Simulation of the transient atmospheric eddies in the Southern Extratropics

The Southern Hemisphere (SH) extratropical circulation is strongly dominated by high frequency transient perturbations. The capability of a low-resolution atmospheric global model to represent the high frequency transients and their interaction with the mean flow is investigated. The model is a version of the LMD-Z general circulation model installed at CIMA. The resolution corresponds to a grid of 64 points in longitude, 33 points in latitude and 11 vertical layers. In order to enhance model resolution in the southern midlatitudes we adopt a meridional zoom centred at 45°S.

Relevant diagnostics of the transient eddy activity are evaluated. As in most low-resolution models, the eddy kinetic energy is somewhat underestimated. The baroclinicity of the mean flow is rather well simulated and even the model tends to overestimate the values in certain regions. However, eddies do not develop enough due to the underestimation of the baroclinic conversion term in the middle latitudes. As the model resolution is too coarse, progress is expected in the simulation of both synoptic features and magnitude of eddy statistics with the use of higher resolution. But also improving the simulation of eddy properties requires a better representation of the physical processes governing the SH storm track.