Why waves go rogue — a vexing mystery of wind and currents
San Luis Obispo Tribune
Published: Sunday, Jun. 20, 2010
Last week three boaters were rescued unharmed from their overturned fishing skiff off the coast of Cambria. According to the fisherman, their boat was hit by a rogue wave close to the surf line.Maybe because of this incident I received a number of e-mails from readers asking about rogue waves and just how big wind-driven ocean waves can get. This is a fascinating question and is often fraught with mystery, arguments and misconceptions.
In 1933, the officer of the deck on watch aboard the U.S. Navy tanker Ramapo measured a wave with a height of 112 feet. This was truly a monster. Even swell trains with much smaller heights and shorter periods contain a fearsome amount of power. In December 1942, the Queen Mary was carrying 16,082 American troops across the North Atlantic Ocean. She was hit broadside by a rogue wave that may have reached a height of 92 feet. It was calculated later that the ship tilted 52 degrees. Let me tell you, I was on a Navy frigate, the USS McCandless, that took an estimated 38 degree roll off the Azores Islands in the North Atlantic and it scared the heck out of us.
As I wrote last week, oceanographers operate a vast network of buoys that dot the Pacific Ocean.
The buoys, like the wave-rider buoy at Diablo Canyon nuclear power plant, measure "significant wave height," which is defined as the average height of the waves in the top third of the wave record.
This turns out to be very close to what an experienced mariner — "old salt" — would perceive the wave heights to be.
Rogue waves (also known as sneaker or freak waves) are relatively large ocean surface waves. In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height. For example, if the wave-rider buoy at Diablo Canyon reported the wave height at 10 feet, there is a chance, statistically speaking, that one wave in 1,175 could reach 20 feet in height.
I have seen "significant swell heights" reach more than 55 feet at the SE PAPA marine buoy, moored about 600 nautical miles west of Eureka. That means rare waves over 110 feet in height could have occurred under those conditions. On the outer fringe of probability, one wave in 300,000 can be up to 2.5 times the significant wave height. These would certainly count as rogue waves.
The three main factors that determine wave height are wind speed, wind duration and the fetch — the distance over which the wind blows across the ocean without significant change in its direction.
Waves are created by the friction or the dragging motion of the wind over the water.
Rogue waves occur in the open ocean in a number of ways. One of the most common causes is when wind waves with different periods or wavelengths meet in a single spot and complement each other as part of a general process called interference.
Destructive interference occurs when the different wave trains are 180 degrees out of phase and cancel each other out. Constructive interference occurs when the different wave trains are in phase and two smaller waves coalesce to produce a very large wave for a short time. This additive formation of large crest and deep troughs can cause waves to suddenly form of enormous size.
Another cause of rogue waves is when wind-driven waves run into strong ocean currents that are running counter to the direction of the swell. When waves hit these currents they can suddenly increase in height, producing extremely steep forward wave faces and deep troughs. Old salts often refer to these waves as "holes in the sea."
Bottom topography on the ocean floor can also play a part in producing large waves. While stationed on San Clemente Island with HS-85, a U.S. Navy helicopter squadron, we would sometimes fly our trusty SH-3H Sea King helicopter out to the Cortes Bank located about 45 miles southwest of the island. Two buoys indicate the location of the bank and when the conditions were right, enormous waves would suddenly erupt in the middle of the ocean.
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