Perched atop the crow’s nest of a ship, one can see that waves are the most visible manifestation of the dynamism, chaos, and power of the seas. All ocean surface waves start out in the same way: with wind. Picture the ocean as a calm, flat surface. As winds blow and swirl over the water, tiny wavelets are born — just like the ripples that streak across your coffee when you blow on it to cool it down. The wavelets move in the same general direction as the wind, and if the wind is strong enough — faster than about two miles per hour — they’ll form into stable waves that travel along with the wind. (It is the shape of the wave, not any individual water molecule, that progresses across the seas. The water particles trace vertical circles within the wave, with each returning after a circuit to their original position.)
The height of a wind-generated wave from the peak, or crest, to the bottom, or trough, is determined by the speed of the wind, how long the wind has blown in the same direction, and the width of the open water, or fetch, over which it is blowing. Winds that blow strong and steady for great distances — the trade winds and other prevailing winds, for example — will create steeper, more powerful waves than will brief gusts. The average height of an ocean wave is about 12 feet; but far larger waves — 50 feet or higher — are sometimes born of raging storms in the open sea, particularly in the northern Atlantic and off the coast of Antarctica.
Waves travel with little change across the vast expanses of the open ocean, but eventually all waves must reach shore. As they do and the ocean gets shallower, interference with the sea floor causes the waves to slow down while increasing in height. Finally the front of the wave collapses, or breaks, as surf.
|A Coast Guard ship cuts through a towering wave.|
Breaking waves unleash a tremendous amount of energy. A breaking storm wave, which throws thousands of tons of water at the coastline, can hurl rocks and bust apart concrete breakwaters. In areas where the shores are not composed of hard rock, breaking waves can rapidly wear back the land. In certain areas of Britain, for instance, waves have eaten back several miles of the shore since Roman times; some parts of Cape Cod are being eroded at a rate of more than three feet per year.
Crashing waves aren’t the only force that shapes coastlines. As waves break on shore, a churning froth of turbulent water, called swash, moves as a sheet up the slope of the beach toward shore. Once it runs out of energy, it flows back toward the surf zone, as backwash. Depending on the strength of the surf, swash can scour sand, pebbles, and even rocks off the surface of the beach; backwash deposits the debris back onto the beach, slightly displaced from its original position. Even more sand and gravel is transported by a type of flow called a longshore current. When waves — particularly steep waves — approach a straight beach at an angle, alternating swash and backwash can transport water along the beach in addition to sand and gravel. The water flows parallel to the shore, in the form of a longshore current. Longshore currents can haul vast amounts of sand and gravel along the shore, destroying beaches in some spots, and creating new ones in others. Longshore currents off Sandy Hook, New Jersey, move an average of 750,000 tons of sand per year along the shore.
At the shores, the dramatic action of the waves is superimposed over the graceful ebb and flow of the tides, the twice-daily fluctuations in the height of the ocean surface. Tides, as Isaac Newton first explained in the late 17th century, owe themselves to gravitational attraction between the Earth and the moon (and, to a lesser extent, the sun). That attraction has little effect on the solid Earth, but the fluid oceans are easily deformed. On the side of the Earth nearest the moon, then, the waters are pulled toward the moon, creating a tidal bulge. On the other side of the Earth, a similar bulge is formed for the opposite reason: since gravitational pull from the moon is weakest, the ocean waters move slightly away from the Earth’s center to create a comparably sized bulge.
|This computer model has accurately predicted currents in the North Pacific Ocean.|
Over the course of a day, the moon’s position varies little, so the bulges stay fairly stationary. But the Earth, of course, rotates, and so any one spot on the planet will pass through both bulges in a 24-hour period. This is manifested as the day’s two alternating low and high tides. The elevation difference between high and low tide depends on the phase of the moon. When the moon is full or new, the sun and moon are aligned, and their combined gravitational pull makes for bigger bulges — that is, higher high tides, and lower lows (or spring tides). When the moon is at the first or third quarter phase, the sun and moon sit at right angles to one another relative to Earth, and each partially counteracts the gravitational effect of the other. This leads to a smaller difference between high and low tide (neap tides).
