Simulations of Southern Hemisphere climate using a variable resolution stretched grid atmospheric GCM

Different simulations of the Southern Hemisphere (SH) climate using a variable resolution atmospheric general circulation model are analysed. The grid-point global model used is LMD-Z, version 1. Its horizontal discretization is a function of latitude and longitude, so a stretched grid (zoom) can be applied to any region of the globe. A stretched grid with a smoothly varying mesh size has been defined to improve the meridional resolution in a latitudinal band in the middle southern latitudes. In order to study the sensitivity to the horizontal resolution three simulations were performed: low (64x33), medium (96x49) and high (144x73) resolution, all of them with zoom centred at 45°S and 11 vertical layers. Sea surface temperatures were fixed climatological (1979-1995) and all simulations have been run for 10 years.

Higher resolution allows a more accurate simulation of the time evolution of finite amplitude baroclinic waves and, consequently, the eddy meridional fluxes of heat and momentum are improved. Higher resolution also implies a better representation of the main topographic features of the Southern Hemisphere (SH), that is the Andes and the massive ice dome of East Antarctica. The simulation of wintertime dynamical fields in the SH generally improves with model resolution, but not universally. The low-resolution simulation is not expected to provide correct representation of complex features like the dual jet structure in the western Pacific Ocean. However, we found that a very high horizontal resolution does not warrant a better simulation of the SH dynamics (as compared with the medium-resolution run). The tendency for the subtropical jet to weaken and for the polar jet to strengthen with resolution is quite apparent. Our results suggest that while there are of course grounds for expecting progress with the use of higher resolution and variable resolution grids, there are also difficulties with the simulations more directly connected to subgrid-scale physics than to resolution.