Towards a diagnostic framework unifying different perspectives on blocking dynamics: insight into a major blocking in the North Atlantic-European region

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 blocking, namely the 5 commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of mid-latitude 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 blocking 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 Atlantic-European region. All three perspectives highlight the importance of moist processes 10 during the onset and 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 sector are associated with 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 15 that a large fraction of air parcels that end up in the negative PVA experiences 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 blocking life cycle. cloud diabatic processes could amplify initial condition error of the midlatitude flow and might be responsible for forecast errors of the large-scale circulation in the North Atlantic-European region. By unifying the three perspectives on blocking dynamics within the theoretical concept of PV thinking, our novel 610 framework enables - for the first time - a holistic view on the dynamics of blocked weather regimes in particular including a quantification of the contribution of moist processes in different life cycle stages. Therefore, a next step will be a climatological study which will also allow to investigate whether the predictability of blocked regime life cycles is related to the contribution of moist processes in the onset and maintenance stage of a block.