夏季孟加拉湾低频振荡特征及其对中国南方降水的影响

Atmospheric low-frequency oscillations have two major cycles, namely the quasi-biweekly oscillation of 10-20 days (QBWO) and the intra-seasonal oscillation (ISO) of 30-60 days. The structures and propagation characteristics of QBWO and ISO are quite different. The low-frequency oscillations in summer monsoon activity have significant phase-locked characteristics associated with precipitation. The Bay of Bengal is the main source of water vapor for the summer monsoon, and its warm and humid southwesterly airflow plays an important role on the precipitation over southern China. However, the understanding of the low-frequency oscillations in the monsoon region, especially the QBWO, is still limited. Therefore, this project will apply a variety of mathematical statistics and diagnostic analysis methods to the 10-20-day oscillation and the 30-60-day oscillation over the Bay of Bengal to understand the different structural characteristics of the two low-frequency oscillations. The project will explore the role of the low-frequency oscillations over the Bay of Bengal at different stages (establishment, maintenance, and retreat) for the summer monsoon, analyze its impact on the precipitation over southern China, and clarify the specific physical processes and mechanisms. The proposed work aims to improve the understanding of the different structures and mechanisms of low-frequency oscillations and the subseasonal variability of Asian summer monsoon. It will provide useful theoretical information for enhancing the short-term climate prediction in southern China.

大气低频振荡有两个主要的周期，分别是 10-20 天的准双周振荡（QBWO）与 30-60 天的季节内振荡（ISO），两者从结构到传播特征都有较大区别。夏季风活动中的低频振荡与降水具有显著的锁相特征。孟加拉湾作为夏季风主要的水汽源地，其温暖潮湿的西南气流对我国南方降水有重要的影响。而我们对孟加拉湾季风中低频振荡特征，尤其是QBWO的了解仍然不够充分。鉴于此，本项目采用多种数理统计和诊断分析方法，从孟加拉湾10-20天QBWO与30-60天ISO不同的结构特征入手，研究并比较这两种低频振荡活动规律。在此基础上探讨孟加拉湾低频振荡在夏季风建立、维持和撤退不同阶段的作用，并分析对我国南方降水的影响，阐明其中具体物理过程和机制。以上工作的开展有助于加深我们对低频振荡不同结构及机理的认识，进一步理解亚洲夏季风季节内变化规律，并可为延伸期预报与短期气候预测提供新的理论依据。

本项目开展对孟加拉湾地区季节内振荡（包括10-20天和30-60天）的研究，尤其针对10-20天振荡的传播特征进行深入分析。利用跟踪方法计算夏秋季（8月至10月）亚洲季风区准双周振荡（QBWOs）事件的频率，对比了季风降雨中的两种类型的QBWO。这两种QBWO一个起源于110°-140°E，向西传播到中国南部，对东亚降雨有显著影响，另一个从160°E开始到日界线，不会对中国南方降雨产生太大影响。对水分通量垂直结构的分析表明，1型QBWO事件的水汽主要受纬向通量分量的支配，2型QBWO事件中经向水汽通量分量占主导地位。结果还表明，南亚高压和西太平洋副热带高压的强弱不同地调节了两种类型的QBWO的产生。具体而言，较强（较弱）的两个高压有利于第一（第二）类型的QBWO事件出现。 QBWO与西太平洋海表温度（SST）异常之间也存在密切关系。以上工作对进一步理解亚洲夏季风的结构、活动机理及我国南部地区夏季降水过程都有重要科学意义。并能够应用于实际，为短期气候预测以及延伸期降水预报提供理论依据和参考。