Drivers of the Northern Extratropical Eddy-Driven Jet Change in CMIP5 and CMIP6 Models

The wintertime midlatitude atmospheric circulation is evaluated in CMIP6 models. The biases have been reduced since CMIP5 although the low-level flow is still too zonal. CMIP5 and CMIP6 projections of 850 hPa zonal wind are then analyzed and are consistent under the RCP8.5 and the SSP5-8.5 scenarios, respectively. A poleward shift is identified in the Pacific, while a tripole structure is found in the North Atlantic: The zonal wind strengthens over Western Europe and decreases north and south. A multiple linear regression allows us to quantify the contribution of different drivers to the intermodel spread in zonal wind projections. It supports the importance of projected tropical warming and changes in the stratospheric vortex but also suggests a contribution of the asymmetry in the projected surface warming of the equatorial Pacific and of the present-day biases in the eddy-driven jet position. The North Atlantic warming hole plays a weaker role. Plain Language Summary The projection of the midlatitude atmospheric circulation is highly uncertain. Climate models exhibit a significant ensemble dispersion although some robust signals seem to emerge at the end of the 21st century: a poleward shift of the 850 hPa zonal wind in the Pacific and a strengthening over Northern Europe. This response is consistent between the former and current-generation global climate models. Several drivers have been proposed to explain this response. Here, it is confirmed that the amplified warming of the tropical high troposphere and variation of the stratospheric vortex play a more important role than the amplified warming over the Arctic. In addition, it is suggested that the West-East temperature gradient in the tropical Pacific and the biases of the jet position have a significant contribution, as opposed to the minimum warming in the North Atlantic.