| Summary: | Global warming is amplified in the cold and white Arctic, where strong positive feedback mechanisms associated with, e.g., changes in surface conditions and the vertical structure of the Arctic atmosphere enhance the warming. The Arctic sea ice cover is described as a sensitive indicator for global warming, despite substantial internal climate variability in the region. The contribution to Arctic climate change from the oceanic heat source – the deep Atlantic layer – is to a large extent unknown, as there are only sparse measurements of the upward heat fluxes from the ocean, and it is not well understood which factors make the heat fluxes vary, laterally and temporally. This is perhaps one of the last big unknowns in the Arctic climate puzzle. The Arctic warming has a distinct regional maximum where the winter sea ice decline and the surface warming are greatest. The northern Barents Sea is in this ‘Arctic warming hotspot’ and here the warming extends high up into the lower atmosphere and deep down into the water column. The Arctic warming hotspot has been linked to large-scale changes in the atmospheric circulation and mid-latitude weather extremes. As a consequence of the warming, structural changes are observed in the Barents Sea ecosystem, a productive and complex Arctic-boreal shelf ecosystem, inhabiting both valuable commercial fish stocks and vulnerable sea iceassociated marine mammals. The varying position of the sea ice edge in the Barents Sea is a complicating factor for activities across a range of sectors, including research, ecosystem management, fisheries, petroleum, shipping and tourism, and is therefore both a national and a geopolitical issue. The triggering factors and governing mechanisms for the ongoing rapid warming are not well understood, although increased heat losses to the atmosphere in autumn and winter is a likely consequence of the reduced sea ice cover. It is not known what role the ocean plays in the Arctic warming hotspot, and what the ongoing processes here can tell us about how ...
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