强La Nia背景下的东亚夏季风异常与1989年和1999年中国夏季降水的对比分析

East Asian Summer Monsoon Anomalies in Strong La Nia Years and Comparison of Summer Precipitation in China between 1989 and 1999

选取两个强La Nina年（1989年和1999年）,对比分析了强La v背景下的东亚夏季风异常和中国夏季降水分布。结果表明,受南极海冰分布异常的影响,这两年6-7月间南极涛动呈现不同的位相,进而改变了南方涛动的位相。1989年,南极涛动为正时,南方涛动为正,马斯克林高压（简称马高）偏强,澳大利亚高压（简称澳高）偏弱。与1989年相反,1999年南极涛动和南方涛动均为负位相,马高偏弱,澳高偏强,这与一般La Nina年的情况正好相反。与马高和澳高强度变化相对应,西太平洋副热带高压在1989年偏西、1999年偏东,并影响到6-7月间中国降水的分布。8月副高北抬后,南半球环流变化的影响减弱,东亚夏季风环流主要受热带环流和中高纬度环流的影响。1989年8月,受中高纬度冷平流的影响,副高偏弱,长江流域降水偏多。1999年8月,由于热带西风异常偏强,副高偏强,长江以南降水偏多。本文的研究结果表明,即使在两个相似的强La Nina事件影响下,由于其他因子对La Nina信号的调制作用,中国夏季降水仍呈现不同的分布,1989年为中间型,而1999年为南方型,这与一般La Nina年雨带偏北正好相反。最后,对中国夏季降水的季度预测提出了一些建议。

Two strong La Nina years, i.e. 1989 and 1999, are selected to compare the East Asian summer monsoon （EASM） anomalies and summer precipitation distribution in China. It is shown that, during June through July, the Antarctic oscillation （AAO） exhibits different phases due to the anomalous Antarctic sea ice distribution. Accordingly, the phase of the southern oscillation （SO） is also changed. In 1989, the positive phase of the SO corresponds to that of the AAO, the Mascarene high （MH） tends to be intensified while the Australian high （AH） tends to be weakened. In 1999, by contrast, both the AAO and SO exhibit a negative phase, and the MH becomes weaker whereas the AH becomes stronger, this distribution is opposite to that in the usual La Nina years. Corresponding to the anomalies of MH and AH, the western Pacific subtropical high （WPSH） tends to extend westward in 1989 and retreat eastward in 1999, as a result, precipitation in China during June- July exhibits a different distribution. In August when the WPSH extends northward, the influence from the southern circulation on EASM becomes weaker. On the other hand, the tropical circulation and the circulation from mid-high latitudes exert a major impact on the EASM circulation. In August 1989, the WPSH tends to be weaker because of the cold advection from high lati-tudes, resulting in more rainfall along the Yangtze River valley. By comparison, in August 1999, the WPSH tends to be intensified due to a stronger tropical westerly anomaly, and more rainfall is found in the region to the south of the Yangtze River. The result in this study shows that, due to the modulation of some other factors on La Nina signals, summer precipitation in China may exhibit a different pattern even in the background of two similar La Nina events. The rainfall distribution corresponds to the intermediate pattern in 1989 while it corresponds to the southern pattern in 1999, which is opposite to the northern pattern in the usual La Nina years. Finally, some suggestions about the seasonal forecast of summer precipitation in China are proposed.