Notes on a booklet describing the behaviour of the tides around New Zealand

The Rise and Fall of Cook Strait

A look at New Zealand's unusual tidal patterns


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Cover of rise & Fall

This story originated in some work in the late 1970s when I was involved in controlling oil tanker traffic on the New Zealand coast. It is a subject on which no plain-language presentation is available.

I was surprised, early in that work, to discover that the port of Wellington has only one spring-tide bulge each month and that a graph of its tidal predictions looked totally different from graphs of other ports that had the normal pair of spring-tide events each month. It was a striking abnormality.

My curiosity then took me a further step, into comparing the timings of high water at various ports. I had noticed in my work that there was a large gap between some of the high water times and I quickly found that high water times seemed to advance progressively around the coast in an anti-clockwise pattern. There was little or nothing available at that time by way of commentary on the tides. There is now Internet information from NIWA that describes just that circulation of the tides, but with nothing to explain it.

Taking this work further in 2006, I found that the tides not only circulate. The intervals between HW at successive ports around the coast become longer and shorter in a cyclcal pattern of about a week. It gives the impression that the speed of circulation systematically speeds up and slows down in different parts of the circuit. For each port-to-port part of the coast, the brakes seem to be on for about a week and then the accelerator takes over. Some parts of the coast seem to be speeding up while others are slowing down, and vice versa, all fitting together neatly so that the overall circuit takes almost exactly the same time.

We can see that it is a circuit that seems to originate when the moon is aligned to Bluff, and the subsequent high waters around the coast occur when the moon is further and further distanced from New Zealand. It was not, it seemed, in line with the general belief that tide times depend directly on the moon. New Zealand's tide times are very different.

It does seem that a possible explanation for the variable speed of the tidal circuit is that the profile of our giant circulating tidal wave changes, altering the position of the crest in such a way that the leading and trailing faces of the wave alter in length. That would have much the same effect of speeding up and delaying the times of high water. Going back to the tide tables reveals that the flood and ebb flows at coastal locations constantly change by becoming longer or shorter. Thus the profile of the tidal wave does indeed keep changing back and forth. All these aspects of the tides are described and discussed in my book.

Intervals between HW times

The picture that we are looking at in this discussion is that New Zealand's tides are created by a single tidal wave, with one trough and one peak, that stretches right round the whole country, a distance of around 2500 miles. This is known as a captive Kelvin wave and makes one complete circuit approximately every 12 hours and 20 minutes, raising and lowering our tides as it circulates. At any moment during the day, the crest of the tidal wave is at one point on the coast and the trough is at an opposite point on the circuit. As this wave has a height varying between about two metres and about three and a half metres, it is not visible to any observer and we only see its presence in the steady rise and fall of our tides. Even though the wave covers 2500 miles in 12 hours, at a speed of more than 200mph, we cannot possibly see it. Though it is a fascinating phenomenon, I have never found a plain-language description or explanation. So far as I am aware, "The Rise and Fall of Cook Strait" is the only discussion that can be obtained anywhere.

I already knew that tidal ranges on the west coast were much higher than those on the east coast. No explanation is offered in any of the tide tables. It is the fact that high and low waters at either end of Cook Strait are more or less opposed that explains the "to and fro" of the tidal streams in the strait. But tidal information says nothing of the possible effects of the changing speeds of the tidal circulation (i.e. the changing profile of the tidal wave). It is clear that it must have an effect on the strength of the Cook Strait tidal streams by slightly altering the E-W gradient, and it seems that it probably explains the very strange tidal graphs that emerge when streams like those in Tory Channel are plotted. It continually changes the degree to which the tides at each end of the strait are opposed. My graphs illustrate some of these tidal patterns.

NIWA bathymetry charts clearly show the wide continental shelf on the west coast and the narrow shelf on the east coast with its deep trenches near by. Waves, both sea waves and tidal waves, are affected by differences in water depths and it seems clear that the large ranges on the west coast must be created by the extensive continental shelf. This is yet another tidal feature that is not explained to the user of the tide tables.

