Work

Observed Circulation Trends in Boreal Summer Linked to Two Spatially Distinct Teleconnection Patterns

Year: 2025

Type: article

Abstract: Various regions in the Northern Hemisphere 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 U.S.–Atlantic pattern and a Eurasian trend pattern. The circulation trend can explain on average 15% and 26% of the observed regional temperature trends in the U.S.–Atlantic and Eurasian regions, respectively. The circulation trend in the CMIP6 multimodel 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 (U.S.–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 3 weeks leading up to the atmospheric configuration, resembling known teleconnection patterns. CMIP6 models appear to lack these tropical–extratropical 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-multidecadal time scales. Significance Statement With climate change, several regions around the world are experiencing trends in both atmospheric and surface variables, such as temperature. Some of these trends in observational datasets lie outside the range of the trends as simulated in state-of-the-art climate models. In this research, we show that there are two separate regions in which the regional atmospheric trends behave consistently with each other (the U.S.–Atlantic and Eurasian sectors). These atmospheric circulation trends can explain significant parts of the observed regional surface temperature trends. Critically, the state-of-the-art climate models do not have the same circulation trend patterns and physical pathways as found in the observations. This can be problematic for regional climate projections on decadal-to-multidecadal time scales.

Source: Journal of Climate

DOI: 10.1175/jcli-d-24-0750.1

Authors: Tamara Happé, Chiem van Straaten, Raed Hamed, Fabio D’Andrea, Dim Coumou