Shallow and Deep Eastern Boundary Currents in the South Atlantic at 34.5 degrees S: Mean Structure and Variability

The first in situ continuous full-water-column observations of the eastern boundary currents (EBCs) at 34.5 degrees S in the South Atlantic are obtained using 23months of data from a line of Current and Pressure recording Inverted Echo Sounders (CPIES) spanning the Cape Basin. The CPIES are used to evaluate the mean structure of the EBC, the associated water masses, and the volume transport variability. The estimated northward time-mean Benguela Current absolute geostrophic transport is 24Sv, with a temporal standard deviation of 17Sv. Beneath this current the time-mean transport is southward, indicating the presence of a deep-EBC (DEBC), with a time-mean transport of 12Sv, and a standard deviation of 17Sv. Offshore of these currents, the shallow and deep flows are more variable with weak time means, likely influenced by Agulhas Rings transiting through the region. Hydrographic data collected along the CPIES line demonstrate that the DEBC is carrying recently ventilated North Atlantic Deep Water, as it flows along the continental slope. This is consistent with a previously hypothesized interior pathway bringing recently ventilated North Atlantic Deep Water from the Deep Western Boundary Current across the Atlantic to the Cape Basin. The observations further indicate that much of the DEBC must recirculate within the basin. Spectral analyses of the shallow and deep EBC transport time series demonstrate that the strongest variability occurs on timescales ranging from 30 to 90days, associated with the propagation of Agulhas Rings. Plain Language Summary Heat and freshwater transported by the overturning circulation in the Atlantic have been shown to have significant influences on global climate. Boundary current variations in the Southeast Atlantic have furthermore been shown to play a major role in the overturning circulation. This study presents, for the first time, full-depth, daily observations of the eastern boundary currents at 34.5 degrees S based on moored instruments. The 23months of observations (velocity and water mass properties) reveal the presence of two distinct eastern boundary currents, with equatorward flow in the upper layer and poleward flow in the deep layer. The variability of both currents is characterized, in unprecedented detail, and is shown to be dominated by large anticyclonic eddies transiting across the moorings. The observations further indicate that much of the deep current must recirculate within the Cape Basin, with a smaller portion of the flow participating in the global overturning circulation.