Quantitative methods to pinpoint the origin of atmospheric temperature anomalies (T’) associated with heatwaves are pivotal for the construction of physically plausible synoptic storylines of heatwave formation and their evaluation in models. Here, we combine a Lagrangian T’ decomposition with concepts from moisture tracking techniques to identify where and when the principal physical processes generate T’ and to attribute these sources to synoptic weather systems. Applying this framework to near-surface and free-tropospheric T’ associated with the record-shattering 2021 heatwave in the Pacific Northwest shows that ascending, diabatic air streams in North Pacific cyclones contribute more than 50% of free-tropospheric T’, whereas near-surface T’ is mainly produced by local subsidence and diabatic heating with only marginal upstream contributions. Since free-tropospheric T’ facilitates near-surface accumulation of locally produced T’ by rendering the atmosphere stable to moist convection, our findings corroborate the notion of top-down induced heatwave formation fuelled by upstream diabatic processes.

Persistent dry and wet spells can arise from stationary weather situations or recurrent flow patterns and result in significant socio-economic impacts. Here, we study the effects of recurrent synoptic-scale transient Rossby wave packets (RRWPs) on the persistence of dry and wet spells using the ERA-Interim reanalysis data. RRWPs significantly alter (decrease and increase) dry and wet spell persistence across the globe. Spatial patterns of statistically significant links between RRWPs and spell durations arise from the superposition of a zonally symmetric component and a wave-like component that is modulated by local factors such as orography and the position relative to major moisture sources. The zonally symmetric component is apparent during the Northern Hemisphere winter and dominates the Southern Hemisphere signal in winter and summer. The wave-like component appears primarily in the Northern Hemisphere, changes its wavenumber with the season and is thus, conceivably related to stationary wave dynamics.