On double Rossby wave breaking in the North Atlantic

We discuss the dynamical features associated with double Rossby wave breaking (DWB, concurrent cyclonic and anticyclonic breakings) over the North Atlantic, with a focus on the North Atlantic Oscillation (NAO), the midlatitude jet stream, and surface wind extremes over continental Europe. Objective automated algorithms for detecting wave breaking and determining the location, intensity, and direction of the jet are adopted. The analysis is performed on the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis and the Max Planck Institute Earth System Model (MPI-ESM). We find that DWB events can project onto both phases of the NAO, albeit showing no strong preference for either. Wave-breaking pairs occurring in the northern North Atlantic project onto the positive NAO, while the opposite holds for pairs occurring farther south. DWB also affects the direction and intensity of the jet stream. Events in the eastern half of the basin (EWB) intensify and zonalize the jet, while events farther to the west (WWB) weaken the westerly flow over Europe. An analysis of destructive wind storms over Europe in the last three decades suggests that these are typically associated with a very intense, zonal jet—similar to the case of EWB. Indeed, EWB corresponds to an enhanced likelihood of destructive windstorms over the continent, although there is not a one-to-one correspondence. The MPI-ESM model does not capture this statistical relationship. On the contrary, WWB corresponds to a decreased likelihood of destructive weather.