Interdecadal increase of summer precipitation in North China in the early 2010s and its association with atmospheric circulation anomalies
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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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Key words:
- North China /
- Summer precipitation /
- Interdecadal variability /
- East Asian westerly jet
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图 1 1961—2021年夏季华北降水量时间序列 （a，绿色实线表示1999—2011年和2012—2021年两个时段的平均降水量，黑色实线表示1961—2021年夏季平均降水量）及其滑动t检验 （b，黑色实线表示95%置信区间）
Figure 1. Time series of summer rainfall over North China from 1980 to 2021 （a，green lines indicate mean summer rainfall during 1999—2011 and 2012—2021，respectively） and its moving t-test （b，black dashed lines indicate the 95% confidence interval）
图 2 湿润期与干旱期的夏季降水量差值 （矩形区域表示华北，黑点区表示超过95%置信度）
Figure 2. Difference in summer rainfall between 1999—2011 and 2012—2021 （black dot indicates differences passing significance test at the 95% confidence interval；black box indicates North China）
图 3 （a） 湿润期与干旱期的夏季850 hPa风 （风矢，单位：m/s） 差值及其散度 （色阶，红色箭头表示风差异超过95%置信度，绿色和紫色等值线分别表示湿润期和干旱期850 hPa西太平洋副热带高压特征线，矩形区域表示华北）；（b） 同 （a），但为250 hPa
Figure 3. （a） Difference in summer 850 hPa wind （vectors，unit：m/s） and its divergence （shaded，red vectors indicate differences passing significance test at the 95% confidence interval；green and purple contours represent the western Pacific subtropical high at 850 hPa during the wet period and the dry period，respectively；black box indicates North China）；（b） as in （a），but for 250 hPa
图 4 （a） 湿润期与干旱期沿38°N的纬向辐散风差和垂直速度差 （色阶） 的纬向环流 （矢量箭头）（垂直速度的单位为10−1 Pa/s，纬向辐散风和经向辐散风的单位为m/s，矢量箭头是由水平辐散风与垂直速度合成，斜线表示垂直速度差异超过95%置信度，蓝色粗实线表示华北地区的位置，黑色区域表示地形）；（b） 同 （a），但为沿118°E的经向辐散风差和垂直速度差的经向环流
Figure 4. （a） Longitude-height cross section （vectors） of differences in zonal divergent wind and vertical velocity （shaded） along 38°N （the unit of vertical velocity is 10−1 Pa/s，and for zonal divergent wind and meridional divergent wind is m/s，vectors are composed of horizontal divergent wind and vertical velocity，hatching indicates differences passing significance test at the 95% confidence interval；blue thick solid line indicates the location of North China；black shading indicates topography）；（b） latitude-height cross section of differences in meridional divergent wind and vertical velocity along 118°E
图 5 （a） 夏季整层积分水汽通量气候态 （单位：kg/（m·s）），（b） 湿润期与干旱期夏季的整层积分水汽通量差值 （红色和紫色箭头分别表示差异超过95%和90%置信度，矩形区域表示华北），（c） 同 （b），但为800—700 hPa积分水汽通量
Figure 5. （a） Climatology of summer vertically integrated water vapor flux （unit：kg/（m·s））；（b） difference in summer vertically integrated water vapor flux （red and purple vectors indicate differences passing significance test at the 95% and 90% confidence intervals，respectively；blue box indicates North China）；（c） same as （b），but for vertically integrated water vapor flux at 800—700 hPa
图 6 湿润期与干旱期夏季的600 （a）、650 （b）、700 （c）、750 （d）、800 （e） 和850 （f） hPa假相当位温 （色阶） 及其垂直变化 （等值线，单位：10−2 K/hPa） 差值 （打点和叉号分别表示
$ {\theta }_{\mathrm{s}\mathrm{e}} $ 和$ \mathrm{d}\theta_{\mathrm{s}\mathrm{e}}/\mathrm{d}p $ 差异超过95%置信度）Figure 6. Differences in summer Pseudo-equivalent potential temperature （shaded） and its vertical variation （contours，unit：10−2 K/hPa） at 600 （a），650 （b），700 （c），750 （d），800 （e），and 850 （f） hPa （dots and cross marks represent
$ {\theta }_{\mathrm{s}\mathrm{e}} $ and$ \mathrm{d}\theta_{\mathrm{se}}/\mathrm{d}p\, $ differences passing significance test at the 95% confidence interval，respectively）
图 7 （a） 标准化东亚夏季风指数 （EASMI） 时间序列 （黑实线表示干旱期平均和湿润期平均）；（b） 华北夏季降水与东亚夏季风指数的21 a滑动相关 （黑实线表示相关系数达到95%置信度）
Figure 7. （a）Standardized time series of the East Asian summer monsoon index （black solid lines indicate the mean values of the East Asian summer monsoon index during 1999—2011 and 2012—2021，respectively）；（b）21-year sliding correltion coefficient between the regionally averaged summer precipitation over North China and the East Asian summer monsoon index （black solid line indicates correlation coefficient passing significance test at the 95% confidence interval）
图 8 （a）干旱期夏季平均200 hPa纬向风距平 （等值线，单位：m/s，深浅灰阶分别表示纬向风速≥30 m/s和≥25 m/s，粗线表示急流轴，矩形区域表示华北）；（b） 同 （a），但为湿润期
Figure 8. （a）Averaged summer zonal wind anomalies at 200 hPa during 1999—2011 （contours，unit：m/s ，deep and shallow shadings indicate that the zonal wind speed is greater than or equal to 30 m/s and greater than or equal to 25 m/s；thick line indicates the jet stream axisblack box indicates North China）；（b） as in （a），but for 2012—2021
图 9 湿润期与干旱期夏季250 hPa涡度方程各项 （a. ζhadv，b. ζvadv，c. ζdiv，d. ζtilt，e. fpadv，单位：10−11 s−2） 和相对涡度 （f，单位：10−6 s−1） 差值 （黑色粗实线表示干旱期急流轴，打点表示差异超过95%置信度，f中流线表示旋转风场的差值）
Figure 9. Difference of 250 hPa vorticity equation （a. ζhadv，b. ζvadv，c. ζdiv，d. ζtilt，e. fpadv，unit：10−11 s−2） and relative vorticity （f，unit: 10−6 s−1） between the summers of 2012—2021 and the summers of 1999—2011 （black thick solid lines represent the position of the jet axis during 1999—2011，dots represent differences passing significance test at the 95% confidence levels，the streamlines in f represent the difference of rotating wind）
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