Relationships between ebbs and flows of waterways such as Tory Channel are found to have unexpected facets in relation to high tides within the sounds, such as those at Picton.

"The Rise and Fall of Cook Strait" also looks at the way that actual tides can differ from the predicted tides. I was able to obtain actual tide data for Wellington from the then Harbour Board, located in the building that many years later came to house Wellington's City and Sea Museum. Those deviations have been correlated with data on winds and barometric pressures from the Meteorological Office.

All these curiosities make New Zealand tides a fascinating and often puzzling phenomenon. It is a situation where very little is known by the general public.

"The Rise and Fall of Cook Strait" is a plain-language discussion of the New Zealand tidal process, illustrated by some 35 graphs and other diagrams. The graphs show striking and unexpected patterns. I have continued to find, in conversation with friends and even with many people who are well acquainted with New Zealand's coastal waters, that the remarkable patterns and mechanisms of our tides are almost wholly unknown.

NIWA (the National Institute for Water and Atmospheric Research) has given me a research paper describing NIWA's tidal model. My book has been able to draw some valuable information from it but many questions remain unanswered. I have been able to draw some basic information from textbooks used in my degree course in London many years ago, information that I did not fully appreciate at that time. More recently the library at GNS Science bought a copy of my "Rise and Fall", and decided a little later to buy two more copies; it seems to confirm that it is of real academic interest.

Most of the actual data for New Zealand tidal patterns can be found in the Tide Tables, once published by the Marine Department and later by LINZ. Those tables are intended simply to provide information on the times of high and low waters and the heights of the tides. It is the analysis and display of the data that reveals so many unexpected and remarkable relationships. The great Einstein once remarked that discoveries come from looking differently at things that people have long been aware of. Most people, even mariners, know well that tides rise and fall; generally they know little or nothing of their remarkable patterns and relationships.

One of the very few books to comment on New Zealand tides, and then only briefly, is "Greater Cook Strait: form and flow" by Professor Thomas Harris. My story is fully compatible and Professor Harris read it at my request and fully endorsed it.

In this new century when climate change and the need to conserve energy and to develop renewable sources have suddenly swept into the public consciousness, one of those renewable and environmentally friendly resources is tidal and wave power. It has therefore felt very appropriate and timely to dust off my early work on the tides and to take it further into what has now become a much more detailed and informative little book. It may well be that an ice-free Arctic will transform ocean currents and add yet another huge element to climate change. Will our captured tidal wave survive such fundamental changes?

Those who read this book will have a much greater appreciation of the tidal mechanisms on which we may all depend in a few years' time. They may also need to reflect that the immutability that King Canute was so keen to demonstrate to his subjects may one day be changed by the actions of mankind.

FURTHER RESEARCH
I would very much like to see research carried out to answer further questions that interest me.
Having found great variations between actual tidal readings for Wellington and the corresponding tidal projections, how do such comparisons look for other ports around the coast? Just as variable? More or less variable? Behaving differently at different ports?
Is there any trend in this variability as our climate changes? Are there any plans to measure such changes?
I am sure that NIWA could very easily answer such questions and would have ready access to the necessary tidal records around the coast




Availability

"The rise and fall of Cook Strait" is available from the author, Michael Whitfield Foster, 30 Campbell Street, Karori, Wellington 6012.

It is in a comb-bound or wire-bound format of A4 size, a format that opens flat on the table. The text is double-sided and the diagrams are on individual folios.

The original price of $10 per copy was little more than the cost of printing and postage, giving no profit margin. It was only made possible by very favourable copying rates from Victoria University and could only remain so while those rates were available. Commercial printing rates for a recent reprint have put the cost up to $13 a copy and thus about $15 - $16 by post (where the cost over the counter can vary at times when weight and dimensions are subject to measurement and gauges).

The book has ISBN 978-0-473-11956-0

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