南半球中纬度偶极模态与亚洲—非洲夏季降水

南半球大气环流第一模态为南极涛动, 它表现出较好的纬向对称结构。本工作利用经验正交分解方法研究发现, 在南半球的东半球, 夏季(6~9月)大气环流还存在一个重要的模态。不同于南极涛动, 该模态表现为显著的纬向偶极分布, 本文定义其为偶极模态。在海平面气压场上, 该模态的解释方差可以达到20%以上, 表现出显著的年际变化特征。进一步的研究发现, 该偶极模态与亚非夏季降水存在密切联系, 尤其与我国华南、南亚以及热带非洲东部地区的夏季降水存在显著的正相关关系。机制分析表明, 南半球这一偶极模态的异常会影响东半球越赤道气流的变化, 从而造成向上述三个地区的水汽输送的多寡, 并最终导致这三个地区夏季降水发生变化。本工作揭示的偶极模态变化独立于南极涛动, 研究结果不但可以深化对南半球环流系统变化特征及影响的认识, 而且对亚非夏季降水的变化特征和机制研究也具有重要意义。

Dipole Pattern in Southern Hemispheric Mid-latitude and Asian-African Summer Precipitation

As the first middle and high latitude circulation mode in the Southern Hemisphere, the Antarctic Oscillation (AAO) has a zonally symmetric structure.Using the empirical orthogonal function method, this study reveals existence of another dominant mode of atmospheric circulation in the eastern Southern Hemisphere boreal summer (June-September).Different from the AAO, this mode shows a significant zonal dipole distribution, or dipole pattern.This dipole pattern accounts for more than 20% of the total variance in the regional sea-level pressure field, and shows a strong interannual variation.Further research has demonstrated that the dipole pattern is significantly positively correlated with summer rainfall in Asia and Africa, especially in South China, South Asia, and tropical eastern Africa.Analysis suggests this pattern anomaly can influence the variation of cross-equatorial flow in the Eastern Hemisphere, which can affect the transportation of water vapor into South China, South Asia, and tropical eastern Africa, and ultimately result in summer precipitation anomalies over the three regions.The dipole pattern revealed in this study is independent from the AAO, and the results of this study are not only helpful for better understanding the variation and influence of the atmospheric circulation in the Southern Hemisphere but also for examining summer precipitation variations and mechanisms in Africa and Asia.