Know Your Product: Why the tides?

Stu Nettle
Swellnet Analysis

Welcome to the latest instalment of Know Your Product, where we explain surfing theories and concepts. This week it's tides, the inexorable ebb and flow of water that inspires poets, makes for handy metaphors, and causes your local reefbreak to roar.

Given a few years surfing, most surfers develop an intuition for tidal movements. We understand how high and low tides effect the waves, how long it is between tidal shifts, and how they all link to lunar phases. This knowledge helps us get good waves but when it comes to explaining what tides are, how they form, and how they propagate around the world, it's a vastly incomplete body of knowledge.

Getting right back to basics, tides are bulges of water that circle the globe caused by the gravitational pull of the moon, and to a lesser extent the sun. The sun is the greater body – it's diameter is 400 times that of the Moon's - but because of its distance from Earth its effect is greatly diminished. Hence, the moon is the main driver of our tides.

If Earth were totally covered with water it'd be relatively easy to track and map the movement of the tides. Two bulges of water would follow the moon, one on the side facing the moon, the other on the exact opposite side due to centrifugal force. The bulges would rotate with rhythmic consistency and the changes in tide level would clearly mesh with the moon phase – from full moon to new moon and back again.

final_tide_image.jpg
In simplest terms, the tides are bulges of water that follow the moon

The tidal range - calculated in vertical metres - changes with the monthly lunar phase. The range is greatest during the full moon and new moon because the Sun, Moon, and Earth are all in a line and hence working together. The tidal range is lowest when the Sun is at 90° to the Earth and Moon (when half the Moon is visible)

Imagining the Earth as a watery planet is more than a mental exercise, it shows that for all their complexities the basis for tides is really quite simple - amphidromic points notwithstanding, but we'll come to that later.

Tides begin to get complicated when those bulges of water begin to interact with coastlines, the underlying bathymetry, and with water from other ocean basins. It's then that water starts behaving unusually and tides can no longer be explained in simple terms.

Hawaii is one place where tides are easy to understand and predict. The simple explanation is that the Hawaiian Islands are small, have no continental shelf, and are located far away from any land mass, meaning there is very little interference with the passing tidal bulges. The tidal difference in Hawaii is surprisingly small, just a few feet separate the highest and lowest tides of the year and average daily differences are in the order of two feet.

At the other extreme are coastlines that have long continental shelfs which both slow the tides and and exaggerate the tidal difference. In Australia's north-west lies the resource rich North West Shelf which extends hundreds of kilometres out into the Timor Sea. The North West Shelf augments the tidal flow so by the time it reaches the coast it's far greater than it would otherwise be. Tides of several vertical metres are not uncommon in north-west Australia, same as other places with lengthy continental shelfs such as England, France, and parts of the US east coast.

Continental shelfs aren't the only geographic feature that can increase tides. Bays, estuaries, rivers, and sounds can do the same. The common link of the aforementioned features is that they're semi-confined bodies of water; linked to the ocean and thus tide dependent, but small enough to create their own tidal environment.

Just as an inclining shelf amplifies the tide, so to does a narrow body of water, particularly a funnel-shaped bay or sound. The top spots for tidal difference in the world are the Bay of Fundy in eastern Canada and Derby in north-west WA. Both have large continental shelfs and large bays – Derby is located at the southern end of King Sound – and both have tidal differences that peak over ten vertical metres.

adobestock_33945475.jpeg
Fishing boats dry docked by the outgoing tide in the Bay of Fundy

Between the barely-there tides in Hawaii and the three storey tides in Derby lies a wide range of differences, and they're all due – or at least mostly due – to nearby geographic features. Water acts weird when it's interfered with...and that brings us to the timing of the tides.

Most locations around the world have two highs and two lows in a day – what's known as semi-diurnal tides – though some coastlines, generally in regions with large tidal differences, have just one tide – called a diurnal tide. Things get complicated when regions of semi-diurnal and diurnal tides meet. When that happens you can forget trying to predict the tides day by day as they rarely follow a calculable path and instead are dictated by a seemingly random methodology. Western Australia's South-west is one place with a mix of diurnal and semi-diurnal tides.

It's not random of course, rather there are multiple drivers influencing the timing of the tides so the pattern they follow is harder to ascertain.

And speaking of difficult patterns: when it comes to understanding tides, the elephant in the room is the amphidromic system, and this is where things get really weird.

