New Zealand tides are moderate by world standards. The tidal range is 1–2 metres, and tidal currents are generally about 2 kilometres per hour (1 knot). The exception is Cook Strait, where tidal currents can be much stronger.
The moon’s gravitational pull
Tides are mainly driven by the gravitational pull of the moon and sun on the ocean. The moon has the most influence as it is the closest to earth.
The moon rotates around the earth in about one day. Its gravity attracts a bulge of water (high tide) that travels around the earth. The reason there are two high tides is the common centre of mass about which the moon–earth system rotates. The centre of rotation is about two-thirds out from the centre of the earth (rather than in the centre itself). The earth acts like a centrifuge, causing a second bulge in the ocean opposite to the moon.
When the sun, moon and earth line up, their gravities act together and cause especially high and low tides known as spring tides (which have nothing to do with the season). When the sun and moon are at right angles their gravities cancel each other out, causing small tides known as neap tides.
Tides are the result of complex natural influences, and are produced by as many as 62 constituents. The most important of these is the gravitational pull of the moon and sun. The moon does not revolve around the equator, but at an angle to it. So the two bulges of water that travel around the earth are above and below the equator. This means that some places have two high and low tides a day (semi-diurnal), while other places have daily (diurnal) tides.
A diurnal tide occurs in the Ross Sea around Antarctica. The tide returns once a day (every 24.84 hours), and its height reduces to almost zero every 13.66 days.
New Zealand has semi-diurnal tides. This twice-daily rise and fall of sea level is primarily caused by the main lunar tide (known as the M2 – the ‘M’ stands for moon and the ‘2’ for twice a day). The time between high tides varies from day to day because the orbits of the moon around the earth, and the earth around the sun, are not exactly circular. On average, the moon rotates around the earth once every 24.8 hours, so that the M2 occurs in half this time – 12.4 hours.
Prediction and measurement
Because the orbits of the moon and earth are regular, it is relatively straightforward to predict the tides far in advance. All one needs is a month-long record of sea level at the site of interest. Tide gauges can be used to measure the height of the tide every few minutes. However, not all places have gauges, so computer models have been developed to compute tidal constituents for all coastal locations. A New Zealand-specific tidal computer model is run by the National Institute of Water and Atmospheric Research (NIWA) in Wellington.
French Pass is a narrow gap that separates D’Urville Island from the South Island. The tides flowing through it can create a maelstrom. When the tides change, the currents are sometimes strong enough to stun fish, which float to the surface. Māori living on the island could collect the fish and enjoy their easy meal.
Cook Strait tides
Despite having a smaller tidal range (the height difference between high and low tide) than most places in New Zealand, Cook Strait has some of the strongest tidal currents in the world. The reason is that the main lunar tide is out of phase on either side of the country. High tide arrives on the Pacific Ocean side of the strait five hours before it arrives at the Tasman Sea side – when it is high tide on one side it is nearly low tide on the other. This difference in water level drives very fast tidal currents – up to 1.4 metres per second (3 knots) – through Cook Strait and into the Marlborough Sounds. Tory Channel and French Pass have currents that can reach 2 metres per second (4 knots).