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

We propose a novel diagnostic framework within a unified monsoon coordinate system to quantify the variability of the East Asian Summer Monsoon (EASM). This framework introduces two new concepts: Monsoon Vector Projection (MVP), which quantifies monsoon intensity, and Directed Angle (DA), which captures directional variability. The newly developed MVP and DA indices exhibit highly significant correlations with summer precipitation over the middle–lower Yangtze River basin and outperform traditional EASM indices. Moreover, they offer a clearer and more comprehensive representation of the spatial pattern of the Meiyu–Changma–Baiu rainbelt.

Strong EASM years are characterized by pronounced convergence along the Meiyu front, as indicated by enhanced MVP, and are accompanied by anomalous cyclonic shear reflected in DA deflection. This circulation pattern is associated with enhanced rainfall in the Meiyu region, a westward extension and southward shift of the Western Pacific Subtropical High, and suppressed precipitation over northern China, collectively forming a north–south dipole in rainfall anomalies. In contrast, weak EASM years display the opposite pattern. These circulation features are closely linked to the Indo–Asian–Pacific (IAP) teleconnection, as revealed by horizontal Rossby wave ray trajectories and the newly introduced Rossby wave ray flux (Li-Yang WRF). Furthermore, the monsoon coordinate framework is extendable to other monsoon regions, offering a promising tool for better capturing monsoon variability and improving our understanding of its relationship with broader climate dynamics.