The Pacific Meridional Mode (PMM) exhibits a marked seasonal variability, with the strongest (weakest) variance in northern spring (fall). Such a phase locking feature is investigated through a combined observational and modeling study. Given the PMM perturbation, the wind induced latent heat flux anomaly leads to a strongest (weakest) heating on local sea surface temperature anomaly (SSTA) in MAM (SON) through positive wind-evaporation-SST feedback. The difference between MAM and SON lies on the strength and area of mean northeasterly trades. Experiments with a simple air-sea coupled model further demonstrate that a PMM-like SSTA perturbation grows much faster in MAM than in SON. The difference is primarily attributed to the seasonal mean wind, not mean SST condition. It is greatest strength and area of the mean northeasterly trade in MAM that leads to most efficient wind-evaporation-SST feedback and thus fastest PMM growth rate.