ASTTEX science goal: provide a quantitative, multi-year Eulerian measurement of the strengths and characteristic scales of Agulhas-South Atlantic (ASA) mass and themohaline fluxes, at mesoscale resolution.

Figure 1. The thermohaline complexity of the Cape Basin hydrography reflects its many distinct sources. Agulhas rings bring in Subtropical Surface Water (SSW) and Central Water from the Indian Ocean. These water masses are both warmer and saltier than their Atlantic counterparts at equivalent density horizons (see Valentine et al., [1993] Table 1).
Strong evaporation, cooling and mixing can further modify the water mass properties, enhancing the signature of South Indian Subtropical Mode Water (STMW) [Olson et al., 1992]. Three rings in the process of cooling, mixing and propagating into the South Atlantic Ocean are sketched in red-orange in the figure above.

Anticyclonic rings with a distinctly different core of Subantarctic Mode Water (SAMW) have been observed on several occasions. While indistinguishable from warm Agulhas rings on the basis of their sea level signature, SAMW rings carry a thermohaline signature completely unlike the typical warm Agulhas ring. SAMW rings probably also originate in the Agulhas Retroflection while the water at their cores comes from ~60E in the Indian Ocean [McDonagh and Heywood, 1999].

Retroflection and ring formation processes can also inject Subantarctic Surface Water (SASW) and the relatively colder and fresher Antarctic Intermediate Water (AAIW) into the region [Shannon and Hunter, 1988]. This process is represented by the blue arc above.

Warm filaments entering the Cape Basin inshore of the Agulhas Current have been observed [Lutjeharms and Cooper, 1996]. The short red line symbolizes these mesoscale features.

Lastly, in the Benguela upwelling regime near the African coast, Atlantic surface waters (SSW) are replaced by warmed and upwelled water from further down in the water column [Stander 1964; Shannon 1966]. Filaments and eddies of upwelled water are advected to the west and have been detected 700 km from the coast. This water then mixes into the South Atlantic [Lutjeharms and Stockton, 1987]. This process is represented by the green curve in the figure above.

Our principal interest is to measure the contributions of the larger features of Indian Ocean origin to the thermohaline balance of the South Atlantic. The ASTTEX array will span the trajectories of over 90% of Agulhas rings that have been tracked from the Retroflection [e.g., Byrne et al., 1995; Witter and Gordon, 1999, Goni et al, 1997, O. Boebel, pers. comm.] plus the observed location of the Benguela Current [Garzoli and Gordon, 1996; Garzoli et al, 1996]. We estimate up to 1/2 of the inter-ocean transport may be accomplished by mesoscale features (e.g., rings, eddies, filaments) [Byrne, 2000]. ASTTEX also contains a retrospective component, which is the main subject this work.

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