SEA-LEVEL VARIABILITY AND SEMIANNUAL ROSSBY WAVES IN THE SOUTH-ATLANTIC SUBTROPICAL GYRE

We derived the sea level variability in the South Atlantic subtropical gyre between 15-degrees and 35-degrees-S from 2 years of the Geosat exact repeat mission altimeter data. On the mesoscale, along-track wavenumber spectra are those of nonlinear dynamics with a steeper slope and stronger energy along the eastern boundary. Frequency/wavenumber spectra reveal a significant semiannual signal at a wavelength of about 500 km. West of the Walvis ridge (5-degrees-E), this signal appears to correspond to semiannual Rossby waves of 2- to 3-cm amplitude, propagating westward at 3 cm s-1, and showing latitude refraction effects. The pattern is very similar to that of annual Rossby waves as modeled by Reason et al. (1987). These waves are probably related to the semiannual wind component. However, understanding the exact mechanism for their excitation requires dynamical modeling. The large-scale variability also shows a significant semiannual component (about 6-cm amplitude and 15% of the yearly variance) in phase with climatological winds.