DECEMBER 1999 GARREAUD 2823 Cold Air Incursions over Subtropical and Tropical South America: A Numerical Case Study RENE D. GARREAUD Department of Atmospheric Sciences, University of Washington, Seattle, Washington (Manuscript received 6 June 1998, in ®nal form 24 November 1998) ABSTRACT Synoptic-scale incursions of cold, midlatitude air that penetrate deep into the Tropics are frequently observed to the east of the Andes cordillera. These incursions are a distinctive year-round feature of the synoptic climatology of this part of South America and exhibit similar characteristics to cold surges observed in the lee of the Rocky Mountains and the Himalayan Plateau. While their large-scale structure has received some attention, details of their mesoscale structural evolution and underlying dynamics are largely unknown. This paper advances our understanding in these matters on the basis of a mesoscale numerical simulation and analysis of the available data during a typical case that occurred in May of 1993. The large-scale environment in which the cold air incursion occurred was characterized by a developing midlatitude wave in the middle and upper troposphere, with a ridge immediately to the west of the Andes and a downstream trough over eastern South America. At the surface, a migratory cold anticyclone over the southern plains of the continent and a deepening cyclone centered over the southwestern Atlantic grew mainly due to upper-level vorticity advection. The surface anticyclone was also supported by midtropospheric subsidence on the poleward side of a jet entrance±con¯uent ¯ow region over subtropical South America. The northern edge of the anticyclone followed an anticyclonic path along the lee side of the Andes, reaching tropical latitudes 2± 3 days after its onset over southern Argentina. The concomitant cold air produced low-level (surface to ;800 hPa) cooling on the order of 108C over the subtropical part of the continent (as far north as 108S). Based on the observations and model results, a three-stage evolution of the cold air incursion is suggested. The initial cooling to the south of 308S and far from the Andes is mainly produced by the geostrophic southerly winds between the continental anticylone and the developing low off the coast of Argentina. As the surface pressure increases over southern Argentina, a large-scale meridional pressure gradient is established between the migratory anticyclone and the continental trough farther to the north. The blocking effect of the Andes leads to an ageostrophic, low-level southerly ¯ow that advects cold air into the subtropics. Finally, as the cold air moves to the north of 188S, the blocking effect of the Andes weakens (because the adjustment back to geostrophy is quite slow at these low latitudes) and the cold air spread out over the Tropics. In the last two stages of the incursion the strong pressure (temperature) gradient drives the northward accelaration of the low-level winds, while horizontal advection of cold air by southerly winds maintains the strong temperature gradient against the dissipative effects of the strong surface heat ¯uxes. 1. Introduction in¯uence on the weather over the western side of the Synoptic-scale incursions of cold air that penetrate great plains of North America (e.g., Tilley 1990; Blue- deep into the Tropics are frequently observed to the east stein 1993; Colle and Mass 1995) and their subsequent of major north±south-oriented mountain ranges, pro- impact on Central America and the Caribbean [see ducing dramatic weather changes over a wide range of Schultz et al. (1997, 1998) for an extensive review of latitudes. Particular emphasis has been placed on cold the literature on Central American cold surges]. Surges surges bounded by the Himalayan Plateau over south- of cold air of somewhat smaller scale have also been east Asia and their in¯uence on the Tropics during the documented along the east side of the Appalachians winter monsoon (e.g., Ramage 1971; Lau and Chang (e.g., Bell and Bosart 1988) and along the east coast of 1987; Boyle and Chen 1987; Wu and Chan 1995, 1997). Australia (e.g., Baines 1980; McBride and McInnes Cold surges along the east side of the Rocky Mountains 1993). have also attracted considerable interest due to their In South America, episodic incursions of midlatitude air to the east of the subtropical Andes (also referred to as South American cold surges) are a distinctive year- round feature of the synoptic climatology, whose ex- Corresponding author address: Dr. Rene D. Garreaud, Departa- mento de Geo®sica, Universidad de Chile, Casilla 2777, Santiago, istence and relevance has been recognized by synopti- Chile. cians for a long time. Statistical analyses by Kousky q 1999 American Meteorological Society 2824 MONTHLY WEATHER REVIEW VOLUME 127 FIG. 1. Topography of South America. Elevation scale in meters above sea level. The outer and inner boxes indicate the outer and inner domain used in the numerical simulation. and