On precursors of South American cyclogenesis

Cyclogenesis over the southern region of South America is studied in relation to the evolution of dynamic and thermodynamic fields over the continent, using storm tracking and derived composite analysis over a 25 yr period. Results show that, irrespective of the season, there is always a moist-entropy reservoir northwest of the cyclone formation region. One day before cyclone formation, moist entropy over North Argentina is anomalously high and increasing, due to an intensification of the northerly flow along the eastern flank of the Andes, peaking at the time of cyclogenesis. The new cyclone is fed by warm and moist air coming from the continent but, as it intensifies and moves eastwards, imposes an anomalously southerly flow reducing the moist-entropy anomaly over South America. The presence of the Andes Cordillera plays a major role controlling the location of the main cyclone formation area in the region by (i) channelling the warm moist northerly flow in the lower troposphere and (ii) inducing cyclonic circulation in the lee side, when upper troughs travelling eastwards move over the mountain range at the middle latitudes.