Assimilation of Geosat Altimeter Data into a Quasigeostrophic Model of the Antarctic Circumpolar Current

Abstract

To study the relationship between mesoscale variability and the mean circulation of the Antarctic Circumpolar Current, the authors have assimilated Geosat altimeter measurements into a quasigeostrophic model of the Southern Ocean. The model is eddy resolving and includes realistic lateral boundaries and bottom topography. The method of ?nudging? is applied for a continuous assimilation of the satellite data. It is shown that assimilation of vorticity into the surface layer is insufficient to drive the model toward observations. On the contrary, modes are excited that are not constrained by the vorticity in a periodic ocean domain. These modes soon dominate the flow and the mean current changes direction. Nonlinear effects prevent a convergence of the model vorticity toward the satellite measurements. When sea surface height is nudged into the model instead of vorticity, a fast convergence toward observations is found. However, when the technique that was proven successful in identical twin experiments is used with real data, it is observed that there is only slight improvement. One reason for this failure could be an unrealistic reference mean that is incompatible with observations. A second reason could be the scale dependence of the formulation of the nudging term. Long waves are constrained more than the short ones. The success of the assimilation is restricted mainly to driving the variability of the model closer to the observations. Additionally, in the ice-covered domain of the Southern Ocean, which is modeled but not constrained by Geosat observations, the authors find a major improvement. The overestimation of the westward currrent around Antarctica is reduced significantly.