Cavalcanti (1997), Compagnucci and Salles (1997), or so of moderate low-level southerly winds (;10 m Garreaud and Wallace (1998), Vera and Vigliarolo s21) characterize the passage of cold surges over sub- (2000), and Garreaud (1998) also demonstrate that tropical South America during wintertime. Extreme ep- South American cold surges dominate the synoptic var- isodes produce widespread freezing from central Ar- iability of the low-level circulation, air temperature, and gentina to southern Brazil, which has motivated some rainfall over much of the continent to the east of the case studies (Hamilton and Tarifa 1978; Fortune and Andes. The prevalence of this phenomenon seems at Kousky 1983; Marengo et al. 1997; Bosart et al. 1998). least partially related to the favorable continental-scale Summertime episodes produce less dramatic ¯uctua- topography: the Andes cordillera extends continuously tions in temperature and pressure, owing to the smaller from the southern tip of the continent (;508S) to the seasonal temperature gradient between mid- and low north of the equator with an almost straight north±south latitudes, but they are accompanied by a band of en- orientation, elevations in excess of 3000 m along most hanced convection and rainfall at the leading edge of of its extension, and a steep eastern slope (;300 km the cool air (Ratisbona 1976; Parmenter 1976; Kousky wide). Along its central portion (158±228S) the Andes 1979; Kousky and Ferreira 1981; Garreaud and Wallace holds a high-level plateau known as the Altiplano, about 1998). These synoptic-scale bands of organized deep 300 km wide and at a mean elevation of 3800 m (;620 convection account for nearly half of the summertime hPa). To the east of the Andes, the terrain is low and rainfall over the subtropical plains of the continent and ¯at, at an average elevation of 500 m, with the exception produce some of the heaviest rainfall episodes during of the Brazilian highland near the eastern tip of the summer (Garreaud and Wallace 1998). continent (see Fig. 1 for a topographic map of South The above cited case studies and statistical analyses America and Fig. 16 for a west±east cross section along provide a comprehensive picture of the mean, synoptic- 208S). scale structure of South American cold surges. The up- Moderate surface pressure rises (;10 hPa day21), per-level circulation is typically characterized by a de- low-level air temperature drops (;58C day21) and a day veloping midlatitude wave, with a ridge immediately to DECEMBER 1999 GARREAUD 2825 the west of the Andes and a downstream trough over 208S) and tropical (208±108S) South America to the east eastern South America, which provides the large-scale of the Andes that took place in the autumn of 1993 forcing of the system predominantly in the form of vor- between 11 and 16 May. The analysis is based on con- ticity advection (Marengo et al. 1997; Garreaud and ventional synoptic data (surface and radiosonde station Wallace 1998; Garreaud 1998). At lower levels, the reports stored by NCAR), and the National Centers for equatorward incursion of cold air is initiated by the Environmental Prediction (NCEP)±NCAR gridded (2.58 movement of a migratory cold anticyclone from the lat 3 2.58 long) reanalyzed ®elds (Kalnay et al. 1996). southeastern Paci®c onto the southern plains of the con- Because the 6-hourly reanalyses were produced using tinent (to the east of the Andes between 408 and 308S) a state-of-the-art weather prediction model and data as- and the deepening of a low-pressure center over the similation system, and an enhanced observational da- Atlantic off the coast of southern Argentina. Along 358S tabase (including conventional, aircraft, and satellite the anticyclone moves slowly to the east, while its north- data), they are considered to compose one of the most ern leading edge becomes increasingly detached from complete and physically consistent datasets of the at- the eastward drifting upper-level ¯ow and moves north- mospheric circulation (Kalnay et al. 1996). Furthermore, ward to reach the subtropical and tropical (208±108S) the most important features of South American cold part of the continent within 2±3 days, collocated with surges take place over a continental area to the north a sharp transition in low-level meridional wind and a of 408S, where the assimilation of conventional data low-level baroclinic zone. should strongly damp any errors that may propagate in The mesoscale structural evolution and dynamics of from the data-void southern Paci®c. We were unable to this phenomenon, however, are largely unknown. Two gather enough precipitation data, but GOES-7 infrared important issues need to be examined. First, is the north- images (from a Geostationary Operational Environ- ward advance of cold air produced by orographically trapped waves (such as Kelvin waves or shelf waves) mental Satellite) were used as a proxy for convective or, alternatively, is horizontal advection the dominant cloudiness.