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Two-way interactions between ocean heat transport and Arctic sea ice

Periodic Reporting for period 1 - OSeaIce (Two-way interactions between ocean heat transport and Arctic sea ice)

Berichtszeitraum: 2019-11-01 bis 2020-10-31

The Arctic sea-ice area has decreased by about 2 million km² over the last 40 years, mainly due to the current anthropogenic global warming. Based on current observations and climate model projections, the Arctic Ocean could be ice free in summer by the middle of this century. The fast rate of melting from Arctic sea ice could lead to important adverse impacts on the weather and climate, biosphere and society. Therefore, it is of crucial importance to understand the specific processes behind the Arctic sea-ice loss. One of these processes is the melting of Arctic sea ice via ocean heat transport, both from the Atlantic and Pacific Oceans.

The objectives of our project were:
1) To improve the understanding of the impact of ocean heat transport on Arctic sea-ice melt.
2) To quantify the impact of Arctic sea-ice reduction on the large-scale oceanic circulation and ocean heat transport.
3) To assess the effect of increased model resolution on the interactions between ocean heat transport and Arctic sea ice.
4) To reduce the uncertainties in future sea-ice projections.

Our main conclusions related to these four objectives are:
1) Through sensitivity experiments with the EC-Earth3 global climate model, we showed that the ocean heat transport strongly affects Arctic sea ice. We also showed that not only the Atlantic Ocean but also the Pacific Ocean is a major driver of Arctic sea-ice melt.
2) The Atlantic ocean heat transport slightly increases north of 65°N and decreases south of 65°N following a drastic Arctic sea-ice loss simulated by EC-Earth3.
3) The relative decreases in Arctic sea-ice area and volume as a response to an increase in ocean heat transport do not depend on the model resolution in EC-Earth3.
4) Selecting the Coupled Model Intercomparison Project 6 (CMIP6) models closest to observations and reanalysis in terms of the present Arctic sea-ice area / volume and Atlantic / Pacific ocean heat transport provides a stronger sea-ice reduction through the 21st century compared to the CMIP6 multi-model mean without model selection.
"1. We performed a wide range of sensitivity experiments with EC-Earth3 at standard resolution to test the impact of ocean heat transport on Arctic sea ice. In these sensitivity experiments, we artificially increased the sea-surface temperature with different levels of warming in different regions of the Atlantic and Pacific oceans. This led to an increase in the ocean heat transport, and consequently to a decrease in Arctic sea-ice area and volume. We found that for a same amount of ocean heat transport increase, the reductions in Arctic sea-ice area and volume were stronger when the sea-surface temperature increase was imposed in the North Pacific, compared to the North Atlantic. The peer-reviewed paper associated with this work has been published in Climate Dynamics: ""Docquier et al. (2021). Impact of ocean heat transport on the Arctic sea-ice decline: a model study with EC-Earth3. Climate Dynamics, https://doi.org/10.1007/s00382-020-05540-8"". This work has also been presented at the European Geosciences Union (EGU) General Assembly 2020.

2. We performed different sensitivity experiments with the standard resolution of EC-Earth3 in which we strongly reduced the sea-ice albedo in different ways. All these experiments resulted in a strong reduction of the Arctic sea-ice area and volume, especially in the summer, when a complete disappearance occurred within the very first years. We found that due to the drastic Arctic sea-ice loss, the Atlantic ocean heat transport slightly increased north of 65°N, following the temperature rise of the northward water masses associated with sea-ice loss, and decreased south of 65°N, suggesting an impact of Arctic sea ice on the Atlantic Meridional Overturning Circulation (AMOC) in our experiments.

3. We performed model experiments with EC-Earth3 at high resolution and compared the results to the corresponding experiments performed with the standard resolution of EC-Earth3 (see point 1). We found that the relative decreases in Arctic sea-ice area and volume following the increase in ocean heat transport were relatively similar at high and standard model resolutions.

4. We investigated the present and future Arctic sea-ice area and volume and ocean heat transport in CMIP6 model simulations. We defined a series of selection criteria based on sea-ice area / volume and ocean heat transport, and we used these selection criteria to retain the CMIP6 models closest to observations over the historical period. We found that the reduction in Arctic sea-ice area and volume over the twenty-first century is stronger when applying selection criteria compared to the case without model selection (for both SSP1-2.6 and SSP5-8.5). This work is now being summarized into a peer-reviewed publication to be submitted soon: ""Docquier, D., T. Koenigk (in prep.). Refinement of future Arctic sea-ice projections"". This work has also been presented at the Bolin Days 2020 (organized by the Bolin Centre for Climate Research, Sweden).

5. Finally, a broad range of outreach activities have been performed during the project lifetime, including 2 dissemination articles written for the EGU Cryosphere Blog, a school event in Belgium, a talk with the general public at a Belgian municipality, the participation to Pint of Science Sweden, online lectures given for the students of the Stockholm University, a radio interview with RTS Switzerland, the participation to the Science is Wonderful! online exhibition, and a dissemination article written for the European Dissemination Media Agency (EDMA) Project Repository. More information can be found here: https://sites.google.com/view/daviddocquier/outreach".
The OSeaIce project allowed to carry out in-depth analyses of the interactions between ocean heat transport and Arctic sea ice using global climate models. It allowed to make new discoveries that have been (and will be) published in peer-reviewed publications.

Two main contributions to the current state of the art have been achieved in the framework of the OSeaIce project:
1) We demonstrated that Arctic sea ice is strongly influenced by the ocean region (e.g. North Atlantic, North Pacific, Barents Sea Opening, Bering Strait, ...) in which the warming occurs. In particular, we found that for a same amount of ocean heat transport increase, the Arctic sea-ice loss is greater when the sea-surface temperature is increased in the Pacific Ocean compared to the Atlantic Ocean.
2) We used the CMIP6 model outputs to refine model projections of Arctic sea ice by using a series of selection criteria based on the present Arctic sea-ice area / volume and Atlantic / Pacific ocean heat transport. These results are highly relevant in the quest for improving global climate models as a preparation for CMIP7, especially in terms of Arctic sea ice.

The OSeaIce project fits well within the new European Green Deal, as it brought results that can be used by the climate modeling community, especially in the framework of the future Horizon Europe projects. The work on constraining the future Arctic sea-ice projections provides stakeholders and users with more robust information on the evolution of Arctic sea ice.

The interested users can easily get access to the results of the project, as these have been made available through open-access peer-reviewed publications, open-access scripts and data, and a public website.
Changes in March Arctic sea-ice volume as a function of changes in ocean heat transport
Future evolution of Arctic sea-ice area in March (a) and September (b) using CMIP6 models