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11.7: Amphidromic Points and Co-tidal Lines

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    Amphidromic points are locations where there are little or no tide in the ocean. (This is also related to influence of continental land masses interfering with the westward movement of tidal bulges and the influence of the Coriolis effect.)

    • The closer to the amphidromic the lower the tidal range.
    • There are about 1 dozen amphidromic points in the oceans (Figure 11.15).
    • About five in the Pacific Ocean.
    • One near Hawaii - there is little tide change there, so beaches tend to be narrow.

    A cotidal line is a line on a map connecting points at which a tidal level, especially high tide, occurs simultaneously.

    • Cotidal lines are hypothetical tidal crest rotating around an amphidromic point (Figure 11.15).
    • Cotidal lines rotate around amphidromic points about every 12 hours.
    • They rotate left in Northern Hemisphere, and rotate right in Southern Hemisphere.
    Amphidromic points and co-tidal lines
    Figure 11.15. World map showing locations of amphidromic points and cotidal lines. Note: this map also shows where the highest tides occur around the world.

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