One of the most important modes of interannual climate variability over the Americas is related to El Niņo/Southern Oscillation (ENSO). In general, during boreal winter the presence of El Niņo reflects as enhanced precipitation over the subtropical Americas. Anomalous convective activity in the central-eastern Pacific forces the so-called Pacific-North American (PNA) pattern, i.e., a quasi-stationary Rossby wave. The cyclonic circulations in the PNA pattern, one off the coast of California and one extending from Florida to the Gulf of Mexico produce a southward shift in the mean position of the subtropical jet, and ascending motion. As a result anomalous enhanced precipitation occur over California and Florida and the Gulf of Mexico. Over South America, a quasi-stationary Rossby wave train is also triggered. The amplitude of the cyclonic circulations and the intensity of the anomalous ascending motion are weaker. On the other hand, over central America and northeast Brazil, the effects of enhanced subsidence associated with El Niņo, inhibit the development of deep convection and produce precipitation deficits and drought.
In the subtropical and tropical Americas, precipitation anomalies during El Nino, are related to the structure of the mean flow and changes in the amplitude and phase of these waves. These changes result in inter El Nino variability. Numerical experiments with a baroclinic model show that the quasi-stationary waves are in turn affected by the charcateristics of the forcing itself. Results from simulations performed with the Community Climate Model (CCM3), using observed monthly sea surface temperatures are analyzed in the light of the baroclinic model experiments. It is found that, the structure of the NH winter mean flow leads to erroneous position and intensity of the quasi-stationary circulations over the Americas, and therefore, to deficient simulations of precipitation. The importance of these analyses for seasonal climate prediction of regional climate in the subtropical and tropical Americas is discussed