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North Atlantic summer storm tracks over Europe dominated by internal variability over the past millennium


Certain large, sustained anomalies in European temperatures in the past millennium are probably the result of internal variation. Such internal variations can modulate regional temperatures away from the expected response to greenhouse gas forcing. Here we assess the causes of European summer temperature variability over the past millennium using temperature observations, simulations and reconstructions. We find multidecadal-mean summer temperatures have varied within a span of 1 K, largely controlled by external forcing. By contrast, we find subcontinental variations, described by the temperature contrast between northern and southern Europe (the meridional temperature gradient), vary with a span of 2 K, and are controlled by internal processes. These variations are the result of redistributions of precipitation and cloud cover linked to oscillations in the position of the summer storm track. In contrast to recent twentieth-century winter-time trends, variations of the summer storm track over the past millennium show a weak response to external forcing, and instead are dominated by stochastic internal variability. We argue that the response of European summer temperatures to anthropogenic greenhouse forcing is likely to be spatially modulated by the same stochastic internal processes that have caused periods of cool, wet summers in northern Europe over the last millennium.

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Figure 1: Spatiotemporal structure of simulated (MPI-ESM-P, AD 850–2005) and proxy mean continental and meridional temperature gradients (meridional gradient of European JJA near-surface temperature).
Figure 2: Spatiotemporal structure of the European summer (JJA) near-surface temperature meridional gradient from observational data.
Figure 3: Climate patterns linked to simulated summer (JJA) MTG (MPI-ESM-P, AD 850-2005).


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M.H.G., D.M., G.H.F.Y. and I.R. were supported by European Union FP6 project Millennium 017008 and the Climate Change Consortium of Wales C3W. The work of E.Z. and M.Z. is part of the German Cluster of Excellence CLISAP. M.H.G., D.M. and E.Z. were supported by the PAGES initiative (EuroMed2k), which in turn received support from the US and Swiss National Science Foundations, US National Oceanographic and Atmospheric Administration and the International Geosphere-Biosphere Programme. T. D. James is thanked for support with figures.

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M.H.G.: project planning and design, data analysis, manuscript preparation. E.Z.: data analysis and manuscript preparation. D.M.: project planning and design, data analysis, manuscript preparation. M.Z.: data analysis, manuscript preparation. G.H.F.Y.: project planning, data analysis, manuscript preparation. I.R.: project planning, data analysis, manuscript preparation.

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Correspondence to Mary H. Gagen.

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The authors declare no competing financial interests.

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Gagen, M., Zorita, E., McCarroll, D. et al. North Atlantic summer storm tracks over Europe dominated by internal variability over the past millennium. Nature Geosci 9, 630–635 (2016).

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