Identification of key processes in bridging the Arctic warming impact and its variation on decadal timescale (D3.2)

Summary: Observational analysis of the Arctic warming impacts: The key driver bridging the winter Arctic warming (1980 to 2014) impact to the Northern Hemisphere has been identified, my means of an advanced multi-variable statistical analysis, to be a tropospheric pathway, linking interannual variability in Arctic warming to the Northern Hemisphere lower atmosphere variability with one month lag. Clearly, the analysis has shown, that the response to the pan-Arctic sea-ice changes does not involve the stratosphere. A covariation of sea-ice variability with Siberian snow cover may be responsible of previously proposed pathways of influences involving the stratosphere. In addition, the analysis suggests that the mechanism of the tropospheric pathway may include the intensification of the Ural anticyclone. Coordinated experiments on Arctic warming impact and its variation on decadal timescale: Warm Arctic Cold Eurasia in winter surface air temperature. Making use of the ensembles of atmospheric model experiment with and without Arctic sea ice forcing it has emerged that the large scale pattern of winter surface air temperature variability is an internal mode of atmospheric variability. At shorter time scales (interannual) the project models capture this internal mode of atmospheric variability. At longer time scales (multi-annual, decadal), however, the models fail to capture the variability/trend of the winter surface air temperature (over 1980-2014). Arctic sea-ice driven variability. Within the Arctic Circle, the sea-ice driven variability explains about 3% of the total variance for sea level pressure and about 23% for surface air temperature in boreal winter at interannual and longer time scales. Regionally, the sea-ice driven variability is 1-1.5 times as large as the variability driven by the other forcings over the Arctic and northern Eurasia. Contrasting Summer and Winter Impact of Arctic sea-ice loss. Large scale features of atmospheric circulation trends over the period 1980-2014 are not reproduced by models, both in winter and summer. While in winter internal atmospheric variability likely plays a role, the difference in summers may point to structural model deficiencies. Multidecadal variability in sea surface temperatures and Arctic warming. The role of variations of the Pacific Ocean surface temperatures on Arctic warming and its impacts many be hard to be identified, given that preliminary results suggest sensitivity to structural model differences.The Blue-Action project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 727852