Wednesday, 12 January 2005: 1:45 PM
An Appraisal of Coupled Climate Model Simulations: Variability Diagnostics
As part of PCMDI's first appraisal of coupled climate models we have analyzed daily and monthly output from Coupled Model Intercomparison Project 2+ control simulations that were collected at the behest of the CLIVAR Working Group on Coupled Modelling. The variability analyzed is a small, but important, subset of variations that models should strive to represent. We have analyzed the El Nino/Southern Oscillation (ENSO) and the Madden Julian Oscillation (MJO), the most important modes of tropical variability on interannual and intraseasonal time scales, respectively. Importantly, both of these phenomena force extratropical variability, and are therefore of global importance. With respect to ENSO, our results indicate that improvement in the simulation of ENSO has been realized through model development. In the case of the MJO a high quality simulation is presently possible, including its 3-dimensional space-time evolution. More generally, the space-time variability of the tropics is analyzed using frequency-wavenumber plots that show the ability of the models to represent equatorially trapped waves, originally derived from shallow water theory. The models have difficulty in generating the observed level of variability as a function of space and time scale. This is particularly apparent for synoptic timescales (2-6 days) and higher order wave numbers (5-15). In the extratropics we have examined the North Atlantic Oscillation (NAO), which is associated with weather changes over portions of North America and Eurasia on a multitude of time scales. The North Atlantic Oscillation is well represented by the models. The spatial pattern of the large-scale surface air temperature response to the sea-level pressure perturbation over the Atlantic is akin to that observed, with the spatial error indicating the response was not as strong as observed. This shortcoming was systematic across the models analyzed, and it is suggested that improvement in the extratropical flow will improve this aspect of the NAO response.
Acknowledgments. This work was performed under the auspices of the U.S. Department of Energy Office of Science, Climate Change Prediction Program by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.