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).
摘要
全球变暖背景下,北极海冰消融的同时,北极开阔海洋面积增加。这种变化使得冬季北极海冰的生长空间增大。与多年冰的衰退相比,北极冬季新生冰的变化更能反映与北极海-冰-气相互作用以及北半球中高纬大气遥相关有关的动力和热力学信息。随着北极持续增暖,有必要研究北极冬季新生冰的变化特征。因此,本文使用第六次国际耦合模式比较计划(CMIP6)的多模式数据,探究当前与未来北极冬季新生冰的变化特征。多模式等权重分配方案是比较常用的多模式集合平均方案;该方法基本假设所有模式是相互独立的,并且是同等可靠的。然而,这一假设与许多模式相互之间高度依赖以及模式间性能存在较大差异的事实并不相符。因此,与采用传统的多模式等权重分配方案的研究不同,本文使用了一套学术界最新发展的非等权重分配方案。该新方案能够同时兼顾模式间的模拟性能差异以及模式间的独立性,通过对各个模式进行非等权重分配以达到改进模式预估结果的目的。结果表明:传统的多模式等权重分配方案导致CMIP6模式集合平均对北极冬季新生冰的历史模拟存在较大偏差,这极大地影响了北极冬季新生冰未来预估的可信度。多模式非等权重分配方案能够较好地约束上述历史模拟偏差,并有效地减小了北极冬季新生冰的未来预估的不确定性,显著改进CMIP6模式对新生冰的未来预估结果。非等权重方案的预估结果表明:在所有排放情景下,北极冬季新生冰将会在本世纪中期之前持续增多;此后至本世纪末,在SSP1-2.6和SSP2-4.5排放情景下,新生冰将会保持稳定不变,在SSP3-7.0和SSP5-8.5排放情景下,新生冰将会持续减少。
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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 topic on 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. 41, 1465–1482 (2024). https://doi.org/10.1007/s00376-023-2393-2
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DOI: https://doi.org/10.1007/s00376-023-2393-2
Key words
- wintertime newly formed Arctic sea ice
- model democracy
- model weighting scheme
- model performance
- model independence