Abstract
A series of real data experiments is performed with a general circulation model in order to ascertain 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 particularly significant drying influences over North America in our prior simulations. The heating imposed in the prior experiments is compared to the condensation heating rates that naturally occur in the forecast model, and shown to be excessive by approximately a factor of two. Present experiments reduce the imposed anomaly by a factor of three, and also incorporate sea-surface temperature decreases over the eastern tropical Pacific Ocean. The new experimental results are in many ways consistent with our 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 dominant, and allowing increased rainfall over the Amazon Basin.
Highlights
The purpose of this paper is to describe the sensitlvity of the response of a general circulation model to variatiCX1S in the structure and magnitude of the tropical forcing
Such heating rates are probably excessive with respect to latent heating occurring naturally in the National Center for Atmospheric Research (NCAR) general circulation model (GCM) on seasonal time scales, and it is not clear how relevant they may be for shorter periods
The responses for ensemble 4 are similar to those for ensemble 3 and not shown. Comparasion of these cases with the ex periment in wh1ch only the East Pacif1c tropical SST was chang ed suggest that the tropical Atlantic heating provides the pr1nc1pal influence on the Eastern North American rainfall response of Fig. 3 and 4
Summary
The purpose of this paper is to describe the sensitlvity of the response of a general circulation model to variatiCX1S in the structure and magnitude of the tropical forcing. Thls work bullds upon a series of real data experiments incofllOrating tropical heating modifications with the National Center for Atmospheric Research (NCAR) general circulation model (GCM). Ln particu~ lar, modifications of Tropical Atlantic heating have supresingly strong and repeatable effects upon rainfall forecasts over North America, Buchmann et aI. Such heating rates are probably excessive with respect to latent heating occurring naturally in the NCAR GCM on seasonal time scales, and it is not clear how relevant they may be for shorter periods. This complicates the interpretatlon of the earlier results.
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