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Aglaé Jézéquel, Pascal Yiou, Sabine Radanovics, and Robert Vautard
Open access
David Rodrigues, M. Carmen Alvarez-Castro, Gabriele Messori, Pascal Yiou, Yoann Robin, and Davide Faranda

Abstract

It is of fundamental importance to evaluate the ability of climate models to capture the large-scale atmospheric circulation patterns and, in the context of a rapidly increasing greenhouse forcing, the robustness of the changes simulated in these patterns over time. Here we approach this problem from an innovative point of view based on dynamical systems theory. We characterize the atmospheric circulation over the North Atlantic in the CMIP5 historical simulations (1851–2000) in terms of two instantaneous metrics: local dimension of the attractor and stability of phase-space trajectories. We then use these metrics to compare the models to the Twentieth Century Reanalysis version 2c (20CRv2c) over the same historical period. The comparison suggests that (i) most models capture to some degree the median attractor properties, and models with finer grids generally perform better; (ii) in most models the extremes in the dynamical systems metrics match large-scale patterns similar to those found in the reanalysis; (iii) changes in the attractor properties observed for the ensemble-mean 20CRv2c are artifacts resulting from inhomogeneities in the standard deviation of the ensemble over time; and (iv) the long-term trends in local dimension observed among the 56 members of the 20CR ensemble have the same sign as those observed in the CMIP5 multimodel mean, although the multimodel trend is much weaker.

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Philippe Naveau, Aurélien Ribes, Francis Zwiers, Alexis Hannart, Alexandre Tuel, and Pascal Yiou

Abstract

Both climate and statistical models play an essential role in the process of demonstrating that the distribution of some atmospheric variable has changed over time and in establishing the most likely causes for the detected change. One statistical difficulty in the research field of detection and attribution resides in defining events that can be easily compared and accurately inferred from reasonable sample sizes. As many impacts studies focus on extreme events, the inference of small probabilities and the computation of their associated uncertainties quickly become challenging. In the particular context of event attribution, the authors address the question of how to compare records between the counterfactual “world as it might have been” without anthropogenic forcings and the factual “world that is.” Records are often the most important events in terms of impact and get much media attention. The authors will show how to efficiently estimate the ratio of two small probabilities of records. The inferential gain is particularly substantial when a simple hypothesis-testing procedure is implemented. The theoretical justification of such a proposed scheme can be found in extreme value theory. To illustrate this study’s approach, classical indicators in event attribution studies, like the risk ratio or the fraction of attributable risk, are modified and tailored to handle records. The authors illustrate the advantages of their method through theoretical results, simulation studies, temperature records in Paris, and outputs from a numerical climate model.

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Matteo Zampieri, Fabio D’Andrea, Robert Vautard, Philippe Ciais, Nathalie de Noblet-Ducoudré, and Pascal Yiou

Abstract

Drought in spring and early summer has been shown to precede anomalous hot summer temperature. In particular, drought in the Mediterranean region has been recently shown to precede and to contribute to the development of extreme heat in continental Europe. In this paper, this mechanism is investigated by performing integrations of a regional mesoscale model at the scale of the European continent in order to reproduce hot summer inception, starting with different initial values of soil moisture south of 46°N. The mesoscale model is driven by the large-scale atmospheric conditions corresponding to the 10 hottest summers on record from the European Climate Assessment dataset. A northward progression of heat and drought from late spring to summer is observed from the Mediterranean regions, which leads to a further increase of temperature during summer in temperate continental Europe. Dry air formed over dry soils in the Mediterranean region induces less convection and diminished cloudiness, which gets transported northward by occasional southerly wind, increasing northward temperature and vegetation evaporative demand. Later in the season, drier soils have been established in western and central Europe where they further amplify the warming through two main feedback mechanisms: 1) higher sensible heat emissions and 2) favored upper-air anticyclonic circulation. Drier soils in southern Europe accelerate the northward propagation of heat and drying, increasing the probability of strong heat wave episodes in the middle or the end of the summer.

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Cheikh Dione, Fabienne Lohou, Marjolaine Chiriaco, Marie Lothon, Sophie Bastin, Jean-Luc Baray, Pascal Yiou, and Aurélie Colomb

Abstract

The relative contribution of the synoptic-scale circulations to local and mesoscale processes was quantified in terms of the variability of midlatitude temperature anomalies from 2003 to 2013 using meteorological variables collected from three French observatories and reanalyses. Four weather regimes were defined from sea level pressure anomalies using National Centers for Environmental Prediction reanalyses with a K-means algorithm. No correlation was found between daily temperature anomalies and weather regimes, and the variability of temperature anomalies within each regime was large. It was therefore not possible to evaluate the effect of large scales on temperature anomalies by this method. An alternative approach was found with the use of the analogs method: the principle being that for each day of the considered time series, a set of days that had a similar large-scale 500-hPa geopotential height field within a fixed domain was considered. The observed temperature anomalies were then compared with those observed during the analog days: the closer the two types of series are to each other, the greater is the influence of the large scale. This method highlights a widely predominant influence of the large-scale atmospheric circulation on the temperature anomalies. It showed a potentially larger influence of the Mediterranean Sea and orographic flow on the two southern observatories. Low-level cloud radiative effects substantially modulated the variability of the daily temperature anomalies.

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Robert Vautard, Augustin Colette, Erik van Meijgaard, Frederik Meleux, Geert Jan van Oldenborgh, Friederike Otto, Isabelle Tobin, and Pascal Yiou
Open access
Aglaé Jézéquel, Vivian Dépoues, Hélène Guillemot, Amélie Rajaud, Mélodie Trolliet, Mathieu Vrac, Jean-Paul Vanderlinden, and Pascal Yiou

Abstract

Extreme event attribution (EEA) proposes scientific diagnostics on whether and how a specific weather event is (or is not) different in the actual world from what it could have been in a world without climate change. This branch of climate science has developed to the point where European institutions are preparing the ground for an operational attribution service. In this context, the goal of this article is to explore a panorama of scientist perspectives on their motivations to undertake EEA studies. To do so, we rely on qualitative semi-structured interviews of climate scientists involved in EEA, on peer-reviewed social and climate literature discussing the usefulness of EEA, and on reports from the EUCLEIA project (European Climate and Weather Events: Interpretation and Attribution), which investigated the possibility of building an EEA service. We propose a classification of EEA’s potential uses and users and discuss each of them. We find that, first, there is a plurality of motivations and that individual scientists disagree on which one is most useful. Second, there is a lack of solid, empirical evidence to back up any of these motivations.

Open access
Gabriele Messori, Emanuele Bevacqua, Rodrigo Caballero, Dim Coumou, Paolo De Luca, Davide Faranda, Kai Kornhuber, Olivia Martius, Flavio Pons, Colin Raymond, Kunhui Ye, Pascal Yiou, and Jakob Zscheischler
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Pascal Yiou, Julien Cattiaux, Davide Faranda, Nikolay Kadygrov, Aglae Jézéquel, Philippe Naveau, Aurelien Ribes, Yoann Robin, Soulivanh Thao, Geert Jan van Oldenborgh, and Mathieu Vrac
Free access
Gabriele Messori, Rodrigo Caballero, Freddy Bouchet, Davide Faranda, Richard Grotjahn, Nili Harnik, Steve Jewson, Joaquim G. Pinto, Gwendal Rivière, Tim Woollings, and Pascal Yiou
Open access