The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

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.

On the Variability of the DWBC Transport Between 26.5°N and 16°N in an Eddy‐Rich Ocean Model

“…Temporal variability on the northern side is weaker in the model, although this could be a bias of VIKING20X. According to a recent study by Biló and Johns (2020), temporal variability is mainly driven by southward (downstream) propagating meanders and (less frequent) northward propagating eddies, which is in agreement with our results. Temporal variability further south shows different patterns and is associated with southward propagating mesoscale eddies between 24°N and 20°N, and upstream propagating meanders (along the coast of Puerto Rico).…”

Wind-driven and buoyancy-driven circulation in the subtropical North Atlantic Ocean

“…The net southward transport below 1100 m depth averages 12 Sv for UNADW (1100 to 3000 m depth) and 6 Sv for LNADW (3000 to 5000 m depth). These net southward transports are each about 5 Sv smaller than the nearboundary DWBC transports estimated by Biló & Johns [3] suggesting a northward mid-ocean recirculation of deep water of about 10 Sv. The Rapid end stations estimates do not define the zonal extent of the recirculation.…”

“…In the mean, it is confined within 150 km of the western boundary defined by the Bahamas. From moored current metre observations, Biló & Johns [3] estimated a timeaveraged southward transport of 28.3 Sv in the boundary current; 60% (17 Sv) of this transport is in the UNADW between 1100 and 3000 m depth and 40% (11 Sv) in the LNADW between 3000 and 5000 m depth. Offshore beyond 150 km from the Bahamas, there is northward recirculation of the deep waters.…”

“…From available current metre moorings, Bryden et al [40] estimated a northward recirculation of 13 Sv from 160 to 580 km offshore. Biló & Johns [3] used Argo profiles plus shipboard CTD and LADCP profiles to suggest a northward recirculation of 8 Sv between 150 and 500 km offshore with much of the recirculation just to the east of the boundary current in the deep Abaco gyre. Because of the uncertainty in the zonal extent of the recirculation, the overall southward transport of UNADW and LNADW across the 26°N section is not well defined by near-boundary observations.…”

“…At a latitude of 26°N, the principal northward flows are in the Gulf Stream through Florida Straits above 800 m depth and Antilles Current in a 45 km zone east of the Bahama Islands also above 800 m depth and in the midocean wind-driven surface Ekman layer. The principal southward flows are in the deep western boundary currents (DWBCs) between 1100 and 5000 m depth in a 150 km wide zone just east of the Bahamas [3] and in the mid-ocean geostrophic thermocline flow from the surface to 800 m depth. The southward mid-ocean thermocline flow is generally considered to recirculate northward in the western boundary currents.…”