Towards a holistic understanding of blocked regime dynamics through a combination of complementary diagnostic perspectives

Abstract

Abstract. Atmospheric blocking describes a situation in which a stationary and persistent anticyclone blocks the eastward propagation of weather systems in the midlatitudes and can lead to extreme weather events. In the North Atlantic–European region, blocking contributes to life cycles of weather regimes which are recurrent, quasi-stationary, and persistent patterns of the large-scale circulation. Despite progress in blocking theory over the last decades, we are still lacking a comprehensive, process-based conceptual understanding of blocking dynamics. Here we combine three different perspectives on so-called “blocked” weather regimes, namely the commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of midlatitude potential vorticity (PV) thinking, the joint consideration of the three perspectives enables a comprehensive picture of the dynamics and quantifies the importance of dry and moist processes during a blocked weather regime life cycle. We apply the diagnostic framework to a European blocking weather regime life cycle in March 2016, which was associated with a severe forecast bust in the North Atlantic–European region. The three perspectives highlight the importance of moist processes during the onset or maintenance of the blocked weather regime. The Eulerian perspective, which identifies the processes contributing to the onset and decay of the regime, indicates that dry quasi-barotropic wave dynamics and especially the eastward advection of PV anomalies (PVAs) into the North Atlantic–European region dominate the onset of the regime pattern. By tracking the negative upper-tropospheric PVA associated with the “block”, the quasi-Lagrangian view reveals, for the same period, abrupt amplification due to moist processes. This is in good agreement with the Lagrangian perspective indicating that a large fraction of air parcels that end up in the negative PVA experience diabatic heating. Overall, the study shows that important contributions to the development take place outside of the region in which the blocked weather regime eventually establishes, and that a joint consideration of different perspectives is important in order not to miss processes, in particular moist-baroclinic dynamics, contributing to a blocked regime life cycle.

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

“…This mechanistic link between upstream WCB activity and blocking over Europe, which we found and described here in a qualitative way based on composites has recently been confirmed by Hauser et al. (2023b) and Hauser (2023) in a quantitative potential vorticity framework.…”

“…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.…”

“…Our qualitative result is further confirmed quantitatively in a quasi‐Lagrangian potential vorticity framework recently established by Hauser et al. (2023b) and Hauser (2023).…”

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.…”

A New Atmospheric Background State to Diagnose Local Waveguidability

“…We do not attempt to recover localized versions of these theorems in this work, but note that the localized zonalized state can be changed both by non‐conservative processes (diabatic, frictional or mixing, as for the hemispherically zonalized state; Methven & Berrisford, 2015) and by zonal rearrangement of PV. These processes can lead to significant local changes in the background state, for example, during episodes of atmospheric blocking which are often associated with horizontal PV advection (Hauser et al., 2023; H. Nakamura et al., 1997), cross‐isentropic PV transport due to latent heat release (Pfahl et al., 2015; Steinfeld et al., 2020) and irreversible mixing of PV during wave breaking (Berrisford et al., 2007; Masato et al., 2012).…”