M6 Tides

Topic

Tides are the periodic oscillations of sea level—and the accompanying currents—produced by the gravitational pull exerted by the Moon and the Sun on the ocean's entire water mass. These astronomical tides are perfectly periodic and, therefore, predictable: in most oceans, a lunar semidiurnal component dominates—with a period of approximately twelve hours and twenty-five minutes—followed by a diurnal component with a period close to twenty-three hours and fifty-six minutes, alongside other minor components. Harmonic analysis allows the tide observed at any coastal point to be broken down into these various components, each with an amplitude and phase characteristic of that specific location.

Internal tides—previously mentioned in the context of planetary ocean waves—are generated when the barotropic tide (which moves the entire water column as a unit) interacts with underwater topography, such as ridges and continental slopes, within a stratified ocean. It is this interaction with the seabed that converts a portion of the barotropic tide's energy into internal waves oscillating at the exact same frequency as the tide that spawned them. These internal waves can propagate thousands of kilometers from their point of origin and serve as the primary energy source for diapycnal mixing within the ocean, involving a total dissipated power of approximately one trillion watts. Resonance occurring within certain coastal basins enormously amplifies tidal amplitude: when the natural oscillation period of a bay or estuary matches the period of the dominant tidal component, this coincidence causes the tidal amplitude to multiply—increasing ten- to twenty-fold compared to open-ocean levels—as seen in the Bay of Fundy, Canada, where the tidal range reaches up to sixteen meters.

The expected behavior of the global tidal system involves the propagation of amphidromic tidal waves—waves that rotate around points where tidal amplitude drops to zero—within each ocean basin; while the typical amplitude of these waves ranges from just three-tenths of a meter to one meter in the open ocean, it can reach up to sixteen meters in coastal resonance zones, as previously described.

Thus, from the gravitational pull of the Moon and Sun—with which this document began—to the coastal resonance zones where it concludes, and encompassing the internal tides that mix the ocean's interior and the amphidromic waves rotating within each basin, tides ensure that the very same periodic force driving sea-level changes at the coast simultaneously and silently stirs the depths of the entire ocean.

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