the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jun 2025
The contribution of circulation changes to summer temperature trends in the northern hemisphere mid-latitudes: A multi-method quantification
Abstract. The increase in summer temperature and heat extremes is well documented and attributed to anthropogenic climate change. There is, however, still a vivid debate about the influence of atmospheric circulation changes on summer temperatures and heat extremes. Over the northern hemispheric mid-latitudes, considerable regional differences in summer temperatures have been observed. These differences have been linked to atmospheric circulation changes using statistical methods, but it remains challenging to evaluate such methods on multi-decadal time scales. Here, we evaluate different decomposition methods and systematically investigate circulation-induced summer temperature trends. For the evaluation of statistical and machine learning decomposition methods we use climate model simulations without external forcing but with atmospheric circulation nudged towards the circulation of a freely running simulation forced by anthropogenic emissions and land use changes. We train the decomposition methods on the free-running forced simulation and compare its circulation-induced trends to the trends simulated in the nudged circulation simulation. Most decomposition methods show skill in estimating the sign of circulation-induced trends but all methods underestimate the magnitude of these trends. The application of tested decomposition methods confirms that circulation changes have contributed substantially to an increase in summer heat over several mid-latitude regions, including Europe. In this hotspot region, we estimate that up to half of the warming over the period 1979–2023 is due to circulation changes.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2397', Robin Guillaume-Castel, 27 Jun 2025
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RC2: 'Comment on egusphere-2025-2397', Anonymous Referee #2, 07 Aug 2025
Review of WCD paper “The contribution of circulation changes to summer temperature…” by. Pfleiderer et al.
Overall, the paper is well structured and well documents a thorough study on different methods of estimating the role of atmospheric circulation changes to trends in the northern midlatitudes.
My overall recommendation would be publication after some revisions, which are generally minor.
General remarks:
My main query with this paper is the interpretation of the nudged simulations as a “benchmark”. This is an excellent approach to include but I’m not wholly convinced that this is necessarily a gold standard in term of attributing the changes due to circulation.
The nudging approach is elegant, and the demonstration in Figure 2 clearly shows that impact of the thermodynamic forcing on the global scale. However, on smaller scales I am less sure that the nudging strictly represents the contemporaneous circulation driven anomalies. A couple of conceptual examples of the potential issues are as follows:
- In Figure 3 the nudged anomalies are consistently higher than those predicted by the individual methods over the Eurasian continent in the summer. The nudging constrains all seasons (not just summer) so there are likely to be other factors that are modified by the nudging that contribute to these – particularly soil moisture but also other factors such as vegetation, snow melt etc.. These depend on seasons preceding the summer in question. In general these are small but there is the potential for these to have a local influence over time that systematically enhances the temperature response. I suppose these can be summarised as being model “feedbacks” (from other seasons and any associated integrated response) that are explicitly not captured by any of the statistical estimates but are implicitly included in the nudged “benchmark”.
- The second point is regarding the nudging to winds in the lower troposphere – here the nudging is performed on short timescales and, despite only forcing winds, the dominance of thermal wind balance on synoptic scales means that the nudging will have an effective local temperature forcing. This adjustment will be fast but, for example, any thermodynamic feedbacks that occur between say the land and the atmosphere (for example the strengthening of an anticyclonic high over continents during summer) will show up as being due to the “circulation” when in reality there is a non-negligible impact from thermodynamics. These should not be as well captured by the statistical methods as the information and feedbacks are not directly included but must be elucidated from the data output. Of course, on global scales (i.e. in terms of GMST) this will have no impact but in terms of estimating the “circulation” contribution to local temperature changes, the thermodynamic contribution to thermal wind balance adjustment will be attributed to “circulation”.
Neither of these examples particularly undermines the nudged simulations but do highlight how they are fundamentally different from the statistical approaches, as they implicitly include more thermodynamic effects adn feedbacks.
This may explain why the distibutions of the trends are systematically underestimated (e.g. the distrubutions on the right of Figure 3 and in Figure D2) in all the statistical approaches as they do not include the feedbacks and adjustments that are implicit in the nudged runs. At present there is only a discussion of the limitations of the statistical methods but I think discussing the limitation fo the nudged approach would also be useful to include.
I am sure the authors can directly discuss and address these differences and I think this would strengthen the interpretation of the results.
Minor comments:
Figure 3: This is a bit messy in the version I have– more details on the KDE plots on the right would be usefukle (along with axis labels etc).
Section 2.3.4: I don’t quite follow why the SLP would not be detrended as the “forced response” is small. It this important? Surely it would be better to include, unless the results are sensitive to this? I also may have misunderstood this, in which case a brief clarification might help.
I want to end on a positive: I think this is a very interesting paper!
Citation: https://doi.org/10.5194/egusphere-2025-2397-RC2
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Overall review
This paper by Pfleiderer et al. aims to improve our ability to decompose climate trends into thermodynamic and dynamical components, with a focus on surface temperature trends in the Northern Hemisphere. The first one is to determine whether statistical methods are able to quantify dynamically induced trends in climate model data by comparing their outcomes to a set of nudged climate models experiment, considered to be the ground truth. Once this is validated, the statistical methods and another set of nudged experiments are applied to ERA5 data to actually determine the contribution of dynamical changes to the surface temperature trends in the northern mid-latitudes.
The paper is highly relevant and timely, and it provides an important assessment of dynamical adjustment techniques. Beyond its specific results, the framework developed could be applied to a wider range of climate variables, such as precipitation or extreme events.
The use of nudged simulations with no external forcing is a particularly smart approach to isolate the dynamical influence on surface temperature trends. Since such experiments are difficult to construct for observational datasets (though the AMIP + nudging above 700 hPa approach seems promising), validating statistical methods is crucial, and this paper does so effectively.
The manuscript is generally well written, although it can be hard to follow at times. Some sections, particularly on the analogues method, would benefit from clearer explanations. Also, the two main objectives, though related, are presented somewhat independently and could be more tightly connected in the structure of the paper. For example the authors could emphasize that the first objective is used to strengthen our confidence in the second objective.
Despite some concerns I have about the paper (detailes below), I think this paper is almost suitable for publication in WCD, but requires some work, notably to improve clarity. For these reasons I suggest to accept this paper with minor revisions.
Main comments
1. Comparability between methods
One of my main concerns is the comparibility between the different statistical methods. Indeed, each method uses a different set of predictor variables:
This makes it difficult to assess whether differences in performance are due to the method itself or the choice of input variables. It would be helpful for the authors to comment on this explicitly. If the best predictor was chosen for each method, this should be clarified.
2. Lack of information on trend estimation, significance and uncertainty
Maybe I have missed it but I couldn't find a mention on how the trends were computed. In addition, such a study would benefit from statistical tests on trend significance and uncertainty, especially for the second objective which aims to provide robust estimates. As all methods provide an estimate of surface temperature directly, trends statistics could be computed for all cases. Moreover, it might make sense to evaluate skill metrics only for statistically significant trends.
3. Section 2.3.2 (circulation analogues) lacks clarity
The description of the analogue method is quite confusing. As someone who is not familiar with circulation analogues, I cannot say I have understood what it is from that section. Please revise it to make it clearer. Here are the points that made it unclear to me:
Maybe this is also the case for the UNET paragraph, but as I am more familiar with UNETs it was easier to follow.
4. Are the UNET Predictions Truly Circulation-Induced Temperature Changes?
Also, it is unclear what “CESM2 transient simulations” refers to. Do these include historical + SSP runs?
Minor comments