ENSO Phase-Dependent Modulation of the Interannual Relationship Between Summer Rainfall and Intraseasonal Oscillation Intensity over the Yangtze River Basin in China

Based on gridded rainfall data and reanalysis datasets during the period 1979–2021, this study investigates the phase-dependent modulation of ENSO (El Niño–Southern Oscillation) on the interannual relationship between summer rainfall and intraseasonal oscillation (ISO) intensity over the middle-lower reaches of the Yangtze River Basin (YRB), together with the associated physical mechanisms. The results show that summer rainfall over the YRB exhibits prominent intraseasonal variability and is significantly positively correlated with ISO intensity at the interannual timescale. This interannual correlation is strongly dependent on the phase of ENSO. During the developing phase of El Niño summers, both summer rainfall and ISO intensity over the YRB are significantly suppressed, and their interannual relationship becomes statistically insignificant. In contrast, during the decaying phase of El Niño summers, both rainfall and ISO intensity are remarkably enhanced, with their positive interannual correlation being substantially strengthened compared to the climatological mean. Further analysis indicates that ENSO influences YRB summer rainfall and ISO intensity primarily by modulating the structure and amplitude of the East Asia–Pacific (EAP) teleconnection pattern. These EAP-related circulation anomalies alter the large-scale atmospheric circulation and moisture transport conditions over the YRB, leading to adjustments in both summer mean rainfall and its intraseasonal variability. Such adjustments not only modify the magnitudes of rainfall and ISO anomalies but also reshape their interannual covariability, resulting in the distinct characteristics of their relationship observed between the developing and decaying phases of El Niño. Therefore, ENSO acts as a key regulator of summer rainfall, ISO intensity, and their interannual relationship in the YRB through its phase-dependent modulation effects.