<p>It remains uncertain whether precipitation oxygen isotopes (δ¹⁸O) reliably capture East Asian Meiyu monsoon variability. Analyzing daily δ¹⁸O across the Yangtze-Huai River Basin from 28-34°N, we reveal a distinct spatial dichotomy. In the middle and northern Meiyu regions, δ¹⁸O robustly tracks Meiyu precipitation. Conversely, the southern Meiyu margin is decoupled from Meiyu variability, primarily reflecting upstream convection processes further south. We identify the western Pacific subtropical high (WPSH) as the central driver, creating a dynamic dipole: its northwestward extension enhances moisture transport and deep convection along its northwestern flank (driving isotopic depletion in the northern Meiyu region), while imposing subsidence and convective inhibition under its body (suppressing isotopic depletion in the southern Meiyu region). Importantly, these mechanisms persist on interannual timescales. Consequently, while northern δ¹⁸O records effectively capture Meiyu variability, southern records reflect distinct vertical constraints, necessitating spatially differentiated paleoclimate interpretations.</p>