Quantification of the interocean exchange of intermediate water masses around southern Africa

Abstract

Abstract A large collection of historic and recent hydrographic data has been used in a water-mass mixing model to quantify the remote sources of intermediate water that pass through the region around southern Africa. Special emphasis has been given to deriving the fractional transports of water with Indian Ocean origins that enter the South Atlantic and that thus contribute to the overturning circulation of the Atlantic. The model includes five remote sources: Antarctic Intermediate Water (AAIW) from the Drake Passage (dAAIW), a component of AAIW from the South Indian Ocean (siAAIW), intermediate water from the Indonesian Seas (IIW) and from the Red Sea (RSIW), and a transformed end member of the mixture of these sources (aAAIW). The study is performed on five neutral density surfaces chosen to follow the core of AAIW and to span the AAIW layer, encompassing a depth range between about 700 and 1200 m . To resolve the mixing of water masses from the five remote sources, the six physical–chemical tracers, potential temperature, salinity, dissolved oxygen, silicate, initial phosphate, nitrate (NO) and potential vorticity, have been used as input information. The model-derived mixing fraction gives a quantitative description of the remote sources when they are mapped onto neutral density surfaces. Results show that dAAIW is a dominant source, with a large proportion of its water extending to the southwestern Indian Ocean in a continuous distribution, while the Indian Ocean sources spread to the southeastern South Atlantic mostly in a patchy distribution. Given the mixing fractions, the geostrophic velocity fields and the RAFOS velocity fields, the volume transport can be separated into individual contributions from each source. It is found that dAAIW provides most of the transport (63% or 1.9 Sv , 1 Sv =10 6 m 3 s −1 ) crossing the 25°S section (10°W–14°E). The rest of the equatorward transport is contributed by siAAIW (16% or 0.5 Sv ), by IIW (10% or 0.3 Sv ) and by RSIW (12% or 0.4 Sv ), respectively. The net eastward transport of dAAIW passing south of Africa into the Indian Ocean is estimated to be 13.5 Sv at 20°E (50°S–36°S). Its westward return transport from the Indian Ocean is 2.1 Sv , amounting to about one-seventh of its total transport into the Indian Ocean. The Benguela Current transport from RAFOS observations for the AAIW layer at 6°W (34°S–22°S) is 12.9 Sv westward. Of this 80% is contributed by dAAIW, 11% by siAAIW, 3% by IIW, and 6% by RSIW, respectively. This means that of the 12.9 Sv Benguela Current transport, 10.3 Sv is contributed by dAAIW alone, which is composed of 8.8 Sv directly fed from the South Atlantic and 1.5 Sv returned after a loop in the Indian Ocean. Due to a weak baroclinic shear the corresponding geostrophic transport is 6.5 Sv westward, only half of the RAFOS transport. Of this 80% is contributed by dAAIW, 15% by siAAIW, 2% by IIW and 3% by RSIW.