Effects of Cold Fronts on the onset of the South American Monsoon

The interactions between the southward-moving intertropical convergence zone with cold fronts that progress equatorward contribute to the complexity of the timing and regional differences in South American Monsoon onset. The goal of this paper is to study the impact of cold fronts on the onset of the South American monsoon. Previous studies have shown that cold fronts undergo a seasonal transition in structure, intensity and propagation. In fact, whereas wintertime cold fronts bring relatively little rainfall but strong temperature anomalies to South America, summertime cold fronts affect the rainfall patterns well into the tropical portions of the continent but cause very small changes in temperature. In this study the 1979-2007 NCEP/NCAR reanalysis data will be used to produce a composite analysis of the dynamic and thermodynamic structures, intensity and propagation of cold fronts that occur before, during, and after monsoon onset. Those composites will be used not only to study the effect of cold fronts on onset in the Amazon basin, but also to study the effect of cold fronts on the abrupt onset of the monsoon season in the region of the South Atlantic convergence zone. The relative importance of the roles of gradual seasonal thermodynamic priming of the atmosphere versus the potentially rapid dynamical triggering of onset associated with cold fronts will also be evaluated on a regional basis.