Polar Outbreaks And Their Significance In The Climate Of The Americas

3.3.1. South American Polar Outbreaks

Synoptic-scale incursions of midlatitude air moving into tropical and subtropical latitudes east of the Andes (leeward side) are observed all year long in two preferred regions: the first is located close to the mountains between 20° and 30°S, and the second region is located in southern Brazil, extending into the adjacent Atlantic.

In summer (December-February), cold air from higher southern latitudes moves into tropical latitudes behind a cold front and organizes tropical convection. This movement has a large influence on climatological rainfall patterns over central Brazil and eastern Amazonia. At this time of the year, there is a strong southward advection of warm and humid air coming from the northern Amazon that creates favorable conditions for the development of mesoscale convective systems linked to abundant rainfall over Paraguay and northern Argentina. On the leeward side of the Andes, incursions of polar air toward lower latitudes accompanying a cold front are favored by the channeling effect of the mountains. During these incursions, cold fronts move northward and merge with the South Atlantic Convergence Zone (SACZ), a region of convergence of moist air from the Amazon and cold air from the south, generating convection that is enhanced by the assimilation of the cold fronts. Because of this convergence, the SACZ is more intense in summer (Garreaud, in press a,b,; Liebmann et al., 1998; Seluchi and Marengo, in press).

In winter (June-August), polar air masses move from higher to lower latitudes and the cold front sometimes becomes stationary, allowing an intense nocturnal cooling of the surface, associated with low humidity and a lack of clouds. This cold air has undergone modifications along its path, but still may be cold enough to produce freezing conditions and snow in the subtropical parts of Brazil. Freezing weather in southeastern Brazil is caused by these polar air outbreaks in May-August. Events like these are known by the Portuguese name of friagem (plural: friagems) or Surazo in the Amazon region, where they have a marked effect on tropical and extratropical weather. These cold surges occur several times per year (from zero to eight times), producing low temperatures in the midlatitudes, and are sometimes so strong that extensive freezes affect southeastern Brazil (see reviews in Marengo et al., 1997a). In some episodes, the cold surges produce considerable cooling in central and northern Amazonia (Morize, 1992; Serra and Ratisbona, 1942; Parmenter, 1976; Fortune and Kousky, 1983; Satyamurty et al., 1990; Seluchi and Nery, 1992; Marengo et al., 1997a,b; Seluchi and Marengo, in press).

3.3.1.1. Conceptual Model of South American Polar Outbreaks

Most of the literature on South American polar outbreaks has focused on descriptions of the circulation and dynamics of individual episodes (see reviews in Marengo et al. 1997a; Garreaud, in press a,b; Marengo, submitted), while more recent work has focused on the

Cold Surge The Northern Hemisphere
FIGURE 3 Conceptual model of wintertime polar outbreaks in South America. (Adapted from Marengo et al., 1997a; Garreaud, in press b.)

mean structure, evolution, and dynamics of these phenomena (Garreaud, in press a,b; Seluchi and Marengo, in press; Kousky and Cavalcanti, 1997). Based on literature published from early work by Morize (1922) to the most recent work by Garreaud (in press a,b) and Marengo et al. (1997a) describing general features and discussing individual case studies, a simple conceptual model for wintertime polar outbreaks is shown in Fig. 3.

Several case studies have shown that cold outbreaks and some freeze events in southern Brazil are preceded by a slow eastward-moving long wave in the South Pacific Ocean, which is amplified greatly some days before (see review in Marengo, submitted) and exhibits large meridional displacements of the flow (see Table 1 for some cold events in the state of Sao Paulo, Brazil). When at maximum amplitude, the wave ridge is usually located near the southern Andes, while the next downstream trough lies over the South Atlantic in Brazilian longitudes. This typical configuration allows air originally from higher latitudes to be channeled equatorward. Frontogenesis is associated with the wave amplification, and a freeze occurs east of the polar anticyclone. In South American cold waves, an anticyclone-cyclone couplet causes the associated southerlies to transport colder air from high latitudes, contributing substantially to the cooling. The conceptual model (Garreaud, in press b; Marengo et al., 1997a) shows the evolution of near-surface circulation patterns. Key elements include the cold core anticyclone that moves from the southeastern Pacific into southern Argentina near the extreme southernmost region of South America, as well as the cyclone over the southwestern Atlantic. South of 30oS these two pressure systems grow due to upper level vorticity advection. During the pre-polar outbreak period, before the coldest day in southeastern Brazil (Fig. 3A), the geostrophic southerly winds between the high- and low-pressure systems produce low-level cooling along the east coast of South America and farther inland as far north as 250S.

Closer to the tropical Andes, the low-level flow is blocked during the developmental period, leading to ageostrophic, mountain parallel flow (Fig. 3B) and cold

TABLE 1 List of Cold Events in the State of Sao Paulo, Brazil and Their Intensity

Date

Intensity (min. temp., °C)

Damage

14 July 1892

0 0

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