The Atlantic Deep Western Boundary Current: Water masses and transports near the equator

Abstract

The distributions and transports of deepwater masses at the western boundary in the tropical Atlantic off Brazil have been studied on three surveys along 35°W and 5°S and one at 10°S. Transports are obtained from direct measurements of the velocity fields (Pegasus profiling system and lowered acoustic Doppler current profiler) and from geostrophic computations. Using chlorofluoromethane (CFM) and hydrographic distributions, four water masses could be identified forming the North Atlantic Deep Water (NADW) system. Two of these have a high CFM content, the “shallow upper NADW” (SUNADW) and the “overflow lower NADW” (OLNADW). These exhibit the highest velocity signals at 35°W, where distinct flow cores seem to exist; most of the southeastward flow of the SUNADW (centered around 1600 m) occurs 320 km offshore between 3°09′S and 1°50′S (9.7±3.3 Sv); farther north in that section, a highly variable reversing flow is found in a second velocity maximum. The transport of OLNADW (centered around 3800 m) of 4.6±2.6 Sv is guided by the Parnaiba Ridge at 1°45′S, 35°W. The water masses located between the two CFM maxima, the Labrador Sea Water (LSW) and the LNADW old water mass (LNADW‐old), did not show any persistent flow features, however, a rather constant transport of 11.1±2.6 Sv was observed for these two layers. The total southeastward flow of the NADW at 35°W showed a transport of 26.8±7.0 Sv, if one neglects the reversing SUNADW north of 1°50′S. At 5°S the flow of all deepwater masses shows vertically aligned cores; the main southward transport occurred near the coast (19.5±5.3 Sv). The boundary current is limited offshore by a flow reversal, present in all three surveys, but located at different longitudes. At 10°S a southward transport of 4.7 Sv was observed in November 1992. However, the section extended only to 32°30′W, so that probably a significant part of the flow has been missed. An important result is the large transport variability between single cruises as well as variability of the spatial distribution of the flow at 35°W, which could lead to large uncertainties in the interpretation of single cruise observations. Despite these uncertainties we suggest a circulation pattern of the various deepwater masses near the equator by combining our mean transport estimates with other observations.