THE IMPACT OF THE ANDES ON TRANSIENT ATMOSPHERIC SYSTEMS - A COMPARISON BETWEEN OBSERVATIONS AND GCM RESULTS

The high and low pressure transient systems that cross the Andes at va rious extratropical latitudes are strongly affected by this. natural b arrier. Available analysis from operational weather services, such as the European Centre for Medium-Range Weather Forecasts (ECMWF), provid e a description of these migrating systems, which are often associated with regional features of orographic origin, such as the Northwestern Argentinean Depression. Their propagation is subject to a particulari ty: for distinct reasons both low and high pressure systems are common ly deflected to the north when crossing the Andes. The purpose of the paper is to test the ability of an atmospheric general circulation mod el, the LMD (Laboratoire de Meteorologie Dynamique) GCM, to simulate c orrectly the behavior of these systems. This is done through the analy sis of conventional meteorological parameters, but also by duplicating , from the output fields of the model, a series of composite analyses that were previously carried out using both the ECMWF analysis and mea surements at local weather stations. The model appears generally succe ssful in reproducing the large-scale features of the circulation. The precipitation patterns are less realistic: they extend too broadly ove r the eastern side of the mountains, with too little precipitation alo ng the Atlantic coast. Sensitivity experiments are carried out to test the impact of the orography and the role of the model resolution. An experiment with enhanced, and probably more realistic, orography, give s much improved results. An experiment with a lower model resolution s hows a degraded but persistent skill in reproducing the observed patte rn.