Cold Surges in Tropical and Extratropical South America: The Strong Event in June 1994

Abstract

The authors investigate one case of surges of polar air that occasionally propagate into southeastern Brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them. The cooling is also observed in southern and, with less intensity, in western Amazonia. The event of 26 June 1994 is studied. The frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices.The event was characterized by the presence of an anticyclonic perturbation off the coast of Chile that enters South America and crosses the Andes Mountains south of 45ЊS hours later. The anticyclone center tracks equatorward until it reaches 20Њ-30ЊS and then shifts eastward toward the Atlantic.Based on a diagnostic analysis using the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, a two-stage process can be distinguished. In the starting period, midtropospheric troughing is established east of the Andes, over central Argentina, due to stretching and/or vorticity advection. This troughing is responsible for cold advection along the eastern flank of the Andes, which produces substantial temperature drops and deepens the upper trough locally. The increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hPa is another result of the cooling process. This feature would ensure that the cold advection be extended eastward, producing an upper-level trough local deepening there. This intensification would produce an increase in the cyclonic vorticity advection, which would tend to produce sea level pressure drops underneath. This near-surface low pressure area would eventually contribute to the southeastern Brazil (SB) cooling due to the associated southerly winds.Another feature is the near-surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions, low-level divergence, and anticyclonic growth. Eventually this high pressure would also contribute to produce southwesterlies to transport cold air to SB.The paper is divided in two sections. The first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations. The purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four-times-daily surface and upper-air NCEP-NCAR reanalysis, as well as to look for possible predictors.

Cold surge episodes over southeastern Brazil – a potential vorticity perspective

“…Finally, cold surge days are compiled based on data and information given in Marengo et al (1997aMarengo et al ( ,1997b and Krishnamurti et al (1999). In total, 27 cases between 1983 and 1996 are identified and the 'verifying' day is taken to be the one with the largest temperature drop over southeastern Brazil.…”

“…Cold surge events used in the composite analysis. This compilation is based on data and information given in Marengo et al (1997aMarengo et al ( ,1997b and Krishnamurti et al (1999) America (June 26 event in Table 1). Shown are equivalent potential temperature θ e and geopotential height at 850 hPa from 1200 UTC, 24 June to 1200 UTC, 27…”

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…However, these features do not mean that there is a lack of convective organization. The main synoptic systems that approach the region and alter weather conditions are (a) the Intertropical Convergence Zone, mostly affecting the northern half of the Amazon; (b) the easterly waves coming from the tropical Atlantic (Diedhiou et al, 2010); (c) the upper tropospheric cyclonic vortices originating on the eastern coast of north-east Brazil and the associated upper-air Bolivian High (Silva Dias et al, 1983;Kousky and Gan, 1981); (d) the South Atlantic Convergence Zone, which affects the southern half of the Amazon and has a major effect on Amazonian convective activity as a whole ; and (e) the northward propagation of convective clouds (Siqueira and Machado, 2004) and the remnants of mid-latitude cold frontal systems that may propagate northward, sometimes beyond the Equator, resulting in so-called "friagem" events (Marengo et al, 1997). Within the basin, convection is often organized into squall lines (Cohen et al, 1995) that frequently occur as large systems originating at the northern coast and are triggered by local sea breeze circulation .…”

Dynamical conditions and synoptic tracks associated with different types of cold surge over tropical South America

“…In the literature, this issue has been analysed mainly in terms of case studies affecting the tropical and subtropical areas, known by the Portuguese word friagem (Hamilton and Tarifa, 1978;Fortune and Kousky, 1983;Girardi, 1983;Vera and Vigliarolo, 2000;Marengo et al, 1997;Garreaud, 2000). Composites of atmospheric fields are also found (Vera and Vigliarolo, 2000 and references therein); however, no synoptic climatology of the cyclone and anticyclone tracks related to strong cold surges has been presented yet.…”