Explained: The New Zealand beach where the moon doesn’t move the sea – Stuff

The moon makes the tide go in and out at every beach in New Zealand, bar one. But to understand Oteranga Bay, you have to understand the rest of the tides too. Chris Hyde reports.

Fun fact: On a stretch of the southwest coast of Norway there are no tides. The locals build properties right down to the shoreline, and even over it, because the sea doesn't move in and out. It's what's known as an amphidromic point.

A second fun fact for the day: Some say there is another amphidromic point on the other side of the globe. That point is known as New Zealand.

I was confused as well because I was pretty certain we have tides so I asked around. Few knew what I was going on about.

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But those who did were able to help connect the puzzle pieces of what is surely one of nature's most intriguing pieces of water the moana around Aotearoa.

In te ao Mori, the incoming and outgoing tides are the inhale and exhale of a taniwha by the name of Te Parata.

The taniwha, who was encountered in Moana-nui--Kiwa by some of the first voyagers from Hawaiki, is not to be angered, for obvious reasons.

Bill Kearns/Stuff

A high tide at Haumoana in Hawkes Bay in 2006.

Te Parata's powers represent the gravitational pull of the moon, which plays a huge part in our tides and all tides. So too, to a lesser extent, does the pull of the sun and the curvature of the Earth.

But our oceans are far more complicated than just a bunch of liquid being pulled one way or another by the gravity fields of rocks and balls of gas in the sky.

You can thank the coriolis effect that drives our weather for the extra confusion.

Think of the oceans not as if they're giant ponds of water in the traditional sense, but instead as a collection of really deep glasses of water.

These glasses of water aren't sitting on the surface of the earth like it's a table.

Uncredited/AP

The coriolis effect, which causes hurricanes and tropical cyclones to swirl, also makes our oceans slowly turn and creates amphidromic points with no tides, the equivalent of a hurricanes eye.

Because of the coriolis effect - read more on that here if you like - it's as if the glasses are being swirled and sloshed around like giant calm hurricanes, or a tropical cyclone if you're in the Southern Hemisphere.

At the edge of these glasses the water level changes from high, and then back low, as it's spun around.

These are places with large tides.

But in the middle of the glasses, the water level stays stable, even as other all points around it rise and fall. These are amphidromic points, where there are no tides.

Simon Williams/Unsplash

In parts of the southwestern coast of Norway, builders know the tide won't come in much, if at all.

I'm guessing that before you clicked on this article you hadn't heard of many places that had no tides, right? So amphidromic points must be rare too?

Not really.

They exist in every ocean on the planet, and it's possible to have several in a single ocean. Roughly five exist in the Pacific alone.

But to have one that actually intersects with land is rarer.

And one of those landforms, like Norway, that keeps getting name-dropped in science books, in reports, and even in viral Tik Toks about places with no tide, is little ol' New Zealand.

Clearly Aotearoa does have tides, and some rather large ones on a world scale, especially on its western coastlines.

Unlike Norway, the exact middle of New Zealand's amphidromic point, if it even was a true one, would be somewhere inland, not right on the shoreline.

So instead of our beaches being in the middle of the glass of water, theyre actually sitting somewhere between the middle and the edge of the glass, hence, tides.

Which leaves the question of where on land the amphidromic point would be located. The answer is that it is impossible to pinpoint. It would change.

That's because, as Niwa tidal scientist Glen Reeve notes, despite the literature that keeps including us in the amphidromic point conversation, we're not actually one in the truest sense of the science.

R Ray/NASA

A map of the world's amphidromic points. They are shown as the points where the white lines meet. Most are surrounded by dark blue, indicating low or no tides.

Its a shame, but you cannot say for certain that either Ashburton or Levin is the metaphorical plughole of New Zealand.

What is true though is that our tidal pattern makes us look like an amphidromic point because it propagates or swirls around the country very close to every 12 hours.

Bizarrely it does so anticlockwise, completely contradicting the coriolis effect. That's right, our tides are as weird as a tropical cyclone hitting New Orleans.

It's actually offshore amphidromic points that are to blame for this. There's two of them around us in the Pacific Ocean, one to the northwest of us and one to the southeast, that pull the sea in such a way that it has no choice but to go against its natural instincts and travel anticlockwise around us.

