Observed circulation trends in boreal summer linked to two spatially distinct teleconnection patterns

Various regions in the Northern Hemispheric midlatitudes have seen pronounced trends in upper-atmosphere summer circulation and surface temperature extremes over recent decades. Several of these regional trends lie outside the range of historic CMIP6 model simulations, and they might constitute a joined dynamic response that is missed by climate models. Here, we examine if the regional trends in circulation are indeed part of a coherent circumglobal wave pattern. Using ERA5 reanalysis data and CMIP6 historical simulations, we find that the observed upper-atmospheric circulation trends consist of at least two separate regional signatures: a US-Atlantic and a Eurasian trend pattern. The circulation trend in these two regions can explain up to 30% of the observed regional temperature trends. The circulation trend in the CMIP6 multi-model-mean does not resemble the observed trend pattern and is much weaker overall. Some individual CMIP6 models do show a resemblance to the observed pattern in ERA5, although still weak. We show that the regional wave patterns in ERA5 resemble known teleconnection patterns, while CMIP6 models appear to lack these teleconnections. Our findings highlight the limitations of CMIP6 models in reproducing teleconnections and their associated regional imprint, creating uncertainty for regional climate projections on decadal to multi-decadal timescales.