The tide schedule that never changes

In a recent piece on tides (“Tidal patterns” Issue No. 54) we stated, “There are locations in Indonesia and other South Pacific islands where the tide adheres to the solar day, with high and low tide occurring at the same time each day.” Since we didn’t give any explanation for this bizarre effect, I thought I would expand on it a bit.

To understand the effect, it is necessary to view tides within entire ocean basins. Rather than marching east to west across oceans following the moon’s shadow, high tides often rotate through oceans around some central point or points. These central points are called amphidromic points. (This does not occur in the South Atlantic where every 12 hours, a high tide moves steadily south to north from Antarctic waters to the Caribbean.)

The amphidromic point for the North Atlantic is some 350 miles east of Newfoundland. In the North Pacific, there is a major amphidromic point about halfway between California and Hawaii. In the South Pacific, there are at least two amphidromic points: one about 200 miles southeast of New Zealand and another centered near Tahiti.

It is this last amphidromic point that is most interesting. Tide level at a lunar amphidromic point is unaffected by lunar tides. High tide radiates from an amphidromic point, so it is always in the high region. As a result, the small solar tide, which is usually overwhelmed by the lunar tide, is evident. On an island near Tahiti there is a high tide every midnight. Tahiti, then, is one of these exotic South Sea isles mentioned in the article. I have been unable to turn up any tidal information on Indonesian waters, but I would appreciate hearing from any readers with experience in those areas.Cameron Bright

By Ocean Navigator