Christophe Cassou | Searcharxiv

arXiv2020

Modulation of the occurrence of heat waves over the Euro-Mediterranean region by the intensity of the Atlantic Multidecadal Variability

The influence of the Atlantic Multidecadal Variability (AMV) and its amplitude on the Euro-Mediterranean summer climate is studied in two climate models, namely CNRM-CM5 and EC-Earth3P. Large ensembles of idealized experiments have been conducted in which North Atlantic sea surface temperatures (SSTs) are relaxed towards different amplitudes of the observed AMV anomalies. In agreement with observations, during a positive phase of the AMV both models simulate an increase (decrease) of temperature and a decrease (increase) of precipitation over the Mediterranean basin (northern half of Europe). An increase in the number of heat wave (HW) days is also found over the continental Mediterranean region. In terms of atmospheric circulation, an anomalous ridge (trough) over the Mediterranean basin (northern Europe) induces a thermodynamical response explaining the increase in the occurrence of HW. The anticyclonic anomaly over the Mediterranean basin is associated with drier soils and a reduction of cloud cover, which concomitantly induce a decrease (increase) of the latent (sensible) heat flux and an increase of the downward radiative fluxes over lands. The source mechanisms leading to the mid-troposphere anomalies over North Atlantic and its adjacent continents are discussed. It is found that both the tropical and extra-tropical parts of the AMV modulate the atmospheric circulation over the Euro-Atlantic region. The number of HW days increases linearly with the amplitude of the AMV. However, the strength of this relationship differs between the models, and depends on their intrinsic biases, raising questions regarding the robustness of the mechanisms of teleconnections associated with the AMV.

physics.ao-ph↗

arXiv2019

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

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.

physics.ao-ph↗