What Causes Rogue Waves?
To understand what causes a rogue wave, first you must learn a little about regular waves. Think about waves you're familiar with -- such as the waves you bodysurf in at the beach or at the local water park's wavepools. A wave has several characteristics that can be used to define it.
- The crest is the highest portion of the wave.
- The trough is the lowest portion of the wave (the "dip" in between waves).
- The distance from the trough to the crest represents a wave's height.
- The distance between crests represents a wave's length.
- The amount of time that passes between one crest and the next is the wave period or wave speed.
- The amount of kinetic and potential energy carried by the wave is known as wave energy [source: Bryant, 156].
A huge number of variables influences these factors, including the depth of the water, tidal forces, wind blowing across the water, physical objects such as islands that reflect waves, and interaction with other waves and ocean currents. At any given moment, thousands of waves are passing and interacting through a specific area of ocean. The faster the wind is and the longer it blows, the stronger and larger the waves. Fetch is the unobstructed distance of ocean over which the wind can blow on the water -- it's how much ocean the wind is blowing on. More fetch means bigger waves.
Weather reports list the significant wave height, which is the height of the highest one-third of the waves. Why do rogue waves exceed the significant wave height by so much? Scientists aren't completely sure, but they have some good theories.
One possibility is that ocean currents cause waves to "pile up" when waves run into currents head on. Powerful storms can cause significant wave heights of 40 to 50 feet (12 to 15 meters). When such waves run into a strong current, the current can increase wave heights and cause the waves to break. This would explain monster waves 98 feet (30 meters) high or more, and account for the "wall of water" effect. Rogue waves frequently occur in areas known for strong ocean currents. For example, he Agulhas Current runs southward along the east coast of Africa. Storm waves moving up from the south crash into the current -- mathematical predictions suggest rogue waves there could reach 190 feet in height, and 20 ships have reported rogue wave strikes in that area since 1990 [source: Smith, 188]. The Gulf Stream, which runs up the east coast of the United States, is another potential rogue wave source. Rogues originating in the Gulf Stream could be responsible for much of the legend of the Bermuda Triangle.
Not all rogue waves occur in strong ocean currents, however. Scientists think some waves may be caused by randomly occurring wave reinforcement. Whenever two waves interact, their wave height is added together. If a 5-meter wave passes over a 10-meter wave, the result is a briefly occurring 15-meter wave. This can happen in the opposite manner as well. A 15-meter wave moving across a 10-meter trough results in a 5-meter wave. Dozens of waves could be interacting and reinforcing each other. Once in awhile, several waves may come together at just the right moment and create one huge wave in relatively calm seas. If 10 waves that are only 5 feet high come together, they will result in a 50-foot wave. This fits descriptions of rogue waves that seem to appear out of nowhere and disappear after just a few minutes.