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

Two years of Geosat data are used to investigate the space and time sc ales of the mesoscale variability in the Brazil-Malvinas Current confl uence region. The mesoscale activity is highly inhomogeneous and aniso tropic. The inhomogeneity is characterized by low values of sea level variability in the Malvinas Current (less than 8 cm, i.e., eddy kineti c energy less than 150 cm2 s-2), intermediate values in the Brazil Cur rent (typically 16 cm/800 cm2 s-2) and high values in the Brazil-Malvi nas frontal region (30 cm/1700 cm2 s-2). The anisotropy is marked with meridional variances of velocity that are typically three times large r than zonal ones in the region of maximum variability. Mesoscale vari ations are dominated by relatively large spatial scales and low-freque ncy fluctuations. Contrary to similar spectra derived from Geosat data over the North Atlantic (Le Traon, 1991; Le Traon et al., 1990) or ov er the whole southern ocean (Chelton et al., 1990) there is very littl e 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 peri od 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 wav e that dominates the atmospheric circulation in the southern hemispher e. However, the mechanism responsible for this semiannual signal in th e frontal region remains unclear. Beyond this dominant semiannual freq uency, the altimetry-derived mesoscale fluctuations exhibit other ener getic signals, especially signals at periods between 75 and 150 days a nd spatial scales of 500-600 km with a westward propagation which have characteristics consistent with the dynamics of barotropic Rossby wav es.