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This amplification is characterized by Arctic temperature trends which are approximately twice as strong as the hemispheric trends (Semenov 2016).The Eurasian variant of this phenomenon has been termed 'warm Arctic–cold Siberia,' or WACS—similar definitions such as 'warm Arctic–cold Eurasia' are essentially describing the same pattern (Petoukhov (2009) first demonstrated that the WACS pattern can be attributed to sea ice loss over the BKS, which in turn triggers increased turbulent heat fluxes from the ocean to the atmosphere.We checked using different autumn months between September and December and found that regression with September sea ice best represents the DJF temperature trend, especially so for the ETCAW (see supplementary figure 1 available at org/ERL/13/025009/mmedia).However, we did not find a large difference among all autumn months.The Warm Arctic–cold Siberia surface temperature pattern during recent boreal winter is suggested to be triggered by the ongoing decrease of Arctic autumn sea ice concentration and has been observed together with an increase in mid-latitude extreme events and a meridionalization of tropospheric circulation.However, the exact mechanism behind this dipole temperature pattern is still under debate, since model experiments with reduced sea ice show conflicting results.The ECMWF integrated an ensemble of ten Integrated Forecast System (IFS) atmospheric simulations for the years 1899–2009 at a horizontal resolution of 1 degree with 91 vertical levels reaching from the surface up to 1 Pa, known as the final ERA-20 cm version (ERA20CM).Specified sea ice concentration as well as sea-surface temperature boundary conditions come from Had ISST.2 (same as ERA20C) and the radiation scheme follows the CMIP5 protocol exactly, including aerosols, ozone and greenhouse gases (Hersbach (2016) reported no differences between these two datasets.

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The goal of this study is to demonstrate the consistency between both periods, of the sea ice decline, the covariability of the sea ice with atmospheric temperature and heat fluxes, and their impact on the Arctic region.Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Together with this sea ice decline, a strong near-surface Arctic warming has been recorded, especially during boreal wintertime (Screen and Simmonds 2010), attributed to the Arctic amplification of anthropogenic global warming (Serreze and Barry 2011).We define the BKS as region bounded by 30–90° E, 65–85° N and focus on September sea ice.

In September, sea ice reaches its annual minimum and open-water regions provide a strong heat and moisture release to the cold Arctic atmosphere (Jaiser 2016).

For the ETCAW period we investigate five different SIC datasets, namely the three products used in the long-term reanalyses (see table 1) as well as the independent sea ice concentration data by Walsh (2016) found a substantial decrease of September Arctic sea ice between 1930–1940 in their assessment of Arctic sea ice in the 20th century.