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冬季北半球对流层极涡天气型的长期变化特征及其成因

Long-Term Variability and Causes of the Tropospheric Polar Vortex over the Northern Hemisphere in Winter

  • 摘要: 本文首先基于1979~2019年ERA5逐日再分析数据集,采用自组织映射(SOM)神经网络方法对冬季北半球对流层极涡进行客观分型,分析了极涡天气型的时间变化特征,揭示了冬季北半球极涡天气型长期变化的成因。结果表明:(1)根据极涡中心位置,极涡可分为绕极型、偶极型、偏欧亚型和偏北美型,其中绕极型和偶极型为主导环流型。绕极型和偶极型的出现频次分别呈现显著减少和增多的趋势,并具有明显的年际和年代际变化。(2)绕极型的长期减少和偶极型的长期增加主要是由于北极地区快速升温导致北半球中高纬度区域间的经向温度梯度不断减小,大气斜压性减弱,进而引起绕极西风环流减弱,使得北半球极涡的强度减弱,极涡极易分裂。随后基于第六次国际耦合模式比较计划(CMIP6)中一个海气耦合模式(CESM2)的工业革命前参照试验(piControl)和CO2浓度每年增加1%的强迫试验(1pctCO2)数据集,采用上述同样的方法进一步探讨了极涡绕极型和偶极型的长期变化与全球增暖的关系,发现无论在piControl还是在1pctCO2试验中绕极型和偶极型极涡仍为主导环流型,但在piControl试验中绕极型和偶极型的出现频次无显著变化趋势,而在1pctCO2试验中绕极型和偶极型则分别表现为显著减少和增多的趋势,进一步验证了观测中这两类极涡天气型的长期变化与全球增暖密切有关。同时本文也使用了CMIP6中其他11种模式做了对比分析,发现结果可靠。

     

    Abstract: This study uses ERA5 daily reanalysis data from 1979 to 2019 to objectively classify the tropospheric polar vortex over the Northern Hemisphere during winter through SOM (self-organizing mapping) analysis. Further, the temporal variation characteristics of the tropospheric-polar-vortex weather are analyzed, and the causes of its long-term temporal variations are revealed. Results reveal the following: (1) Depending on the center position, the polar vortex can be classified into four types, namely circumpolar, dipole, Eurasian, and North American, with the circumpolar and dipole types being the predominant circulation types. The circumpolar type shows a significant long-term decrease, while the dipole type exhibits a notable increase, with both demonstrating obvious interannual and interdecadal variability. (2) The long-term decrease in the circumpolar type and the long-term increase in the dipole type are mainly attributed to the rapid warming of the Arctic region, which continuously reduces the meridional temperature gradient and weakens baroclinicity between the middle and high latitudes of the Northern Hemisphere. This results in a weakened circumpolar westerly circulation, making the tropospheric polar vortex in the Northern Hemisphere more prone to splitting. Using data from the pre-industrial control simulation (piControl) and a simulation with a 1% a−1 CO2 increase (1pctCO2), which leverages the ocean–atmosphere coupled model (CESM2) for CMIP6 (the coupled model intercomparison project phase 6), SOM explores the relationship between the long-term temporal variability of circumpolar and dipole types and global warming. The analysis reveals that in both piControl and 1pctCO2, the circumpolar and dipole types remain the dominant circulation patterns. While there is no significant variability trend for these types in piControl, 1pctCO2 shows a significant decrease in the circumpolar type and an increase in the dipole type. This indicates a strong link between these long-term temporal variability trends and global warming. Concurrently, a comparative analysis using eleven other CMIP6 models confirms the reliability of the obtained results.

     

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