Deep Western Boundary Current variability at 26.5ºN and its connection with local abyssal recirculation.

Tiago Carrilho Bilo, University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL, United States and William E Johns, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, United States
Deep Western Boundary Current (DWBC) mesoscale activity within the Abaco Gyre (i.e., ~24-30ºN and 1000-5000 m depth) is investigated using velocity observations obtained in the past 20 years at 26.5ºN and outputs from an eddy-resolving numerical simulation. A wavelet analysis of the current meter derived DWBC transport variability (2008-2017) indicates that the transport variance is mainly dominated by two kinds of variability: (1) perturbations with periods of 250-270 days that occur regularly throughout the time series; and (2) an energetic oscillation with period of approximately 540 days that occurs sporadically and with long intervals between successive events (i.e., 2-5 years). Combining the observational results with the outputs from a 30-year eddy-resolving simulation, we find that the shorter-period transport variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24-30ºN. In contrast, the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward along the deep continental slope between ~18-27ºN, counter to the mean direction of the DWBC. The consistency of the model results with the available observational record provides a new understanding of the mesoscale dynamics within the DWBC that differs from previous studies. An assessment of the modeled mean potential vorticity (PV) budget additionally shows that the convergence of eddy-PV fluxes – generated by these two types of oscillations – is responsible for forcing the DWBC mean flow to locally recirculate and form the deep Abaco Gyre.