Climate response to a Bluer Arctic with increased newly-formed winter Sea ICe (BASIC)

For years, scientists have debated whether – and how – Arctic warming and sea-ice loss influence weather and climate further south. The BASIC project is helping to resolve this debate by studying the “new Arctic”, a region increasingly defined by more open water in summer (a 'bluer' Arctic) and by greater amounts of newly formed ice each winter. While the summer decline of sea ice is well known, the increase of thin, freshly formed ice in winter has only recently been recognized – and it may have far-reaching consequences. Why does this matter? Changes in sea ice affect the salinity of the ocean and, in turn, the strength of the Atlantic Meridional Overturning Circulation (AMOC) – a major current system that carries warm water northward, shaping climate in Europe and beyond. Historically, sea-ice melt has freshened the North Atlantic and contributed to AMOC weakening. But as thick multi-year ice disappears, the seasonal cycle is shifting. New winter ice forms in larger areas and melts again in summer, creating a different kind of freshwater input. BASIC has documented this increasing winter ice-growth and is studying how it could alter ocean circulation. Another focus is the “threshold” in Arctic change. As the Arctic marches toward ice-free summers, its influence on climate will not vanish smoothly. Model experiments suggest that once ice cover falls below a critical level, the role of sea ice fades, and the ocean’s heat uptake becomes the dominant driver. BASIC has designed and run experiments to pinpoint this threshold. Recent results show that beyond this point, Arctic Ocean temperatures – not the ice itself – will shape future atmospheric circulation and extreme events. A further challenge has been the so-called “deep Arctic warming”. Observations show that Arctic warming extends downward into the ocean interior and upward into the mid-troposphere (about 5 km high). Yet many climate models have struggled to capture this vertical depth of warming, leading to conflicting conclusions about its impacts. BASIC has tackled this by applying new methodologies in the Norwegian Earth System Model (NorESM2). The results confirm that realistic, vertically deep Arctic warming produces stronger and more consistent linkages to Eurasian winter climate – a key step in resolving the “warm Arctic, cold Eurasia” debate. BASIC works closely with its sister project MAPARC, which emphasizes seasonal prediction and atmospheric processes. The two projects provide a more complete picture: BASIC reveals the long-term ocean–ice mechanisms, while MAPARC develops tools to predict year-to-year variability. A shared PhD position and international collaborations – particularly with partners in China – ensure strong integration and knowledge exchange across the projects. In short, BASIC has moved the field forward by showing that: - newly formed winter ice is a critical and overlooked piece of the climate system, - a threshold exists where the ocean overtakes ice as the main driver, and - the vertical extent of Arctic warming must be represented correctly to understand its global impacts. As the Arctic continues to transform, these insights are vital not only for climate science but also for predicting how future changes may ripple far beyond the polar regions.

The rapid decline of Arctic sea ice has led to a ‘bluer’ Arctic. While at the same time, the extent and the volume of newly-formed sea ice in winter are increasing; the climate science community has largely overlooked the latter unforeseen change, but it has potentially profound and lasting impacts on the Arctic and the Eurasian climate. We anticipate that: (1) ‘Bluer’ Arctic is changing the respective roles of Arctic ocean temperature and sea ice in impacting the climate. (2) The increase of newly-formed wintertime sea ice year-after-year will promote the slowdown of the Atlantic Meridional Overturning Circulation (AMOC). (3) Vertical extent of Arctic warming is critical in resolving the debates on mechanisms driving the "warmer Arctic, colder Eurasia" winter climate pattern. In BASIC we will identify the mechanisms on the vertical propagation of Arctic ocean warming and assess the Eurasian winter climate response to deep Arctic warming (from ocean interior upwards to mid-troposphere) using coupled simulations. We will further demonstrate that there is increased newly-formed Arctic sea ice in winter in the context of Arctic warming. We will then quantify the freshwater fluxes anomalies due to the increased newly-formed sea ice and assess their impacts on the Arctic ocean stratification and the AMOC. BASIC's results will therefore directly address (i) the grand challenges of the WCRP: ‘Melting Ice and Global Consequences’ and (ii) the United Nations (UN) Sustainable Development Goal 13 and Goal 14. Through running large ensemble simulations, BASIC will identify (1) a 'threshold' of future Arctic sea ice state where there will be a role switching between the Arctic ocean temperature and sea ice in impacting the climate, and (2) the dominant effects of Arctic open seawaters on the climate variability due to their increasing capability to absorb and store energy.