Autumn snow cover variability over northern Eurasia and roles of atmospheric circulation
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This study analyzes the variability of northern Eurasian snow cover (SC) in autumn and the impacts of atmospheric circulation changes. The region of large SC variability displays a southward shift from September to November, following the seasonal progression of the transition zones of surface air temperature (SAT). The dominant pattern of SC variability in September and October features a zonal distribution, and that in November displays an obvious west–east contrast. Surface air cooling and snowfall increase are two factors for larger SC. The relative contribution of SAT and snowfall changes to SC, however, varies with the region and depends upon the season. The downward longwave radiation and atmospheric heat advection play important roles in SAT changes. Anomalous convergence of water vapor flux contributes to enhanced snowfall. The changes in downward longwave radiation are associated with those in atmospheric water content and column thickness. Changes in snowfall and the transport of atmospheric moisture determine the atmospheric moisture content in September and October, and the snowfall appears to be a main factor for atmospheric moisture change in November. These results indicate that atmospheric circulation changes play an important role in snow variability over northern Eurasia in autumn. Overall, the coupling between autumn Eurasian snow and atmospheric circulation may not be driven by external forcing.
Key wordsAutumn Eurasian snow cover surface air temperature surface heat fluxes atmospheric circulation water vapor transport
欧亚中高纬度雪盖的变化能影响印度夏季风和东亚夏季气候变异. 秋季欧亚雪盖对冬季北极涛动这一北半球重要气候系统的变化可能是一个贡献因子. 因此,研究秋季欧亚地区雪盖的变化及其因子对理解随后季节欧亚的气候变异会有重要帮助. 香港中文大学的叶坤辉博士和大气物理研究所的吴仁广教授近年来致力于研究秋季欧亚地区雪盖变异,以提高对欧亚积雪变化的因子和其对冬季北极涛动影响的理解. 他们的研究表明,降雪和地面气温对欧亚积雪的变化都有贡献. 其影响的途径和重要性在不同地区是不一样的. 进一步分析揭示伴随的表面热通量和大气环流变化. 大气中的水汽输送和其辐合辐散对于降雪和长波辐射的变化有重要作用. 向下的异常长波辐射和异常风温度平流共同引起表面温度的变化. 他们的研究结果说明大气环流和与之相联系的地面状况变化是秋季欧亚积雪变化的基本因子. 这使得秋季欧亚积雪异常和其气候影响的预报变得复杂.
关键词秋季欧亚雪盖 地面气温 表面热通量 大气环流 水汽输送
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RW is supported by the National Key Basic Research Program of China (Grant No. 2014CB953902) and the National Natural Science Foundation of China (Grant Nos. 41530425, 41275081 and 41475081). We would like to thank the NSIDC, the Met Office Hadley Center, the CRU (University of East Anglia, UK), and the Japan Meteorological Agency, for providing data. We are grateful to Prof. Gabriel LAU for his comments on an earlier version of the manuscript.
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