The mysteries of the tide

The ancient mariners of Malaysia believed that the rise and fall of the ocean tides were caused by the movement of a gigantic crab. This is not correct. Nor are they caused by the ‘Flood Tide Woman' lifting her skirt, as believed by the Haida people of Canada's Queen Charlotte Island. The main cause of the motion of the tides is, of course, the moon, as was first suggested in ancient India, where a classical Sanskrit poem describing this lunar influence has been dated to between 1,500 and 1,200 BC. It may have been known for millennia, but the connection between the tides and the moon still feels utterly magical. How mysterious, that something as ‘up there' as the phase of the moon - the extent to which its face is bathed in sunlight - should influence something as ‘down here' as the rise and fall of water in the bay.

The tug of the moon

I have to confess that, beyond this basic lunar connection, my understanding of the tides has always been a little hazy. Like most people, I knew precious little else about how tides work. I realised I'd better find out when I began writing a book about waves, for the ebb and flow of the tides are the result of ocean waves - not the normal ones rolling up the beach, but much, much larger waves: ‘tide waves'. These reach many hundreds of miles from crest to crest, though they are just a few feet in height. And while normal ocean waves are formed by winds blowing over the water's surface, these enormous tide waves are driven by gravitational attraction towards not just the moon, but also the sun. High tide occurs when the crest of the tide wave sweeps by; low tide, with the arrival of the trough. I soon realised why many of us are ignorant about tides: they're complicated.

Why, for instance, do the British Isles get two high tides and two low tides within any 24-hour period, known as a ‘semi-diurnal' tidal regime, while around the Gulf of Mexico, or some shores of the South China Sea, they have just one high and one low tide - known as a ‘diurnal' regime? The pull of the sun and moon is exerted on the waters of the whole globe, after all. The main reason for all this variation is that the changing tug of the moon as it orbits around us, and of the sun as we orbit around it, leads seawater to slosh around the ocean basins rather like water in a frying pan. If you put an inch or two in a pan and knock it, you'll notice the water sloshes back and forth at a certain rate. This natural rate of sloshing depends on the amount of water and the shape of the pan. Now gently rock the pan back and forth. How much the water rises and falls at the edges depends on whether you do it in sync with this natural rate of sloshing. It won't rise and fall nearly as much if you rock it out of sync with its natural rate than if you're in time with it.

Like frying pans of water, oceans and seas also have natural rates of sloshing about. The principle is exactly the same, though being so much larger, the rates are much, much slower. The degree and frequency of tides around an ocean or sea basin depend largely on whether its natural rate of sloshing happens to chime with the waxing and waning of the gravitational forces towards the sun and moon.

British tides explained

The tides around the British Isles may share similar frequencies, but they don't half differ in height. At one end of the spectrum is somewhere like Bournemouth, on the south coast, where the tidal range - the difference between the high and low water on any given day - rarely exceeds a metre. Portrush, in Northern Ireland, and Lowestoft, in Suffolk, both have maximum ranges of little more than two metres. Compare those to Morecambe Bay and the Solway Firth. For both, the tidal ranges during spring tides (those times in the month when they are greatest) exceed eight metres. At Avonmouth, in the Bristol Channel, the average range on a spring tide is more than 12 metres, which is the second highest tidal range in the world.

Why so much variation? The feeble tides at Lowestoft are related to the fact that tide waves don't just travel from one side of a sea to the other, they rotate around the edges of the basins, rather like the way a ‘high tide' of water sweeps around the edge of that frying pan if you agitate it with a gentle circular movement. In the oceans, the rotation of these tide waves is set up by the spin of the planet. And, naturally, it is not as simple as in the pan. The ocean floors are not flat, smooth and coated in Teflon. In fact, seafloor and coastline irregularities mean that the rotation of the tide waves is much more complex. Rather than rotating around a single point in the middle of a basin, the tide waves generally rotate around several points, which are known as ‘amphidromes'. Close to one of these centres of rotation, the water level barely rises and falls at all. Lowestoft's modest tidal range is because there is an amphidrome nearby - halfway between the Sussex and Dutch coasts. A far more local factor contributes to the very large tidal ranges. These tend to occur in estuaries and bays - places where the coastline is shaped like a funnel. This constricts the incoming tide, causing the level to rise dramatically, as there is less and less room for the water to flow into.

The Severn Bore and neap tides

In some estuaries, the funnelling can cause the tide to bunch up so much that it forms an abrupt wave front, known as a tidal bore. The most famous and dramatic of these in the UK is on the River Severn, where the steep face of the bore can be as high as two-and-a-half metres. The Severn bore progresses up the meandering river channel at a speed of 10mph, often ridden by local surfers, hoping to break the long distance surfing world record. The ride is made more exciting by the roaring sound that accompanies the water's advance. And tides vary over time, too, increasing and diminishing through the lunar month. This is where the phase of the moon comes in, because the crucial factor is the relative positions of the sun and moon. Spring tides happen around full or new moons, when the sun, moon and the earth are in line, so the gravitational pulls of the sun and moon upon the earth are lined up. The name for this alignment is a Scrabble player's dream: ‘syzygy'. Neap tides, on the other hand, when the tidal range is at its least, occur when the positions of the sun and moon mean that their gravitational pulls are not in line but at right angles to each other, so their tidal effects don't add in a constructive way. Set up like this, the sun's light falls on the side of the moon, relative to us, so we see a half-moon.

Oh, and to complicate things further, the changing distances of the moon during the lunar month and the sun through the solar year add yet more variation. Perhaps, in the face of such complexities, those Malaysian mariners didn't have such a bad idea after all. Don't tell anyone, but I'm now of the firm belief that tides are indeed caused by an enormous crab. Clearly, he just times his submarine strolls to correspond exactly with the phases of the moon.