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

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 from 5° 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 Niño–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 Niña 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.