As part of the global thermohaline circulation, some North Atlantic Deep Water (NADW) exits the Atlantic basin to the south of Africa. Observations have shown that there is a quasi-zonal pathway centred at 25°S carrying NADW eastward, connecting the Deep Western Boundary Current to the Cape Basin. However, it has been unclear what sets this pathway. In particular, waters must move southward through the Cape Basin, thereby crossing isolines of planetary vorticity, in order to exit the basin. Here, we find that an eddy thickness flux induced by Agulhas rings moving northwestward, forces a circulation of NADW through the Cape Basin. The pathway at 25°S feeds the southeastward flow of this circulation while conserving potential vorticity. Using Lagrangian floats advected for 300 years in a 1/10° resolution ocean model, we show that the most common pathway for NADW in our model lies directly below the Agulhas ring corridor. By analysing the velocity and density fields in the model, we find that the decay of these rings, and their forward tilt with depth, results in a southward velocity, across isolines of planetary vorticity, of 1 to 2 cm/s in the deep waters. The associated stream function pattern yields a deep circulation transporting 4 Sv of NADW from the Deep Western Boundary Current at 25°S to the southern tip of Africa.
Movie
Tracing North Atlantic Deep Water in the South Atlantic using Lagrangian particles. This is only a subset of all the particles released for the study.
Key figure
Figure 3: A map of the connectivity, the transport-weighted number of crossings of the Lagrangian floats launched at 5°S that reach the Agulhas section at 17°E, on a 0.5° × 0.5° grid. Note that the color bar is non-linear. Two quasi-zonal pathways can clearly be distinguished, a broad pathway around 25°S and a narrower pathway around 35°S. The contours are lines of constant f/h, which the water would follow when it conserves potential vorticity. The southernmost pathway in general follows the f/h lines, especially when crossing the Mid-Atlantic Ridge. The pathway at 25°S follows f/h lines west of the Mid-Atlantic Ridge, but clearly intersects these lines east of the Mid-Atlantic Ridge.
An edited version of this paper was published by AGU. Copyright 2012 American Geophysical Union.