A quasi-Lagrangian perspective on the role of dry and moist processes in the formation of blocked North Atlantic-European weather regimes

Abstract. Atmospheric blocking often triggers extreme events and remains difficult for weather and climate models to represent due to the complex multi-scale processes in its lifecycle. While recent studies highlight the importance of latent heat release in building and maintaining the upper-level anticyclonic anomaly, different perspectives assign varying roles to dry and moist dynamics, and it is still unclear whether their relative roles differ across regions where blocking occurs. This study uses a quasi-Lagrangian potential vorticity (PV) framework applied to ERA5 (1979–2021) to investigate blocking in the North Atlantic-European sector from the perspective of four large-scale blocked weather regimes. We track negative upper-tropospheric PV anomalies (PVAs^-) around blocked regime onset and quantify the processes governing their amplitude changes to assess the roles of dry and moist dynamics. Most PVAs^- linked to blocked regime onset are not formed in situ but follow two main pathways, arriving either from upstream or from downstream. PVAs^- intensify in the days before onset, with moist, divergence-related PV tendencies associated with warm conveyor belt activity and baroclinic PV tendencies contributing strongly to their amplification, independent of blocked regime type or pathway. The position of PVAs^- relative to storm tracks determines the strength of the moist contribution, with moist processes exerting a greater influence within the midlatitude storm track over the North Atlantic. Consequently, the magnitude of PVA^- amplification depends more on whether a PVA^- arrives from upstream or downstream, since the pathway controls the timing, location, and strength of the moist-dynamical processes acting on it, than on the blocked regime type it eventually contributes to. This study highlights the synoptic-scale moist-dynamical evolution of PVAs^- associated with different types of blocked regimes from a quasi-Lagrangian perspective. Complementing the quasi-Lagrangian analysis with previous insights from a Eulerian perspective provides a coherent view of blocked regime evolution, linking the remote moist amplification of PVAs^- with the local formation of the regime pattern by anomaly re-arrangement, which is dominated by dry, quasi-barotropic dynamics. Given the key role of moist processes in PVA^- amplification and the systematic biases of blocking in weather and climate models, our results emphasize the need for better representation of moist baroclinic eddies and scale interactions, from cloud microphysics to the synoptic scale.