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.