Mesoscale Variability, Critical Latitude and Eddy Mean Properties in the Tropical South-East Atlantic Ocean

Equatorward decrease of mesoscale eddy abundance, and meridional variations of their mean properties in the tropical South-East Atlantic Ocean have been investigated on the basis of horizontal length-scales of geostrophic flows, and theories of two-dimensional geostrophic turbulence on a fl-plane. Meridional variations of satellite-derived altimetric eddy kinetic energy density spectra were assessed in bins of 5 degrees, from 5 degrees S to 25 degrees S, zonally averaged between 8 degrees W and 15 degrees E. They revealed two latitudinal bands of minimum eddy energy spectra: one in the northern Angola Basin (5 degrees S-10 degrees S) and the other across the Angola Benguela Front (15 degrees S-20 degrees S). In the latter, both the eddy forcing scale and the Rhines scale were observed at wavelengths of 300 and 350 km respectively, suggesting that only a small spectral range exists for eddies to grow. On the other hand, in the former, the eddy forcing scale and the Rhines scale were observed at wavelengths of 454 and 520 km respectively. The ratio between the Rhines scale and the first baroclinic Rossby radius revealed an onset of a critical latitude at about 11.4 degrees S. Consistently, maps of eddy frequency of occurrence computed from eddies identified in the region has shown a notable decay in signal of the eddy density distribution around that latitude. This could explain the sharp equatorward transition of mesoscale processes observed in this region, which seems to shift from nonlinear eddies to linear Rossby waves, potentially explaining why fewer eddies are observed in the region when compared to other regions of the world oceans.