Preliminary results from the AMIP II experiment

The Atmospheric Model Intercomparison Project (AMIP) is WGNE organized effort established to help develop a community infrastructure for atmospheric modelers and diagnosticians. The first AMIP experiment was a ten year integration (7901-8812) where all atmospheric models were forced with the same observed surface SSTs and sea-ice. A large common database was generated from all (33) participant groups and this was used to help diagnose model errors. With the conclusion and success of the AMIP I experiment a new experiment was supported by the WGNE and named AMIP II. This new experiment integrates over a longer period of time (7901-9603), using improved boundry conditions and each modelling group will submit a larger and more comprehensive data volume to be archived in a common format at PCMDI.

Within this work emphasis will be placed on the examination of surface, pressure-level and deep-layer temperatures (e.g., simulated MSU). Our current work addresses measures of coupling, decadal trends and surface-troposphere coherence from both observational estimates as well as, atmospheric models. These results will illuminate model improvements (from AMIP I to AMIP II) and areas of both model and observed uncertainty.

Simulated temperatures were derived from model data with a comprehensive radiation retrieval scheme that resolves view-angles and incorporates moisture. These simulated temperatures allow us to explore a model’s ability to represent observed surface to tropospheric coupling/decoupling, decadal trends and measures of temperature coherence. Where possible results from both the AMIP I and AMIP II will be presented. Observational quantities are obtained by using data from the NCEP/NCAR, NCEP/DOE and ERA15 reanalyses. Our results indicate that the AMIP II models and reanalyses data have quite different zonal-mean pressure level temperature trends. For the AMIP II models examined to date the (six) models demonstrate a consistent lower tropospheric warming in excess of the surface boundry forcing (SSTs) and this warming is not consistent throughout the atmospheric column. Measures of surface to troposphere temperature sensitivity for the AMIP II models show more sensitivity than derived by the observational estimates. This means that per degree of skin temperature warming the models have a greater tendency to warm in the troposphere than do the observations. Results also show a substantial quantitative difference in the strength of surface/lower-tropospheric temperature coupling for the observations and models. Our results highlight the considerable uncertainty that exists in both observational estimates and modeled measures of temperature change.