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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Overturning circulation in the new Arctic

Alternative title: Omveltningssirkulasjonen i det nye Arktis

Awarded: NOK 11.7 mill.

The Atlantic Meridional Overturning Circulation (AMOC) - carrying warm, salty water to high latitudes - is a key component of the global ocean circulation with profound impacts on climate. Dense-water formation at northern high latitudes is a requirement to sustain the AMOC. This production of dense waters has canonically been thought to take place through deep convection in the Labrador and Greenland Seas. However, recent observations and model results suggest that the main source of dense water to the lower limb of the AMOC is Atlantic Water (AW) that gradually transforms to colder deep water on its path through the Nordic Seas and Arctic Ocean. Whereas the AW transformation in the Nordic Seas is well studied, little is known about the formation of dense water masses and corresponding overturning circulation in the Arctic Ocean. The Arctic climate is currently transitioning to a warmer, seasonally ice-free climate state, and this could have important implications for the Arctic overturning circulation. The recent sea-ice retreat along the margins of the Arctic Ocean has partially uncovered the boundary currents and left them exposed to the atmosphere in winter, allowing further modification of the AW along its pathways. As the transition towards a seasonally ice-free Arctic Ocean continues, enhanced convection can be expected, leading to a possible strengthening of the Arctic overturning circulation. The Arctic overturning circulation could therefore be a stabilizing factor in a weakening AMOC, highlighting the importance of understanding its drivers and its response to climate change. This project (ArMOC) will provide new, fundamental knowledge about key oceanic processes for the Arctic overturning circulation and their sensitivity to climate change. ArMOC will accordingly lead to reduced uncertainties in projections of Arctic climate change and an improved understanding of the role of the Arctic Ocean in global ocean circulation changes.

The Arctic overturning circulation involves the production of water masses that are key to the global ocean circulation associated with the Atlantic Meridional Overturning Circulation (AMOC). Arctic climate is, however, rapidly changing, and it is currently not known how the Arctic overturning circulation is responding. Dense-water formation that is a requirement to sustain the AMOC has canonically been thought to take place through deep convection in the Labrador and Greenland Seas. However, recent observations and model results suggest that the main source of dense water to the lower limb of the AMOC is Atlantic Water (AW) that gradually transforms to denser waters on its path along the Atlantic Water Boundary Current (AWBC) in the Nordic Seas and Arctic Ocean. The recent sea-ice retreat along the margins of the Arctic Ocean has partially uncovered the AWBC, exposing it to the atmosphere in winter and, hence, allowing further modification of the AW. As the transition towards a seasonally ice-free Arctic continues, more years of enhanced convection can be expected, leading to a possible strengthening of the Arctic overturning circulation. The Arctic overturning circulation could therefore be a stabilizing factor in a weakening AMOC, highlighting the importance of understanding its drivers and its response to climate change. ArMOC will accordingly provide: - A step-change in our understanding of the Arctic overturning circulation in present and future climates by providing a first quantification using the approach of thermohaline streamfunctions. - A comprehensive understanding of the entire AWBC system around the Nordic Seas and the Arctic Ocean from observations, including new, unique wintertime data from Argo floats, and high-resolution models. - Novel insights on AW pathways and transformation within the Arctic Ocean, and whether these are expected to change under future global warming, through Lagrangian analysis with high- and low-resolution models.

Funding scheme:

FRINATEK-Fri prosj.st. mat.,naturv.,tek