Abstract
A 1/6° resolution primitive equation model of the Atlantic circulation is analyzed in the Agulhas region. The model has a realistic level of eddy kinetic energy, and produces anticyclonic Agulhas rings as well as cyclonic structures. In the model as well as in the data, ring trajectories undergo a transition between a turbulent character in the Cape Basin and a steady propagation in the rest of the South Atlantic. The topography of the Walvis Ridge does not seem to play a part in generating this contrast in the model. The model shows that cyclones are primarily generated from the negative shear vorticity side of the Agulhas Current as it leaves the coast, and they are most of the time paired with anticyclones in dipolar or tripolar structures. Contribution of Agulhas rings to the transports has been estimated by two methods, either focussing on the amount of water trapped inside the eddies and carried with them, or as a perturbation to the time-mean flow. The second estimate always produces smaller mass fluxes than the first. Even so, the transient eddy flux (2 Sv of warm water over the Agulhas Ridge) is very large when compared to parameterizations of eddy fluxes used in low-resolution climate models.
Highlights
The Agulhas retroflection, south of Africa, is one of the most energetic regions of the world ocean
£ £ The model Agulhas rings reach the western basin (30 W) around 20-25 S. Their path is more to the north than found by Schouten et al, (2000), even more north than the paths of the eddies studied by Garnier et al (2001), but it could be compatible with the trajectories calculated from MODAS for years 1997 to 1999
If the eddies were deeper, they would be advected by a more westward mean flow because of the structure of the South Atlantic subtropical gyre. Another possibility is the lack of influence of Walvis Ridge in the ATL6 model, because the eddies of Matanao and Beier (2002), which seem more influenced by topography, have a more realistic trajectory outside the Cape Basin
Summary
The Agulhas retroflection, south of Africa, is one of the most energetic regions of the world ocean. 1/3 model ATL3 (Treguier et al, 2001), but those models fail to reach the observed amplitude of the eddy variability Another problem in FRAM was that the Agulhas ring formation was much too regular, as demonstrated by the analysis of temperature fronts (Lutjeharms and Webb, 1995, their figure 8). This regularity appears in the time series of barotropic streamfunction from ATL3 (Treguier et al, 2001, their figure 13). ATL3 is described in detail in Treguier et al (2001)
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