Abstract
Precipitous Arctic sea-ice decline and the corresponding increase in Arctic open-water areas in summer months give more space for sea-ice growth in the subsequent cold seasons. Compared to the decline of the entire Arctic multiyear sea ice, changes in newly formed sea ice indicate more thermodynamic and dynamic information on Arctic atmosphere–ocean–ice interaction and northern mid–high latitude atmospheric teleconnections. Here, we use a large multimodel ensemble from phase 6 of the Coupled Model Intercomparison Project (CMIP6) to investigate future changes in wintertime newly formed Arctic sea ice. The commonly used model-democracy approach that gives equal weight to each model essentially assumes that all models are independent and equally plausible, which contradicts with the fact that there are large interdependencies in the ensemble and discrepancies in models’ performances in reproducing observations. Therefore, instead of using the arithmetic mean of well-performing models or all available models for projections like in previous studies, we employ a newly developed model weighting scheme that weights all models in the ensemble with consideration of their performance and independence to provide more reliable projections. Model democracy leads to evident bias and large intermodel spread in CMIP6 projections of newly formed Arctic sea ice. However, we show that both the bias and the intermodel spread can be effectively reduced by the weighting scheme. Projections from the weighted models indicate that wintertime newly formed Arctic sea ice is likely to increase dramatically until the middle of this century regardless of the emissions scenario. Thereafter, it may decrease (or remain stable) if the Arctic warming crosses a threshold (or is extensively constrained).
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Data Availability Statement All the data analyzed in this study are openly available. The monthly mean sea-ice concentration is from the Met Office Hadley Centre Sea Ice and Sea Surface Temperature dataset at https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html. Users should click on the link named “HadISST_ice.nc.gz” to download the compressed nc file. The monthly mean Arctic sea-ice extent is from the NSIDC Sea Ice Index, version 3, at https://nsidc.org/data/g02135/versions/3. CMIP6 simulations provided by ESGF can be found via the following open-source link: https://esgf-node.llnl.gov/search/cmip6/. Users should select the variable as siconc and tas, which stand for sea-ice concentration and surface air temperature, respectively. Select the Frequency as mon; select the Table ID as Simon and Amon; select the Experiment ID as historical, ssp126, ssp245, ssp370 and ssp585; select the CMIP6 models employed in this study (see Table 1); and then download the nc files that appear as the search outputs.
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Acknowledgements
This research was supported by the Chinese–Norwegian Collaboration Projects within Climate Systems jointly funded by the National Key Research and Development Program of China (Grant No. 2022YFE0106800) and the Research Council of Norway funded project, MAPARC (Grant No. 328943). We also acknowledge the support from the Research Council of Norway funded project, COMBINED (Grant No. 328935), the National Natural Science Foundation of China (Grant No. 42075030), and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX23_1314).
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Funding Note: Open Access funding provided by University of Bergen (incl Haukeland University Hospital).
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Author contributions S. P. HE and H. J. WANG designed the research. S. P. HE, J. Z. Zhao, K. FAN, and F. LI performed the research. J. Z. ZHAO prepared the manuscript with contributions from all co-authors.
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Article Highlights
• CMIP6 projections of wintertime newly formed sea ice are subject to large bias and uncertainty.
• Both the bias and uncertainty can be effectively constrained by weighting models by their performance and independence.
• Weighted projections indicate that newly formed sea ice will likely increase continuously from the mid-2000s to the mid-21st century.
• Thereafter, newly formed sea ice may decrease (or stabilize) if the Arctic warming crosses a threshold (or is constrained).
This paper is a contribution to the special issue on the Ocean, Sea Ice and Northern Hemisphere Climate: In Remembrance of Professor Yongqi GAO’s key contributions.
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Zhao, J., He, S., Fan, K. et al. Projecting Wintertime Newly Formed Arctic Sea Ice through Weighting CMIP6 Model Performance and Independence. Adv. Atmos. Sci. (2024). https://doi.org/10.1007/s00376-023-2393-2
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DOI: https://doi.org/10.1007/s00376-023-2393-2