m s⁻¹ southwesterly winds of about 0.5 d duration. Current reversal at the shelf break occurred soon after the high speed winds ceased, following by abrupt temperature (2.8°C) and salinity (1.2 ppt) increases up to 1 d later. At 167 m depth in the central canyon region isohalines rose at rates of 0.2 - 0.5 cm⁻¹. The reversal was baroclinic: the Labrador Current continued to flow southwestward, but with reduced speeds and the reversal itself was confined to a layer centered at about 130 m depth. The reversal and decreased upper-layer speeds probably arise from the barotropic response to a wind-induced decrease in sea surface slope: the upwelling represents the slower baroclinic response. Steady upwelling in the presence of a shelf-break jet in the upper layer is modeled using a two-layer system and a vertical wall at the shelf break. The model results differ from the usual (no jet) case: upwelling is suppressed by 26% at the coast, but extends to greater distances offshore.

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