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

Abstract

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 SummaryThe 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.

A regime view of future atmospheric circulation changes in Northern mid-latitudes

“…The projected changes in the regime frequencies in the future scenarios give a clear picture of the evolution of the variability in the mid-latitudes dynamics. For the EAT sector, the results shown in Section 3.3 are consistent with a zonalization of the midlatitude circulation and a squeezing of the eddy-driven jet distribution around its central position (Peings et al, 2018;Oudar et al, 2020). There is generally a good correspondence between the Euro-Atlantic WRs and the North-Atlantic jet latitude index (Madonna et al, 2017;Fabiano et al, 2020).…”

“…A significant fraction of the spread of the model response over the EAT sector is explained by the Polar Stratospheric Temperature and the Arctic Amplification (Section 4.2). The increase of the NAO+ regime is consistent with a squeezing of the jet around the central position (Peings et al, 2018;Oudar et al, 2020) and with a reduced variability of the jet (Barnes and Polvani, 2013). In the PNA sector the future trends are characterized by an increase in the PT regime occurrence, with a concomitant decrease in the BR regime frequency.…”

“…A set of indices -also known as "remote drivers" (e.g. Zappa and Shepherd, 2017;Peings et al, 2018;Oudar et al, 2020) -that (in principle) have the potential to affect WR frequencies can be selected: -NAW: the surface warming in the North Atlantic (averaged between 80W and 10E, 20N and 75N).…”

“…However, the picture appears more complex than that, and the jet response strongly depends on the region (Barnes and Polvani, 2013;Peings et al, 2017) and season considered (Barnes and Polvani, 2015;Shaw et al, 2016). Whilst the northward shift of the jet is evident in the North-Pacific (Oudar et al, 2020), the trend over the North-Atlantic shows rather a squeezing of the time-mean jet, with intensification and eastward elongation of the westerlies over Europe (Oudar et al, 2020;Peings et al, 2018). The eastward elongation of the North-Atlantic jet is also consistent with results obtained from analysis of the stationary waves response to climate change, which show an eastward shift in phase, produced by a decrease in the stationary zonal wavenumber (Wills et al, 2019;Simpson et al, 2014).…”

“…This picture is further complicated by the emerging role of the polar stratosphere, which is strongly coupled with the North-Atlantic jet stream position (e.g. Baldwin and Dunkerton, 2001), and might contribute to the dynamical response to the UTW (Peings et al, 2017;Manzini et al, 2014;Beerli and Grams, 2019), therefore explaining the large inter-model spread (Peings et al, 2017;Zappa and Shepherd, 2017;Oudar et al, 2020).…”