To understand the amphidromic system we have to revisit that image of the watery planet with tidal bulges circling it. Imagine that those rotating bulges are actually waves with massively elongated wavelengths inexorably moving around the planet. When the continents are accounted for and the unseen bathymetry below, all of which diverts and influences the rotating waves of water, the result is scattered points on the ocean where there is almost no vertical movement. They're called amphidromic points and they're best described by the mathematical field of wave harmonics: amphidromic points are created by the cancelling out – or the interference – of opposing waves.

Even better, when viewed on a global scale the tides no longer move east to west, they actually rotate around amphidromic points. Check the next image and the following video to make sense of it.

First the image, which shows amphidromic points around the globe - see the converging lines in areas of blue - and arrows showing the direction the tides rotate around them.

m2_tidal_constituent.jpg
(With Permission R. Ray, TOPEX/Poseidon)

Now the video. The colour yellow denotes zero height and note how the amphidromic points stay yellow the whole time. Choose an easy one such as the point offshore from south-west WA. As surrounding waters turn blue (low tide) and orange (high tide) it stays yellow. If our imaginary watery planet model were real everywhere would experience high and low tides, yet it isn't the case.

 

Earlier I gave you the simple explanation for Hawaii's meagre tidal range: no continental shelf, no land mass. The other reason is that it lies fairly close to an amphidromic point in a region that sees little vertical movement.

Another amphidromic point worth looking at is New Zealand. The whole country is considered an amphidromic point as the tidal bulge rotates around the North and South Island (see video) in a counter-clockwise direction. Unlike, say, Australia's East Coast where places on the same longitude share similar tide times, neighbouring regions in NZ can have greatly differing tides. Best you keep a tide chart handy.

On Australia's East Coast we're a little spoilt for tides. The range is great enough to service an array of waves yet not so great that conditions change radically. We have permanent semi-diurnal tides, and, as mentioned above, most of the coast experiences similar tide phases making it easier to keep track of tides while on a road trip.

Before we make for the exit it's worth noting that the tidal predictions you read in the newspaper/website/fishing chart are just that - predictions. Aside from the influence of the Sun and Moon, plus the coastal geography and the bathymetry, tides are also effected by the Coriolis Force, the tilt of the Earth's equator, the inclination of the lunar orbit, and the elliptical shape of the Earth's orbit of the sun. Then there are the real time factors such as barometric pressure which can lower or rise the sea surface, and local winds that can 'pile up' water against a coast, both of which can alter tidal readings.

It's a chaotic system with many different agents influencing the outcome, and it's a marvel that scientists have managed to maintain the accuracy they have.

Past articles:
Know Your Product: Real Buoys vs Virtual Buoys
Know Your Product: Wave Period

Comments

crg's picture
crg's picture
crg Tuesday, 18 Oct 2016 at 1:46pm

Fascinating... And I imagine that was a very basic summary that barely scratched the surface.

stunet's picture
stunet's picture
stunet Tuesday, 18 Oct 2016 at 2:28pm

You got it. When I began this article I figured it'd be a breeze, but it quickly became clear you can follow the study of tides as far as you wanna go and it extends way beyond what general readers could comprehend (or care about).

There's some amazing reading out there for those so inclined.

freeride76's picture
freeride76's picture
freeride76 Tuesday, 18 Oct 2016 at 2:29pm

If anyone could explain the central west WA tides to me I'm all ears.

benski's picture
benski's picture
benski Tuesday, 18 Oct 2016 at 2:55pm

Cute story but it's an absolute crock. Believe whatever you want but Bill O'Reilly explained the facts much clearer than you.

Tide goes in, tide goes out, never a miscommunication: 

https://youtu.be/wb3AFMe2OQY?t=110

t-diddy's picture
t-diddy's picture
t-diddy Tuesday, 18 Oct 2016 at 11:41pm

thank you! so good

t-diddy's picture
t-diddy's picture
t-diddy Tuesday, 18 Oct 2016 at 11:47pm

well done stu! i love this stuff but I agree with Benski - if you're implying that there is no invisible man in the sky with this convoluted explanation i'm going back to the church surf forecasting website thing

rooftop's picture
rooftop's picture
rooftop Wednesday, 19 Oct 2016 at 2:31am

Good read, but I'm puzzled by how the centrifugal force in the left of the first diagram relates to the pull of the moon.

