How do Euro-Atlantic regimes respond to climate change?

The regional impacts of anthropogenic climate change over Europe are still very unclear, in large part due to uncertainties in the dynamical response of the large-scale circulation. Particularly in Boreal winter, the structure of the Euro-Atlantic circulation is strongly nonlinear, and is dominated by Rossby wave breaking, persistent atmospheric blocks and latitudinal meandering of the eddy driven jet stream. In the face of this complexity, adopting a regime approach – where the large scale circulation is decomposed into a small number of representative flow state – can shed much needed light on the internal variability and forced response of Euro-Atlantic dynamics.

 

One major open question is how exactly nonlinear regimes respond to a warming climate. While it has been hypothesised that climate forcing should primarily result in a shift in the probability of historically observed regimes, rather than inducing new flow configurations, this has never been tested in complex earth system models. In this work we fill this gap, by analysing regime statistics in 20 CMIP6 models under the SSP5-8.5 scenario. We decompose the forced response into two components: one representing the degree to which the prevailing regime patterns alter, and another representing their changing probability of occurrence and persistence. We show that while robust changes in regime patterns can be detected, these are minor in comparison to changes in the regime life-cycles. The overall picture tends towards a less regime dominated future: we find that anticyclonic regimes associated with persistent blocking are projected to become less persistent in the future and that central jet positions, associated with zonal flows, become more common. However considerable intermodel variability, even in the sign of the prevailing regime trends, shows the need for further investigation, and an improved representation of Euro-Atlantic regimes in state-of-the-art models.