Understanding how oceans move is key to understanding life on Earth.
BY RICHARD HAMBLYN
Sea waves are among the world’s most misunderstood phenomena. When an incoming wave breaks on the shoreline, it appears as though the water has come to the end of a long journey, but in fact the water itself has hardly moved. Most surface sea waves transmit energy, not water, and the turbulence at the surf zone is the result of that moving energy encountering a solid obstruction—usually the shelving sea floor—against which it noisily dissipates. It is at that point that the wave transforms, from an energy-transporting wave of oscillation to a water-moving wave of translation, more commonly known as “swash.” So for most of its life a wave is not a thing so much as an event, a small part of a largescale transfer of energy from one part of the sea to another.
Waves are most commonly generated by the friction between wind and the surface of the water. As wind blows across the sea, the disturbance and perturbation cause a small wave crest to form, and the resulting up and down motion begins to transmit kinetic energy through the water in the form of a series of waves. As the waves grow, the energy (but not the water) passes from crest to crest, as is apparent when a piece of flotsam, say a tin can, can be seen bobbing up and down on the spot as waves pass beneath it.
Waves are classified according to their wave period or wavelength (the distance between two crests), from the smallest capillary waves to the greatest waves of all, the tides, and those who study them are known as kumatologists, from the Greek kumas (wave), a term coined by the wave-obsessed English geographer Vaughan Cornish in 1899. Capillary waves are the small, rippled disturbances that first appear on the surface of wind-blown water, which have been known to mariners for centuries as “cats’ paws.” William Henry Smyth, in his Sailor’s Word-Book (1867), defined the cat’s paw as “a light air perceived at a distance in a calm, by the impressions made on the surface of the sea, which it sweeps very gently,” and noted the widespread superstition of rubbing a ship’s backstay to invoke the lucky cat’s paw, “the general forerunner of the steadier breeze.” Samuel Taylor Coleridge, en route to Malta in April 1804, wrote a finely observed description of the various wave types seen from deck, starting with the hair’s-breadth ripples of capillary waves:
“I particularly watched the beautiful Surface of the Sea in this gentle Breeze! – every form so transitory, so for the instant, & yet for that instant so substantial in all its sharp lines, steep surfaces, & hair-deep indentures, just as if it were cut glass, glass cut into ten thousand varieties / & then the network of the wavelets, & the rude circle hole network of the Foam.”
As is evident from both Smyth’s and Coleridge’s accounts, the characteristically rippled structure of capillary waves is due to light breezes (blowing at speeds of about 10–13 feet per second) that generate wavelengths typically less than 0.6 inch. Such gentle wind is not what sailors need to fill their sails, however, and neither is it enough to cause traveling waves to form. The threshold wavelength at which surface waves begin to travel is above 0.7 inch: Anything shorter than that will be suppressed by gravity. But if—as Smyth’s sailors would have hoped—the wind strengthens to blow consistently over a substantial fetch of water, the second class of sea wave, gravity waves, will then begin to form.
Gravity waves occur when wavelength grows to around 5 feet, and gravity joins forces with wind as the main dispersing agent. A slight convexity in the wave shape is needed to give the wind something to work on, and as soon as a wavelet develops a leeward face and a windward back, the wave (as it now is) will begin to climb: The water’s line of least resistance is to go upward as the energy in the wind is transferred to the sea. When longer gravity waves propagate over deep water, they move rapidly away from the generating wind, at which point they are known as swells, with a typical wavelength greater than 855 feet, up to a maximum of 2,950 feet. Swells lose so little energy as they cross open water that it’s possible for one generated in the Antarctic Ocean to travel all the way to Alaska at full strength, taking several days to do so…