Diagnosing Northern Hemisphere Jet Portrayal in 17 CMIP3 Global Climate Models: Twenty-First-Century Projections

The anthropogenic climate change impacts on the eddy-jet system include an intensified midlatitude jet stream and an elevated tropopause, as well as a poleward-shifted jet. While both responses are evident in phase 3 of the Coupled Model Intercomparison Project (CMIP3) ensemble mean twenty-first-century projections, uncertainty in the poleward shift response is large enough that even the sign of the shift is not consistent among all models, especially in the Northern Hemisphere. The present analysis finds that twenty-first-century projections of the ensemble mean zonal wind change at 300 hPa predict a weakening and poleward expansion of the Pacific jet and an overall expansion of the Atlantic jet. In contrast with the direct zonal mean climate change signal of increasing midlatitude upper-level winds, zonal winds are projected to decrease in the core of the Pacific and Atlantic jets, with increasing zonal winds located primarily in the jet exit regions and the meridional flanks of the jets. Uncertainties in SST changes from the twentieth century to the twenty-first century between models are shown to impact modeled Northern Hemisphere jet stream changes. In particular, El Nino-Southern Oscillation-like mean winter SST changes explain 30% of intermodel variance of midlatitude zonal wind compared to the 8% explained by the domain-averaged warming SST signal. This suggests that a reduction of uncertainty in the tropical Pacific SST response to global warming will significantly reduce uncertainty in the Northern Hemisphere zonal wind response to climate change.