The Western Pacific Basin (WPB) is the terminal convergence zone of the Pacific Meridional Overturning Circulation (PMOC), yet how gateway transport variability modulates its interior flow remains poorly understood. Integrating a sixteen-mooring array with GLORYS12V1 reanalysis (2010–2024) and a potential vorticity (PV) budget framework, we identify two principal interior pathways: a West Axis along the basin's western boundaries and a South Axis linking the New Guinea and Yap–Mariana Junction (YMJ) channels. Gateway forcing dominates basin dynamics: the YMJ and Izu–Ogasawara Ridge channels together explains 75.6% of total lateral PV flux variance, varying in antiphase with a one-month lag. YMJ transport variability strongly correlates with the South Axis, confirming a "side-forcing" mechanism that drives anticyclonic-to-cyclonic transitions along axes. Leveraging this relationship, observation-constrained flow field reconstruction corrects systematic sign errors in GLORYS12V1, reducing velocity biases by 20–70% at key mooring sites.