Temperatures over Europe are largely driven by the strength and inland penetration of the oceanic westerly flow. The wind influence depends on season: blocked westerlies, linked to high-pressure anomalies over Scandinavia, induce cold episodes in winter1 but warm conditions in summer2,3. Here, we propose to define the onset of the two seasons as the calendar day on which the daily circulation/temperature relationship switches sign. We have assessed this meteorologically based metric using several observational data sets and we provide evidence for an earlier onset of the summer date by ∼10 days between the 1960s and 2000s. Results from a climate model show that internal variability alone cannot explain this calendar advance. Rather, the earlier onset can be partly attributed to anthropogenic forcings. The modification of the zonal advection due to earlier disappearance of winter snow over Eastern Europe, which reduces the degree to which climate has continental properties, is mainly responsible for the present-day and near-future advance of the summer date in Western Europe. Our findings are in line with phenological-based trends (earlier spring events) reported for many living species over Europe4,5,6, for which we provide an alternative interpretation to the traditionally evoked local warming effect. Based on the Representative Concentration Pathway (RCP) 8.5 scenario, which assumes that greenhouse gas emissions continue to rise throughout the twenty-first century, a summer advance of ∼20 days compared with pre-industrial climate is expected by 2100, whereas no clear signal arises for winter onset.
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The authors thank the CNRM-Cerfacs group for the development of the CNRM-CM5 model and for producing the CMIP5 simulations. C.C. is indebted to L. Coquart and M.-P. Moine for making available the outputs at Cerfacs and J. Boé for helpful discussions. The figures were produced with the NCAR Command Language Software (http://dx.doi.org/10.5065/D6WD3XH5). Both C.C. and J.C. are supported by CNRS and by the MORDICUS grant under contract ANR-13-SENV-0002-01.
The authors declare no competing financial interests.
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Cassou, C., Cattiaux, J. Disruption of the European climate seasonal clock in a warming world. Nature Clim Change 6, 589–594 (2016). https://doi.org/10.1038/nclimate2969
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