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Evaluating AGCM Simulations of Tropical Temperature Trends

Paper Reviewed
Flannaghan, T.J., Fueglistaler, S., Held, I.M., Po-Chedley, S., Wyman, B. and Zhao, M. 2014. Tropical temperature trends in Atmospheric General Circulation Model [AGCM] simulations and the impact of uncertainties in observed SSTs. Journal of Geophysical Research: Atmospheres 119: 13,327-13,337.

Introducing their study, Flannaghan et al. (2014) write that "comparison of temperature trends from CMIP5 model runs with prescribed sea surface temperatures (SSTs) for the period 1981-2008 shows that some models do better than others when compared to observations," citing Po-Chedley and Fu (2012), who found that (1) "nearly all AMIP models" -- i.e. those associated with the Atmospheric Model Intercomparison Project (Gates, 1992) for the period 1981-2008 -- "overestimate warming in the tropical upper troposphere," and that (2) "those models that perform best when compared to the observations use the HadlSST1 data set (Rayner, 2003), whereas the other models use a different data set (Hurrell et al. (2008)."

Thereby prompted to further explore the subject, the six scientists ran the Geophysical Fluid Dynamics Laboratory HiRAM model with the HadlSST1 and Hurrell SST data sets, finding that "subtle (but systematic) differences in the two SST data sets induce an unexpectedly large difference in upper tropospheric temperature trends." And how large a difference, you may ask? How about, as they put it, "a factor 3 larger than expected from moist adiabatic scaling of the tropical average SST trend difference."

In light of this stunning finding, Flannaghan et al. go on to conclude that "systematic uncertainties in SSTs need to be resolved before the fidelity of climate models' tropical temperature trend profiles can be assessed." And we concur, for first things first is a well-worn formula for success, which even the most complex of mathematical efforts must be careful to follow."

Gates, W.L. 1992. AMIP: The atmospheric model intercomparison project. Bulletin of the American Meteorological Society 73: 962-970.

Hurrell, J.W., Hack, J.J., Shea, D., Caron, J.M. and Rosinski, J. 2008. A new sea surface temperature and sea ice boundary dataset for the Community Atmosphere Model. Journal of Climate 2: 5145-5153.

Po-Chedley, S. and Fu, Q. 2012. Discrepancies in tropical upper tropospheric warming between atmospheric circulation models and satellites. Environmental Research Letters 7: 10.1088/1748-9326/7/4/044018.

Rayner, N.A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

Posted 29 April 2015