Abstract

The mean North Atlantic Deep Water (NADW, 1000< z <5000 m) circulation and Deep Western Boundary Current (DWBC) variability offshore of Abaco, Bahamas at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-year long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24-30°N and 72-77°W. The southward flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv. Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5-16 Sv. Current meter records spanning 2008-2017 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: (1) periods of 250-280 days that occur regularly throughout the time-series; and (2) energetic oscillations with periods between 400-700 days that occur sporadically every 5-6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24-30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20-26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby Waves reflecting from the western boundary.

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Footnotes

Current affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1031, USA.