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On the Average Absolute Transport of the Deep Western Boundary Currents East of Abaco Island, the Bahamas

Kevin D. LeamanUniversity of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, Florida

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Jessie E. HarrisNOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida

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Abstract

Data obtained from a line of absolute current profiler (PEGASUS) and CTD stations at 26.5°N, east of Abaco Island in the northern Bahamas, are used to estimate the average transport, and overall level of variability, of the deep (below 800 m) southward flow associated with the so-called “Deep Western Boundary Current”(DWBC) at this latitude. From April 1985 to September 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the Subtropical Atlantic Climate Studies (STACS) program. In all but two cases (when PEGASUS data were obtained over the total depth), absolute velocities were determined from 0 to about 3000 db while CTD derived relative velocities were determined over the total depth.

Using the combined datasets, it is possible to reference the relative geostrophic velocity with PEGASUS data, thereby obtaining an estimate of the absolute southward transport of the deep flow. The average value of transport determined in this manner is about 35 Sv (1 Sv ≡ 106m3s−1. Most of this large transport is located in a deep (2000 m) offshore core, with average southward velocities of ∼0.20 m s−1. This core is not co-located with the “Deep Western Boundary Current” core as defined by tracer studies. It is suggested here that this core may either be part of a southern deep recirculation gyre similar to those proposed farther north (e.g., nearer the Gulf Stream) or alternatively represents the augmented DWBC flow found in some theoretical models.

Abstract

Data obtained from a line of absolute current profiler (PEGASUS) and CTD stations at 26.5°N, east of Abaco Island in the northern Bahamas, are used to estimate the average transport, and overall level of variability, of the deep (below 800 m) southward flow associated with the so-called “Deep Western Boundary Current”(DWBC) at this latitude. From April 1985 to September 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the Subtropical Atlantic Climate Studies (STACS) program. In all but two cases (when PEGASUS data were obtained over the total depth), absolute velocities were determined from 0 to about 3000 db while CTD derived relative velocities were determined over the total depth.

Using the combined datasets, it is possible to reference the relative geostrophic velocity with PEGASUS data, thereby obtaining an estimate of the absolute southward transport of the deep flow. The average value of transport determined in this manner is about 35 Sv (1 Sv ≡ 106m3s−1. Most of this large transport is located in a deep (2000 m) offshore core, with average southward velocities of ∼0.20 m s−1. This core is not co-located with the “Deep Western Boundary Current” core as defined by tracer studies. It is suggested here that this core may either be part of a southern deep recirculation gyre similar to those proposed farther north (e.g., nearer the Gulf Stream) or alternatively represents the augmented DWBC flow found in some theoretical models.

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