Moored Observations of Western Boundary Current Variability and Thermohaline Circulation at 26.5° in the Subtropical North Atlantic

Abstract

A 5.8-year time series of moored current meter observations is used with hydrographic section data, CME model results, and gridded wind fields over the North Atlantic to describe the mean structure and variability of circulation and volume transports east of Abaco, Bahamas, at 26.5°N. A mean Antilles Current, with 5 Sv of northward transport, is confined against the Bahamas boundary in the upper 800 m and combines with approximately 19 Sv of Florida Current transport to balance the Sverdrup interior circulation, and does not contribute to interhemispheric exchange. The mean transport of the deep western boundary current (DWBC) off the Bahamas is approximately 40 Sv, of which 13 Sv compensates the upper branch of the thermohaline circulation, requiring a 27 Sv deep recirculation. Robust annual and semiannual cycles of meridional are found in both moored observations and model results with remarkable agreement in amplitude (±13 Sv) and phase. Maximum northward transports occur in winter and summer, and minimums occur in fall and spring due to a predominantly barotropic response to remote and local seasonal wind forcing. Transport variability on timescales less than semiannual is dominated by mesoscale eddies that propagate westward into the Bahamas boundary in the thermocline at periods of 70–100 days, wave speeds of about 4 cm s−1, and wavelengths of about 335 km. These events are frequently correlated with offshore shifts of the DWBC core.