COHERENT LEADING MODES OF PRECIPITATION VARIATION IN LATE WINTER AND EARLY SPRING OVER SOUTH CHINA AND THEIR MECHANISMS
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摘要: 基于1951—2018年再分析资料和观测的降水量资料，采用联合经验正交函数分解的方法，分析了华南冬末春初(2、3月)降水年际变异特征，并讨论了相应的环流背景及物理机制。华南2、3月降水变异的第一关联主模态反映出全区2、3月同相变化，第二模态呈现反相变化。第一模态的降水异常与ENSO关联的热带海温异常分布有关，其导致的西太平洋异常反气旋的维持使得2、3月的降水持续出现同相异常。第二模态的降水异常与中高纬度的大气环流异常有关：2月表现为欧亚遥相关型，3月则表现为北极涛动型。第二模态2、3月位势高度异常型的转变分别与北大西洋的热通量的异常变化及平流层极涡信号的下传有关：当2月北大西洋热通量正异常显著时，500 hPa高度场呈现欧亚遥相关(EU)负位相的分布；平流层极涡异常信号在3月下传达到对流层低层，使得3月对流层极涡增强，有利于北极涛动(AO)正位相的形成。2、3月欧亚大陆上空分别在EU遥相关型和AO型环流异常的影响下，导致了华南地区上空的大气环流的辐合辐散异常，并最终造成2、3月华南的降水量反相异常的出现。Abstract: Based on the reanalysis data and observed precipitation data from 1951 to 2018, the interannual variation of precipitation in late winter and early spring (February to March) over South China was explored by using the joint empirical orthogonal function decomposition, and the corresponding circulation background and physical mechanisms were also discussed. The first leading mode of precipitation variation over South China exhibits the in-phase change patterns in February and March, while the second leading mode presents the out-phase change patterns. The first leading mode of precipitation is attributed to the ENSO-related sea surface temperature anomalies in the tropical Indian-Pacific oceans, and the maintenance of the abnormal anticyclone in the northwestern Pacific caused by ENSO leads to the in-phase anomalies of precipitation continuously in February and March over South China. The second mode is attributed to the atmospheric circulation abnormality at mid-high latitudes, which is presented as the Eurasian teleconnection (EU) pattern in February and the Arctic Oscillation (AO) pattern in March. The geopotential anomalies over Eurasian continents are related to the abnormal heat flux in the North Atlantic in February and the downward propagation of stratospheric polar vortex signal in March, respectively. When the significant positive anomalies of surface heat flux appear in the North Atlantic Ocean in February, the 500- hPa geopotential height presents a negative phase EU pattern. The abnormal signal of stratospheric polar vortex propagates downward to the lowest level of the troposphere in March and strengthens the tropospheric polar vortex, and therefore is beneficial to the formation of positive phase AO pattern. Under the influence of EU teleconnection and AO-related circulation anomalies over Eurasian continents in February and March, respectively, the convergence and divergence anomalies of the atmospheric circulation are forced over South China, and finally give rise to the out-phase precipitation anomalies over South China in February and March.
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图 1 1951—2018年华南地区2、3月年际降水联合EOF(EEOF)前两个模态的空间分布
a. EEOF1，2月；b. EEOF1，3月；c. EEOF2，2月；d. EEOF2，3月；e. PC1；f. PC2。a~d的单位：mm。
图 2 标准化PC1与2、3月各物理量场的回归系数
a. 2月850 hPa风场(矢量，单位：m/s)及500 hPa垂直速度(填色，单位：Pa/s)；c. 2月200 hPa风场(矢量，单位：m/s)及散度(填色，单位：10-6 s-1)；b、d同a、c，但为3月(打点区通过95%信度检验)。
图 4 标准化PC1与2、3月各物理量场的回归系数
a. 2月200 hPa散度风(矢量)、速度势(等值线，单位：105 m2/s)及散度(填色，单位：10-6 s-1)；c. 2月850 hPa散度风(矢量)、速度势(等值线，单位：105 m2/s)及500 hPa垂直速度(填色，5×10-2 Pa/s)；b、d分别同a、c，但为3月(打点区表示通过0.05显著性检验)。
图 5 标准化PC2与2、3月各物理量场的回归系数
a. 2月850 hPa风场(矢量，单位：m/s)及500 hPa垂直速度(填色，单位：Pa/s)；c. 2月200 hPa风场(矢量，单位：m/s)及散度(填色单位：10-6 s-1)；b、d分别同a、c，但为3月(打点区表示通过0.05显著性检验)。
图 8 标准化PC2与2月(a、c、e)和3月(b、d、f)北大西洋海温(a~b，单位：K)、感热通量(c~d，单位：W/m2)、潜热通量(e~f，单位：W/m2)的回归系数
打点区表示通过0.05显著性检验。
图 9 标准化PC2与极区70~90 °N平均位势高度(a，单位：gpm)、60~80 °N平均纬向风(b，单位：m/s)、70~90 °N平均气温(c，单位：K)的回归系数
打点区表示通过0.05显著性检验。
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