| 12 Jun 2026
How atmospheric and surface conditions shape recurrent heatwaves in Europe: From local to continental scales
Abstract. Heatwaves are among the most impactful climate extremes in Europe. Their recurrence, defined as sequences of heatwaves separated by short breaks, is projected to increase disproportionately under climate change, yet remains poorly understood. This is in part because recurrence emerges from interacting processes operating across spatial scales, from local land–atmosphere feedbacks to large-scale atmospheric circulation, and studies often focus on only one scale at a time. We develop a unified framework to investigate heatwave recurrence across local, object, and continental scales over Europe using the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis for 1950–2024.
Our results reveal complementary insights at each spatial scale. Local recurrence, that is heatwaves at one location separated by less than seven days, exhibit weaker daily temperature and soil moisture anomalies than isolated events. However, recurrent heatwave objects, defined as spatially contiguous heatwave objects corresponding to strong local recurrence, show higher soil moisture depletion and higher temperatures compared to single events. They further persist longer, cover larger areas, and are more frequently associated with European blocking in July and August. Continental recurrence reflects summers in which multiple heatwave objects overlap spatially, affecting large areas or striking the same regions repeatedly. It emerges as a distinct dimension of summer heat extremes rather than simply reflecting anomalously warm conditions. Summers with high continental recurrence feature more heatwaves, larger affected areas, higher temperature anomalies, and earlier, more persistent soil moisture depletion compared to summers with isolated events. Further, despite some overlap between high continental and high local recurrence, both represent distinct and complementary perspectives on heatwave recurrence.
These findings demonstrate that recurrence is a cross-scale phenomenon and highlight the importance of integrating complementary spatial perspectives to better understand compound heat extremes and the coupled atmospheric–land surface processes that govern them.
This study investigates recurrent heatwaves over Europe from local, object, and continental perspectives using ERA5 during 1950–2024. The authors develop a multi-scale framework and examine how temperature, soil moisture, heatwave characteristics, and large-scale weather regimes differ between recurrent and non-recurrent events. I find the attempt to connect recurrence across spatial scales interesting, particularly because local recurrence and spatially evolving heatwave systems are usually studied separately.
My main concern is with the definition and physical interpretation of recurrence across the three scales. The local definition is relatively straightforward, but the object and continental definitions represent increasingly different phenomena. This makes it difficult at times to understand what is actually “recurrent” and whether the differences identified later in the manuscript can be specifically attributed to recurrence. I think these conceptual issues need to be clarified before the multi-scale framework can be fully supported. I have three major comments below.
Major comments
1. Definition and physical meaning of recurrence across the three scales
I have some concerns about how the three types of recurrence are connected conceptually.
(1A) The local recurrence definition is clear: two heatwaves affect the same location within a seven-day break, irrespective of whether they belong to the same or different heatwave objects. However, the subsequent analysis does not fully explore this distinction. Local recurrence can result either from the same heatwave object moving away and returning, or from two different objects affecting the same location in close succession. The following heatwave object analysis focuses only on the former. I am not sure why recurrence involving different heatwave objects should be considered less physically meaningful. Different heatwave objects may still be related to the same evolving or propagating large-scale circulation pattern; alternatively, genuinely different weather systems repeatedly affecting the same region within a short interval is itself an interesting form of recurrence. It would therefore be useful to at least distinguish same-object and cross-object local recurrence and discuss whether their characteristics or large-scale drivers differ.
(1B) The definition of object recurrence is less intuitive to me. What is quantified is not the recurrence of heatwave objects, but the fraction of an individual object's area experiencing local recurrence during its lifetime. In this sense, “objects with strong/weak local recurrence” may describe the metric more accurately than “object recurrence”. There is also a physical issue here: the interpretation seems to assume that one tracked heatwave object corresponds to one coherent synoptic-scale system. However, the tracking method allows one-day gaps, splitting and merging, and full-history merging, which can generate very long objects. Strongly recurrent objects indeed last almost 28 days on average. It is not obvious that such an object necessarily represents one coherent circulation system rather than a sequence of evolving systems linked by the tracking algorithm. This point needs more discussion and, if possible, some direct evidence that the identified long-lived objects retain the intended synoptic coherence.
(1C) Finally, I am least convinced by the definition of continental recurrence. Different heatwave objects affecting the same grid cell at any time between May and September contribute to this metric, regardless of their temporal separation. Thus, two events occurring a few days apart and two events occurring in June and August are treated similarly. At this point, it becomes difficult to distinguish “recurrence” from a generally heatwave-active summer. In fact, strongly recurrent summers also contain more heatwave objects and heatwave days and have larger affected areas. Showing that these summers are not exactly the hottest summers does not fully resolve this issue. I suggest that the authors clarify what continental recurrence measures beyond conventional seasonal heatwave activity, for example by examining its relationship with the total number of heatwave objects, heatwave days, affected area, and cumulative heatwave intensity.
Overall, I think the distinction between local recurrence within the same object, local recurrence across different objects, recurrence/repeated exposure over a season, and simply high seasonal heatwave activity needs to be made clearer throughout the manuscript.
2. Are the object-scale differences driven by recurrence or simply by object duration and size?
Strongly recurrent objects are very different from weakly recurrent objects even before temperature, soil moisture, or circulation are considered: their mean duration is 27.91 versus 8.58 days, and their mean spatial extent is 2.6 × 10⁶ versus 0.9 × 10⁶ km².
This raises a confounding issue. A longer object naturally has more opportunity to move away from and return to the same grid cells, and therefore to satisfy the local-recurrence criterion. At the same time, a long-lived and large heatwave is also more likely to produce stronger soil-moisture depletion and to be associated with persistent circulation anomalies. The authors themselves note that the higher temperatures of strongly recurrent objects may be related to their longer duration.
I therefore wonder how much of the difference between strongly and weakly recurrent objects is actually related to recurrence itself, or is it more one of the characteristics of persistent and large heatwaves. Could the authors control for object duration and/or spatial extent, for example by comparing objects with similar duration/area or through a regression analysis? This would help determine whether soil moisture, temperature, and weather-regime differences remain once the very large differences in object lifetime and size are accounted for.
3. Soil-moisture preconditioning needs to be separated from generally dry background conditions
The manuscript motivates recurrent heatwaves partly through the idea that an earlier heatwave depletes soil moisture and thereby affects the following event. However, the local results provide rather limited support for this sequence. Soil moisture is somewhat lower during subsequent recurrent heatwaves, but these events are not systematically hotter than the first heatwaves.
At the continental scale, soil moisture is already lower at heatwave onset during strongly recurrent summers. This could reflect preconditioning by previous heatwaves, but it could also simply indicate that these summers have a generally drier background state that favors both stronger soil-moisture deficits and more heatwave activity. The current composites cannot clearly distinguish these possibilities.
For the local analysis, it would be useful to show the evolution of soil moisture across the first heatwave, the break period, and the following heatwave. This would provide more direct evidence of whether the first event leaves a soil-moisture memory into the subsequent event. More generally, I suggest being careful in distinguishing soil-moisture depletion caused by preceding heatwaves from antecedent dry conditions that may favor recurrence in the first place.
Minor comments