What is a Rogue Wave?

| Tuesday, 05 Mar. 2013

The last few weeks brought two fatal cruise ship accidents. First the Costa Europa was windblown against a dock in Sharm El Sheik on the Sinai Peninsula killing two crewmen. Then the Greek/Cypriot cruise ship Louis Majesty was hit by a series of three rogue waves, killing two European passengers when sea water shattered the glass in a public observation room. These two accidents were among the worst in modern cruise history - and the amazing coincidence that they both occurred within days of the other may lead some people to believe that cruise ship-related accidents occur far often than history tells us.

Well, in fact history is correct. Fatal accidents for passengers are extremely rare in the cruise industry - there have been entire years in recent history when no passengers died of an accident on a CLIA-member cruise ship at all. (CLIA is the Cruise Line Industry Association based in the U.S.)

Both of the two recent accidents were the result of bad weather. A strong wind blew the Costa Europa against a dock hard enough to breach her hull. The captain quickly put ballast water into the opposing tank to make the ship lean away from the hole, but it is believed two crewmembers were unfortunately in the vicinity of the impact.

The Louis Majesty was hit by rogue waves while sailing in the Mediterranean Sea close to Marseille, France. I have often encountered strong winds in this area. I was on the new MSC Splendida last May when the ship had to skip a scheduled stop in Marseilles due to high wind speeds. As we sailed on to Toulouse the gusts were so strong the entire ship was shaking.

I recall another cruise with a stop in Monte Carlo where hundreds of passengers were tendered ashore in the calm morning weather, but the wind picked up in the afternoon. Getting the same passengers back onto the ship from the tenders was extremely difficult. We watched crewmen literally lifting people back onto the ship platform as the tenders pitched up and down by up to four feet. No one was hurt, but I saw a two-inch thick rope break when a tender suddenly lurched away from the platform.

Sometimes I have to laugh when I hear people saying cruising is for little old ladies. In truth, it can be dangerous at very rare times - like any travel experience. But I don't want to leave you with the wrong impression, cruising is still by far the safest form of travel available in modern life in terms of the very low percentage of accidents resulting in injuries and especially fatalities.

So - what causes high seas and rogue waves in particular? Air motion over the surface of the ocean creates waves; but waves then take on a momentum of their own once they are set into motion.

Waves will continue to oscillate until something happens to change the momentum. One thing that changes the momentum of a wave is when it meets another wave, usually coming from a different direction. Two waves meeting each other will combine their momentum to create a single wave roughly twice the size of the two waves. The combined amplitude will only last for the exact moment when the waves are essentially passing through each other, but sometimes that is enough to cause "freak waves" crashing into ships like the Louis Majesty.

I just read an article by a Weather.com meteorologist somewhat mocking the term "rogue wave", as if it implies an evil and lurking vampire-like phenomenon. His premise is that they are not "rogue" at all; they are actually quite predictable sea conditions.

He is correct, of course, that wave motion is regular and predictable in any most conditions, but conditions change. Two weather fronts can meet and create opposing waves of considerable amplitude which can create abnormally large waves. That is the definition of a "perfect storm", a phrase which ironically is now used to describe any confluence of factors leading to an unusual result. But the term was first used to describe the exact conditions that result in "rogue waves".

So, how big must a wave become before it is considered a rogue wave? Oceanographers define a rogue wave as more than twice the significant wave height (SWH), which is defined as "the mean of the largest third of waves in a wave record." In other words, a rogue wave is over twice the size of the largest waves occurring under predictable conditions.

Rogue waves are typically the result of what is called a "focusing effect," or a combination of conditions including weather and the geographic conditions in the location. Rogue waves occur most often in deep water combined with converging currents (two factors which tend to remain constant) along with unusual weather conditions.

In other words - there are certain areas that are prone to high seas, and as we are just beginning to learn, there are even areas that can be prone to "rogue waves." Perhaps it is not surprising that recorded rogue waves have also hit ships within the area known as the "Bermuda Triangle." As we eventually discover more areas prone to rogue waves they could become the oceanic mythological equivalent to Transylvania.

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