An overview of the results of the Atmospheric Model Intercomparison Project (AMIP I)

The Atmospheric Model Intercomparison Project (AMIP), initiated in 1989 und er the auspices of the World Climate Research Programme, undertook the syst ematic validation, diagnosis, and intercomparison of the performance of atm ospheric general circulation models. For this purpose all models were requi red to simulate the evolution of the climate during the decade 1979-88, sub ject to the observed monthly average temperature and sea ice and a common p rescribed atmospheric CO2 concentration and solar constant. By 1995, 31 mod eling groups, representing virtually the entire international atmospheric m odeling community, had contributed the required standard output of the mont hly means of selected statistics. These data have been analyzed by the part icipating modeling groups, by the Program for Climate Model Diagnosis and I ntercomparison, and by the more than two dozen AMIP diagnostic subprojects that have been established to examine specific aspects of the models' perfo rmance. Here the analysis and validation of the AMIP results as a whole are summarized in order to document the overall performance of atmospheric gen eral circulation-climate models as of the early 1990s. The infrastructure a nd plans for continuation of the AMIP project are also reported on. Although there are apparent model outliers in each simulated variable exami ned, validation of the AMIP models' ensemble mean shows that the average la rge-scale seasonal distributions of pressure, temperature, and circulation are reasonably close to what are believed to be the best observational esti mates available. The large-scale structure of the ensemble mean precipitati on and ocean surface heat flux also resemble the observed estimates but sho w particularly large intermodel differences in low latitudes. The total clo udiness, on the other hand, is rather poorly simulated, especially in the S outhern Hemisphere. The models' simulation of the seasonal cycle (as repres ented 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 press ure in the tropical Pacific is reasonably close to that observed (except fo r its underestimate of the amplitude of major El Ninos), while the interann ual variability is less well simulated in midlatitudes. When analyzed in te rms of the variability of the evolution of their combined spacetime pattern s 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 co nsidered. Analysis of the subset of the original AMIP models for which revised versio ns have subsequently been used to revisit the experiment shows a substantia l reduction of the models' systematic errors in simulating cloudiness but o nly 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 ex perimental infrastructure.