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

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 Sao Paulo (USP) meteorological station in Sao Paulo City (Brazil) are used to select cold events with different intensities: extreme (T < 0degreesC plus frost), strong (0 < T < 2.5 degreesC plus frost), strong without frost (0 < T < 2.5 degreesC without frost) and moderate (T > 2.5 degreesC 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 Sao 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 Sao 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 (C) 2005 Royal Meteorological Society.