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Speaker: Dr. Karen McKinnon
Title: “Internal variability, uncertainty, and the Observational Large Ensemble.”
Abstract: Our observationally-based inferences about the forcedf behaviors of the climate system, whether in response to anthropogenic influence or to a mode such as the El Nino-Southern Oscillation (ENSO), will always be confounded by the presence of internal atmospheric variability. Internal atmospheric variability is sufficiently large that it can influence the magnitude and even sign of multi-decadal regional temperature and precipitation trends for both the past and next 50 years. The magnitude of this influence was perhaps first illuminated by initial condition ensembles from climate models, but they suffer from biases that can limit their use for regional climate studies. As a complementary tool, I will present the Observational Large Ensemble (OLENS), which is based on a statistical model fit to the observations. The OLENS allows for more realistic simulation of the surface climate impacts of internal variability, and can be used to assess the range of climate trends consistent with the same underlying climate change signal. Similar ideas can be applied to the extratropical response to ENSO, revealing that - given the current observational record length - we have large uncertainties regarding the surface climate response to a given ENSO event