14.1.2.1.3. Large-scale atmospheric circulation
Latin America's climate is influenced mainly by the northern Atlantic
anticyclone and the migration of the inter-tropical convergence zone, which
also affects large areas of tropical South America. The southern part of the
continent is more affected by Atlantic and Pacific anticyclones, the thermic
low pressure of northwestern Argentina, and mid-latitudes westerlie. All of
these circulation features interact strongly with the complex topography of
Latin America.
Analysis of ice cores in west Antarctica indicate that meridional atmospheric
circulation intensity between middle and high latitudes has experienced substantial
strength variability, increasing in the Little Ice Age (Kreutz et al.,
1997; Leckenbush and Speth, 1999). At paleoclimatic time scales, analyses of
fossilized pollen and lake sediments have shown more intense and frequent incursion
of polar air from the Antarctic region 12,000-8,000 years before the present
(BP) (Ledru et al., 1994).
In southern Brazil, there has been a tendency over the past 20 years for fewer
wintertime cold fronts and polar outbreaks (Marengo and Rogers, 2000), which
is somewhat consistent with reported interdecadal variations in the mean position
and intensity of the south Atlantic anticyclone (Venegas et al., 1998).
For mid-latitude South America, important changes in zonal circulation have
been observed between 1899 and 1986, with wintertime circulation weaker for
the period 1939-1949 and strong during 1967-1977—suggesting interdecadal
changes. Over Paraguay, southern Brazil, Uruguay, and northeast Argentina, northeasterly
circulation associated with the subtropical Atlantic anticyclone increases after
1954 (Hoffman et al., 1987; Minetti and Sierra, 1989; Cantañeda
and Barros, 1993; Barros et al., 1999).
Instrumental records, sounding information, and satellite data show changes,
fluctuations, and "sudden jumps" in some features of atmospheric circulation
over Latin America and its adjacent oceans, in connection with detected changes
in the global climate system.
|