SPATIAL AND TEMPORAL SCALES IN ALTIMETRIC VARIABILITY IN THE BRAZIL-MALVINAS CURRENT CONFLUENCE REGION - DOMINANCE OF THE SEMIANNUAL PERIOD AND LARGE SPATIAL SCALES

Two years of Geosat data are used to investigate the space and time scales of the mesoscale variability in the Brazil-Malvinas Current confluence region. The mesoscale activity is highly inhomogeneous and anisotropic. The inhomogeneity is characterized by low values of sea level variability in the Malvinas Current (less than 8 cm, i.e., eddy kinetic energy less than 150 cm2 s-2), intermediate values in the Brazil Current (typically 16 cm/800 cm2 s-2) and high values in the Brazil-Malvinas frontal region (30 cm/1700 cm2 s-2). The anisotropy is marked with meridional variances of velocity that are typically three times larger than zonal ones in the region of maximum variability. Mesoscale variations are dominated by relatively large spatial scales and low-frequency fluctuations. Contrary to similar spectra derived from Geosat data over the North Atlantic (Le Traon, 1991; Le Traon et al., 1990) or over the whole southern ocean (Chelton et al., 1990) there is very little energy at the annual period. In the mean spectrum, the energy is at least 3 times smaller at the annual period than at the semiannual period or at shorter periods. The semiannual signal, with length scales of about 400-500 km, has a clear northward propagation and is maximum in the frontal region. It is probably associated with the semiannual wave that dominates the atmospheric circulation in the southern hemisphere. However, the mechanism responsible for this semiannual signal in the frontal region remains unclear. Beyond this dominant semiannual frequency, the altimetry-derived mesoscale fluctuations exhibit other energetic signals, especially signals at periods between 75 and 150 days and spatial scales of 500-600 km with a westward propagation which have characteristics consistent with the dynamics of barotropic Rossby waves.