Influence of the Frontal Systems on the Day-to-Day Convection Variability over South America

Abstract

Cold cloud-top fractions derived from International Satellite Cloud Climatology Project images and latitude? time diagrams are used to study the interaction of frontal systems with tropical convection over South America (SA). An 11-yr climatology for three frequent types of frontal system?tropical convection interaction is built, and the associated day-to-day convection variability is described using satellite images, complex principal components, and wavelet transforms. Type 1 is frequent throughout the year, especially in austral summer, and is characterized by the penetration of a cold front in subtropical SA that interacts with tropical convection and moves with it into lower tropical latitudes. Type 2 is also more frequent in austral summer and is characterized by Amazon convection and enhancement of a quasi-stationary northwest?southeast-oriented band of convection extending from the Amazon basin to subtropical SA along the passage of a cold front in the subtropics. When the type 2 pattern remains longer than 4 days over SA, it often characterizes the South Atlantic convergence zone. Type 3, which is more frequent in austral winter, is represented by a quasi-stationary cold front in subtropical SA and midlatitudes without significant interaction with tropical convection. Predominant day-to-day fluctuation time scales of convection associated with the three types were identified, ranging from 5 to 7 days in the Tropics (types 1 and 2) and subtropics (type 3). By evaluating circulation patterns over SA using National Centers for Environmental Prediction analysis at 850 and 200 hPa, the northeastward propagation of a transient cyclonic vortex organized by a cold front in southeast SA and Amazon moisture flows is the main feature of the type 1 pattern at low levels. A cyclonic vortex similar to the one in type 1 but quasi-stationary in the subtropics is remarkable for the type 2 pattern, while upper-level cyclonic vortices in northeast Brazil and the existence of a subtropical jet seem to contribute to the blocking configuration of cold fronts in subtropical SA that characterizes the type 3 pattern.