Know Your Product: Why The Tides?
As it's Christmas, a time of year when much of the Australian coast sees its biggest tides, we figured it was high time to run this old favourite.
Also, next week Craig will have another article on tides, though peering a little deeper into how they interact with swell.
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.
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.
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.
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.