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Abstract:
Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. Due to their role in weather extremes and atmospheric predictability, regimes that feature a blocking anticyclone are of particular interest. This study investigates the dynamics of these ''blocked'' regimes in the North Atlantic-European region from a year-round perspective. For a comprehensive diagnostic, wave activity concepts and a piecewise potential-vorticity (PV) tendency framework are combined. All occurrences of four types of blocked regimes (namely Atlantic Ridge, European Blocking, Scandinavian Blocking and Greenland Blocking) during the 1979-2021 period of ERA5 reanalysis are considered. Distinct differences between blocked regimes are found in their wave activity characteristics after regime onset: Greenland Blocking is associated with a suppression of wave activity flux, whereas Atlantic Ridge and European Blocking are associated with a northward deflection of the flux without a clear net change. During onset, the envelope of Rossby wave activity retracts upstream for Greenland Blocking, whereas the envelope extends downstream for Atlantic Ridge and European Blocking. From the piecewise PV tendencies perspective, the dynamics governing regime onset exhibit a large degree of similarity. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset, dominated by nonlinear PV eddy fluxes and linear Rossby wave dynamics, respectively. This dynamics-centered variability does not merely reflect that associated with seasonality or different types of regime transitions, but appears to be of more fundamental, dynamical significance than phenomenologically defined variability.
