National Geographic Explorer-in-Residence Dr. Robert Ballard has discovered some of the ocean’s most fascinating treasures, from the Titanic to hydrothermal vents on the seafloor.
On his very first ocean expedition, as a 17-year-old National Science Foundation scholar, Ballard also encountered one of the sea’s most amazing, and dangerous, natural marvels: a rogue wave.
“We were 500 miles out to sea off Eureka, California, on a Scripps [Research Institute] ship called the ORCA,” Ballard writes by email. “We were in a storm with 30-foot swells when a rogue wave over 50 feet high hit us, blowing out the windows of the bridge, blowing out the portholes in the galley, destroying the mast and splash rail, and flooding the engineer room with water. We were unable to head for shore since we would be rolled over by the swell, so we slowly steamed into the sea until a Coast Guard cutter could reach us and escort us back to shore while telling us over the radio how to treat two crew members who were badly injured when the wave hit us.”
Ballard is not the only seaman who has encountered these huge waves. Capt. Joshua Slocum, who completed the world’s first solo sail around the world, probably encountered a gigantic wave that submerged the hull of his sailboat in 1895. In 1966, the Italian cruise ship Michelangelo was traveling to New York when it was hit by a wave estimated to be 24 meters (80 feet) high. More recently, in 2005, the cruise ship Norwegian Dawn had its ninth and 10th floor windows smashed by a wave that rose to near 21 meters (70 feet) high.
A rogue wave is usually defined as a wave that is two times the significant wave height of the area. The significant wave height is the average of the highest one-third of waves that occur over a given period. Therefore, a rogue wave is a lot bigger than the other waves that are happening in its vicinity around the same time.
Tim Janssen, a research scientist who studies physical oceanography in Half Moon Bay, California, says one of the best examples of a rogue wave is the so-called New Year’s Wave of 1995. On Jan. 1, a 26-meter (85-foot) wave struck the Draupner oil rig in the North Sea off Norway.
“It was one of the first observations [of a rogue wave] with a digital instrument,” Janssen says.
These so-called “freak waves” are not confined to the Atlantic Ocean or North Sea. One of the places rogue waves appear to happen most frequently is off the southeast coast of South Africa. A professor of applied mathematics at the University of Colorado at Boulder, Dr. Bengt Fornberg, studied this phenomenon with Marius Gerber of the University of Stellenbosch, South Africa. Fornberg believes there is a particular reason extremely large waves occur there.
“It’s the interaction of wave swell with the current,” he says.
Specifically, it’s when a large ocean swell hits the fast-moving Agulhas current. When this happens, the curved current narrowly focuses the wave’s energy, like an optical lens can powerfully focus light into a single beam.
Dr. Libe Washburn, a geography professor at the University of California, Santa Barbara, explains what occurs to waves interacting with a current like the Agulhas.
“It shortens their wavelength and makes them steeper,” he says.
Danger at Sea
Obviously, these huge waves are able to inflict damage on oceangoing vessels.
“The waves are pretty dangerous, especially for bigger ships because they can crack their hulls,” Washburn says.
Fornberg, the mathematician, says rogue waves may also form from eddies, currents that flow in a different direction than the main current.
“Eddies are often generated along the edges of currents, but they can survive for long times and are able to drift across oceans, forming very extensive eddy fields,” he says. “These eddy fields in fact contain far more kinetic energy than the currents do. Within, and in the immediate vicinity of currents, rogue waves tend to be somewhat predictable—and they are confined to relatively small areas. On the other hand, energy focusing due to the chaotic, irregular and widely distributed eddies is somewhat less likely, and is essentially unpredictable, as these can occur almost everywhere.”
While there are many oceanographers and other scientists who forecast rogue waves, there is a lot more to be learned.
“These waves occur everywhere, all the time,” Janssen says. “The question is how much they happen.”
“It would be interesting to learn if the frequency of rogue waves is changing over time,” says Washburn.
While rogue waves still hold scientific mysteries, Ballard has some simple advice for those who are involved in designing ocean vessels:
“Build safer ships,” he writes.
