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Tuesday, December 12, 2017

POINTS OF VIEW

Scientists have been monitoring the oceans using two European Space Agency satellites to detect rogue waves, monitor their frequency and investigate their causes. In a three week timespan they tracked more than ten waves of at least 25 metres (80 feet) in height – more than twice the maximum anticipated wave height.

Rogue waves are typically very steep and have an equally steep trough in front of them, described by eye-witnesses as a hole in the ocean, in which a ship finds it difficult to get her bow up fast enough to rise on the monster wave. The chief officer of a container ship struck by such a wave off Australia said that he could see the 80 foot wave coming when it was still two miles off.

Shipowners and insurers have been taking an interest in this phenomenon believing that increasing sightings and vessel damage and loss might be linked to climate change. The three-year study, Max Waves, was launched in December 2000, headed up by eminent German scientist Professor Wolfgang Rosenthal. “Having proved they exist, in higher numbers than anyone expected, the next step is to forecast them,” he said.

Susanne Lehner, a professor of marine physics at the University of Miami, moved on from this project to a follow-up one called WaveAtlas, which uses the 30,000 images from Max Waves to understand how these waves work.

One pattern discerned involves normal waves encountering ocean currents which can concentrate wave energy like an optical lens. Concentrations of rogue waves are found in the Agulhas Current off South Africa and in the North Atlantic Gulf Stream.

Another pattern involves weather fronts and low pressure areas where a steady wind from a long-lived storm can boost a single wave as it moves over hundreds of miles.

These two patterns are known to combine off eastern South Africa where freak wave warnings are given by the weather service.

The Southampton Oceanography Centre believes in another pattern where freak waves can occur when one wave takes on power from other waves. “Waves steal energy from each other all the time,” it says, “and freak waves are an extreme case of that.” Wave specialists claim that in certain unstable conditions waves around the focus wave shrink, helping the focus wave to grow to an enormous size.

Other evidence of huge waves can be seen in parts of Scotland where rocks the size of a Transit van, weighing some five tons each, have been found at the top of 65 foot cliffs. Big waves usually lose power as they approach land as the shelving seabed slows them down. But where there is deep water close to the shoreline, very little of a wave’s force is dissipated before it hits the land.

Just how bad will this problem get? How should the seafarer respond? A conference entitled “The Mariner and Climate Change”, organised by The Honourable Company of Master Mariners on 29 November 2004 in London, will try and come up with some answers – the key speaker is Professor Wolfgang Rosenthal.

More information is available from the HCMM website www.hcmm.org.uk or by e-mail from enquiries@conferencebusiness.co.uk or from INTERTANKO’s Howard Snaith.