Seasonal variability of the M₂ ocean tide can be detected at many ports. The physical mechanisms underlying seasonality, in the broadest terms, are astronomical, frictional–advective interactions, and climate processes. Some of these induce annual modulations, some semiannual, in amplitude, phase, or both. This note reviews how this occurs and gives an example from each broad category. Phase conventions and their relationship with causal mechanisms, as well as nomenclature, are also addressed.