Tropical-midlatitude exchange of air masses during summer and winter in South America: climatic aspects and examples of intense events

Abstract

Meridional transport of air masses between the tropics and midlatitudes in South America are the most intense in the entire Southern Hemisphere, mainly due to the presence of the Andes. The incursions of tropical air into midlatitudes occur on the eastern side of the Andes in two preferred regions. The first is located in the tropical latitudes, close to the mountains between 20° and 30°S, and the second is a function of the position of the South Atlantic Convergence Zone (SACZ). All year long, the two preferred regions maintain their behaviour, with only small variations of their position or relative importance in different seasons of the year. The variability of the meridional transport is larger on the eastern side of the Andes, due to the presence of the mountain barrier, which favours baroclinic activity and allows an active exchange of air masses in both senses, especially during winter. The importance of the air mass transport is evident in the precipitation and surface temperature fields. During summertime, the Chaco Low (25°S and 65°W) intensifies due to the positive net radiation, favouring the transport of tropical air masses towards the south and the presence of strong convective activity, which is fed by moisture from tropical regions. During winter, the penetration of tropical air towards higher latitudes is more sporadic. The displacement of midlatitude air towards tropical latitudes occurs on both sides of the Andes. On the western side, the air associated with the subtropical Pacific anticyclone flows northward channelled by the Andes. On the eastern side, incursions of polar air towards lower latitudes are linked to cold fronts whose trajectory and movement is also favoured by the presence of the Andes. In particular, during wintertime the cold fronts are more intense and faster, and sometimes even reach tropical and equatorial latitudes which produces freezes in subtropical regions, such as the coffee growing areas in southeastern Brazil. In contrast, the incursions of cold air are notably weaker and less frequent in summer, and during these events the active cold fronts move northwards merging with the SACZ, which becomes more intense. Copyright © 2000 Royal Meteorological Society