April 1998, 47 pp.
Although there are apparent model outliers in each simulated variable examined, validation of the AMIP models' ensemble mean shows that the average large-scale seasonal distributions of pressure, temperature and circulation are reasonably close to what are believed to be the best observational estimates available. The large-scale structure of the ensemble mean precipitation and ocean surface heat flux also resemble the observed estimates, but show particularly large intermodel differences in low latitudes. The total cloudiness, on the other hand, is rather poorly simulated, especially in the Southern Hemisphere. The models' simulation of the seasonal cycle (as represented by the amplitude and phase of the first annual harmonic of sea-level pressure) closely resembles the observed variation in almost all regions. The ensemble's simulation of the interannual variability of sea-level pressure in the tropical Pacific is reasonably close to that observed (except for its underestimate of the amplitude of major El Ninos), while the interannual variability is less-well simulated in mid-latitudes. When analyzed in terms of the variability of the evolution of their combined space-time patterns in comparison to observations, the AMIP models are seen to exhibit a wide range of accuracy, with no single model performing best in all respects.
Analysis of the subset of the original AMIP models for which revised versions have subsequently been used to revisit the experiment, shows a substantial reduction of the models' systematic errors in simulating cloudiness, but only a slight reduction of the mean seasonal errors of most other variables. In order to understand better the nature of these errors and to accelerate the rate of model improvement, an expanded and continuing project (AMIP II) is being undertaken in which analysis and intercomparison will address a wider range of variables and processes, using an improved diagnostic and experimental infrastructure.(pdf file).