Similarity and variability of blocked weather-regime dynamics in the Atlantic–European region

Abstract

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. 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.

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

“…Our findings are in line with the climatological evidence regarding the chain of physical processes by which LHR can affect blocking (e.g., Grams & Archambault, 2016; Hauser et al., 2023b; Pfahl et al., 2015; Steinfeld & Pfahl, 2019; Teubler et al., 2023). We conclude that there is a strong link between the correct representation of local WCB activity in the North Atlantic‐European region and the correct representation of the large‐scale circulation around EuBL onsets in NWP models.…”

The Dynamical Footprint of Year‐Round North American Weather Regimes

“…Overall, our findings further motivate the relevance of weather regimes diagnostics for subseasonal‐to‐seasonal prediction and climate model analyses. A particular focus on understanding the processes that govern the behavior of the North American regimes and their lifecycles, as has been performed for the Atlantic‐European regimes (e.g., Hauser et al., 2023; Michel & Rivière, 2011; Teubler et al., 2023), would help to link the dynamical systems analyses herein with physical atmospheric variability.…”

“…Michel & Rivière, 2011;Teubler et al, 2023), would help to link the dynamical systems analyses herein with physical atmospheric variability.…”

Life cycle dynamics of Greenland blocking from a potential vorticity perspective

“…Rivière and Drouard, 2015), which is closely related to blocking over Greenland (e.g., Woollings et al, 2010). The breaking of upper-level Rossby waves (further referred to as RWB) has been found as a formation mechanism for blocking over Greenland with predominantly cyclonic RWB towards Greenland (Benedict et al, 2004;Woollings et al, 2008; results on the importance of moist-baroclinic processes by adopting the Eulerian perspective of Teubler et al (2023) into a new quasi-Lagrangian perspective. Previous studies have partly used quasi-Lagrangian approaches to investigate the role of eddy feedback on blocks and, in particular, the role of transient anticyclonic eddies (e.g., Shutts, 1983;Yamazaki and Itoh, 2013;Suitters et al, 2023).…”

“…Previous studies have partly used quasi-Lagrangian approaches to investigate the role of eddy feedback on blocks and, in particular, the role of transient anticyclonic eddies (e.g., Shutts, 1983;Yamazaki and Itoh, 2013;Suitters et al, 2023). By tracing back negative, upper-tropospheric PV anomalies linked to a block over Europe and investigating the relative contributions of dry and moist dynamics using the same piecewise PV tendencies of Teubler et al (2023), the quasi-Lagrangian approach of Hauser et al (2023b) revealed a non-local development of the negative PV anomaly over the western North Atlantic. This negative PV anomaly propagated eastward and was pulse-like amplified by moist processes over the North Atlantic -a development the Eulerian perspective missed as the PV anomaly was located and associated processes beyond the blocking region which the Eulerian perspective focuses on.…”