| description 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?confluent flow 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 10°C over the subtropical part of the continent (as far north as 10°S). 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 30°S 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 flow that advects cold air into the subtropics. Finally, as the cold air moves to the north of 18°S, 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 fluxes. | |
