Amphidromic

Amphidromic is an adjective used to describe a tidal pattern in which there exist points in an ocean basin where the tidal range is effectively zero. An amphidromic point is the location around which the tides rotate as the tidal cycle progresses. The term comes from Greek roots meaning “running around” or “around twice.”

In a given basin, the water level at an amphidromic point is minimal during tidal cycles, and the crest and trough of tides circulate around the point. As one moves away from the amphidromic point, the tidal range generally increases, and the phase of the tide changes in a rotating manner. After one complete tidal cycle, the phase around the point returns to its starting position. The overall pattern consists of amphidromic points connected by lines of constant phase, often forming a network that reflects the basin’s geometry and coastline.

The direction and sense of the rotation around amphidromic points are governed by the Coriolis effect and the shape of coastlines and ocean floor. Consequently, the rotation can vary between hemispheres and different ocean basins, producing characteristic, basin-specific amphidromic patterns.

Significance and study: Amphidromic patterns help explain observed tidal ranges and times across coastlines, informing navigation, coastal engineering, and tidal energy planning. They are analyzed using tide gauge records, harmonic analysis, and numerical models based on the Laplace tidal equations, which describe how tides propagate in a rotating, bounded ocean.