The northern Barents Sea during 1970–2016: From seabed to surface in the Arctic warming hotspot

Abstract

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 the Arctic climate will develop as the planet become warmer. The northern Barents Sea has been monitored annually in joint Norwegian-Russian ecosystem surveys since the early 1970’s, resulting in unique data documenting the changes in the Arctic. The data describes the entire water column’s interannual variability and development through five decades of climate change. In this thesis, the data set is employed to investigate the interaction between the sea ice cover and the three layers of the water column, the Surface-, the Arctic and the Atlantic layers, and how changes relate to other factors such as surface air temperature, surface wind patterns, upstream Atlantic Water temperature and sea ice import from the interior Arctic Ocean. With focus on the vertical water column structure, the objectives are to identify the key factors that control the vertical heat fluxes from the deep Atlantic layer towards the surface, and the key factors that maintain the stratification of the water column. The role of the ocean for the Arctic warming hotspot, the wider Arctic, and the global climate system is further discussed. The main findings of this thesis are that the intermediate, cold and fresh Arctic layer plays a key role in limiting the vertical heat exchange in the water column, and that the interannual variability of the Arctic layer salinity is a controlling factor for the strength of the vertical mixing and thus the upward fluxes of heat and salt from the deep Atlantic layer. The salt flux implies a freshwater input is needed to maintain the stratification. The sea ice import from the interior Arctic domain is the main freshwater input to the area, affecting the Barents Sea ice cover directly through adding ice, and indirectly through adding freshwater that maintains the stratification and makes the conditions for new sea ice growth favourable. A sharp decline in the sea ice inflow to the Barents Sea after ~2005 has resulted in a large, significant and comprehensive shift in the water column structure in the northern Barents Sea. By 2016, the water column had lost about one meter (40 %) of its normal freshwater content and was weakly stratified, much warmer, and with a very limited winter sea ice cover. If the sea ice import does not soon recover, the stratification in the northern Barents Sea will likely break down completely, and make the entire Barents Sea ice free all year round, with unknown consequences for the circulation, the water mass distributions and the ecosystem in the Barents Sea. The exact timing of such a breakdown is hard to predict due to substantial variability in the atmospheric forcing and the sea ice import, but given the rapidly declining Arctic sea ice thickness and extent and reduction in thick, old ice in the Eurasian Basin, sufficiently large sea ice inflows to recover the stratification in the northern Barents Sea become less and less probable. The results of this thesis highlight the importance of the stratified boundary layers in the Arctic air-ice-sea column, limiting vertical exchange of properties. Local positive feedback mechanisms, in addition to less import of fresh water in the form of sea ice, have been found to be essential for the changes that have led to the Arctic warming hotspot. The mechanisms outlined in the thesis will continue to play a key role, when larger areas with cold, stratified and sea ice covered Arctic waters will transform to warmer, weaker stratified, seasonally ice-covered waters. The thesis shows that the present northern Barents Sea is in an unstable state that depends on regular freshwater input, and that stratification and distribution of freshwater are crucial aspects of the new Arctic on a warmer planet.