西风与季风在青藏高原的耦合模态及其季节性变化特征

Coupling Mode of Westerly–Monsoonal Flow over the Tibetan Plateau and its Seasonal Variation

青藏高原是我国的水塔,西风与季风及其相互作用是导致亚洲天气和气候变化最重要的环流系统。本文基于1981~2020年大气再分析资料,采用经验正交函数分解方法(Empirical Orthogonal Function,EOF)提取了西风与季风季节循环分量在青藏高原的耦合模态,并对其季节变化特征进行分析。研究发现,第一主模态方差贡献率高达78.39%,主要反映的是东亚季风、南亚季风和对流层高层中纬度西风的季节循环特征及各个季节的年际变化特征。夏季在对流层高层高原及其南侧主要为东风气流,范围从北纬5°至35°,对流层低层则表现为典型的绕高原气旋式季风环流系统,热带和副热带地区为西南季风控制,冬季的环流结构刚好相反。耦合模态的冬、夏季节转换节点与东亚季风和南亚季风的季节转换时间基本一致。从年际变化的角度来看,各个季节耦合模态的强度偏强时,东亚季风和南亚季风均偏强,西风带位置偏北;反之,季风偏弱,西风带位置偏南。厄尔尼诺—南方涛动(El Ni?o–Southern Oscillation,ENSO)是影响西风与季风耦合模态年际变化的关键外强迫,拉尼娜(La Ni?a)事件发生的前夏、前秋和次年夏季耦合模态的强度均增强,冬季至次年春季耦合模态的强度均减弱。西风与季风耦合的第二主模态主要表现为对流层高层高原上的东风及其南侧西风,以及低层南亚季风区的西南季风和西北太平洋反气旋的协同变化特征。该模态的方差贡献率为4.68%,表现出明显年际差异的同时还呈现显著减弱的长期趋势,尤其是在冬季。

Tibetan Plateau(TP) is regarded as the “ Chinese Water Tower.” Interaction between the westerly and monsoon flows around the TP has an important impact on the Asian climate. Based on the atmospheric reanalysis dataset during 1981–2020, we extracted the coupling modes of the seasonal cycle component of westerly wind and monsoon flow over the TP via the empirical orthogonal function(EOF) method, and discussed their seasonal variation characteristics.The first mode accounts for 78.39% of the total variances, mainly reflecting the seasonal cycle characteristics of the East Asian monsoon, South Asian monsoon, and midlatitude westerly wind, as well as their interannual variation in each season. In summers, easterly winds prevail on the TP and the southern side of TP at the upper troposphere, ranging from5° to 35°N. Simultaneously, the lower troposphere is characterized by a typical cyclonic monsoon circulation around the TP, and the tropical and subtropical areas are controlled by the southwest monsoon. Notably, the circulation structure is opposite in the winter. The transit timing of this mode from winter(summer) to summer(winter) is consistent with the onset(ending) of East Asian and South Asian summer monsoon. On the interannual timescale, the enhancement of the coupling mode is correlated with the intensity of East Asian and South Asian monsoons, as well as the northward movement of westerly in each season. For a weak coupling mode, the monsoon and westerly display opposite characteristics. El Ni?o–Southern Oscillation is the key external force affecting the interannual variation of the westerly monsoon coupling mode. Its impact is strengthened in summer, autumn, and the following summer, while weakened in winter and the following spring of the La Ni?a event. The second coupling mode of westerly monsoon is characterized by the coordinated variation of easterly wind over the TP and westerly wind in the south of the TP in the upper troposphere,and the southwesterly in the South Asian monsoon region and anticyclone in the Northwest Pacific at the lower troposphere. The variance contribution rate of this mode is 4.68%, showing the interannual variation with a significant long-term weakening trend, especially in winter.