The tide in Ross Sea is completely different to the tide around New Zealand, or indeed in most parts of the world. For a start, there is only one tide per day compared with two in most places. And there is no relationship whatsoever with Moon’s phase. This means there are no spring tides at New and Full Moon and no neap tides at First and Last Quarter. Instead, the tide disappears almost completely every 13.66 days, corresponding to when Moon crosses the equator (zero declination).
The plots below compare the tide at Dog Island in Foveaux Strait with the tide at Ross Island in Ross Sea for April 2003. Dog Island, like all New Zealand sites, has a semidiurnal (twice daily) tidal regime and the tidal range varies with Moon’s phase and orbital position. Largest tides occur fortnightly at Full and New Moons (spring tides) and monthly at lunar perigee when Moon is closest to Earth (perigean tides). In contrast, Ross Island has a diurnal regime and neither Moon’s phase nor its orbital position have any significant effect on the tidal range. Instead, when Moon crosses the equator every 13.66 days (zero declination), the tides in Ross Sea diminish to almost zero.
In the New Zealand region, the main tide is the semidiurnal lunar tide, semidiurnal meaning twice daily and lunar meaning it is caused by the direct gravitational attraction of Moon on Earth’s waters. Its symbol is M2: M for Moon and subscript 2 for twice daily. M2 is so strong around New Zealand compared to the dozens of other tides, that it governs the timing of high tides that occur every 12 h 24 m. But in Ross Sea there are two equally important tides: K1 and O1. They are called the diurnal declinational tides because they occur once daily and they arise from Moon’s declinational path, where it moves from 29.1 degrees above the equator to 29.1 degrees below the equator over a period of 13.66 days, on average.
In Ross Sea, K1 has an amplitude (half range) of about 0.4 m, while O1 has an amplitude of about 0.3 m, but as we travel north towards New Zealand the amplitude of these two tides decreases and for most of the New Zealand region, they are less than 0.1 m in amplitude. Contrast this with M2, which is less than 0.1 m in Ross Sea, but increases as we go north to be as high as 0.6 m in the northern New Zealand region.
The figure beside shows the variation with latitude of the amplitude of various tides along the 180º meridian. The results are from two models: EEZ is a model for tides in New Zealand’s Exclusive Economic Zone, and CATS is a model of tides around Antarctica. The models overlap from 58 to 65º S.
For more information have a look at:
Goring, D. G.; Pyne, A. 2003: Observations of sea-level variability in Ross Sea, Antarctica. New Zealand Journal of Marine and Freshwater Research 37: 241-249.
Last Updated: 16 November 2003
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