INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 25: 215–241 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1080 DYNAMICAL CONDITIONS AND SYNOPTIC TRACKS ASSOCIATED WITH DIFFERENT TYPES OF COLD SURGE OVER TROPICAL SOUTH AMERICA ALEXANDRE BERNARDES PEZZA a * and T ´ ERCIO AMBRIZZI b a School of Earth Sciences, The University of Melbourne, Parkville, Victoria, Australia b Department of Atmospheric Sciences, Institute of Astronomy, Geophysics and Atmospheric Sciences, University of S˜ ao Paulo, S˜ ao Paulo, Brazil Received 28 November 2003 Revised 2 June 2004 Accepted 9 June 2004 ABSTRACT A 1973–2000 synoptic climatology (every 12 h) is presented of the surface cyclone and anticyclone tracks associated with cold surges in tropical South America, complemented by the corresponding atmospheric circulation for the period of 1950–2000. The aim of this study is to provide a new insight into the synoptic paths and the dynamics associated with distinct cold-wave categorization in the tropics. Extreme minimum temperatures and frost occurrence from the University of S˜ ao Paulo (USP) meteorological station in S˜ ao Paulo city (Brazil) are used to select cold events with different intensities: extreme (T< 0 ° C plus frost), strong (0 ≤ T ≤ 2.5 ° C plus frost), strong without frost (0 ≤ T ≤ 2.5 ° C without frost) and moderate (T> 2.5 ° C plus frost). Atmospheric variables at low and upper levels derived from the National Centers for Environmental Prediction – National Center for Atmospheric Research reanalysis are calculated from day -10 to day 0 with regard to the coldest day in S˜ ao Paulo. In addition, an automatic tracking scheme is applied to diagnosing and tracking the cyclones and anticyclones at the surface associated with cold surges. Through a superposition technique, climatological ‘clouds’ showing all tracks on the same map are produced, adding some new insights into the synoptic patterns of propagation and improving the Southern Hemisphere climatology. For all composites the mean cold front crosses the equator, and the extratropical cyclones also play an important role in favouring frost occurrence in S˜ ao Paulo. The lagged composites indicate that most of the cold events may be tracked up to 9 days before their occurrence, with a persistent upper level signal in the eastern Pacific. Copyright  2005 Royal Meteorological Society. KEY WORDS: frost; cold surges; cyclones; anticyclones; tracking schemes 1. INTRODUCTION Polar air outbreaks associated with severe frosts and snowfall are economically important for South America, a continent in which most countries depend strongly on agriculture and primary products. 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. Vera and Vigliarolo (2000) studied the dynamics of wintertime polar outbreaks from a climatological per- spective, emphasizing the differences between cases associated with frosts in the subtropical area and cases confined to mid latitudes. In this study, the principal component analysis (PCA) technique is used to obtain the most relevant physical patterns. At high levels, some Rossby wave dispersion to the northeast was shown, * Correspondence to: Alexandre Bernardes Pezza, School of Earth Sciences, The University of Melbourne, McCoy Building, SE Corner of Swanston and Elgin Streets, Parkville, Victoria 3010, Australia; e-mail: apezza@unimelb.edu.au; ambrizzi@model.iag.usp.br Copyright  2005 Royal Meteorological Society