Waves and Tides

Jan Chaiken and Marcia Chaiken

So many ways to enjoy the ocean along the Oaxacan Riviera! Maybe you like to sit or lie on the sand and listen to the waves while you read or daydream. Maybe you enjoy snorkeling while bobbing up and down on the waves. Perhaps you go further out from shore in a boat, for fishing or diving or just cruising. In all of these activities you are affected by the waves and tides but may not give much thought to them – they are just a kind of gentle background to your activities. But if you have ever wondered where the waves come from, or why the tides occur when they do, read on…

Tides and waves are basically the same thing – a rise and fall of the surface of the water. Tides just happen a lot slower than waves, and they rarely make a sound here on the Oaxacan Riviera. It may appear from the shore that waves move in toward you, while tides just go up and down, but actually the water in waves also just goes up and down – you can check this out by watching the motion of the bright red lifejacket of a snorkeler as the waves come in. Some force is propelling the crest of the wave in a certain direction – that force is provided for the enjoyment of surfers, of course – but surfers are scooting across the surface of the water, they are not moving the same way the water is moving.

Think about it – the water near the sand is warm, while out past the coral reef it is cold; so the cold water in a wave that you see far out does not actually move from there and reach the beach.

We have probably all heard, or learned in school, that tides are caused by the gravitational pull of the moon and sun. This is true up to a point, but tides are actually much more complicated than you might guess if you think that when the moon is directly overhead, it must be high tide. In the idealized world of textbooks, the earth can be imagined as a perfect sphere (no continents, thank you) and the oceans completely cover all of the earth. In this imaginary world (which also, by the way, has no winds or precipitation), the part of the ocean that is directly under the moon bulges out toward the moon because the moon’s gravity is pulling stronger there toward the moon than anywhere else on earth – that is a high tide. On the opposite side of the earth, there is also a high tide at the same time, because the moon’s pull of gravity is weakest there, and the water that is not being pulled to the moon is drifting away from the moon, resulting also in a bulge of water away from the earth’s surface. Halfway between the two tides it is low tide because the water has been pulled somewhere else.

What does the sun have to do with it? The sun is much bigger than the moon and has much stronger gravity, but it is farther from us than the moon, so the force of the sun’s gravity on the ocean water is less than that of the moon. The sun does not much affect the time of high tide in our idealized spherical world, but it does affect the size of the tide. If the moon and sun are in line (which happens at full moon or at new moon), the difference in height of the water between high tide and low tide is greater than at other times of the month. Mathematical equations can calculate the exact height of the tide at any place in the imaginary spherical earth, based just on knowing where the sun and moon are at that moment.

Now in the real world with continents and islands and bays and seas, the tide does not actually agree with these wonderful equations. Imagine that you put a plastic membrane in the center of your swimming pool and pulled it up and down slowly, imitating the moon. The waves would be circles until they hit the sides and corners of your pool. After a while, the pattern would settle down and you would see that the time when the water is highest at a corner of the pool is quite different from the time when it is highest in the center or along the edge. So it is in Huatulco – the time of high tide in one bay can be quite a bit different from the time of high tide in another bay – and neither of these may agree with the location of the moon.

So how do oceanographers calculate tide tables? (Or you may have a GPS or smartphone app that will show you the height of the tide at your location for any date and time that you select – how do they work?) Remarkably, the answer is that the oceanographers take measurements at each location and compare the results with the ideal equations mentioned above. For example, if the high tide is always 42 minutes later than the equations say in Santa Cruz, then this 42-minute correction factor is stored (along with a bunch of other correction factors – these are called the coefficients of the harmonics, for those of you who remember your Fourier transforms).

The measurements are averaged over many years, so that the effects of winds and storms and docking cruise ships are washed out (ho ho). Years ago, the measurements were taken in vertical tubes that had a float inside. Someone had to write down the readings, or buy a machine that kept a record on paper. The floats would get corroded or fouled up by seaweed, requiring constant maintenance. Nowadays the height is measured optically by a reflected light beam, and the information is transmitted electronically to a central recording station – very little human intervention is required.

How big can the correction factors get? In some places the corrections due to the shape of the sea or basin completely overwhelm the direct effect of the moon. For example, in the Gulf of Mexico there is sometimes only one tide in a day. In Veracruz, on May 6, there is predicted to be a high tide at 4:47am, and the next high tide will not be until May 7 at 5:53am. Don’t you agree it is nicer here on the coast of Oaxaca, with its two high tides a day?

Back to the surfers. Where do those waves come from? Remarkably, waves may start from winds or storms many days earlier and in distant locations. The swells created by the winds travel unimpeded through thousands of miles of clear, calm weather and turn into breakers only when they approach the shore. Waves that started out in various places all over the world are hitting the beach near you right now, but the largest and strongest swells are the only ones that you can notice. If you go to the website ( www.surfline.com/surf-forecasts/mainlandmexico/huatulco-area_16175 y ) you can find forecasted surf heights up to 14 days from now. The predictions for the next few days are based on storms that already happened, and the wave swells are already on their way here. But forecasted surf heights for Huatulco more than 7 days from now are based on forecasted weather all over the globe – the predicted surf levels will come from storms that haven’t even happened yet! Avid surfers keep their boards and other equipment packed, and they follow the forecasts on the internet for their favorite beaches. The forecasts are good enough that they can purchase airline tickets, finish up their urgent work at home, take a flight to their destination, and enjoy the highest swells – along with hundreds of others who used the same websites!

The very highest waves should be avoided by surfers at all costs – and every other living being. Those are tsunamis or, as they are incorrectly called, tidal waves. When a strong earthquake occurs on the ocean floor, the seizures of the earth below the ocean can start a massive amount of water waving outward from the epicenter.

On the surface of the ocean, the swells produced by millions of kilograms of moving water may not appear to be any larger than those produced by winds. But they are moving with tremendous force, often at extraordinarily high rates and can travel across thousands of miles. When they reach land, the force propels much of the water close to the shore far inland. The water than sweeps virtually anything in its path relatively far back to sea and then, after a seemingly endless pause, rushes back inland again once or twice more. Fortunately, tsunami monitors are being placed over more and more of the oceans and the size, speed and directions of these seismic waves can be gauged. The tsunami’s exact destination and size are determined by other factors as well, such as the shape of the coastline, so prediction is not an exact science Tsunamis are very rare events on the Oaxacan Riviera. Most of our waves range from gentle to large swells enjoyed by surfers. So settle down with a cervaza on one of our glorious beaches to watch the waves and the tides rise and fall. But be careful, this wonderful activity can be addictive.

Jan and Marcia Chaiken live in view of the ocean in Huatulco and far inland in Ashland, Oregon.