EDDY AND WAVE DYNAMICS IN THE SOUTH-ATLANTIC AS DIAGNOSED FROM GEOSAT ALTIMETER DATA

Almost 3 years of Geosat data were analyzed to ascertain the characteristics of the sea surface height variability in the South Atlantic. The nature of the sea surface height field is shown to be nonstationary, heterogeneous, and anisotropic. In particular, the wavenumber spectral characteristics of the sea surface height field, such as spectral slopes, shape, breaks in spectral slopes, and energy density values, vary with latitude and longitude within the basin. The spectral slopes exhibit a spectral dependence approaching k-5 in the high energy areas and k-3 in the low energy areas. Along-track characteristic wavelengths are found to decrease from north (500 km) to south (300 km) and from west to east. The (eddy available potential) energy exhibits maximum values in the highly energetic eddy shedding regions of the Malvinas-Brazil confluence and the Agulhas leakage region. Mean frequency spectra, calculated from time series of sea surface height variability at crossover points, reveal evidence of annual and semiannual signatures with energy levels reaching 1000 cm2 in the high-energy western and eastern systems. These values represent 20% of the energy found for periods of 100 days or less, which is the time scale of the mesoscale variability. Baroclinic Rossby waves with periods of 400-500 days are found to propagate away from the eddy-shedding region of the Agulhas leakage into the Atlantic. The phase (energy) of these planetary waves propagates toward the west-southwest (west-northwest) from their source near the southern tip of Africa. Correlation functions in space (both in the zonal and along-track directions) and time lag corroborate these findings. Zonal and meridional wavenumbers were calculated using a simple wavenumber projection technique which takes into account the anisotropic property of the wave field. The resultant wavelengths (approximately 250 km) are consistent with those calculated from the dispersion relation for baroclinic Rossby waves. These waves propagate with a group speed of 0.4 cm s-1 and provide a net energy flux from the Agulhas leakage into the Atlantic toward the western boundary and the equator across 25-degrees-S.