Equatorward cold surges in South America

Relatively cold air sometimes surges northwards east of the Andes, as explained by Nocera et al (1). These result from displacement of the southeastern Pacific anticyclone onto South America, particularly when there is a trough over the east coast and a ridge over Chile. A large pressure gradient builds up along the Andes, and trapped by the mountains, air from Argentina (sometimes Patagonia) moves towards the equator. Extreme cold surges are known as friagems in Brazil (Section 13.3). There were 46 extreme cold surges during 1882-1994.

Closer examination of weather maps shows that five types of cold surges can be distinguished, as follows -

1. The high from the Pacific never establishes itself east of the Andes, and a ridge of high pressure lies east-west, rather than parallel to the mountains. The result is only a weak surge.
2. A ridge is parallel to the Andes and east of them, with its nose pointing to the equator. The anticyclonic motion collects high-latitude air, which is then carried northwards by low-level winds channeled by the range. This type provides the strongest cold surges (friagems).
3. Normally brief cold surges occur when there is a cyclone in the western Atlantic, usually offshore from Buenos Aires, directing air westwards at high latitude and then northwards. Such flows are prolonged only while the cyclone remains stationary.
4. A high near south Brazil sends winds westwards against the Brazilian coastal mountains, and they deflect a cold surge along the coast towards Brazil's Northeast. This type may last for a week or more, bringing cool, wet weather from Sao Paulo to Recife.
5. Cold surges also happen when southerlies blow between the Andes and the Brazilian coastal mountains towards a deeper-than-normal trough in the SW quadrant of the Amazon Basin.

 The most common types during 1992-6 were 4 and 5, occurring throughout the year. The third type was rare. Types 1 & 2 happened in May - September, the cool season.

A friagem on 26 June 1994 caused record-breaking low minimum temperatures from Buenos Aires to southern Brazil. It damaged the coffee crop to the extent that its price rose by 70% (2). A 5-day sequence of surface charts (Fig 1) shows that this outbreak is a combination of types 2 and 3. A high off Chile crossed the Andes then tracked north to about 25° S, and then east towards the Atlantic. This unusually strong high came in the wake of a frontal low that intensified first in the lee of the Andes (on 23 June) and then over the southwestern Atlantic (on 24-25 June). The strong high and later deep low produced a strong southerly wind, guiding cold air towards the state of Sao Paulo. The cold air advection, followed by radiational cooling on the calm, clear night of 25-26 June, caused widespread frosts.

Fig 1. Synoptic charts for southern South America at Greenwich noon on 23-27 th June 1994 (2).

1. Nocera, J.J., L.F. Bosart and D.L. Knight 1998. A study of cold surges in South America. Paper to 8th Amer. Meteor. Soc. conference on Mountain Meteorology, Flagstaff, 2 pp.
2. Marengo, J. et al. 1997. Cold surges in tropical and extra-tropical South America. Monthly Weather Review, 125, 2759-86.