A three-stage conceptual model for South American monsoon onset

The evolution of monsoon onset across South America has a complex temporal and regional variability caused by the complex interactions between the southward propagation of the land and ocean-based intertropical convergence zone (ITCZ) and frontal systems that propagate equatorward from higher latitudes.

In this study a three-stage conceptual model for the onset of the South American monsoon season is proposed. This conceptual model is obtained using a rain threshold analysis and a rotated empirical orthogonal function analysis of Global Precipitation Climatology Project v2 dataset, with precipitation given in pentads at 2.5° horizontal resolution from 1979-2007. According to this model, the first stage of onset starts around pentad 56 (3-7 Oct) when precipitation begins in the northwestern part of the continent and gradually progresses south and southeastward. The second stage is marked by the abrupt onset of the South Atlantic convergence zone brought about by changes in the characteristics of frontal systems passing through South America. This second stage of monsoon onset occurs on average around pentad 61 (28 Oct-1 Nov). The third stage of monsoon onset involves the late arrival of the monsoon to the mouth of the Amazon River, associated with the slow migration of the Atlantic ITCZ. The third stage of onset occurs on average by pentad 72 (22-26 Dec). This three-stage model of onset proves to be an useful framework for the study of regional differences in monsoon onset mechanisms, a subject that is further explored in two companion posters.