Statistics of geostrophic turbulence in the southern ocean from satellite altimetry and numerical models

Abstract

We have used satellite altimetry to measure the statistics of geostrophic turbulence in a 3 500 km segment of the Antarctic Circumpolar Current in the Southeast Pacific. The statistics were compared with comparable statistics of variability computed from a numerical model of the ocean circulation. We found that the distribution of eddies observed from space is very similar overall to the distribution calculated from the model. The two differ on small scales, especially near the crest of the East Pacific Ridge. The model output tends to be more patchy, with large variability just upstream of the ridge crest, while space data are less patchy with large variability just downstream of the crest. The flow further downstream of the ridge was similar to that of a free jet. Variance of topography seen from space was about 80% of the variance from the model; variance of current speed was about 120% of the variance from the model. Downstream of the ridge the typical size of eddies increased with distance downstream consistent with theories for two-dimensional turbulence. The decay rate of turbulence downstream of the ridge crest calculated from the model output was nearly the same as that observed from space. The similar decay of turbulence seen in the satellite and model data indicate that the overall influence of viscosity may be well modeled in the numerical calculations. The zonal mean velocity downstream of the ridge agrees well with hydrographic data relative to a depth of 3000 m collected from 1963 to 1968. The position of the current maximum agrees within 0.5° of latitude, and speed is 25% faster than that calculated from hydrographic data. The differences are consistent with interannual variability of the current.