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

Meridional transport of air masses between the tropics and midlatitudes in South America are the most intense in the entire Southern Hemisphere, mainl y due to the presence of the Andes. The incursions of tropical air into mid latitudes occur on the eastern side of the Andes in two preferred regions. The first is located in the tropical latitudes, close to the mountains betw een 20 degrees and 30 degrees S, and the second is a function of the positi on of the South Atlantic Convergence Zone (SACZ). All year long, the two pr eferred regions maintain their behaviour, with only small variations of the ir position or relative importance in different seasons of the year. The va riability of the meridional transport is larger on the eastern side of the Andes, due to the presence of the mountain barrier, which favours baroclini c activity and allows an active exchange of air masses in both senses, espe cially during winter. The importance of the air mass transport is evident in the precipitation an d surface temperature fields. During summertime, the Chaco Low (25 degrees S and 65 degrees W) intensifies due to the positive net radiation, favourin g the transport of tropical air masses towards the south and the presence o f strong convective activity, which is fed by moisture from tropical region s. During winter, the penetration of tropical air towards higher latitudes is more sporadic. The displacement of midlatitude air towards tropical lati tudes occurs on both sides of the Andes. On the western side, the air assoc iated with the subtropical Pacific anticyclone flows northward channelled b y the Andes. On the eastern side, incursions of polar air towards lower lat itudes are linked to cold fronts whose trajectory and movement is also favo ured by the presence of the Andes. In particular, during wintertime the col d fronts are more intense and faster, and sometimes even reach tropical and equatorial latitudes which produces freezes in subtropical regions, such a s the coffee growing areas in southeastern Brazil. In contrast, the incursi ons 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 (C) 2000 Royal Meteorological Society .