26 Oct 2022
26 Oct 2022
Status: this preprint is currently under review for the journal WCD.

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

Franziska Teubler1, Michael Riemer1, Christopher Polster1, Christian M. Grams2, Seraphine Hauser2, and Volkmar Wirth1 Franziska Teubler et al.
  • 1Institute for Atmospheric Physics, Johannes Gutenberg-University Mainz, Mainz, Germany
  • 2Institute of Meteorology and Climate Research (IMK-TRO), Department Troposphere Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

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, we combine wave activity concepts and a piecewise potential-vorticity (PV) tendency framework. The latter essentially quantifies the well-established PV perspective of mid-latitude dynamics. All blocked regimes during the 1979–2021 period of ERA5 reanalysis are considered.

Wave activity characteristics exhibit distinct differences between blocked regimes. After regime onset, one regime (Greenland Blocking) is associated with a suppression of wave activity flux, whereas two other regimes (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. The fourth regime (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. We further discuss the limitations of projected tendencies in capturing the importance of moist processes, which tend to occur at the fringes or outside of the regime pattern. Finally, we stress that this study investigate the variability of the governing dynamics without prior empirical stratification of data by season or by type of regime transition. We demonstrate, however, that our dynamics-centered approach does not map predominantly on 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.

Franziska Teubler et al.

Status: open (until 01 Jan 2023)

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  • RC1: 'Comment on wcd-2022-56', Gwendal Rivière, 25 Nov 2022 reply

Franziska Teubler et al.

Franziska Teubler et al.


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Short summary
Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. The year-round dynamics of blocked regimes in the Atlantic European region are investigated in over 40 years of data. We show that the dynamics between the regimes are on average very similar at regime location. However, the average picture experiences some cancellation due to different pathways especially before onset of the regime and some processes maximize outside of that location.