Novel monsoon indices based on vector projection and directed angle for measuring the East Asian summer monsoon

This study presents a novel framework within a unified monsoon coordinate system for quantifying variability of the East Asian Summer Monsoon (EASM) through integrating two new concepts: the monsoon vector projection (MVP), quantifying monsoon intensity and directed angle (DA), capturing monsoon directional variability. The newly introduced MVP and DA indices based on the framework show very significant correlations with summer precipitation in the middle-lower Yangtze River, much more surpassing previous EASM indices and can also capture the Meiyu–Changma–Baiu belt's spatial pattern more clearly and comprehensively. Strong EASM years are characterized by convergence, indicated by the MVP along the Meiyu front, coupled with anomalous cyclonic shear, reflected by the DA deflection, being associated with wetter conditions in the Meiyu belt and the westward extension and southward shift of the Western Pacific Subtropical High, alongside drier conditions in northern China, which together manifest the north–south precipitation dipole pattern. In contrast, weak EASM years exhibit the opposite characteristics of strong EASM years. Moreover, this paper also proposes a creative conception—the Rossby wave ray flux (WRF), which can reflect the pathway of Rossby wave activity of the Indo-Asian-Pacific teleconnection very well. The monsoon coordinate system can be extended to other monsoon domains and could provide valuable insights for better capturing variabilities of monsoons and understanding their relationships with other climate factors in the future. Meanwhile, the WRF may provide a new tool for diagnosing local activity and propagation direction of Rossby waves.