Intercomparison of the Northern Hemisphere Winter Mid-Latitude Atmospheric Variability of the IPCC Models by Wave Activity Performance MetricsPrimary Author: Lucarini, Valerio
Additional Authors: Sandro Calmanti, Alessandro Dell\'Aquila, Paolo M. Ruti, Antonio Speranza
Intercomparison of the Northern Hemisphere Winter Mid-Latitude Atmospheric Variability of the IPCC Models by Wave Activity Performance Metrics
Valerio Lucarini, Sandro Calmanti, Alessandro Dell'Aquila, Paolo M. Ruti, Antonio Speranza
email: valerio.lucarini@unicam.it
We estimate the northern hemisphere mid-latitude winter atmospheric variability within the available XX century (1961-2000) and 720 CO2 ppm stabilization (2181-2200) simulations of the GCMs included in the IPCC-4AR. We compute the Hayashi spectra of the 500hPa geopotential height fields and introduce an ad hoc integral measure of the variability observed in the Northern Hemisphere on different spectral sub-domains. The total wave variability is taken as a global scalar metric describing the overall performance of each model, while the total variability pertaining to the eastward propagating baroclinic waves and to the planetary waves are taken as scalar metrics describing the performance of each model phenomenologically in connection with the corresponding specific physical process. When considering the XX century simulations, only two very high-resolution global climate models have a good agreement with the NCEP-NCAR and ECMWF reanalyses for both the global and the process-oriented metrics. Large biases, in several cases larger than 20%, are found in all the considered metrics between the wave climatologies of most IPCC models and the reanalyses, while the span of the climatologies of the various models is, in all cases, around 50%. In particular, the travelling baroclinic waves are typically overestimated by the climate models, while the planetary waves are usually underestimated, in agreement with what found is past analyses performed on global weather forecasting models. When comparing the results of similar models, it is apparent that in some cases the vertical resolution of the model atmosphere, the adopted ocean model, and the advection schemes seem to be critical in the bulk of the atmospheric variability. The models ensemble obtained by arithmetic averaging of the results of all models is biased with respect to the reanalyses but is comparable to the best 5 models. Nevertheless, the models results do not cluster around their ensemble mean. When considering the XXII century simulations, the first notable change is that in all cases the spectral densities tend to shift towards longer waves. Relatively small changes are observed in all GCMS for the global scalar metric. In virtually all GCMs eastward propagating baroclinic waves increase (up to about 20%), even if disagreement exist on the amount of the change, whereas for the long standing waves not all GCMs agree on the sign of the change. In general, GCMs with low variability in the 1961-2000 run tend to have less pronouned sensitivity with respect to climate forcing. This study suggests caveats with respect to the ability of most of the presently available climate models in representing the statistical properties of the global scale atmospheric dynamics of the present climate and, a fortiori, in the perspective of modeling climate change.
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