John Hale, of Dunedin, asks :-

When crossing a causeway in the harbour the tidal flow direction is quite clear as water rushes through the constriction. How is tidal flow direction determined where there is no constriction?

Ross Vennell, an oceanographer with the Ocean Physics Group, Marine Science, University of Otago, responded.

An insightful question! In shallow water the answer is simple. In harbours and estuaries up to 100m deep, or through large channels like Cook Strait, it is the shape of the sea floor which largely determines the direction of tidal flows. So that in shallow water tidal currents flow back and forth along the channels that they help to maintain by scouring sediment. Offshore in the deep ocean there are few well defined channels, so the answer is much more complex.

Though we think of the tide as water levels rising and falling, the tide is actually ocean water sloshing back and forth around the globe due to the differential gravitational attraction of the moon on the ocean. The water sloshes back and forth as a wave. High tide occurs when the crest of the tide wave passes by and low tide when the trough passes by. The tide wave is not like the ones you see at the beach. Though two metres high, there are hundreds of kilometres between crests and these crests only pass by every 12 hours and 24 minutes! Like tsunamis, the tide wave hurtles across the ocean at up to 800 kmph. Fortunately the tidal currents associated with the tide wave are only a tiny fraction of this, around 1-4 kmph.

In the deep ocean, tidal currents don't flow back and forth in the direction that the tide wave travels, but flow around elliptical paths as a result of the Coriolis effect due to the earth's rotation. So tidal flow directions in deep water are determined by both the direction of propagation of the tide wave and the earth's rotation. Of course this leaves the question of what determines which way the tide wave travels across the ocean? This is largely determined by the shape of the sea floor as the tide wave bounces back and forth between landmasses.

The pattern of the tide wave near NZ is peculiar, in that the crest and trough chase each other anticlockwise around NZ at 300 kmph. As a result, NZ is one of only two landmasses which has a high tide, a low tide and everything in between somewhere on its coast at the same time.