Search

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. The latter essentially quantifies the well-established PV perspective of mid-latitude dynamics. The four blocked regimes (namely Atlantic ridge, European blocking, Scandinavian blocking, and Greenland blocking) during the 1979–2021 period of ERA5 reanalysis are considered. Wave activity characteristics exhibit distinct differences between blocked regimes. 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. Scandinavian blocking exhibits intermediate wave activity characteristics. From the perspective of piecewise PV tendencies projected onto the respective regime pattern, the dynamics that govern regime onset exhibit a large degree of similarity: linear Rossby wave dynamics and nonlinear eddy PV fluxes dominate and are of approximately equal relative importance, whereas baroclinic coupling and divergent amplification make minor contributions. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset. The retrograde pathway is dominated by nonlinear PV eddy fluxes, whereas the upstream pathway is dominated by linear Rossby wave dynamics. Importantly, there is a large degree of cancellation between the two pathways for some of the mechanisms before regime onset. The physical meaning of a regime-mean perspective before onset can thus be severely limited. Implications of our results for understanding predictability of blocked regimes are discussed. Further discussed are the limitations of projected tendencies in capturing the importance of moist-baroclinic growth, which tends to occur in regions where the amplitude of the regime pattern, and thus the projection onto it, is small. Finally, it is stressed that this study investigates the variability of the governing dynamics without prior empirical stratification of data by season or by type of regime transition. It is demonstrated, however, that our dynamics-centered approach does not merely reflect variability that is associated with these factors. The main modes of dynamical variability revealed herein and the large similarity of the blocked regimes in exhibiting this variability are thus significant results. [ABSTRACT FROM AUTHOR]

Topographically forced Rossby waves shape the upper-level waveguide over the midlatitudes, affecting the propagation of transient waves therein, and have been linked to multiple surface extremes. The complex interplay between the forcing and the background flow in shaping the Rossby wave response still needs to be elucidated in a variety of configurations. We propose here an analytical solution of the linearized barotropic vorticity equation to obtain the stationary forced Rossby wave resulting from arbitrary combinations of forcing and background zonal wind. While the onset of barotropic instability might hinder the applicability of the linear framework, we show that the nonlinear wave response can still be retrieved qualitatively from the linearized solution. Examples using single- and double-jet configurations are discussed to illustrate the method and study how the background flow can act as a waveguide for Rossby waves. [ABSTRACT FROM AUTHOR]

Upper tropospheric Rossby wave packets (RWPs) have implications for the predictability of the atmosphere. Recently it has been shown that there is a link between high forecast skills in the medium and long range and long lived RWPs propagating from the Pacific region towards Europe across North America and the North Atlantic. On the other hand, this was not the case for such wave packets that deeply interacted with mesoscale phenomena in the lower and mid-troposphere over the North American continent. This contribution presents a novel diagnostic for RWPs based on local finite amplitude wave activity (LWA) in the primitive equations/isentropic coordinates framework and its application to an episode in which a propagating RWP was associated with a reduced predictability horizon. LWA is proportional to the local (in longitude) meridional displacement of contours of potential vorticity (PV) from zonal symmetry. The advantage of using LWA consists in the fact that its formulation does not make any small amplitude assumption and it is able to identify nonlinear phenomena such as Rossby wave breaking, blocking, PV streamers, or cutoffs. Furthermore, LWA has an exact conservation relation which allows one to formulate a budget equation for its evolution, distinguishing between conservative propagation and the impact of non-conservative processes. The latter is quantified as a residual from the LWA budget equation. In addition, the conservative propagation is partitioned into the tropopause near, tropopause deep and divergent wind contribution using the Helmholtz partitioning and a piecewise potential vorticity inversion technique.This new diagnostic is applied first to RWPs simulated in a dry general circulation model (the Portable University Model of the Atmosphere, PUMA) showing that nonconservative processes in general play a non-negligible role in their evolution. The LWA diagnostic and its budget are also applied to a specific episode containing a large amplitude wave packet propagating from North America towards Europe which was associated to very low forecast skills. This was done by using the forecasts from the European Center for Medium range Weather Forecast and the respective analyses and comparing the two. The results show that nonconservative processes played a considerable role in the RWP dynamics. In particular the operational model misrepresented the wave packet amplitude during its growth stage, leading to a wrong amplitude throughout the whole episode. This fact was associated to the very low forecast skills during that episode. [ABSTRACT FROM AUTHOR]