Impact of the radiative forcing on the winter North Atlantic-European atmospheric circulation

Abstract

The increase in the global-mean temperature linked to climate change is associated with atmospheric and oceanic circulation changes. In this work, we assess the impacts of the (anthropogenic) radiative forcing on the North Atlantic-European atmospheric circulation during boreal winter (DJF). We use two approaches: three target experiments with the European Consortium {Earth System model version 3.3 (EC-EARTH) and fixed radiative forcing, and three large-ensemble historical/scenario simulations with the Institut Pierre-Simon Laplace Coupled Model version 6 (IPSL-CM6), characterizing Past, Present-day and near-Future climate conditions. First, a comparison between the Present simulations and observations shows that both models simulate a too zonal eddy-driven jet. Then, differences between the radiative forcings are analysed. Results show that both approaches yield a consistent forced response, and that it scales linearly with radiative forcing, increasing in amplitude from Present-minus-Past to Future-minus-Present. At low latitudes, in the tropical Atlantic, the forced atmospheric response is characterized by a Gill-type baroclinic structure, where the anomalous anticyclonic circulation at upper levels reinforces the westerly wind at the equatorward ank of the North Atlantic jet. At high latitudes, the forced response is reminiscent of the `Arctic Amplification' linked to sea-ice reduction, and a thermally-driven baroclinic structure can be seen west of Greenland (EC-EARTH) and over the Hudson Bay (IPSL). At midlatitudes, the forced response shows a barotropic pattern, with a cyclonic (anticyclonic) circulation in the North Atlantic (Euro-Mediterranean) sector, pointing to a key role for non-radiative, eddy-related effects.