Technical note: On seasonal variability of the M2 tide

Seasonal variability of the M-2 ocean tide can be detected at many ports, perhaps most. Examination of the cluster of tidal constituents residing within the M-2 tidal group can shed light on the physical mechanisms underlying seasonality. In the broadest terms these are astronomical, frictional-advective interactions, and climate processes; some induce annual modulations and 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.