On seasonal to secular M2 variability in the Gulf of Maine

The lunar semidiurnal (M₂) tide of the Gulf of Maine and Bay of Fundy is remarkable not only for its large amplitude but also for its spatially coherent temporal changes of ∼1–3 cm on secular to seasonal time scales. Previous work suggests a role for ocean stratification in causing the tide's seasonal modulation, while the forcing factors for lower-frequency M₂ variability are yet unknown. Here I show, using a regional baroclinic modeling framework, that changes in ocean stratification also matter on interannual time scales and account for ∼40% of the observed M₂ changes 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 ∼7% (0.30 GW) drop in the area-integrated conversion rate inducing a 1-cm amplitude increase along the Massachusetts coast. Evidence is given for the same process to have caused the near-monotonic M₂ amplitude decrease throughout the 1980s, as slope waters warmed due to a northerly shift of the Gulf Stream. I present results from model-based M₂ projections for the end of the 21^st century and highlight possibly competing roles of stratification changes and sea level rise in driving the tide's response to future climate change.