Under Way

Author:
Publish date:

Here, there really do be dragons

Here, there really do be dragons

There was a time when giant or rogue waves fell into the category of sea monsters, ghost ships or other seemingly implausible sea stories. But too many encounters over the last 20 years by professional mariners who made it back to port to tell their stories and display their damaged ships put an end to any debate over their existence.

And now science is adding a new wrinkle to the story: These monster waves might actually be larger than anyone previously imagined. British scientists recently reported in a journal on their encounter with what may be the largest waves ever recorded by instruments in the open ocean.

Interestingly, the ship carrying the researchers wasn’t struck by a single rogue wave but rather by sets of enormous winter storm waves, which pummeled the vessel for hours. In a severe gale about 155 miles west of Scotland, the team of scientists aboard the 297-foot RRS Discovery battled giant waves that measured as high as 96 feet (29.1 meters) on a shipboard wave recorder, a record height.

While the researchers knew the storm system was approaching, they didn’t anticipate just how bad things were about to become. The RRS Discovery was forced to heave-to for several days in huge seas and high winds. The system was the equivalent of a severe storm on the Beaufort scale. “We were prepared for bad weather,” says Penny Holliday, an oceanographer with the National Oceanography Centre in Southampton, England. “But not for what we encountered. Usually, people run away when they see a forecast like that.”

The worst battering came during a 12-hour period that lasted from mid-afternoon to mid-morning of the next day. During that time, 23 waves larger than 66 feet (20 meters) hammered the research vessel, including the one that measured nearly 100 feet from crest to trough.

“We kept looking at the computer screen saying, ‘Shoot, they can’t get any bigger,’ ” recalls Holliday, 38, who was the lead researcher. “But they did. They were nothing like we’d ever seen before.”

It turns out they were nothing like anyone had ever seen or even anticipated, including the computer program models that predict wave heights. The significant wave height — the average of the highest 1/3 of the waves — was a remarkable 61 feet, which also was greater than anything previously recorded.

What caused the huge seas?

The waves were spawned by what the researchers described as a deep, complex low near Iceland and two days of strong west winds, which averaged 40.8 knots, across the entire North Atlantic. The long fetch was a significant factor in building wave height. Also, the wind and waves crossed the ocean together and at roughly the same speed, with the seas building continually under these so-called “resonant” conditions, Holliday says.

Ironically, Holliday told me that she and her fellow researchers were in that area of the ocean measuring currents, temperature and salinity, not searching for storms or giant waves. They just happened to be in the right place — or wrong place, depending on your perspective — at the right time.

And it is a testament to the ship, her captain and crew that everyone came through as well as they did.

Despite the heavy weather, Holliday says she was able to maintain her focus on her work and her confidence in the ship and its crew.

“Staying seated was a challenge, as was staying in your bunk,” she says. During one severe roll, Holliday recalls, “The chair in my cabin wound up in the bunk with me. And a 33-degree roll to starboard caused the [50-person] lifeboat to break loose.”

Holliday says the accurate wave measurements they obtained should help improve computer forecast models and better calibrate satellite-based wave sensors.

The storm took place in February 2000; the research paper was published this spring in the journal Geophysical Research Letters.

A final note on large waves. It’s very difficult to get accurate measurements of huge waves because of the paucity of buoys, ships and sensors to measure them. In that respect, U.S. researchers were fortunate two years ago when Hurricane Ivan (as a Category 4 storm) passed directly over six wave-tide gauges located on the sea floor about 75 miles south of Gulfport, Miss.

Scientists with the National Research Laboratory recorded one wave that measured 91 feet from crest to trough. And out of 146 large waves, 24 exceeded 50 feet. The researchers reported in the journal Science that they suspect individual wave heights near Ivan’s eyewall exceeded 40 meters. That’s 132 feet … or roughly the size of a 15-story building.