Rogue Waves
The imaginative cartographers of the Middle Ages labeled much of the Atlantic Ocean as the Sea of Darkness and warned mariners: "Here there be monsters." The oceans are mostly charted now, imagination has succumbed to knowledge, but the monsters remain; brief, fleeting beasts with an existence as ephemeral as a mayfly but savage enough to sink a modern ship with a single blow. Rogue waves. Even the name connotes rampage and destruction like the charge of an outcast elephant. Like the rogue elephant, their appearance can be abrupt and devastating.

A rogue wave is the coincidence of several wave trains, the crest of one train superimposed and amplifying others. The typical life span of a rogue is measured in seconds. Abruptly they heave themselves above the surrounding waves and abruptly dissipate but within that brief lifespan a rogue can rise to monstrous heights. The existence of a rogue wave is predictable only as a statistical probability--the percentage of probability that a wave of a specific height will occur within a specific area during a specific period of time, typically a period of years - not much comfort for a sailor looking warily over his shoulder for the sudden onslaught of a rogue. One thing is obvious: big seas breed bigger rogues. And the sears are getting bigger.

For the last 20 years a British ship has been anchored off Land's End, Cornwall, its crew patiently measuring the heights of endless waves. In those 20 years the average height of storm waves buffeting the anchored ship has risen from 39 feet to an intimidating 57 feet. (God alone knows how they manage to keep a crew onboard.) More typical wave heights have increased from 7.4 feet to 9 feet. The cause of this increase is still unknown.

How big are really big waves? In 1966 the passenger ship Michaelangelo was struck by a wave that stove-in ports 81 feet above her waterline. The bow section was flooded and three passengers killed. In 1965 the heavy cruiser USS Pittsburgh had 90 feet of her bow torn off by a wave. In 1933, the USS Rampao reported the largest wave ever accurately measured at sea--112 feet. The largest wave until Halloween, 1991.

It was the storm expected once every 10,000 years. A low pressure system stalled off the coast of Nova Scotia. All the elements necessary to create massive waves were present--a wind of 70 knots blowing across unobstructed water for a prolonged period. These elements--wind strength, fetch and duration--resulted in waves reliably measured at 150 feet! Over a thousand miles away the residents of Melbourne, Florida, gathered on their boardwalk to watch 15 foot surf thunder against the shore.

The height of a wave is determined by three factors--wind speed, fetch and duration. In the Southern Ocean the wind's fetch is almost endless, encircling the entire planet. The waves generated by that ocean are formidable. When these waves are opposed by the powerful Agulhas Current, the results are phenomenal.

The Agulhas Current is similar to the Gulf Stream and the Black Current (Kuroshio) of Japan. All are western boundary currents, among the most intense of ocean currents. The Agulhas can attain a velocity of eight feet per second driving south along the eastern coast of South Africa. When it rounds the Cape of Good Hope, the current is channeled by the steeply sloping continental shelf of Africa into the prevailing westerly winds of the Roaring Forties and their attendant waves. Large waves driven by relentless winds across a vast ocean are suddenly opposed by a strong current. Conditions are ideal for the formation of rogue waves of staggering height.

 
Where the troughs of numerous wave trains coincide, huge holes may suddenly appear in the ocean. Rogue troughs are secretive beasts and rarely seen. Their inverted profile make them even more elusive than rogue waves. A ship's bow may fall into an enormous hole without warning, the deck falling beneath the crew's feet, leaving the forward lookout with barely enough time to whisper "Madre de Dios" before the bow, driven by gravity and the ship's own machinery, buries in the trough and a wall of water sweeps the deck. Enough water shipped on deck may overwhelm the vessel's buoyancy. She may be unable to rise again. A modern ship with all the imaginable safeguards can die without time enough to broadcast a Mayday. It's a terrifying but plausible scenario.

A new theory suggests that rogue waves are likely to form when a swell is opposed by a current following a curved path—an eddy. A field of random eddies can apparently focus the energy of a waves much like light is focused by a lens. Focused energy can apparently produce waves of remarkable size. For a summary of the new scientific thinking, see Rogue Waves Revisited.