Diagnosing Inter-model variability of Northern Hemisphere Jet Stream Portrayal in 17 CMIP3 Global Climate Models

The present study focuses on diagnosing inter-model variability of non-zonally averaged Northern Hemisphere (NH) jet stream portrayal in 17 global climate models (GCMs) from the CMIP3 dataset.;Compared to the reanalysis, the ensemble mean 20th century 300 hPa Atlantic jet is too zonally extended and located too far equatorward in GCMs. The 20th century Pacific jet varies significantly among modeling groups, with large biases in the vicinity of the jet exit region that cancel in the ensemble mean. It is found that 20th century NH biases in upper-level winds are strongly related to an ENSO-like pattern in winter mean tropical Pacific SST biases. Also, the temporal variability of the upper-level zonal winds in the 20th century is found to be accurately modeled in nearly all 17 GCMs. The anthropogenic climate change impacts on the eddy-jet system include the direct response of an intensified mid-latitude jet stream and a secondary response of a poleward shifted jet. While both responses are evident in the ensemble mean 21st century projections, uncertainty in the poleward shift response is large enough that even the sign of the shift is not consistent among models, especially in the NH.;Future 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 addition, 300 hPa zonal winds are projected to decrease in the core of the Pacific and Atlantic jets with increasing 300 hPa zonal winds located primarily in the jet exit regions and the meridional flanks of the jets.;Uncertainties in SST changes from the 20th century to the 21st century between models are shown to impact modeled NH jet stream changes. In particular, ENSO-like mean winter SST changes explain 26% of inter-model variance of mid-latitude zonal wind compared to the 8% explained by the domain-averaged warming SST signal. This suggests that reduction of uncertainty in the ENSO response to global warming is very important because it will significantly reduce uncertainty in projections of the NH zonal wind response to climate change.