The Combos of Anomalies in the East Asian Westerly Jet and Western North Pacific Subtropical High: Determinants of the Intraseasonal Precipitation Anomalies over Eastern China during the Flood Season

Abstract The lower-level western North Pacific subtropical high (WPH) exerts a significant influence on the water vapor transportation, while the upper-level East Asian subtropical jet (EAJ) contributes to vertical motion through the secondary circulation in the jet–streak exit region. Together, these two circulation systems serve as the primary determinants of the spatial distribution and intensity of monsoonal rainfall, thereby shaping intraseasonal precipitation anomalies over eastern China. Building upon this physical understanding, we define three critical circulation indices: the meridional displacement index of the EAJ, the intensity index of the EAJ, and the pattern index of the WPH. The meridional displacement of the EAJ is influenced by intraseasonal Rossby wave trains propagating along the Africa–Asia subtropical westerly jet stream, which is associated with convection in the tropical Indian Ocean. In contrast, the intensity of the EAJ is predominantly determined by intraseasonal Rossby wave trains originating from northern Europe and propagating toward East Asia. The combined effects of EAJ meridional displacements and changes in the WPH pattern contribute to significant north–south migrations of rainfall belts across eastern China. Conversely, variations in the intensity of the EAJ, together with alterations in the WPH pattern, control the intensity of rainfall anomalies in the Yangtze River basin. The various combos of EAJ and WPH provide a basis for understanding and predicting intraseasonal rainfall variability in eastern China during flooding season. Significance Statement The upper-level East Asian subtropical jet (EAJ) and lower-level western North Pacific subtropical high (WPH) are critical circulation systems influencing precipitation anomalies over eastern China during the flood season. However, the dynamics underpinning the varying configurations of these systems and their distinct impacts on 10–40-day precipitation anomalies are not yet fully understood. Here, we show that combinations of the EAJ meridional displacements and the WPH pattern changes govern north–south swings of precipitation anomalies over eastern China, whereas combinations of the EAJ intensity variations and the WPH pattern changes modulate rainfall intensity in the Yangtze River basin. By leveraging these identified physical connections, we propose two sets of circulation indices aimed at enhancing the monitoring and predictive capabilities of intraseasonal precipitation over eastern China during the flood season.