Characteristics of the South Atlantic subtropical frontal zone between 15°W and 5°E

In this paper we present data from a number of crossings of the boundary between subtropical and subpolar water in the 15°W–5°E region of the South Atlantic and discuss the implications. The previous paucity of synoptic data sets near 40°S between 25°W and the Greenwich Meridian meant that up to now it has not been possible to fix the position of the boundary in the mid-South Atlantic or to deduce the effects of the midocean ridge on the strength of the South Atlantic Current (SAC). Using hydrographic and chemical tracer data we confirm that the transition from subtropical to subpolar waters occurs within a Subtropical Frontal Zone (STFZ), which constrains the South Atlantic Current flow and is bounded on each side by a distinct front. The northern one, the Northern Subtropical Front (NSTF), varies by only 1.5° of latitude, whereas the southern one, the South Subtropical Front (SSTF), the traditional Subtropical Convergence (STC, defined by Deacon, 1937), migrates over 2.5° of latitude and remains south of the island of Tristan da Cunha. This finding goes some way in resolving the disparity in the literature to the position and seasonal migration of the STC. The data confirm the existence of a subsurface salinity maximum lobe, which appears to be formed at the NSTF rather than at the SSTF. By closely investigating the mesoscale structure and comparing it with historical data from a number of meridional cruises, we have shown strong seasonality in the frontal structure between 30 and 45°S in the South Atlantic. Having resolved the fine structure, we have made an estimate of the geostropic flow of the SAC and suggest that there are east to west differences, which may be related to recirculation in the Argentine Basin rather than to a slowing down by the midocean ridge.