In some parts of the world, the combination of tidal forcing with the shape of the coastline and estuaries leads to huge tidal fluctuations. During spring tides in Nova Scotia’s Bay of Fundy, for example, the difference between low and high tide can be a staggering 70 feet — the world’s largest tidal variation. At high tide, the incoming surge of Atlantic waters rushes up the Petitcodiac River, reversing the flow of the river and forming a tidal bore. About a hundred or so tidal bores exist worldwide (including the bore on China’s Qiantang River, as described in the SAVAGE SEAS episode “Killer Waves”). Tides like these carry extraordinary energy, and for centuries people have been coming up with ways to tap into it. As far back as the 12th century, people used tide-driven water wheels to provide power for sawmills and gristmills, for example. More recently, power plants have been constructed in areas with large tidal variations — at the mouth of the Rance River in the Brittany region of France, for example — where strong tidal currents can drive turbines and generators.
The oceans’ great currents are no less powerful, although they are far more difficult to visualize from a vantage point on the ocean surface. (Unseen ocean currents can carry even experienced sailors toward dangerous shoals.) Sailors have known about currents for thousands of years; as early as 800 BC, the Phoenicians and Greeks were aware of the presence of currents at the mouth of the Mediterranean sea. Only in the past century or so, however, have oceanographers figured out what drives the currents and been able to chart their courses throughout the world’s great oceans. Like waves, many currents are set in motion by wind — in particular, strong, steady prevailing winds like the trade winds — blowing across the ocean surface. Other currents can be driven by differences in water density due to differences in temperature or salinity of water masses, or by slight differences in sea surface elevation. The paths those currents take is determined, in part, by the effect of the Earth’s rotation (the Coriolis effect), which deflects currents to the right (or clockwise) in the Northern Hemisphere and to the left (counter-clockwise) in the Southern Hemisphere, and in part by the morphology of the ocean basins.
In the Northern Hemisphere, ocean waters circulate in two great, clockwise gyres. The North Pacific subtropical gyre is divided into the westward-flowing Pacific North Equatorial current, the northeast-flowing Kuroshio, the eastward North Pacific current, and the California current, which moves south. The North Atlantic subtropical gyre consists of the Atlantic north equatorial current, the Gulf Stream, and other currents. The Southern Hemisphere has four counter-clockwise gyres.
The Gulf Stream, first charted back in 1770 by Benjamin Franklin, is still the best-studied ocean current. At its origins in the Straits of Florida, the current is a remarkable 50 miles wide and a quarter-mile deep, and moves 10 cubic miles of water every hour; it flows so fast that its warm waters don’t mix with the surrounding colder seas. The current eventually slows down as it crosses the Atlantic and approaches Europe — but it still carries enough warm tropical water to help keep the climate of northern Europe relatively mild.
Early studies of ocean circulation relied on drift bottles released into the current. These bottles contained message cards asking the finder to return the card, along with a description of where and when the bottle was recovered. Today, many oceanographers use satellites and other technology to monitor the flow of currents. But drifting objects that wash up on shore, including those that fall off cargo ships — such as the Nike golf shoes and hockey equipment described in the SAVAGE SEAS episode “Killer Waves,” a large spill of rubber duckies and other bathtub toys in 1992, and an enormous new spill of Christmas-themed items swept into the mid-Pacific in October, 1998 — can still play a role in studies of ocean circulation.
To predict the course of these drifting objects, researchers like oceanographer Curt Ebbesmeyer (featured in “Killer Waves”) rely on a unique computer model of Pacific Ocean circulation called the Ocean Surface Current Simulator, or OSCURS, created by oceanographer James Ingraham of the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center in Seattle, Washington. It was with this model that Ingraham determined the pace of the North Pacific subtropical gyre, which takes about seven years to complete one circuit.