Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Voyage of the Odyssey Voice from the Sea
What is the Voyage of the Odyssey Track the Voyage Interactive Ocean Class from the Sea Patrick Stewart
> Odyssey Logs -
Search by Region
- Atlantic Ocean
- Mediterranean Sea
- Mauritius
- Sri Lanka
- Maldives
- The Seychelles
- Indian Ocean
- Australia
- Papua New Guinea
- Kiribati
- Pacific Passage
- Galapagos Islands
> Odyssey Logs - Search by Topic
> Odyssey Video
> Current Location - Map
> A Day in the Life
> Meet the Crew
site map  
LatestPhoto
Waves along a beach.
Photo: Chris Johnson

September 6, 2002
Waves
  Real Audio
  28k


Log Transcript

Life at sea is governed by many elements. In rough seas our ability to find and track whales is seriously compromised. But in calm seas, our ability to work efficiently increases significantly. If you were to ask me what single factor has the greatest impact on our work and on our personal lives aboard Odyssey, there is no question it would be the kinds of rough seas that we have been experiencing of late.

Most of us are familiar with ocean waves by having experienced them at the beach, or while surfing (or being seasick on a rolling boat). So what are they; how are they formed; where do they come from; and where are they headed?

A wave is what happens when energy moves through water. The water molecules in a wave do not travel forward with it, instead each molecule moves in a circular motion, ending up after the passage of each wave in roughly the same position in which it began. In the majority of cases, waves are caused by the action of wind blowing across the ocean. More wind means more energy, which results in larger waves. Waves are generated and affected by several factors and by combinations of such factors. These can include wind strength, water depth, and the size of the unobstructed body of water across which the winds are blowing.

Waves are measured by their height, while the distance between adjacent waves is known as the wavelength of those waves. Capillary waves are the smallest waves, a mere rippling on the surface. As the wind increases, capillary waves can increase in size too to become wavelets, then waves. Wind generated waves get longer and steeper as the wind continues to blow. The size of a body of water determines the size of the waves that can form while crossing it during any constant wind. The larger the crossing distance, the larger the waves can become. For example when sailing across the open ocean, the Odyssey occasionally experiences winds of fifty knots or higher. When this happens wave size increases dramatically over time. Interestingly, winds of the same speed blowing for the same period of time across the water in a bucket would generate only tiny capillary waves. This is because the accumulative effect of the wind on already existing waves is much smaller in the confined space of the bucket.

So why do waves increase in height as they approach the beach? It is because water depth also affects wave height. As the wave approaches shore, there comes a point when the depth of the water is about half the wave length and the wave starts to 'feel the bottom'. This means that the circular energy of the wave starts dragging water against the seafloor which changes the circular motion of the water molecules and makes them follow an oval path which makes the wave higher and narrower. As it moves across progressively shallower water, the wave drags more and more against the bottom. This, combined with the forward push of wave energy produces 'breakers' that dissipate their energy in the surf line by violently stirring the water into a frenzy of turbulence. These releases of wave energy are releases of accumulated wind energy that were stored in the waves. When they are strong enough, they may eventually change the shape of a beach.

Another type of wave which is rare but fascinating, is the tsunami, also referred to as a tidal wave - an unfortunate usage of the term, since tsunamis have nothing to do with tides. Tsunami waves are not produced by wind or tides but by some sudden undersea event such as an earthquake, a landslide or the eruption of an undersea volcano. Movement of the seafloor produces a surge of water that flows out as a long sea wave, and this becomes one or more waves of very long wavelength-up to 120 miles or longer-that can travel across an entire ocean at speeds of up to 800 kilometers per hour (approximately 450 miles per hour- a speed comparable to a jumbo jet). Far from land, tsunamis are barely detectable, appearing only as fast-moving, low waves that rarely exceed two feet in height. If you encountered such a wave at sea you would not experience it as a wall of water hurtling toward you at jet airplane speed, instead, the water surface would simply rise slowly, remain up until the 120 mile-long wave had passed, and then slowly subside. You would hardly notice it.

However, upon reaching shallow water tsunamis sometimes rise to tremendous heights causing devastating damage and destruction. The eruption of Krakatoa in Indonesia in 1883 triggered a tsunami that reached over 40 meters in height and drowned more than 36,000 people who lived along the surrounding coastline. The largest tsunami on record was in 1971 off Ishigaki Island in Japan-an incredible 278 feet high.

You may have noticed that the waves that crash against a rocky headland or point of land are bigger than the breakers entering the bay next to it. This is due to wave refraction (the bending of wave fronts) and is a phenomenon well known to any serious surfer. Wave refraction occurs when a relatively straight wave front hits an indented seashore, causing the wave front to flex into the inlets and to bend around the headlands, thereby roughly mimicking the curves of the coastline. Wave refraction results in more intense wave action on projecting areas where the water becomes shallow over shorter distances and thereby produces larger breakers (and happier surfers). It is against headlands that waves expend more energy than against straight stretches of shore, and this means that the forces of wave erosion are concentrated along headlands.

The southwest monsoon season is still upon us, so that when we are working in the open sea beyond the shelter of islands we have to face big waves that have been built up by the constant pressure of the wind that pushed them across the entire Indian Ocean. These 12 to 15 foot waves make both work and life uncomfortable (and sometimes dangerous). In these latitudes and at this time of year the winds normally die out, then come up from the opposite direction in a few weeks time. But this year the changeover appears to be delayed and so we have to put up with a bigger dose of rough seas. It is clear that what we are experiencing is yet another example of the more frequent El Niņo occurrences generated by global warming-something that has plagued us all across the Pacific and is now doing the same in the Indian ocean.

LatestPhoto
A diagram of waves forming on a beach.
Illustration: Chris Johnson

Because we live among waves that consistently reach such heights we feel global warming. It is no longer something remote; it is already knocking at humanity's door. And since the majority of us live within a few miles of the seashore, global warming is already making life both uncomfortable and dangerous for the majority of us. When will our species pay attention and take steps to make our lives quieter and safer again? When will we take seriously the need to reduce the human addiction to fossil fuels that is the ultimate genesis of the current frequent episodes of El Nino and therefore of the outsize waves they generate?

Links:

  • Read more about Roger Payne discussing Global Warming.
  • To learn more about Global Warming and its affect to Kiribati, click here.

Log by Dr. Betsy Johnson & Genevieve Johnson

 
 
> Home > Voice from the Sea > What is the Voyage? > Track the Voyage > Interactive Ocean > Class from the Sea > Patrick Stewart > Help with Plugins? > Site Map