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A quasi-Lagrangian perspective on the role of dry and moist processes in the formation of blocked North Atlantic–European weather regimes
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DOI：10.5194/wcd-7-937-2026.png)
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Abstract
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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.
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