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There are many kinds of waves in the ocean, and they differ greatly in form, velocity, and origin. There are waves too long and low to see and waves that travel below the sea surface within water layers of different densities. Waves may be raised by ships, or landslides, the passage of the Moon and Sun, by earthquakes, or changes in atmospheric pressure. Probably there are kinds of waves that have not yet been discovered. But most waves, and the waves that are most important on a daily basis, are those raised by the wind.

Let us begin the life story of a wave with a perfectly smooth water surface such as a mirror-like pond. Suddenly a breeze begins to blow. Waves are born as the air pressure on the surface changes and the frictional drag of the moving air against the water creates capillary waves then ripples. When a ripple has formed, there is a steep side against which the wind can press directly. Now the energy can be transferred from air to water more effectively, and small waves grow rapidly. In the ocean the same thing happens, but with no nearby shore to limit wave development the waves soon develop into a sea.

Of course, it would be rare indeed for a constant wind to blow on an entirely undisturbed ocean surface. Usually there are “old seas,” or waves generated earlier by winds elsewhere. If the new wind and these existing waves are moving in about the same direction, the old waves are rapidly enlarged. If the two are opposed, the wind will flatten the sea surface as the new waves cancel the old. For the moment, though, let us ignore this complexity.

Because winds are by nature turbulent and gusty, there are local variations in the air velocity and the pressure on the surface. As a result, wavelets of all sizes are created simultaneously.

As the wavelets continue to grow into larger waves, the surface confronting the wind becomes higher and steeper, and the process of wave building becomes more efficient—up to a point; there is a limit on how steep a wave can be. Steepness is the ratio of the height of a wave to its length, and the limit is about 1:7. A wave 7 feet (2.1 m) long can be no more than 1 foot (0.3 m) high. When small, steep waves exceed this limit, they break, forming whitecaps (a sea surface covered with such waves is said to be “choppy”). When the wind blows the top off a wave, causing a breaking wave at sea, some of the energy goes into turbulence but most is contributed to longer, more stable waves. The result is that a long wave can accept more energy and rise higher than a shorter wave passing under the same wind. Therefore, as the sea surface takes energy from the wind, the small waves give way to larger ones, which can store the energy better. But new ripples and small waves are continually being formed on the slopes of the existing larger waves. Thus, in the zone where the wind is moving faster than the waves, there is a wide spectrum of wave lengths. At the same time, however, the longest waves continue to accumulate energy from the smaller waves. Although the wind produces waves of many lengths, the shortest ones reach maximum height quickly and are destroyed, while the longer ones continue to grow.

Three factors influence the size of wind waves: (1) the wind velocity; (2) the duration of the time the wind blows; and (3) the extent of the open water across which it blows (otherwise known as the fetch). A simplified idea of the development of waves in the generating area is given in figure 12. In this generating area (often a storm), wind waves are called sea. At the upwind end of the fetch, the waves are small, but with distance they develop—their period and height increase and eventually they reach the maximum dimensions possible for the wind that is raising them. The sea, then, is said to be fully developed; the waves have absorbed as much energy as they can from wind of that velocity. An extension of the fetch or a lengthening of the time would not produce larger waves.

FIGURE 12: The fetch, inside the dashed line, is the area of water on which a wind blows to generate waves, transforming them from ripples to swell.

Drake Passage Crossing

In the austral spring of 2002, we left Punta Arenas, Chile, near the southern tip of South America. We headed toward the Southern Ocean, into the Drake Passage, bound for the Antarctic Peninsula. The experienced captain of the ice-strengthened ship said the winds were moderate—“only” 50 knots. We exited the Strait of Magellan, left sight of Tierra del Fuego, and headed farther south. The small, choppy wind waves of the channel were replaced by larger, long-period rolling swells emanating from deeper water. Beyond the continental shelf, the Southern Ocean’s unlimited fetch produced waves over 35 feet (11 m) high. The decks were constantly awash. The winds and waves increased, torturing the vessel, crew, and scientists for a few days. I went to my bunk in high seas and when I awoke all was calm, I was confused. The ship had ceased its twisting rolls and pitches. The ship’s engines were still running strong. I peered outside; we were surrounded by mountains covered with glacial ice—we had entered the Neumayer Channel. Protected from wind and deep-water waves, it was calm again. A limited fetch and sheltered waters will brighten your day. – KM

The description of how the wind transfers its energy to the waves derives from the work of Harald Sverdrup and Walter Munk of the Scripps Institution of Oceanography. During World War II their attention was attracted to the problem of predicting the waves and surf that would exist on an enemy-held beach during amphibious landing operations. Wind, Sea, and Swell gave the first reasonably quantitative description of how waves are generated, become swell, and move across the ocean to a distant shore. Today’s changing climate provides increased energy for transfer into ocean waves and allows temperature, air pressure, and weather patterns to respond with larger fluctuations.