华北夏季降水年代际变化与东亚夏季风、大气环流异常

利用华北夏季降水资料和NCEP/NCAR再分析资料,对华北夏季降水、东亚夏季风年代际变化特征及大气环流异常进行研究,发现一些有意义的结果:华北夏季降水变化存在明显的8a、18a周期,东亚夏季风变化18a、28a周期性比较明显,二者年代际变化特征明显,但华北夏季降水变化和东亚夏季风变化的周期不完全一致.华北夏季降水量变化在60年代中期发生了突变,东亚夏季风变化在70年代中期发生了突变.华北夏季降水与东亚夏季风变化存在很好的相关关系,强夏季风年,华北夏季降水一般偏多,弱夏季风年,华北夏季降水一般偏少,但又不完全一致.东亚夏季风减弱是造成华北夏季降水减少的一个重要因素,但不是唯一因素,华北夏季降水减少还与环流异常密切相关.在地面上,青臧高原地区、华北地区气温下降造成华北低压系统活动减少,不利于降水.在850 hPa层上,东亚中纬度的西南季风和副热带高压南部的偏东风、西北部的西南风异常减弱,使得西南气流输送水汽很多难以到达30°N以北的地区,而副热带高压西部外围偏东南、偏南气流输送到华北地区的水汽也大量减少,水汽不足造成华北夏季降水偏少.在500 hPa高度场上,80年代欧亚遥相关型表现与50年代相反,变为欧洲( )、乌拉尔山(-)、中亚( )形势,这种环流使得乌拉尔山高压脊减弱,贝加尔湖至青藏高原高空槽变浅,纬向环流表现突出,不利于冷暖空气南北交换.同时在500 hPa气温场上,80年代,西伯利亚至青藏高原西北部的冷槽明显东移南压到蒙古至华北地区,锋区位于华北以东以南位置,使得华北地区冷暖空气交汇减少,降水也因此减少.华北夏季降水减少是由于东亚夏季风减弱和大气环流异常造成的.

Interdecadal Variability of Summer Precipitation over North China and East Asian Summer Monsoon and Atmospheric Circulations Anomaly

Based on NCEP/NCAR data and summer rainfall data collected from 20 stations over north China,by using trend analysis,wavelet transform,Mann-Kendall test and contrast analysis,we investigated the interdecadal variability of summer precipitation and the cause of summer rainfall reduction over north China.The summer rainfall varied with period of eight years and eighteen years over north China,and an abrupt change occurred in the middle 1960s.The East Asian summer monsoon had periods of eighteen and twenty-eight years.Their interdecadal variability is obvious,but the change periods are different in summer rainfall and in summer monsoon.The summer rainfall has obviously linear decreasing tendency in recent 55 years,15.5mm in a decade during the past 55 years.The summer monsoon has also obviously linear weakening tendency.There is a close relation between the East Asian summer monsoon and summer rainfall over north China.If the East Asian summer monsoon is strong,the summer rainfall also is above normal.Or else,if the East Asian summer monsoon is weak,the summer rainfall will be below normal,but both are not all coincident.The surface air temperature in the region of Tibetan Plateau and north China were lower in the 1980s than in the 1950s,causing Mongolia low pressure to weaken and activity of low pressure to become less.The water vapor to transport to north China by the East Asian summer monsoon was less in the 1980s than that in the 1950s.The front was located more southward and more eastward in the 1980s than in the 1950s,and exchange of cold and warm air was less frequent in the 1980s than in the 1950s.The reduction of summer rainfall over north China was possibly due to weakening of East Asian summer monsoon and atmospheric circulation anomaly.