Further Experiments on the Effect of Tropical Atlantic Heating Anomalies upon GCM Rain Forecasts over the Americas

A series of real-data experiments is performed with a general circulation model to study the sensitivity of extended range rain forecasts over the Americas to the structure and magnitude of tropical heating anomalies. The emphasis is upon heat inputs over the tropical Atlantic, which have shown significant drying influences over North America in the author's prior simulations. The heating imposed in the prior experiments, that is, shown to be excessive by a factor of 2, is compared with the condensation heating rates that naturally occur in the forecast model. Present experiments reduce the imposed anomaly by a factor of 3 and also impose sea surface temperature decreases over the eastern tropical Pacific Ocean. The new experimental results are in many ways consistent with the author's prior results. The dry North American response is statistically more significant than the South American response and occurs at least as frequently in the different members of the experimental ensembles as in our prior experiments. The drying effect is accentuated by the presence of east Pacific cooling, but this does not appear to be the dominant influence. Over tropical South America, the Pacific and Atlantic modifications produce compensating influences, with the former dominating, and allow increased rainfall over the Amazon Basin.