May 5, 2013

More killer waves than we thought

THE EUROPEAN SPACE AGENCY has confirmed that freak waves exist in the world’s oceans “in higher numbers than anyone expected.” Wolfgang Rosenthal, senior scientist with the GKSS Research Center in Geesthacht, Germany, says two of the agency’s Earth-scanning satellites used radar to monitor the oceans for three weeks. They identified more than 10 individual giant waves around the globe that were more than 81 feet high.

Laurence Draper, of the British National Institute of Oceanography, has long insisted it’s no old wives’ tale that every fifth, or seventh, or ninth wave is larger than the others. Sea systems are composed of many different wave trains, he says, each with its own speed and height.

So, at random intervals, waves can ride on each other’s backs to form an exceptionally high wave — and it doesn’t have to be blowing hard. Draper estimates that one wave in 23 is twice the average height; one in 1,175 is three times higher; and one in 300,000 is more than four times higher.

But it’s the height of the breaking crest that’s the greatest threat to small sailboats. Luckily, the size of the crest does not necessarily relate to the size of the wave in deep open water.

Watch out for more frequent giant waves when you’re in a strong ocean current. Winds blowing against the current create the biggest and steepest waves.

Today’s Thought
Under every deep a lower deep opens.
— Emerson, Essays, First Series: Circles

According to the cynics among us, a platonic relationship is the interval between the introduction and the first grope.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

1 comment:

Matt Marsh said...

This is fascinating stuff, John, and it's neat to see how much scientific interest has been directed at "rogue" waves in the last decade or so.

The nonlinearity aspects in particular are proving to be worth a look; it seems that the biggest waves aren't just a simple summation (wave on top of wave) but are in fact extracting energy from the rest of the wave system to make one individual wave higher and steeper. There are models now that suggest that eddy currents can act as lenses of a sort, causing several wave systems to converge and combine in this fashion. Very cool (as long as you're not caught in the path of one).

(Some interesting further reading: Ying et al 2011, )