New Perspectives on the Role of Internal Variability in Regional Climate Change and Climate Model Evaluation (Invited Presentation)

Natural climate variability occurs over a wide range of time and space scales as a result of processes intrinsic to the atmosphere, the ocean, and their coupled interactions. Such internally generated climate fluctuations pose significant challenges for the identification of externally forced climate signals such as those driven by anthropogenic increases in greenhouse gases, both in the observational record and in climate models. This challenge is exacerbated for regional climate responses evaluated over short (< 50 year) time periods. In this talk, I will highlight the uncertainties in interpreting regional climate change over North America in the recent past and projected for the coming decades. I will also discuss insights gained from novel ensembles of climate model simulations, which allow for accurate determination, and straightforward separation, of externally forced climate signals and internal climate variability on regional scales. These ensembles makes clear that a range of climate trajectories is possible under the same greenhouse gas forcing scenario due to the influence of internal climate variability. Finally, I will discuss new approaches to climate model evaluation that incorporate uncertainty due to limited sampling of internal variability.