Eulerian Measurements of the North Atlantic Deep Water Deep Western Boundary Current at 18°S

Abstract

An 18-month time series of moored current meter observations near 18°S in the Atlantic is used to study the deep western boundary current (DWBC) of North Atlantic Deep Water (NADW). This flow is taken to extend from about the shelf break seaward about 200 km and downward from the σ2 = 36.7 isopycnal (at about 1200-m depth) to the σ4 = 45.8 isopycnal (at about 3600-m depth). The mean transport is estimated at 39 ± 20 ? 106 m3 s?1. Of the ?20 ? 106 m3 s?1 uncertainty about 80% is due to the uncertainty of the measured velocities due to the 18-month duration of the study and the remainder to choices in filling in data gaps and specifying boundaries of the DWBC and to data dropouts. The DWBC is embedded in a flow that extends downward through the underlying Antarctic Bottom Water (AABW) to the bottom, upward into the overlying Antarctic Intermediate Water (AAIW) at least to 900-m depth, and has a width about 200 km. An expected recirculation just seaward of the DWBC was not found and is attributed to the data showing that a previously assumed level of no motion in the region is indeed not such a level. The current does not follow local topography and flow to the south but rather to the southeast, perhaps due to blocking effects of the Trindade?Vitoria Seamount Chain about 200 km south of the mooring array. The current exhibits a seasonal variability of amplitude about 10 ? 106 m3 s?1 with maximum poleward transport occurring in February?March and minimum transport around September. The seasonal variability is nearly barotropic and appears to be due to the seasonal wind stress variability in the tropical South Atlantic. The AABW beneath the DWBC transports ?4 ? 106 m3 s?1 poleward (comparable in magnitude of the transport of the equatorward-flowing DWBC of AABW, which is found to the east). Although the net AAIW flow above the DWBC is poleward (transport ?8 ? 106 m3 s?1), the data suggest a strong equatorward flow of AAIW just seaward of the shelf break.