The gravitational pull of the moon drags a wave of water around the ocean’s surface. It is referred to as the M2 tidal constituent, and it moves continuously anticlockwise around New Zealand. In this diagram red illustrates high tide and blue low tide. Cook Strait's strong tidal currents occur because high water arrives on the Pacific Ocean side of the strait five hours before it arrives at the Tasman Sea side. Because it is high tide on one side and low tide on the other, water is continuously sloshing to and fro through the strait.
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