Polar outbreak in South America and the propagation of extratropical cyclones and anticyclones: a climatological point of view

Polar outbreaks during wintertime in South America (May to August) are part of a complex system that involves a great displacement of air masses not only near de surface but also in the high Troposphere. There are basically two major types of polar waves that normally take place in the South America: the first one is the typical polar outbreak that occur from zero to eight times a year in the tropical/subtropical region (referred to friagem, in Brazil, when it reaches the southern Amazon River Basin), and the other is the dramatically cold temperatures (up to -35oC) that can affect the extreme southern tip of the continent when the cold fronts are blocked around 30oS, with no mixture of the polar air with the tropical air mass. In this case, there is generally a great amount of precipitation associated, especially in the vicinity of Uruguay, extreme southern Brazil and northeastern Argentina. Also, a significant amount of snowfall usually occurs in all elevations south of 40oS and in higher elevations of the west up to 30oS.

From a synoptic point of view, the propagation of cold waves can be monitored by the movement of migratory extratropical cyclones and anticyclones near the surface. In this work, an automated procedure for finding and tracking high and low pressure centers has been applied for mean sea level pressure data set daily for 15 years period (1981-1996) from the NCEP (National Centers for Environment Prediction) Reanalysis to investigate such correlation. The area of interest extends from 0-70oS latitude and 180oW-50oE longitude. A case study was conducted to the extremely strong polar displacement occurred in June and July 1994.

The preliminary results are in agreement with the information found in the literature, showing that a strong polar outbreak in the tropical region requires the action of the two systems of pressure (cyclone and anticyclone) simultaneously, with a strong cold advection preceding a period of marked radiational cooling. The polar anticyclone that can originate a polar outbreak can be detected, in some cases, several days before the beginning of the event, in the west Pacific Ocean.