Cold Air Incursions over Subtropical South America: Mean Structure and Dynamics

Abstract

Synoptic-scale incursions of midlatitude air moving into subtropical South America (to the east of the Andes Cordillera) are observed to occur year-round with a periodicity of about 1?2 weeks. During wintertime, they have a profound impact upon the low-level temperature field, and extreme episodes produce freezing conditions from central Argentina to southern Brazil and Bolivia. Warm season episodes produce less dramatic variations of temperature, but they organize deep convection in the form of synoptic-scale bands of convective cloudiness along the leading edge of the cool air. On the basis of 17 yr of NCEP?NCAR reanalysis and outgoing longwave radiation fields, the mean, synoptic-scale structure, and evolution of these incursions is documented, using a simple compositing technique. The underlying physical mechanisms responsible for the occurrence of these incursions are also investigated by diagnosing the leading dynamic and thermodynamic forcing of their development.