• South Atlantic;
  • AAIW;
  • water mass mixing;
  • mixing ratios;
  • tracers;
  • quantitative estimate of AAIW sources

[1] Recently obtained World Ocean Circulation Experiment (WOCE) bottle sections and a pre-WOCE bottle data set are used in a water mass mixing model. The mixing scheme comprises three intermediate water sources: Antarctic Intermediate Water (AAIW) from the northern Drake Passage, a combination source of the Indian Ocean intermediate waters entering from south of Africa, and a transformed end-member of the former two sources. I call them dAAIW, iAAIW, and aAAIW, respectively. The dAAIW originates from the southeast South Pacific, enters the South Atlantic in the northern Drake Passage, and is modified in the Falkland Current loop. The iAAIW is a combination of the Indian Ocean sources including Red Sea Intermediate Water, Indonesian Intermediate Water, and AAIW formed locally in the south central Indian Ocean and transformed dAAIW that has returned following a loop through the Indian Ocean. The aAAIW is a transformed end-member of a mixture of dAAIW and iAAIW located in the eastern tropical South Atlantic, characterized by an oxygen minimum and nutrient maxima. Although aAAIW is not an import source like dAAIW and iAAIW, it spans property fields to extrema as a result of water mass mixing and transformation processes and therefore must be included in the basin-wide water mass mixing scheme. The study is performed on five neutral surfaces that encompass the AAIW layer from 700 to 1200 dbar in the subtropical latitudes with a distance of about 100 dbar between a pair of surfaces. Four conservative variables of potential temperature, salinity, initial phosphate (PO4o), and NO and one conservative dynamical tracer fN2 (where f is the Coriolis frequency and N2 is the squared buoyancy frequency) are used as input information to the mixing model. The model-derived mixing fraction gives a quantitative description of AAIW sources when they are mapped onto neutral surfaces. The contoured pattern of mixing fraction shows water mass spreading paths, thus implying circulation and ventilation of AAIW in the South Atlantic. Results show that dAAIW is a dominant water mass and iAAIW is about 30–60% of dAAIW in the subtropical latitudes. With the mixing proportion of AAIW sources derived from the mixing model the geostrophic volume transport and dianeutral upwelling transport can be separated into the individual contributions from each AAIW source. It is found that the percentage of the transport contributed by dAAIW and iAAIW to the South Atlantic is almost constant at 64 ± 2% and 36 ± 2%, respectively. Meridional transport in the subtropical latitudes between 30° and 10°S by dAAIW and iAAIW (referenced to 2000 dbar) has a mean of 4.26 Sv northward (1 Sv = 106 m3 s−1) shared between dAAIW at 2.70 Sv (63%) and iAAIW at 1.56 Sv (37%). The mean zonal transport in the western and eastern South Atlantic between 40° and 3°S is −5.13 Sv westward, shared between dAAIW at −3.38 Sv (66%) and iAAIW at −1.75 Sv (34%). The dianeutral upwelling transport across the uppermost neutral surface σN = 27.25 in the northwest South Atlantic (north of 30°S and west of 10°W) is 2.26 Sv shared between dAAIW at 1.40 Sv (62%) and iAAIW at 0.86 Sv (38%).