Are you referring to the centrifugal force from the earth's rotation that causes the equatorial bulge? That is responsible for differences in sea levels, but would be a separate force to the gravitational pull of the moon, especially as the moon's orbit is not aligned with the equator.

The diagram seems to conflate the two.

Or is there another force at work that I'm not aware of?

Not counting bearded sky-men....

stunet's picture
stunet's picture
stunet Wednesday, 19 Oct 2016 at 10:14am

"Good read, but I'm puzzled by how the centrifugal force in the left of the first diagram relates to the pull of the moon."

Well yeah, it is puzzling and I came across a few different explanations for the bulge on the opposite side of the moon. None were definitive so I went on the most common explanation, that the bulge was the balancing effect of centrifugal force.

Another explanation was that the bulge was gravitational, that gravity takes a path of least resistance and so flows through the earth causing another, lesser, bulge on the opposite side. See following pic:

field_tidal.svg_.png

It's possible that it's a combination of the two; gravity causes the bulge on the Moon-ward side and the other bulge is caused by an equal and opposite force.

Apologies for being unclear. Like I said, I couldn't find a definitive explanation for it.

Tfo's picture
Tfo's picture
Tfo Wednesday, 19 Oct 2016 at 6:40pm

The Moon doesn't actually rotate around the centre of the Earth. The Earth and Moon both rotate around the centre of gravity of the Earth-Moon system. This point is known as the 'barycentre' (which is very close to the centre of the Earth in this case).

Think of the two bodies as a binary system.

This rotation around a common centre of gravity is what induces the outward centrifugal force. The centrifugal force is equal to the gravitational force (the inward centripetal force in this system), this balances the system. The result of this balance is that we get a back bulge on the other side of the Earth causing a secondary tide equal in strength to the primary gravitational tide, and, that the Moon doesn't collide with the Earth due to the gravitational pull.

rooftop's picture
rooftop's picture
rooftop Sunday, 23 Oct 2016 at 11:32pm

Nice one Tfo. You dropped my penny. So to speak.

Tfo's picture
Tfo's picture
Tfo Monday, 24 Oct 2016 at 11:10pm

It was a light bulb moment for me too when I learnt this. Something that's really obvious in hindsight.

yorkessurfer's picture
yorkessurfer's picture
yorkessurfer Wednesday, 19 Oct 2016 at 1:16pm

Great article Stu. I was over in the U.K three weeks ago and was staying down at St. Ives, Cornwall which is on the South West tip of England.

It was during the last full moon and the tides were massive! The harbour would go from completely dry to almost flooding the town over a couple of hours. If a huge storm and swell coincided with a full moon tide there I wonder what happens to St Ives?

Here's two pictures taken on the same day.

yorkessurfer's picture
yorkessurfer's picture
yorkessurfer Wednesday, 19 Oct 2016 at 1:16pm

Great article Stu. I was over in the U.K three weeks ago and was staying down at St. Ives, Cornwall which is on the South West tip of England.

It was during the last full moon and the tides were massive! The harbour would go from completely dry to almost flooding the town over a couple of hours. If a huge storm and swell coincided with a full moon tide there I wonder what happens to St Ives?

Here's two pictures taken on the same day.

stunet's picture
stunet's picture
stunet Wednesday, 19 Oct 2016 at 1:47pm

Great shots YS. How's the top one? Get to the bottom of the boat ramp and the water is still waaaay over there.

ojackojacko's picture
ojackojacko's picture
ojackojacko Wednesday, 19 Oct 2016 at 3:21pm

Great article Stu, thanks. A non-surfing Canadian mate who I shared it with wasn't happy about you handing the Bay of Fundy over to the US though.

Yorkesurfer - I've been in that part of the world. Some of the beaches around there are really wide (at low tide) and flat too so you can watch the tides come rolling in literally by the second. Beautiful coast.

stunet's picture
stunet's picture
stunet Wednesday, 19 Oct 2016 at 4:39pm

Heh heh...whoops. Those Canucks are sensitive of misappropriation at any time but with all the election craziness going down they really want to distance themselves from their southern neighbour.

Error fixed.

udo's picture
udo's picture
udo Wednesday, 19 Oct 2016 at 4:59pm

A bit more here : Swellnet - Nick Bone -Tides

Nick Bone's picture
Nick Bone's picture
Nick Bone Thursday, 20 Oct 2016 at 11:29am

Eh?

udo's picture
udo's picture
udo Thursday, 20 Oct 2016 at 11:42am

A thread you started on tides

truebluebasher's picture
truebluebasher's picture
truebluebasher Wednesday, 19 Oct 2016 at 5:28pm

Couldn't agree more Swellnet! Eastcoast spoilt for tides we are. Top read!

