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

The scientific community still has no consensus on if and how Arctic warming and sea ice loss can influence weather and climate in the Northern Hemisphere. The BASIC project sets out to better understand the climate response to Arctic change, especially focusing on the new Arctic characterized by more open water in summer (hence bluer) and increased newly-formed sea ice in winter. The latter change has been mostly overlooked, but it has potentially profound climate impacts. Sea ice change can affect the Atlantic Meridional Overturning Circulation (AMOC) through modulating ocean salinity: AMOC is a large ocean current driven by the sinking of denser water in the northern North Atlantic. It carries tropic warm water into the North Atlantic and thus along the Norwegian coast, but has been weakened by the increase of freshwater due to long-term sea ice melting. As multi-year ice is decreasing rapidly, the recent and future increasing newly-formed ice may change such impacts. A bluer Arctic may change the respective roles of Arctic Ocean temperature and sea ice in impacting climate. Model experiments have shown that the climate responses to an ice-free state are appreciably distinct from an ice-covered state. We expect that, before the Arctic reaches an ice-free state, Arctic sea ice may shrink stepwise and go through a threshold where ocean temperature takes over to impact climate. Identifying this threshold is important for climate prediction. Bluer Arctic with increased newly-formed winter sea ice is concurrent with an Arctic warming extending downwards into ocean interior and upwards to mid-troposphere (~5 km). But the climate models have divergent abilities to simulate the observed deep Arctic warming, which caused debates in this field. BASIC will develop a new methodology to conquer this problem. The BASIC project will analyze available observed and simulated datasets and run new experiments with the Norwegian Earth System Model to address the above issues.

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.