Influence of the Subtropical Andes on Baroclinic Disturbances: A Cold Front Case Study

Abstract

The Andes Cordillera produces a significant disruption to the structure and evolution of the weather systems that cross South America. In particular, cold fronts tend to be ?channeled? to the north immediately to the east of the Andes, fostering the advance of cold air incursions (cold surges) well into subtropical, and sometimes tropical, latitudes. In contrast, active cold fronts hardly reach subtropical latitudes along the western side of the Andes (Pacific sea border). Instead, as a cold front moves equatorward along the east side of the Andes, a marked low-level warming tends to appear along the west side of the subtropical Andes, leading to the formation of a mesoscale coastal low (or trough) in this region. To further understand the processes that lead to a contrasting evolution of the cold front at each side of the Andes, a typical frontal passage is studied in this work, using synoptic observations and a regional model [Eta?Centro de Previs?o de Tempo e Estudos Climáticos (CPTEC)] simulation. The passage of the postfrontal anticyclone over southern South America produces a poleward-pointing pressure gradient and, hence, geostrophic easterly flow at low levels. The tall and steep mountains block the flow, leading to a very small zonal wind component close to the slopes. Convergence (divergence) of the zonal flow to the east (west) of the subtropical Andes is largely compensated for by upward (downward) motion, and the associated cooling (warming) over this region. The weak zonal wind component near the Andes also breaks down the geostrophic balance over this region, giving rise to an acceleration of the southerly winds (i.e., along-barrier flow) and the consequent increase in cold advection. Therefore, to the east of the subtropical Andes both horizontal and vertical advection cool the lower troposphere, fostering the equatorward propagation of the cold front. To the west of the Andes, horizontal advection is largely offset by the strong warming associated with the enhanced subsidence over that region hindering the advance of the cold front into subtropical latitudes.