How best to surf these tides ?

Body language of tide-turning! Ever expanding.....
(Hands on guide for fellow swellnetonians.)
Following applies to Surfing natural RIGHT HAND waves...in either hemisphere .

Many know how to race a high line or stall a tail into a hard 'n' heavy pit.
Lets advance those options utilising full tidal effect.

(Use Incoming tide to):
Accelerate up-face vorticing counter- clockwise down with lip for barrel-roll rushing with incoming tide to fly faster out from out your barrels in a spiral manner.

(Use Outgoing tide to):
Stall 'n' reverse up-face corkscrewing clockwise back up with lip for barrel-roll using outgoing tide to re- enter your barrels.

(Top of the tide-turning) Eye-up outward backwash for your aerial counter-clockwise round the world or over the top comb-over maybe back slam a christmas beetle.

(Back of the tide-turning)Cocoon drop several wave-faces pod deep in the funnel spiralling clockwise to harness more outgoing wave power the deeper and lower you go.

(Reverse osmosis for goofy/LEFT HAND breaks). Hooroo! Happy to share.
.
[BEACHSIGN]
Says- Riding the tides in/out/roundabout/upside down/back to front is dangerous.

gunther's picture
gunther's picture
gunther Wednesday, 19 Oct 2016 at 8:19pm

.

gunther's picture
gunther's picture
gunther Wednesday, 19 Oct 2016 at 6:59pm

That video is great! Its a real shame that sw WA doesn't get more tidal variation.

nelsonk's picture
nelsonk's picture
nelsonk Wednesday, 19 Oct 2016 at 10:16pm

Hey all,

I was under the impression that the tidal high on the opposite side of the earth to the moon was due to the difference in gravitational field strength with distance from the mass of the moon. I.e. The water on the moon side is being pulled towards the moon more than the earth and the earth is being pulled towards the moon more than the water on the far side of the moon... proportional to the inverse square of the distance from the mass of the moon... it also makes sense to me that it could be due to centripetal forces and/or the fairly well established oscillation of the tides.

Ontheroad's picture
Ontheroad's picture
Ontheroad Thursday, 20 Oct 2016 at 3:05am

Sheesh... this whole time i've thought it was pretty much just rotational bulgy movements - now that amphidromic points are in my life i'm gonna sound a whole lot better in the parking lot. ha... thanks for the great article..

Craig's picture
Craig's picture
Craig Thursday, 20 Oct 2016 at 8:25am

Haha, love to hear that dropped in the car park!

Nick Bone's picture
Nick Bone's picture
Nick Bone Thursday, 20 Oct 2016 at 11:31am

Isnt the inverse barometer effect a big feature also?

thermalben's picture
thermalben's picture
thermalben Thursday, 20 Oct 2016 at 11:34am

Much smaller feature, less predictable.

truebluebasher's picture
truebluebasher's picture
truebluebasher Thursday, 20 Oct 2016 at 2:18pm

East coast surfcraft riders and SUPS from 1820's onward mastered local tidal patterns.
When The Big Fresh flooded the The Big Scrub...upon king-tide. It became time for shootin' the rapids. No rules applied !
Next building then anchoring your surfcraft required know-how,so not to have it crushed by tides.
Next surfing the Ebb Tide out thru the Tweed River mouth was a unique skill.
Top of the tide surfing back in thru the point lineup by riding incoming tides.Then banking coastline north off littoral current backed by southerly swell.

(note) Any misreading of the tides here... would see surfers cast out to sea and drifting helplessly south or wiped-out on rocky point breaks.(Todays... Gold Coast surfing reserves)

Flood tides were ridden back in to enable surfing thru South Passage(Nerang Heads... Seaway).
(note) Any misreading of the tides here... would see surfers swept north to Moreton Island or wiped-out on the smashed craft littered heads.

