Amphidromic System

An amphidromic system is the rotary pattern a tidal constituent forms in an ocean basin, with the tide wave circling an amphidromic point where its range is zero. Co-tidal lines, joining points of simultaneous high water, radiate outward from that point like spokes, while co-range lines, joining points of equal range, encircle it with range growing toward the basin boundary. The rotation results from the Coriolis force acting on the tidal wave reflecting within the basin, counterclockwise in the Northern Hemisphere for most semidiurnal systems. Each constituent, M2, S2, K1, has its own amphidromic chart, which underlies global tide models and explains why tidal range varies so sharply along a coast.