INVESTIGADORES
VERA Carolina Susana
artículos
Título:
A Diagnostic Study of Cold-Air Outbreaks over South America
Autor/es:
VERA, CAROLINA; VIGLIAROLO, PAULA
Revista:
MONTHLY ENERGY REVIEW
Editorial:
AMER METEOROLOGICAL SOC
Referencias:
Lugar: Boston; Año: 2000 vol. 128 p. 3 - 24
ISSN:
0027-0644
Resumen:
weather
events over the southern part of South America, particularly during the austral winter months. Behind
cold fronts, cold air from higher latitudes is forced to flow equatorward to the east of the Andes. Occasionally
fronts might produce cold surges in tropical latitudes, with freezing temperatures (referred as to GEADAS in
Portuguese) over crop-growing areas of southern, southeastern, and central Brazil.
The structure and dynamical processes of winter synoptic-scale waves associated with cold surges over South
America are documented in this paper. It is shown that the rotated extended empirical orthogonal function
technique is capable of extracting well-defined regional patterns from 850-hPa meridional-wind perturbation
fields. Over South America, waves behave very differently in the lower and middle troposphere: while upperlevel
waves propagate northeastward as they cross the Andes, evolving in a manner consistent with the concept
of Rossby wave dispersion, at lower levels, waves tend to conform to the shape of the mountain range, in
agreement with the theory of topographic Rossby waves. However, it was found that cold surges do not seem
to result from the generation of rotationally trapped waves.
Significant differences have been found between synoptic-scale patterns associated with cold surges over
extratropical South America (NO GEADA composites) and synoptic-scale patterns associated with major coldair
outbreaks that produce extreme low temperatures at tropical regions (GEADA composites). While both surges
are characterized by a long-wave pattern consisting of a cyclonic perturbation over South America and anticyclonic
perturbation behind over the southern Pacific Ocean, at the early stages of GEADA composites, two
additional features were identified: a subpolar and subtropical short-wave pattern. The presence of the upperlevel
subtropical cyclonic perturbation is associated with the subtropical jetstream location farther north, which
would facilitate the equatorward penetration of frontal systems. In addition, this cyclonic perturbation enhances
rising motion at subtropical and tropical latitudes that is associated with a well-defined secondary circulation
with its descending branch over central Argentina. It is shown that this configuration favors the rapid and
equatorward penetration of the subpolar short-wave trough, the temperature decrease over southern Brazil, and
the anticyclone intensification over central Argentina. Thus the presence of the subtropical upper-level feature
plays a key role on the cold-surge occurrence over tropical regions of South America.