Inside the estuary- tides transported all from as far as Nerang to Ipswich for over a century.
(note) Any cross river ebb current could easily take your surfcraft back out through river mouth.(No Jumpinpin Bar meant water moved faster then)

Tidal Punting once a way of life along all eastcoast rivers.( SUP Couriers are not new! )

wingnut2443's picture
wingnut2443's picture
wingnut2443 Thursday, 27 Oct 2016 at 7:06pm

Awesome stuff, Stu n Swellnet. As I commented before, I hope you are syndicating these articles into the inflight magazines of the various airlines that fly into surf destinations. Hell, they could go into any of them ;)

Be such a shame to invest the time into researching and writing to only have it published once.

Or, are you going to put all these "know your product" articles into a coffee table book, released just prior to a key retail event (i.e. fathers day or Christmas)?

Fuck, I've gotta say it ... MORE OF THESE and less reproduced ABC articles!

truebluebasher's picture
truebluebasher's picture
truebluebasher Sunday, 28 Feb 2021 at 10:15pm

Venice > (3 Extreme record Tides in 15 months) + 2 Aussie Tide Anomalies
Nov 2019 > [ + 1.87cm ]
https://www.abc.net.au/news/2019-11-18/venice-suffers-record-flood-for-t...

Jan 2020 > [ - 45cm ] below sea level
https://www.abc.net.au/news/2020-01-14/venices-canals-dry-due-to-low-tid...

March 2020 - Dolphins & Swans in Crystal Clear Venice Canals (Nope!)
https://www.youtube.com/watch?v=Bpz1V_O3v5c

Feb 2021 >[ - 48cm ] below sea level
https://www.independent.co.uk/news/world/europe/venice-canals-low-tide-v...
................................................................................................................................

Recent Oz E/W coasts Tidal Anomalies (Not Major Storm surges!)
WA (Perth) Canning River Weir Saltwater breaches...
Nov 2009 - Oct 2010 > 3 breaches
Sept 2010 - May 2011 (La Nina) > 66 breaches (Note: Not a lot higher just constant!)
Weir was refurbished 2017 (n/a)

Tweed River ( 2 Significantly Higher than predicted East Cost tides)
Bray Park Weir (1983) Above Murwillumbah- Salt Water breaches
Weir has had some "minor" salt incursions but 2 larger recent episodes.
21/22nd Aug 2017 - Oz Tweed River Weir was well & truly topped
25/26th May 2020 - (320mm above predicted tide / 15ML flow-in before Toppers).

Shows recent record tides repeatedly rise/fall for 2 years approx' at specific sites.
How do these far reaching locales relate....need more research into more sites!
2020 NT / North Qld had a lot of Mega Whales up small creeks.(Not normal)
Anyhow...it gives crew a window into rough mechanics of wild tide anomalies.

Kinda hints that if a site has one tidal anomaly it either returns or over compensates.
That sure seems like natures way...it's not always gonna run by our tide charts.
Certainly would be a revelation if we could link more 1-3 year anomalies.
Science would say our sample is pure coincidence at this stage & they'd be right.

Antartica 2020/21
https://www.nbcnews.com/science/environment/two-giant-icebergs-broke-ant...
https://www.nationalgeographic.com/environment/article/world-largest-ice...
This just happened
https://www.youtube.com/watch?v=HY_6iA0Jp80

batfink's picture
batfink's picture
batfink Sunday, 28 Feb 2021 at 11:33pm

All good Stu, very good article. I’ve been thinking about it for about 30 years, but still haven’t fully understood why some parts get only two tides a day as opposed to our four. Gulf of Carpentaria is a good one to think about because so much water has to move around to get into the Gulf, a fkn large body of water, but don’t fully comprehend why it is still the case in NW WA.

Re the bulge at the back, yeah, better explained above by proper sciency person, but it’s just gravity, although that says nothing because it’s all gravity really, isn’t it.

Funny, I was going to write a long piece on how being a surfer brings you into tune with the natural world, weather, tides, barometric pressure, winds. All the essential things. For those who haven’t considered it, for a certain indigenous group that has been here somewhere around 40 to 60K years, yeah, they passed down some observed knowledge. Freaking goldmine in indigenous knowledge, if only we weren’t so stupid not to look.

Know a thing or two about our local climate too.

batfink's picture
batfink's picture
batfink Sunday, 28 Feb 2021 at 11:35pm

And I’m sure many on here watch where the full moon rises, and see how vastly different it can be. Goes from south of east to quite north of north east, and the wobble is significant.

Also loved it when I learned that the tides they tell us are just predictions, and as TBB offers, can vary substantially.

It’s a freaking wonderful world.