On the water masses and mean circulation of the South Atlantic Ocean

We examine recent observations of water mass distribution and circulation s chemes at different depths of the South Atlantic Ocean to propose a layered , qualitative representation of the mean distribution of flow in this regio n. This furthers the simple upper layer geostrophic flow estimates of Peter son and Stramma [1991]. In addition, we assess how well ocean general circu lation models (GCMs) capture the overall structure of flow in the South Atl antic in this regard. The South Atlantic Central Water (SACW) is of South A tlantic origin in the subtropical gyre, while the SACW in the tropical regi on in part originates from the South Indian Ocean. The Antarctic Intermedia te Water in the South Atlantic originates from a surface region of the circ umpolar layer, especially in the northern Drake Passage and the Falkland Cu rrent loop, but also receives some water from the Indian Ocean. The subtrop ical South Atlantic above the North Atlantic Deep Water and north of the An tarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropi cal gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre e xists, embedded in a large tropical cyclonic gyre. The equatorial part of t he South Atlantic shows several depth-dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy-permitting resolution. The northward ex tent of the subtropical gyre reduces with increasing depth, located near Br azil at 16 degrees S in the near-surface layer and at 26 degrees S in the A ntarctic Intermediate Water layer, while the tropical cyclonic gyre progres ses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolu tion is required to capture the South Atlantic Current north of the ACC. Th e North Atlantic Deep Water in the South Atlantic progresses mainly southwa rd in the Deep Western Boundary Current, but some water also moves southwar d at the eastern boundary.