CHARACTERISTICS OF THE SOUTH ATLANTIC SUBTROPICAL FRONTAL ZONE BETWEEN 15-DEGREES-W AND 5-DEGREES-E

In this paper we present data from a number of crossings of the bounda ry between subtropical and subpolar water in the 15 degrees W-5 degree s E region of the. South Atlantic: and discuss the implications. The p revious paucity of synoptic data sets near 40 degrees S between 25 deg rees 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 Atlanti c or to deduce the effects of the midocean ridge on the strength of th e South Atlantic Current (SAC). Using hydrographic and chemical tracer data we confirm that the transition from subtropical to subpolar wate rs occurs within a Subtropical Frontal Zone (STFZ), which constrains t he South Atlantic Current flow and is bounded on each side by a distin ct front. The northern one, the Northern Subtropical Front (NSTF), var ies by only 1.5 degrees of latitude, whereas the southern one,the Sout h Subtropical Front (SSTF), the traditional Subtropical Convergence (S TC, defined by Deacon, 1937), migrates over 2.5 degrees of latitude an d remains south of the island of Tristan da Cunha. This finding goes s ome way in resolving the disparity in the literature to the position a nd seasonal migration of the STC. The data confirm the existence of a subsurface salinity maximum lobe, which appears to be formed at the NS TF rather than at the SSTF. By closely investigating the mesoscale str ucture and comparing it with historical data from a number of meridion al cruises, we have shown strong seasonality in the frontal structure between 30 and 45 degrees S in the South Atlantic. Having resolved the fine structure, we have made an estimate of the geostropic flow of th e SAC and suggest that there are east to west differences, which may b e related to recirculation in the Argentine Basin rather than to a slo wing down by the midocean ridge. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.