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Meanders Downstream The Agulhas Retroflexion
Jose Ochoa, Peter Niiler
Department of Physical Oceanography/CICESE
jochoa@cicese.mx(Abstract received 05/01/2005 for session D)
ABSTRACT
The surface mean flow, derived from lagrangian trajectories of the Global Drifter Program, shows meanders downstream all separation points of Western Boundary Currents. The most pronounced case is the Agulhas Current (Pazan and Niller, 2004). We do an analysis of the five first meanders, which have wavelength of about 700 km and their amplitude decrease from 170 km in the first one to 50 km in the following. The region of the meanders shows, relative to further south and north, an increase in mean speed (i.e. it is a jet) and variability. This system has been the subject of many studies (see Lutjeharms et. al., 2004). Here we argue that the large meanders in the mean flow follow a simple vorticity balance where the beta effect, or meridional excursion, and curvature of the displacements balance out. This balance has been the core in explaining the trajectories of vertical coherent jets, particularly in the absence of bottom effects (Warren 1963, Niiler an Robinson, 1966, ..). However, in this case solely applying for the 2D, and surface, flow requires non-divergent conditions (as applied by Reid, 1972, for the Loop Current). We argue that such requisite is a known characteristic of frontal jets (Niiler and Reynolds, 1984) and the only remaining plausible terms to balance on the axis of the jet are: the one due to curvature and the beta effect. Calculations along pathlines show remarkable agreement with this balance, although the non-divergence is not captured in the mean flow estimate.
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2005 LAPCOD Meeting, Lerici, Italy, June 13-17, 2005