South Atlantic thermocline ventilation

Abstract

South Atlantic Central Water (SACW) is carried poleward within a narrow longitude interval off the Argentine coast by the Brazil Current after confluence with the Malvinas (Falkland) Current. Hydrographic data obtained from the R.V. Atlantis II in December 1979 and January 1980 indicate that within the SACW poleward extension, convective processes are important in ventilating the main thermocline to the north. A number of relatively warm, salty intrusions 100 m thick are found within the 12 to 17°C layer of the thermocline along 38°S. These features are about 0.2 × 10⁻³ more saline than the regional potential temperature-salinity (θ–S) curve of the SACW thermocline and represent local oxygen maxima and nutrient minima. The characteristics of the intrusion are similar to thick remnant winter-mixed layers within various pockets (warm-core eddies) of SACW found further south. The observations support McCartney's 1977 (A Voyage of Discovery, M. Angel, editor, pp. 103–119, Pergamon) suggestion that the Brazil-Malvinas confluence region is the source of a thermostad in the South Atlantic thermocline. The present data suggest that a family of winter altered, relatively salty mixed layers and intrusions are formed in the poleward extension of the SACW thermocline waters, which then spread northward into the main thermocline. The number of family members and their range of θ–S characteristics depend on the distribution of warm eddies and meanders during the winter period. The lower boundary of the intrusions have density ratios (R = αΔT/βΔS) of 1.2 to 1.3, making them unstable to salt-finger activity. Salt fingers are likely to be the primary process that integrates the excess salt of the intrusions into a broader, but less extreme positive salinity anomaly (relative to the South Atlantic thermocline T-S curve), which is associated with the 12 to 17°C weak thermostad of the southwest Atlantic. It is suggested that deep winter convective processes within the SACW extension not only transfer salt and oxygen to mid-thermocline depths, but it also induces significant downward salt flux by salt-finger activity at mid-thermocline depths. Similar processes may be important in all poleward edges of the world thermocline, particularly along the western sectors.