Over the past decade, severe winters occurred frequently in mid-latitude Eurasia 1,2 , despite increasing global-and annual-mean surface air temperatures 3 . Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline 2,4 , through excitation of circulation anomalies similar to the Arctic Oscillation 5 . In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation 6 . Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents-Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insu cient ensemble size.The Siberian High, a continental surface high pressure prevailing the boreal winter Asian monsoon, causes a breakout of cold air to the mid-latitudes, the fluctuation of which greatly affects a significant part of the population of Eurasia. Northern Hemisphere winters (December-February; DJF) have frequently seen pronounced warming and cooling in recent years, in the Arctic and mid-latitudes, respectively, forming the so-called 'warm Arctic-cold continents' pattern 7 (Supplementary Fig. 1), signifying the intensification of the Siberian High over the Eurasian continent. The Arctic surface warming has been accompanied by a rapid decline of Arctic sea ice 8 , which is therefore argued to represent the Arctic amplification signature of global warming 9,10 . However, the causes of these cold winters observed over mid-latitude Eurasia, apparently counteracting the continuous rise of annual-mean surface air temperature (SAT) over land 3 , are not well understood.Observational studies show a statistically significant relationship between cold SAT anomalies over Eurasia and Arctic sea-ice decline 2,4,11,12 , suggesting that the latter forces the former. However, a robust atmospheric response to sea-ice loss has yet to be obtained by modelling studies 2,13-16 because detection of sea-ice loss impacts on the extratropical atmosphere is hampered by the large internal fluctuations of the atmospheric circulation that are prominent in winter 6,15 .Here, we successfully detected the signature of Eurasian cold winters excited by sea-ice decline in the Barents-Kara Sea (BKS), where a pronounced change has been observed during winters since 2004 ( Supplem...

[1] Influence of low Arctic sea-ice minima in early autumn on the wintertime climate over Eurasia is investigated. Observational evidence shows that significant cold anomalies over the Far East in early winter and zonally elongated cold anomalies from Europe to Far East in late winter are associated with the decrease of the Arctic sea-ice cover in the preceding summer-to-autumn seasons. Results from numerical experiments using an atmospheric general circulation model support these notions. The remote response in early winter is regarded as a stationary Rossby wave generated thermally through an anomalous turbulent heat fluxes as a result of anomalous ice-cover over the Barents-Kara Seas in late autumn, which tends to induce an amplification of the Siberian high causing colder conditions over the Far East. The late-winter cold anomalies over Eurasia are also reproduced in our experiment, which is associated with the negative phase of the North Atlantic Oscillation.

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Sea ice variability over the Barents Sea with its resultant atmospheric response has been considered one of the triggers of unexpected downstream climate change. For example, East Asia has experienced several major cold events while the underlying temperature over the Arctic has risen steadily. To understand the influence of sea ice in the Barents Sea on atmospheric circulation during winter from a synoptic perspective, this study evaluated the downstream response in cyclone activities with respect to the underlying sea ice variability. The composite analysis, including all cyclone events over the Nordic seas, revealed that an anticyclonic anomaly prevailed along the Siberian coast during light ice years over the Barents Sea. This likely caused anomalous warm advection over the Barents Sea and cold advection over eastern Siberia. The difference in cyclone paths between heavy and light ice years was expressed as a warm-Arctic cold-Siberian (WACS) anomaly. The lower baroclinicity over the Barents Sea during the light ice years, which resulted from a weak gradient in sea surface temperature, prevented cyclones from traveling eastward. This could lead to fewer cyclones and hence to an anticyclonic anomaly over the Siberian coast.

The polar regions have been attracting more and more attention in recent years, fueled by the perceptible impacts of anthropogenic climate change. Polar climate change provides new opportunities, such as shorter shipping routes between Europe and East Asia, but also new risks such as the potential for industrial accidents or emergencies in ice-covered seas. Here, it is argued that environmental prediction systems for the polar regions are less developed than elsewhere. There are many reasons for this situation, including the polar regions being (historically) lower priority, with fewer in situ observations, and with numerous local physical processes that are less well represented by models. By contrasting the relative importance of different physical processes in polar and lower latitudes, the need for a dedicated polar prediction effort is illustrated. Research priorities are identified that will help to advance environmental polar prediction capabilities. Examples include an improvement of the polar observing system; the use of coupled atmosphere–sea ice–ocean models, even for short-term prediction; and insight into polar–lower-latitude linkages and their role for forecasting. Given the enormity of some of the challenges ahead, in a harsh and remote environment such as the polar regions, it is argued that rapid progress will only be possible with a coordinated international effort. More specifically, it is proposed to hold a Year of Polar Prediction (YOPP) from mid-2017 to mid-2019 in which the international research and operational forecasting communites will work together with stakeholders in a period of intensive observing, modeling, prediction, verification, user engagement, and educational activities.