|October 9, 2012
1:00 PM – 2:15 PM ET
|Speakers and Topics:||Justin Sheffield (Princeton University)
Evaluation of 20th Century North American Climate in CMIP5
Xianan Jiang (University of California, Los Angeles)
Simulations of the Eastern North Pacific Intraseasonal Variability in CMIP5 GCMs
Julienne Stroeve (National Snow and Ice Data Center)
Evolution of the Arctic sea ice cover in CMIP5, CMIP3 and observations
Zaitao Pan (St. Louis University)
Inter-model variability and mechanism attribution of central and southeastern U.S. “warming hole” in the 20th century as simulated by CMIP5 models
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Justin Sheffield — Evaluation of 20th Century North American Climate in CMIP5 — The Coupled Model Inter-comparison Project phase 5 (CMIP5) provides an unprecedented set of model results for improved understanding of climate processes and feedbacks, anthropogenic impacts and potential future changes. In particular, evaluations of the model historical simulations are crucial for assessing model fidelity and are a necessary prerequisite for instilling confidence in their future projections. This presentation summarizes results from the NOAA CMIP5 Task Force on 20th century model evaluations for the climate of North America and related climate processes. We evaluate the models for a range of features at spatial scales from continental to regional and for a variety of time scales from intra-seasonal to multi-decadal variability and teleconnections with North American climate.
Xianan Jiang — Simulations of the Eastern North Pacific Intraseasonal Variability in CMIP5 GCMs — As a key component of tropical atmospheric variability, intraseasonal variability (ISV) over the eastern north Pacific (ENP) exerts pronounced influences on regional weather and climate. Since general circulation models (GCMs) are essential tools for prediction and projection of future climate, current model deficiencies in representing this important variability leave us greatly disadvantaged in studies and prediction of climate change. In this study, we have assessed model fidelity in representing ENP ISV by analyzing sixteen GCMs participating in the fifth phase of the Coupled Model Inter-comparison Project (CMIP5). Among the sixteen CMIP5 GCMs examined in this study, only seven GCMs capture the spatial pattern of the leading ENP ISV mode relatively well, although even these several GCMs exhibit biases in simulating ISV amplitude. Analyses indicate that model fidelity in representing ENP ISV is closely associated with ability to simulate a realistic summer mean state. The presence of westerly or weak mean easterly winds over the ENP warm pool region could be conducive for more realistic simulations of the ISV. One hypothesis to explain this relationship is that a realistic mean state could produce the correct sign of surface flux anomalies relative to the ISV convection, which helps to destabilize local intraseasonal disturbances. The projected changes in characteristics of ENP ISV under the RCP8.5 projection scenario are also explored based on simulations from four CMIP5 GCMs. Results suggest that, in a future climate, the amplitude of ISV could be enhanced over the southern part of the ENP, while reduced over the northern ENP off the coast of Mexico/Central America and the Caribbean.
Julienne Stroeve — Evolution of the Arctic sea ice cover in CMIP5, CMIP3 and observations — Confidence in climate models to provide reliable projections of future climate is largely built on how well they can reproduce observed features of recent climate. Although all models participating in the 5th Phase of the Coupled Model Intercomparison Project (CMIP5) show declining Arctic sea ice over the period of observations, trends from most models remain smaller than observed. The ability of climate models to capture the observed variability in the sea ice extent depends in part on how well they are able to simulate the observed sea ice thickness distribution, since models with an overly thick initial ice cover tend to lose their summer ice cover later than models with initially thinner ice given the same climatic forcing. Performance metrics are developed for models representations of observed sea ice extent and thickness. Metrics are used as a basis for conditioning probabalistic predictions of sea ice cover in the Arctic. Three approaches are used for conditioning predictions; 1) a selection of a subset of best performing models based on thickness and extent metrics; 2) a weighting of all models, both good and bad performers, based on performance metrics; and 3) a baseline case in which all models are selected and given an equal weighting.
Zaitao Pan — Inter-model variability and mechanism attribution of central and southeastern U.S. “warming hole” in the 20th century as simulated by CMIP5 models — Many parts of the central and southeastern U.S. cooled by up to 2C (“warming hole (WH)”) during the 20th century, while global mean temperature rose by 0.6C (0.76C from 1901-2006). Studies showed that PDO and AMO may be responsible for this cooling, while other works reported that regional processes like the low-level jets and evapotranspiration also contribute to the abnormity. Only a few of 53analyzed simulations by CMIP3 models could reproduce the cooling. This presentation analyzes the CMIP5 20th century simulations from 27 models, totaling 173 ensemble members, to assess model skills in capturing the anomalous regional cooling under global warming environment. We found that (i) the observed cooling occurred largely over the southeastern U.S. in winter during the third quarter and over central U.S. in summer during the fourth quarter of the 20th century, (ii) while a large number of models have difficulty in reproducing the cooling, those with the highest resolutions tend to produce a WH-like summer cooling in the central U.S., (iii) the simulations with forcing only from greenhouse gases (GHG) resulted in strong warming in the central U.S. that may have compensated the cooling, and (iv) the all-forcing historical experiment compared with the natural-forcing-only experiment showed a well-defined WH in the central U.S., implying that land surface processes contributed to the cooling in the 20th century.