Scales of spatial and temporal variability in the Southern Ocean

Spatial and temporal variability in Geosat altimeter data is used to consider whether the Antarctic Circumpolar Current (ACC) responds on the large scale to global changes in wind forcing or whether its variability is primarily mesoscale. The Geosat data indicate a spatial decorrelation scale of 85 km and a temporal e-folding scale of 34 days. Using these scales to define autocovariance functions, the sea surface height variability is objectively mapped. The resulting maps indicate substantial evidence of mesoscale eddy activity. Over 17-day time intervals, meanders of the Polar Front and Subantarctic Front appear to elongate, break off as rings, and propagate. Statistical analysis of ACC variability from altimeter data is conducted using empirical orthogonal functions (EOFs). The first-mode EOF describes 16% of the variance in total sea surface height across the ACC; reducing the domain into basin scales does not significantly increase the variance represented by the first EOF, suggesting that the scales of motion are relatively short and may be determined by local instability mechanisms rather than larger basin-scale processes. Likewise, neither complex nor extended EOFs indicate statistically significant traveling wave modes.