Interannual variability of South Atlantic circulation from 4 years of TOPEX/POSEIDON satellite altimeter observations

Variability of large-scale and regional South Atlantic circulation is investigated using TOPEX/POSEIDON sea level observations. Interannual variations are identified from empirical orthogonal functions of gridded sea level fields, year-to-year fluctuations of root-mean-square sea level variability, and variability of Agulhas eddies evaluated from the along-track data. Two modes of variability are identified. A basin-scale mode indicates that sea level in the eastern South Atlantic underwent a transition from a state of high sea level and enhanced gyre-scale geostrophic circulation in 1993 and 1994, to a state of lower sea level and more sluggish circulation in 1996. The dominant mode of basin-scale zonal wind has the same temporal signature, suggesting a link between the observed variation of gyre-scale circulation and the regional wind forcing. Time variations of this mode also coincide with a transition from a broad Agulhas eddy corridor observed in 1993 and 1994 to a narrower corridor observed in 1996. The input of salt and vorticity to the South Atlantic subtropical gyre via Agulhas eddies may therefore be partially controlled by interannual variations of the wind-forced, large-scale circulation. A second mode isolates interannual variations in the Brazil-Malvinas Confluence region. During 1993, eddy variability along the Brazil Current extension was relatively strong and variability along the continental slope was weak. The opposite pattern was observed in 1995. These variations may be related to interannual variations of the latitude of the confluence. While variations associated with both modes are smaller than those observed on seasonal timescales, these interannual variations contribute significantly to the total South Atlantic variability.