Abstract
Cold season temperatures in Europe have increased rapidly by about 1.2°C in the late 1980s, followed by relatively modest and regionally flat temperature trends thereafter. The abrupt change affected the entire European continent and coincided regionally with abrupt hydroclimatic changes such as a widespread reduction in snow days in Switzerland. However, the drivers and causes of the event are not well understood. Using a dynamical adjustment method based on statistical learning, we find that the continental-scale late 1980s abrupt winter warming and regional decreases in snow days can be attributed to cold conditions in the mid-1980s followed by a few exceptionally warm seasons. Both are caused by random atmospheric circulation variability superimposed upon a long-term and relatively homogenous warming trend, and do not require an external cause or change of the underlying dynamics of the system. This explanation is consistent with simulations from a 21-member regional climate model ensemble, in which four members display comparable abrupt temperature increases regionally driven by circulation and a long-term externally forced response. Overall, our analysis provides an observation-based interpretation of abrupt temperature change at the continental scale, associated hydroclimatic changes regionally, and its drivers. Furthermore, our method might contribute to improved mechanistic understanding of different observed climate phenomena in many regions of the world that experience high variability.
Highlights
Climate variability and change in cold season temperatures and hydroclimatic variables are of high societal interest and relevance
Explaining the abrupt change in European and Swiss cold season temperatures and snow days Dynamical adjustment of cold season (Nov.-March) temperatures reveals that atmospheric circulation accounts for about 76% and 73% of the variance in the detrended cold season temperature series over Europe and Switzerland, respectively (figures 2(a) and (b), blue lines)
We have shown that the observed late 1980s abrupt cold season warming in Europe, and regionally in Switzerland associated with decreases in the number of snow days, is consistent with a combination of a short-term, possibly random, circulation-induced component superimposed upon a slowly evolving warming trend
Summary
Climate variability and change in cold season temperatures and hydroclimatic variables are of high societal interest and relevance. Average cold season temperatures (Nov.-Mar.) in Europe increased abruptly by about 1.2◦ C in the late 1980s (figure 1(a)), and the abrupt change affected almost the entire continent (figure 1(c)). Cold season temperatures increased only relatively modestly in Europe, with flat or even slightly cooling cold season temperature trends over the last three decades in some regions including, for example, Switzerland (figure 1(b)) (Ceppi et al 2012, CH2018 2018, Saffioti et al 2016). The abrupt increase in average cold season temperature coincided regionally with abrupt changes in cold season hydroclimate such as an abrupt decrease in snow days in Switzerland (Marty 2008). The decrease had been pronounced at stations at low or medium elevation that are sensitive to temperature changes (Scherrer et al 2004), and without any clear trends after the change (Marty 2008)
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