Abstract The Gulf of Maine's lunar semidiurnal (M2) ocean tide exhibits spatially coherent amplitude changes of ∼1–3 cm on interannual time scales, though no causative mechanism has been identified. Here we show, using a specially designed numerical modeling framework, that stratification changes account for 3248Pearson coefficient 0.58–0.69) of the observed M2 variability at tide gauges from 1994 to 2019. Masking experiments and energy diagnoses reveal that the modeled variability is primarily driven by fluctuations in barotropic-to-baroclinic energy conversion on the continental slope south of the gulf's mouth, with a 1-cm amplitude increase at Boston corresponding to a ∼70.30 GW) drop in the area-integrated conversion rate. Evidence is given for the same process to have caused the decade-long M2 amplitude decrease in the Gulf of Maine beginning in 1980/81. The study has implications for nuisance flooding predictions and space geodetic analyses seeking highest accuracies.