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

TL;DR

This study identifies two distinct teleconnection patterns linked to summer circulation trends in the Northern Hemisphere, revealing that current climate models inadequately capture these regional dynamics, which impacts regional climate projections.

Contribution

It demonstrates the existence of two separate regional circulation patterns associated with teleconnections and shows that CMIP6 models fail to reproduce these observed patterns accurately.

Findings

Observed circulation trends are linked to US-Atlantic and Eurasian teleconnection patterns.

CMIP6 models underestimate the strength and pattern of observed circulation trends.

Teleconnection patterns are associated with specific SST and radiation anomalies prior to circulation changes.

Abstract

Various regions in the Northern Hemispheric midlatitudes have seen pronounced trends in upper-atmosphere summer circulation and surface temperature extremes over recent decades (since 1979). 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 can explain on average 15% and 26% of the observed regional temperature trends in the US-Atlantic and Eurasian regions respectively. 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 with a maximum pattern correlation of 0.47. The pattern correlation is higher for the two individual regions (US-Atlantic and Eurasia), reaching a maximum of 0.69 and 0.78. We show that both regional wave patterns in ERA5 are associated with distinct sea surface temperature and outgoing longwave radiation anomalies in the three weeks leading up to the atmospheric configuration, resembling known teleconnection patterns. CMIP6 models appear to lack these tropical-extra-tropical teleconnections. Our findings highlight the limitations of CMIP6 models in reproducing teleconnections and their associated regional imprint, creating deep uncertainty for regional climate projections on decadal to multi-decadal timescales.