Introduction to waves in the ocean

I think that one thing most of us will agree upon is that sitting on the shore and listening to the sound of the sun can help soothe even the rockiest of passions. Any moving water can bring temporary relief, but there is something about waves moving around sand at the beach that just seems to do it best. It’s highly likely that it has something to do with life on earth having begun in the ocean, but that’s for a later post.

For the most part, the waves we see at the beach are generated by the wind, and the winds are generated by differences in air pressure, which are in turn generated by the sun.

It’s a simple equation.

The sun gives more energy to some places than it does to others — compare and contrast the equatorial Pacific to the Gulf of Alaska for a good example. This causes temperature differences that lead to variations in air pressure (warmer air has lower pressure and rises, while a cooled, more dense airmass will sink), and these differences cause the wind to blow across the surface of the sea — in our example, from the North Pacific towards the equator (with the North Shore of Oahu smack-dab in the way, but we’ll circle back to that in a bit).

As the winds blow across the surface of the ocean, friction transfers the energy to the water, which leads to the formation of sine waves — fill a cookie sheet with water, bend down and blow across the surface, and you’ll get the general idea. In the case of the ocean this is called the “fetch,” and the longer and stronger the fetch, the bigger the waveforms that are created.

All sine waves — whatever medium they travel through — have the same general form, and use the same terms to describe the salient features. Many of these terms are familiar: crest, trough, wavelength, amplitude, wave height, and so on.

We especially like sine waves in the ocean because the medium they are traveling through is something we can see, as opposed to electromagnetic waves that move through, well, the emptiness of space.

When it comes to how waves in the ocean push me on my surfboard and beat up the beach, probably the most critical factor is the wavelength (represented by the Greek letter lambda λ): the distance between one wave crest to the next (λ could also be measured from trough to trough, or wherever to wherever… just so it’s to the same place on the next wave).

But there’s a catch: the mechanical energy transferred by the wind to the water only goes down to what is called “wave base” — the distance below the surface that is half of the wavelength (for those who are hung up on the physics and math, wave base = ½ λ).

Below wave base the wave energy is essentially gone, and the water is unaffected by the surface waveforms. This is why in the movie “Crimson Tide” the USS Alabama has to dive to “launch depth” before Captain Ramsey (played by Gene Hackman) can turn the launch key — above that depth there is too much motion from the wind-generated waves, and the missiles might veer off-course and nuke a bunch of innocent cows in Kansas rather than the ultra-nationalist rebel leader Vladimir Radchenko that they were aiming for.

Whatever. This whole thing works out pretty well for the surfers. Along with the fact that the only thing moving through the water is the energy (so we don’t have to keep paddling to stay out beyond the breakers), a wind-generated wave doesn’t lose any energy as it moves through the water — a thirty foot wave in the Gulf of Alaska is still a thirty foot wave when it gets to the Banzai Pipeline on the north Shore of Oahu.

So what causes a wave to break? Well, when the depth of the water is shallower than wave base, the bottom part of the waveform slows down while the upper part keeps on going, causing the wave to crest, form a breaker, and give the dudes (and dudettes) on the surfboards a ride to remember.

Enough. This should get us going for now, and give us some basis for exploring additional features of ocean waves in future posts. Some of these will include: rogue waves (a.k.a. sneaker waves); the sound of the sun; wave-cut terraces, Waterworld, and the future of life on earth; summer vs. winter beaches; morning vs. afternoon surf; and, of course, tsunami.

Stay tuned…