Descripción: The classification of El Niño events was performed based on the time evolution of sea surface warming in the tropical Pacific Ocean during the period 1950-2000. Two sets of events were constructed: one in which the warming core migrates eastward along the tropical Pacific until April-June of the following year, and another one in which it evolves westward until November-January. The first type has associated positive precipitation anomalies over southeastern South America during April-June. It results from a favourable combination of cyclonic vorticity advection and humidity convergence. At high levels, cyclonic vorticity advection is explained in terms of an eastward extension of the subtropical jet. Enhanced humidity advection takes place by an increased low-level northwesterly flow to the east of the Andes. It provides enough moisture availability that, in combination with the upper-level cyclonic vorticity advection, supports heavy precipitation during April-June. The second type of event exhibits slight negative or near-normal precipitation anomalies over the same region. Both low and high-level circulation anomalies are also weaker in this case. The 1997-1998 El Niño is analysed separately because it cannot be classified into any of the previously described event types. The observed distribution of both types of events along the analysed period changes after the 1970s. Comparison with other authors' results suggests the influence of low-frequency processes such as the Pacific Decadal Oscillation (PDO). Copyright © 2008 Royal Meteorological Society.

Descripción: We investigate the performance of one stretched-grid atmospheric global model, five different regional climate models and a statistical downscaling technique in simulating 3 months (January 1971, November 1986, July 1996) characterized by anomalous climate conditions in the southern La Plata Basin. Models were driven by reanalysis (ERA-40). The analysis has emphasized on the simulation of the precipitation over land and has provided a quantification of the biases of and scatter between the different regional simulations. Most but not all dynamical models underpredict precipitation amounts in south eastern South America during the three periods. Results suggest that models have regime dependence, performing better for some conditions than others. The models' ensemble and the statistical technique succeed in reproducing the overall observed frequency of daily precipitation for all periods. But most models tend to underestimate the frequency of dry days and overestimate the amount of light rainfall days. The number of events with strong or heavy precipitation tends to be under simulated by the models. © The Author(s) 2009.