A mini-AMIP has been conducted to examine the deterministic vs. stochastic behavior of one AGCM. It was carried out as follows:
Ensemble forecasts of global climate conditions during the seven largest Pacific warm/cold events of the last 23 years were made with a new two-tiered climate forecast technique. The internal model variability (IMV) within the atmospheric model used in the forecasts was large-approximately 1/3 to 1/2 the variability obtained from the same model forced by observed SST over the last 20 years (AMIP). This variability led to a wide range of realistic-looking forecasts even when the equatorial SST forcing was nil. For example, single model forecasts forced by climatological SST produced excellent simulations of the extreme cold event of 1988/89 and the large warm event of 1982/83. Clearly, single simulations are inadequate for forecasting and sensitivity studies such as AMIP. The ten member ensembles used in this study appear a minimum for reliable forecasts/sensitivity studies in midlatitudes, although fewer realizations may be needed in the tropics. The next AMIP should be viewed as a stochastic experiment, one requiring a Monte Carlo framework in order to produce robust results.