The role of cold fronts in the onset of the monsoon season in the South Atlantic convergence zone

The onset of the South American monsoon season culminates with the abrupt establishment of the South American convergence zone (SACZ). The impact of cold fronts on the abrupt establishment of the SACZ is studied using an 11-year composite analysis of the dynamic and thermodynamic structures, intensity and propagation of cold fronts that occur prior to, during, and after monsoon onset in the SACZ. A significant change in the structure and propagation of cold fronts is observed at the time of monsoon onset in the SACZ, with cold fronts suddenly stalling and becoming stationary in southeastern Brazil. It is proposed that this regime change of the structure and propagation of cold fronts causes the abrupt onset of the monsoon in the SACZ. A mechanism for this sudden change in cold front behaviour is suggested by analyzing changes in the observed upper-level structure of midlatitude waves. Our observations show that, at the time of monsoon onset in the SACZ, midlatitude waves develop a 'thinning trough' behaviour, with upper-level troughs becoming thinner, westward tilted, and sometimes forming cut-off lows. This behaviour is characteristic of midlatitude waves that develop in the region of anticyclonic shear on the equatorward side of the upper-level midlatitude jet. During the austral spring, the upper-level midlatitude jet over South America migrates southward leaving subtropical South America in the equatorward, anticyclonic shear side of the jet. It is hypothesized that, as the meridional shear on the equatorward side of the upper-level jet gradually becomes more anticyclonic, it suddenly crosses the critical threshold of horizontal shear that has been shown to produce a sudden transition to a 'thinning trough' midlatitude wave regime. At this time a regime change occurs whereby the character of frontal systems crossing South America changes abruptly, causing the onset of the monsoon season in the SACZ. Copyright (C) 2011 Royal Meteorological Society