Mean flow-transient perturbations interaction in the Southern Hemisphere: A simulation using a variable-resolution GCM

CARRIL A. F., C. G. MENÉNDEZ, M. N. NUÑEZ AND H. LE TREUT

CLIMATE DYNAMICS

Springer

Año: 2002 vol. 18 p. 661 - 673

0930-7575

The ability of an atmospheric general circulationmodel to reproduce fundamental features of thewintertime extratropical Southern Hemisphere (SH)circulation is evaluated with emphasis on the dailyvariability of the SH mean flow and the mean flowtransientperturbations interaction. Two 10-year simulationsusinga new version of the LMDZ GCM with astretched grid scheme centered at 45 S and forced byclimatological SST are performed: a high (144 · 73) andlow (64 · 33) horizontal resolution runs. The performanceof both simulations was determined by comparingseveral simulated fields (zonal wind, temperature,kinetic energy, transient eddy momentum and heatfluxes, Eliassen-Palm fluxes, Eady growth rate andbaroclinic conversion term) against the European Centrefor Medium Range Weather Forecast reanalyses (ERA).High and low-resolution simulations are similar in manyrespects; in particular, both experiments reproduce themain patterns of the southern extratropical large-scalecirculation satisfactorily. Increasingresol ution does notimprove universally some spurious aspects of the lowresolution simulation (e.g. the cold bias in the high polartroposphere, the debilitated subtropical jet, the lowbaroclinic conversion rate). Those aspects present littlesensitivity to the model resolution. The interactionbetween transient eddies and zonal mean flow areexamined. The low-resolution experiment is able toqualitatively represent the acceleration/deceleration ofthe mean flow by transient perturbations, south/northof 30 S with an accuracy similar to that of the highresolutionexperiment. Although both experimentsrepresent the baroclinic structure of the mean flow satisfactorily,the model underestimates some transientproperties due to the underestimation of the baroclinicconversion term in middle latitudes. Such misrepresentationdoes not improve with increasingres olution and isrelated to the relatively weak meridional temperaturegradient and the inadequate geographical distribution ofthe eddy heat fluxes. In particular, the eddy kinetic energyis always underestimated. Eddy kinetic energy doesnot improve convincingly with increasing resolution,suggesting that the adequate representation of the stormtracks is highly influenced by the physical parametrizations