Variability of the Western Boundary Current System at 11°S

The circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flows. The zonally integrated meridional flow is associated with the Atlantic Meridional Overturning Circulation (AMOC) — a major component of the global climate system. In the tropics, the northward, upper branch of the AMOC flow is superimposed by the shallower overturning associated with the wind-driven Subtropical cells (STC).

The western boundary observing system of the TRACOS (Tropical Atlantic Circulation & Overturning at 11°S) array consists of four tall moorings monitoring the strong western boundary current system – more specifically, the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) as part of the AMOC, the STC, and the wind-driven gyre system. More than 15 years of moored current meter observations, collected between 2000 and 2004 and continuously since 2013 within the NBUC and DWBC, are analyzed for their variability across different time scales. NBUC and DWBC transports are calculated from alongshore velocity data by regressing current meter observations onto variability patterns. These patterns are obtained from 15 ship sections, derived by combining underway vessel mounted ADCP measurements with on-station lowered ADCP measurements to provide full-depth alongshore velocity fields. The NBUC transport is dominated by a seasonal signal with only minor interannual variations and no obvious trend, while the DWBC is dominated by strong intra-seasonal variability induced by the passage of deep eddies. Despite these deep eddies being evident throughout the DWBC transport time series, longer term variability is also becoming evident as the time series is prolonging.