Salinity and Cross-Shelf Exchanges

 

To further investigate the connection between SSS anomalies and cross-shelf exchanges we computed the volume balance of the study region (Figure 1a). At the eastern boundary (the shelfbreak) there is an annual mean off-shelf transport of 1.21 Sv. Most of this outflow is drawn from the Patagonian Shelf (1.15 Sv); the contributions from the RdlP discharge (0.024 Sv) and the Brazilian shelf (0.038 Sv) are significantly smaller. 

Figure 1

The seasonal variations of the off-shelf transport are relatively small and out of phase with the variations of the Patagonian mass flux (Figure 1b); it decreases during the winter and increases during the summer. Although there is an increment of the Patagonian volume flux during the winter, this increment is largely funneled towards the Brazilian shelf with the RdlP plume water, thus reducing the net off-shelf flux.

The relation between the salinity and the velocity fields is shown in the corresponding cross-sections (Figure 2).  The southern cross-section (A-B) is characterized by the presence of low-salinity waters against the coast, which are associated with the upstream (southward) extension of the Rio de la Plata plume. The plume is vertically homogeneous in the inner shelf, detaches from the bottom at ~25 m depth and extends offshore as a surface-trapped plume. 

The corresponding velocity field shows the aforementioned upstream flow near the coast that brings the Rio de la Plata plume water south and an intense northward mid-shelf jet, with peak velocities of 0.25 m/s. The northern cross-section (C-D) shows a deep, mid-shelf channel filled with saltier Subtropical Shelf Waters flowing poleward underneath the fresher waters from the Rio de la Plata plume. Farther offshore, this cross-section shows a northward flowing jet, which is an onshore intrusion of the Malvinas Current (Palma et al., 2008; Combes and Matano, 2014). Past the shelfbreak there is the poleward flow of the Brazil Current, which carries saltier waters towards the BMC. The eastern cross-section (B-C) shows a dominant off-shelf flow of 10-20 cm/s in the north (34.5°-35.5°S) and 5-10 cm/s in the south (35.5°-37.5°S). The corresponding salinity field shows the shallow layer (d < 20m) of fresh water from the Rio de la Plata near 35.5˚S, capping the saltier waters of the Brazil Current.