What makes a harbour seiche different from a tide?

The harbour can have its own rhythm

Picture water sloshing in a basin rather than a crest travelling steadily towards a beach. NOAA describes a seiche as a standing wave. Its explanation includes bays and ocean harbours, as well as inland examples. The key idea is the repeated oscillation within a bounded body of water. It is a description of motion, not another name for the regular high and low tide.

A disturbance can start the motion

Strong winds and changes in atmospheric pressure can displace water. When the forcing changes, the water can rebound and continue moving. NOAA notes that seiches may persist for hours or days. This explains why the initiating weather and the later oscillation need not be simultaneous. It does not mean every post-storm fluctuation is a seiche.

Tide and seiche are different questions

NOAA’s tide explanation traces the ocean’s long-period tidal waves to the forces of the Moon and Sun. High and low tide describe their changing surface level at a location. The Canadian Hydrographic Service, by contrast, describes a seiche as a basin’s free oscillation at its natural period. Its account distinguishes that free motion from a tidal response amplified by resonance. Similar-looking rises and falls can therefore have different explanations.

Read the prediction with its limits in mind

The Canadian Hydrographic Service states that its tidal predictions do not include non-tidal effects, although they include average seasonal water-level change. A predicted tide and an observed level answer related questions, but they are not interchangeable. Before comparing a local observation with a table, check the location, time reference and the issuing service’s notes.

For a harbour visit, this is a reason to consult current local information, rather than to name a wave from a photograph. Use harbour-authority notices, weather information and any official warnings for decisions on the day. This explainer cannot diagnose an event or establish that a quay, beach or vessel movement is safe.