When you combine that with a country that's far from immune to being shaken and stirred by severe weather, it's no wonder we get some strange happenings out there in the blue.

As well as being head-scratchingly fun science, our odd tides translate into some fascinating real-world effects.

Auckland is one example. Manukau Harbour and Waitemat Harbour are no more than three or four kilometres apart in some places, yet have tides that are almost 180 degree opposites.

Abigail Dougherty/Stuff

When its high tide in Waitemata Harbour, its low tide in Manukau.

The tide literally has to travel from Auckland around Cape Reinga to reach Manukau Harbour a journey that takes it hours.

Travel further south and there's an even more dramatic tidal effect in the gap between the North and South Islands.

In the infamous Cook Strait, when one side of it is at high tide, the other is at low tide.

It means there's a bunch of water in the middle that needs to rush to fill a void that is up to 2 metres lower on one side.

This happens every time things change a smidgeon, and if there's one thing tides are always doing, it's changing.

It's why the currents in the strait are so strong and so complex.

Just ask anyone who's tried to swim it one minute they're speeding towards the shore, the next the tide has changed, and they're moving backwards.

These currents are then amplified further in parts of the Marlborough Sounds.

The channel separating D'Urbanville Island from the South Island, known as Te Aumiti/French Pass is the finest example.

It's the strongest tidal current in New Zealand, water moving an astonishing four metres per second at its fastest point.

Lloyd Homer/GNS Science

The tide at French Pass is the strongest in New Zealand.

Craig Aston, who's lived beside it and fished it his whole life, says the tidal currents are so strong because of the narrow width of it, and the shallow and uneven depth of the pass.

When the tide changes, the power of it can effectively kill a fish.

Aston says their death is a ruthless one that the tides themselves don't carry out, but do facilitate by rendering fish unable to swim.

What happens is fish get caught in one of the updrafts and their air bladders blow, Aston said.

That's a point of no return for them because they'll sit at the top of the tidal updraft, and they can't get down again because their bladder's blown. Then the seagulls come and peck their eyes out.

Aston says it's a particularly common demise for John Dory, and their carcasses commonly wash up on the surrounding coast.

With the massive tidal power comes powerful eddies, fatal to a group of divers just over 20 years ago.

Aston found out their true power first-hand several years ago when he tied a rope to a bloated cow carcass that had washed up on the beach, aiming to drag it in his boat to a more palatable place to dispose it.

"A whirlpool just grabbed hold of it and pulled it under, snapped the rope, and that was the last of that."

ALDEN WILLIAMS/Stuff

Kaikura has smaller tides because the sun only barely has an effect on it.

The reality is that there is no dream beach that will always have its sand uncovered waiting for you to sunbathe on it.

But there are parts of New Zealand that are degenerated or virtual amphidromes because they don't experience certain types of tides.

Near the Chatham Islands, there is a solar amphidrome - a place where the sun's gravitational pull has no effect on the tide.

This then impacts entire east coast of New Zealand - and probably in a good way too given the potential worries ahead with climate change - reducing neap or spring tides and making the east's overall tidal changes generally lower than the west.

Kaikura's the best example of this on the mainland. Its spring tides are an almost non-existent two to three centimetres.

Tides influenced by the moon are what affect New Zealands coast the most.

But at Oteranga Bay, on the southwest coast of Wellington facing Cook Strait, Niwa has measured the lunar tide as zero.

Reeve says Oteranga Bay is a "degenerated" or "virtual" lunar amphidrome that is unique to Aotearoa, generated by the collision course that is Cook Strait rather than the swirling coriolis effect of most amphidromes.

Because lunar tides are so influential, this is also the place in New Zealand that has been measured as having the smallest overall tide in the country.

But if you take a trip out to the isolated spot, which entirely coincidentally is the landing point of the Cook Strait cable, you will still find yourself at a beach where the tide goes in and out.

The other tides take over.

Transpower general manager of grid delivery Mark Ryall says crews in charge of maintaining the cable still have to take the tides into consideration when deciding the best time to send a boat out.

Though it can be sheltered on a good day, it's a place far removed from the calm and tideless fjords of Norway.

Perhaps the main lesson of all of this is actually really simple: Don't go chasing amphidromes.

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Explained: The New Zealand beach where the moon doesn't move the sea - Stuff