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Cool Season Anomalous Temperature Regimes in CMIP5 Models

Paper Reviewed
Westby, R.M., Lee, Y.-Y. and Black, R.X. 2013. Anomalous temperature regimes during the cool season: Long-term trends, low-frequency mode modulation, and representation in CMIP5 simulations. Journal of Climate 26: 9061-9076.

According to Westby et al. (2013), "during the boreal cool season, anomalous temperature regimes (ATRs), including cold air outbreaks (CAOs) and warm waves (WWs), affect regional economies and human safety over large portions of the United States via their significant impacts on energy consumption, local agriculture and human health." Thus, there is significant interest in determining how well these cold and warm phenomena can be simulated by coupled atmosphere-ocean general circulation models (AOGCMs) in association with the most recently reviewed CMIP5 models. And, therefore, the three researchers from the Georgia Institute of Technology in Atlanta analyzed reanalysis and model data for the period from 1949 to 2011 in order to assess (i) long-term variability in ATRs, (ii) inter-annual modulation of ATRs by low-frequency modes, and (iii) the representation of ATR behavior in models from phase 5 of the Coupled Model Intercomparison project (CMIP5)."

In pursuing this protocol, Westby et al. determined that (1,2) "there have not been any statistically significant trends in either WWs or CAOs over most of the continental United States from 1949-2011," that (3) "the average inter-annual variability of ATRs is larger in the model simulations than found in the observations," that (4) a "significant modulation of ATRs by the PDO [Pacific Decadal Oscillation]-like mode is virtually nonexistent in most models," that (5) "this is most likely due to model inadequacy in representing the underlying physics of the PDO," that (6) "there is no apparent benefit of high-top models in replicating the observed characteristics of ATR behavior and its low-frequency modulation," and that (7) the authors' analysis "does not establish the physical processes or mechanisms connecting ATRs to low-frequency modes."

And so we once again have a situation where the very best of the world's climate models cannot predict, or even replicate, another pair of recurring temperature perturbations (CAOs and WWs), which in this case are unique to the United States.

Posted 3 November 2014