Abstract
Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP–NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field.
The summertime incursions move equatorward at a mean speed of 10 m s−1 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough–ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tends to deepen the semipermanent area of low pressure over the central plains of the continent (25°–10°S). The strong southward advection of warm, humid air creates favorable conditions for the development of deep convection over central and southern Argentina. During the next two days, a migratory cold anticyclone, steered by the ridge aloft, moves onto the southern plains of South America (40°–35°S), establishing a large-scale, southward-directed pressure gradient. Due to the blocking effect of the Andes, the induced low-level circulation assumes the form of an agesotrophic southerly flow that produces the equatorward advection of the cold air. In this stage, enhanced convection occurs in a well-defined band of the intensified low-level convergence at the leading edge of the cold surge. This synoptic-scale banded structure in the convective cloudiness also emerges as the dominant mode of the day-to-day variability of the deep convection, and the contribution of these systems to the summertime precipitation varies from ∼25% in the central Amazonia up to ∼50% over the subtropical plains of the continent. The equatorward incursions of cool, dry air (and their hydrostatically induced surface pressure anomalies) finally vanish as a result of strong surface heat fluxes at low latitudes.
Corresponding author address: Dr. René D. Garreaud, Department of Atmospheric Sciences, University of Washington, P.O. Box 354235, Seattle, WA 98195-4235.
