Teleconnection processes linking the intensity of the Atlantic Multidecadal Variability to the climate impacts over Europe in boreal winter

TL;DR

This study investigates how Atlantic Multidecadal Variability influences European winter climate through complex teleconnection processes, revealing nonlinear responses and the importance of thermodynamical feedbacks in climate modeling.

Contribution

It demonstrates the nonlinear impact of AMV amplitude on European climate and highlights the role of thermodynamical processes and feedbacks in this teleconnection.

Findings

AMV influences European winter climate through both dynamical and thermodynamical processes.

Thermodynamical effects dominate at higher AMV amplitudes, leading to milder, wetter conditions.

Nonlinear response challenges the pattern-scaling technique in climate projections.

Abstract

The teleconnection between European climate and Atlantic Multidecadal Variability (AMV) remains difficult to isolate in observations because of internal variability and anthropogenically-forced signals. Using model sensitivity experiments proposed within the CMIP6/DCPP-C framework, the wintertime AMV/Europe teleconnection is investigated in large ensembles of pacemaker-type simulations in the CNRM-CM5 global circulation model. To evaluate the sensitivity of the model response to the AMV amplitude, experiments with AMV-forcing pattern multiplied by 2 and 3 (hereafter 2xAMV and 3xAMV, respectively) are performed in complement to the reference ensemble (1xAMV). Based on a flow analog method, the AMV-forced atmospheric circulation is found to cool down the European continent, whereas the residual signal, mostly including thermodynamical processes, contributes to warming. In 1xAMV, both terms cancel each other, explaining the overall weak AMV-forced atmospheric signal. In 2xAMV and 3xAMV, the thermodynamical contribution overcomes the dynamical cooling and is responsible for milder and wetter conditions. The thermodynamical term includes the advection of warmer and more humid oceanic air penetrating inland and the modification of surface radiative fluxes linked to (i) altered cloudiness and (ii) snow-cover reduction acting as a positive feedback with the AMV amplitude. The dynamical anomalous circulation combines (i) a remote response to enhanced diabatic heating acting as a Rossby-wave source in the western tropical Atlantic and (ii) a local response associated with warmer SST over the subpolar gyre favorizing an anomalous High. The weight between the tropical-extratropical processes and associated feedbacks is speculated to partly explain the nonlinear sensibility of the response to the AMV-forcing amplitude, challenging thus the use of the so-called pattern-scaling technique.