中国西南地区春季降水的时空变化及其异常的环流特征

利用中国气象局国家气象信息中心整编的中国西南地区97站逐日观测资料及美国国家环境预报中心/大气研究中心(NCEP/NCAR)再分析资料,对中国西南地区1961—2013年春季降水的时空分布特征进行了分析,并研究了造成西南地区春季降水异常的环流成因。结果表明,中国西南地区春季降水的主模态呈现为全区一致型,该模态具有明显的2.5—3.5a及准5a的活动周期。对流层中低层副热带地区的异常气旋式环流波列形成的异常气流将洋面上空的水汽向中国西南内陆地区输送,暖湿气流与异常活跃的北方冷空气活动相配合,加上西南地区大气低层辐合、高层辐散而产生抽吸作用,使得当地对流层中低层出现较强的沿地形抬升的上升气流,从而有利于西南地区降水的形成,反之亦然。造成环流异常的原因除了与西南地区、热带地区的异常辐合/辐散运动造成的位涡扰动能量有关,亦与中高纬度波扰动能量下欧亚大陆下游地区的频散及辐合有关。此外,冬季1月的青藏高原地面加热场特征可作为预测后期西南地区春季降水异常变化的一个前期信号。

Variations of spring precipitation over southwest China and characteristic circulations for precipitation anomalies

Based on station precipitation data from China National Meteorological Center (NMC) and monthly mean reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), the spatial-temporal variations of spring precipitation over Southwest China and characteristic circulations associated with precipitation anomalies during 1951-2013 have been investigated. Results show that the most significant mode of spring precipitation is consistent over Southwest China with periodic cycles of 2.5-3.5 a and quasi-5 a. Atmospheric water vapor is transported from oceans to inland areas of Southwest China by anomalous cyclonic circulation wave trains in the middle and lower troposphere of the subtropical region. The warm moist airmass intersect with cold airmass from the north in Southwest China. Meanwhile, the pumping effect caused by low level convergence and upper level divergence results in strong ascending motion in the complex terrain area. The above circulation features are favorable for rainfall over Southwest China, and vice versa. Circulation anomalyies in Southwest China are partly attributed to potential vorticity energy perturbations caused by anomalous convergence and divergence in Southwest China and the tropics, and partly attributed to dissipation and convergence of wave turbulence from the middle and high latitude to downstream region in Eurasia. In addition, surface heating in January on the Tibetan Plateau would be an early signal for the prediction of spring precipitation anomaly over Southwest China.