21世纪第2个10年初华北夏季降水年代际增加及与大气环流异常的联系

Interdecadal increase of summer precipitation in North China in the early 2010s and its association with atmospheric circulation anomalies

  • 摘要: 华北是中国人口聚集区及主要的农业和工业区,旱涝灾害会造成严重的经济损失和环境影响。基于近61年(1961—2021年)逐月降水量观测数据研究发现,华北地区夏季降水在21世纪第2个10年初发生了明显的年代际变化——在2011/2012年前后经历了从干到湿的变化。选择1999—2011年作为干旱期,2012—2021年作为湿润期,进一步对比分析了华北地区在干旱期和湿润期相关大气环流系统的差异。结果表明,华北地区此次从干到湿的年代际变化与东亚夏季风的变化关系不大,这与20世纪70年代末和90年代末发生的年代际变化有所不同。在2011—2021年湿润期,对流层中低层环流场上在蒙古高原—中国东北地区上空为气旋式环流异常,气旋西侧的偏北风将高纬度的冷空气输送到华北,有利于冷、暖空气在华北地区的交汇;对流层高层则在蒙古高原—贝加尔湖地区上空为气旋式环流异常,有利于东亚西风急流的东伸北抬,由涡度方程诊断可知相对涡度的水平平流项对急流位置偏北东伸有重要贡献;上述环流形势有利于华北地区上升运动的增强。与此同时,湿润期华北地区上空假相当位温升高,且假相当位温垂直变化增强,说明华北上空大气更加暖湿,且大气层结更不稳定,也有利于垂直运动的发展。动力和热力条件异常变化共同导致华北地区在2012—2021年降水较前期降水明显增多。

     

    Abstract: North China is a major agricultural and industrial production area in China with population aggregation, where drought and flood disasters will cause serious economic losses and environmental impact. Based on monthly observation data of summer precipitation in the past 61 years (1961—2021), an interdecadal variation is found in summer precipitation over North China in the early 2010s, that is, it experienced a change from dry to wet around 2011/2012. Differences in the associated atmospheric circulation over North China between the dry and wet periods are further analyzed by selecting 1999—2011/2012—2021 as the dry/wet period respectively. The results show that the change in East Asian summer monsoon bears little to the interdecadal change from dry to wet in North China, which is different from the interdecadal changes in the late 1970s and the late 1990s. During the wet period of 2011—2021, the circulations in the mid-lower troposphere show cyclonic circulation anomalies over Mongolia-Northeast China. The northerly winds on the west side of the cyclone transport cold air from the high latitudes to North China, which is conducive to the intersection of cold and warm air in the area. In the upper troposphere, the cyclone circulation anomalies over the Mongolia-Baikal Lake result in the enhancement of the East Asian westerly jet. The diagnosis of vorticity equation shows that horizontal advection term of relative vorticity significantly contributes to the northward and eastward extension of the jet, which is favorable for the enhancement of upward motion in North China. Meanwhile, the pseudo-equivalent potential temperature increases and its vertical variation enhances, indicating that the atmosphere over North China is warmer and wetter at low level with more unstable atmospheric stratification, which is also favorable for the development of anomaly upward motion. The changes in dynamic and thermal conditions lead to a significant increase in summer precipitation over North China from 2012 to 2021.

     

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