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Western Boundary Current Structure and Variability East of Abaco, Bahamas at 26.5°N

Thomas N. LeeUniversity of Miami, Rosentiel School of Marine and Atmospheric Science, Miami, Florida

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William JohnsUniversity of Miami, Rosentiel School of Marine and Atmospheric Science, Miami, Florida

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Rainer ZantoppUniversity of Miami, Rosentiel School of Marine and Atmospheric Science, Miami, Florida

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Friedrich SchottInstitut f¨Meereskunde, Kiel, Federal Republic of Germany

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Abstract

During April 1987 to June 1988, a four-mooring, transport-resolving current meter array was deployed in a section extending 70 km eastward from Abaco, Bahamas, at 26.5°N. Mean currents in the upper layer (≤800 m) showed a clockwise rotation that appears to be associated with a small scale, quasi-permanent, anticyclonic gyre centered just northeast or the Bahamas. Deep mean currents were persistenly southward and indicated a strong, deep jet, the Deep Western Boundary Current (DWBC), reaching core speeds of 20 cm s−1at 2500 m depth about 25 km seaward of the western boundary.

Vertically integrated meridional volume transport over the section showed a surprisingly large variability, ranging from approximately 20 Sv northward to 70 Sv southward (1 Sv = 106m3s−1). The mean meridional volume transport was 30 Sv to the south, with about 3 Sv flowing northward above 800 m, which could be produced by a weak Antilles Current and 33 Sv flowing, southward below 800 m. The deep southward transport of 33 Sv is at least a factor oftwo larger than previous estimates of interbasin exchange of North Atlantic Deep Water (NADW), and suggests the existence of a deep cyclonic gyre that recirculates NADW into the interior. Temporal variations in the volume transport as well as in individual velocity records were dominated by strong, nearly barotropic events of approximately 100-day time scale that appear to result from meandering of the DWBC core. No significant annual variation in the volume transport was apparent, contrary to predictions by basin-scale wind-driven models of a large annual transport signal in boundary currents east of the Bahamas.

Abstract

During April 1987 to June 1988, a four-mooring, transport-resolving current meter array was deployed in a section extending 70 km eastward from Abaco, Bahamas, at 26.5°N. Mean currents in the upper layer (≤800 m) showed a clockwise rotation that appears to be associated with a small scale, quasi-permanent, anticyclonic gyre centered just northeast or the Bahamas. Deep mean currents were persistenly southward and indicated a strong, deep jet, the Deep Western Boundary Current (DWBC), reaching core speeds of 20 cm s−1at 2500 m depth about 25 km seaward of the western boundary.

Vertically integrated meridional volume transport over the section showed a surprisingly large variability, ranging from approximately 20 Sv northward to 70 Sv southward (1 Sv = 106m3s−1). The mean meridional volume transport was 30 Sv to the south, with about 3 Sv flowing northward above 800 m, which could be produced by a weak Antilles Current and 33 Sv flowing, southward below 800 m. The deep southward transport of 33 Sv is at least a factor oftwo larger than previous estimates of interbasin exchange of North Atlantic Deep Water (NADW), and suggests the existence of a deep cyclonic gyre that recirculates NADW into the interior. Temporal variations in the volume transport as well as in individual velocity records were dominated by strong, nearly barotropic events of approximately 100-day time scale that appear to result from meandering of the DWBC core. No significant annual variation in the volume transport was apparent, contrary to predictions by basin-scale wind-driven models of a large annual transport signal in boundary currents east of the Bahamas.

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