Agulhas retroflection rings in the South Atlantic Ocean: an overview

Abstract

The most energetic western boundary current rings in the world's oceans are shed from the Agulhas retroflection south of Africa. These are responsible for a considerable transfer of heat, salt and energy from the South Indian Ocean into the South Atlantic. This transfer has important implications for global ocean circulation. In addition, the rings have some important local effects (e.g. on weather and on the Benguela upwelling system) in their passage across the South Atlantic. Since 1983, hydrographic studies have begun to examine these rings and their effects. These studies are reviewed in both global and local contexts.Die mees energieke ringe wat van westelike grensslfome ter wereld afgewerp word, is die atKomstig van die Agulhas-retrofleksie besuide Afrika. Hulle is verantwoordelik vir aansienlike oordrag van warmte, sout en energie van die Suid-lndiese na die Suid-Atlantiese Oseaan. Hierdie oordrag het belangrike implikasies vir die wereldoseaansirkulasie. Daarbenewens het die ringe etlike belangrik plaaslike uitwerkings (bv. op die weer en op die Benguela-opwelstelsel) tydens hul deurgang oor die Suid-Atlantiese Oseaan. Sedert \983 is begin met hidrografiese studies van hierdie ringe en hul uitwerking. Hierdie studies word in wereld-en plaaslike verband in oenskou geneem.

On the world ocean circulation. Volume II, the Pacific and Indian Oceans/a global update

“…The specific layers selected for Table ii-is were determined by those chosen by Hall and Fofonoff (1993). So to the extent that the upper layer KE(z) observed in the Agulhas Retroflection Area is due to the passage of meanders (see, for the GSS: Halkin and Rossby, 1985;Hogg, 1994;Kelly, 1991) and pinched-off rings (Duncombe Rae, 1991;Duncombe Rae et ai., 1996;Olson, 1991), perhaps , or KE( a), are so similar (see also Holland et ai., 1991, p. 416). Recirculations are related to eddy rectification (Hogg and Johns, 1995;Holland, 1978).…”

A new Eulerian method to estimate “spicy” Agulhas leakage in climate models

“…It is trapping depth rather than the full dynamic depth of the Agulhas ring that is important for the Agulhas water that leaks into the South Atlantic. Observational studies have estimated the trapping depth of Agulhas rings to reach 670-1100 m [McCartney and Woodgate-Jones, 1991;Duncombe Rae, 1991]. Van Sebille et al [2010] found Lagrangian particles to reveal leakage waters down to 1200 m. Hence, a property that can identify Agulhas water to depths of 1000 m or more is desirable.…”

“…Hence, an Eulerian measure is useful. Previous studies have estimated the Agulhas leakage in an Eulerian framework [Duncombe Rae, 1991;van Sebille et al, 2010] using temperature and salinity thresholds to identify warm and saline waters from the Indian Ocean. Yet singular property thresholds are restricted close to the surface.…”

Transformation of an Agulhas eddy near the continental slope

“…Agulhas rings are known to detach from the Agulhas Retroflection south of Africa in the vicinity of 15 • E to 21 • E and 37 • 30 S to 40 • S (Lutjeharms and Ballegooyen, 1988;Lutjeharms and Valentine, 1988) and are known to contribute warm salty water to the South Atlantic Ocean and influence the Meridional Overturning Circulation (Gordon, 1985;Duncombe Rae, 1991;Duncombe Rae et al, 1996). Improved knowledge of their evolution is needed to understand their scales, translation speeds, and the role they play in the MOC.…”