Pacific Decadal Oscillation modulates the Arctic sea-ice loss influence on the mid-latitude atmospheric circulation in winter

Abstract. The modulation of the winter impacts of Arctic sea ice loss by the Pacific Decadal Oscillation (PDO) is investigated in the IPSL-CM6A-LR ocean-atmosphere general circulation model. Ensembles of simulations are performed with constrained sea ice concentration corresponding to pre-industrial, present-day and future states, and initial conditions sampling warm and cold phases of the PDO. Using a general linear model, we estimate the simulated winter impact of sea ice loss, PDO and their combined effects. In response to sea ice loss, the Arctic lower troposphere warms and a negative North-Atlantic oscillation like pattern appears together with a weak deepening of the Aleutian Low. The two patterns are associated with a weakening of the poleward flank of the eddy-driven jet, while in the stratospheric the polar vortex weakens. Besides, a warm PDO phase induces a large positive Pacific North America pattern, as well as a small negative Arctic oscillation pattern associated with a weakening of the stratospheric polar vortex. However, the effects of PDO and Arctic sea ice loss are not additive. The Arctic sea ice teleconnections in both troposphere and stratosphere are reduced by the PDO, most importantly in the stratosphere. The results are discussed and compared to those obtained with the same model in atmosphere-only simulations, where sea ice loss does not significantly alter the stratospheric polar vortex.