A Comparison of Simulations of Current Climate from Two Coupled Atmosphere-Ocean Global Climate Models Against Observations and Evaluation of their Future Climates

Report to the National Institute for Global Environmental Change (NIGEC)

Ruth Doherty and Linda O. Mearns
Revised 27 April 1999

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A qualitative assessment of climate simulations from two state-of the-art coupled
atmosphere and ocean general circulation models (AOGCMs) over North America is
presented. The models investigated are the Canadian Climate Centre CGCMI and the
British Hadley Centre HADCM2. The focus of this work is to examine seasonal-mean
averages of key surface and atmospheric variables and how these compare to the
observational data sets of Legates and Wilmott and NCEP/NCAR reanalyses.
Comparison of historical control simulations against the Legates and Wilmott surface
temperature climatology reveals both models to have a warm bias over Canada and northern
US in autumn and winter, a cold bias in the West, and a warm bias in summer and autumn.
Both models also display a wet bias over the Rockies, and a wet bias in the Northeast
and Canada in the spring and summer months when compared to the Legates and
Wilmott precipitation climatology. These biases are generally greater in the CGCMI
simulations. Surface pressure and 500 mb geopotential height patterns are also
compared between the two models and NCEP/NCAR reanalyses. Both models
simulate the low-pressure systems in winter to be too deep. In general, the HADCM2
simulations underestimate and the CGCMI simulations overestimate the strength of
the high-pressure systems. Geopotential height patterns are more closely
represented in the CGCMI simulations; however, both models exhibit a cold
tropospheric bias. Future temperature and precipitation estimates of both these
models are also examined for three time slices, 2030, 2060, and 2090. The CGCMI
simulations display much more extensive warming than the HADCM2 simulations.
Precipitation changes (mainly positive) in these future periods are somewhat similar
in both models, but the CGCMI simulates more decreases.

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