The Impact of the Andes on Transient Atmospheric Systems: A Comparison between Observations and GCM Results

Abstract

The high and low pressure transient systems that cross the Andes at various extratropical latitudes are strongly affected by this natural barrier. Available analysis from operational weather services, such as the European Centre for Medium-Range Weather Forecasts (ECMWF), provide 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 particularity: for distinct reasons both low and high pressure systems are commonly deflected to the north when crossing the Andes. The purpose of the paper is to test the ability of an atmospheric general circulation model, the LMD (Laboratoire de Météorologie Dynamique) GCM, to simulate correctly the behavior of these systems. This is done through the analysis 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 measurements at local weather stations. The model appears generally successful in reproducing the large-scale features of the circulation. The precipitation patterns are less realistic: they extend too broadly over the eastern side of the mountains, with too little precipitation along 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, gives much improved results. An experiment with a lower model resolution shows a degraded but persistent skill in reproducing the observed pattern.

Upper Atmosphere Research Satellite (UARS) MLS observation of mountain waves over the Andes

“…As shown in Figure 6b, the near surface zonal winds ( U winds) are stronger and less variable during the 1995 active‐period, whereas during the 1996 active‐period, the surface winds are weaker and more variable. We also find a sharper and stronger meridional V wind field just west of the Andes during the 1995 active‐period, which may result from the wind flow that is redirected meridionally by the Andes topography [see Seluchi et al , 1998].…”

The Andes climate and weather

“…In the Southern Hemisphere, the circumpolar storm track intersect the continent between 40 • -50 • S with only minor seasonal changes (e.g., Nakamura and Shimpo, 2004;Garreaud, 2007) although some systems approach South America at subtropical latitudes during winter. These systems are blocked by the Andes cordillera and experience substantial distortion in their structure (e.g., Berbery and Vera, 1996;Seluchi et al, 1998Seluchi et al, , 2006.…”

Surface circulation associated with frost in the wet Pampas

“…Cold air advection can persist over the continent for several days and reach very low latitudes when the system moves northeastwards. This is a consequence of the interaction with the Andes, which channel the meridional flow in such a way as to enhance latitudinal propagation (Gan and Rao, 1994;Seluchi et al 1998). Dynamic and orographic forcings are essential to the evolution and structure of South American cold air surges (Garreaud, 1999(Garreaud, , 2000.…”