Here be monsters

Long a part of sailors' tales, massive waves that emerge from nowhere were thought to be just another part of seafaring lore until instruments in the North Sea recorded one. Finding an explanation for them could save lives.

Not all waves are made the same, as the Japanese artist Katsushika Hokusai powerfully showed in his 1820's woodcut The Great Waves of Kanagawa.

Mariners have good reason to fear the seas a few hundred kilometres off the Pacific coast of Japan. The area is ravaged by severe storms and powerful currents, and many ships have foundered there over the years. But on 23 June last year, the Japanese fishing vessel Suwa-Maru No 58 seems to have encountered something else there - a monster of the sea which claimed the ship and all 22 on board.

The attack came out of the blue, and took place with such violence that no visual evidence of the culprit survives. But now a team of scientists in Japan has examined the circumstances surrounding the attack and come up with a portrait of the monster: a huge wave that can appear seemingly from nowhere to sink even the hardiest vessel. Stories of giant waves have been a feature of mariners' tales for centuries, where they spice accounts of storms that came within an ace of swamping ships. Science has confirmed the truth of such tales: when hurricane Ivan struck the Gulf of Mexico in September 2004, undersea pressure gauges revealed the presence of colossal waves over 27m tall and 180m long. Experts estimate that waves over 40m high can emerge in such storms.

These monstrous waves may explain many cases of large merchant ships caught in severe storms which sink in seconds. In 1980, the 90,000-tonne British bulk carrier MV Derbyshire sank with all 44 on board in a typhoon off the south coast of Japan. A subsequent inquiry found that the vessel's forward hatch cover had been smashed in by giant waves, causing water to flood in so rapidly that the vessel sank before the crew could send a distress call.

Yet scientists have struggled to credit stories of "rogue waves" prowling the seas in less dramatic conditions. Their scepticism dissolved following an event that took place during stormy conditions in the North Sea on 1 January 1995. Gauges fitted to the Norwegian Draupner gas platform were steadily recording waves just a few metres in height when a huge rogue wave struck, reaching over 18m above sea level.

The event spurred attempts to find explanations for these solitary monsters, which have also been recorded by orbiting satellites. What triggers their emergence - and what sustains them as they cross the sea? The reality of self-sustaining waves was first noted over 170 years ago by an engineer walking along the Union Canal in central Scotland. John Scott Russell watched as a boat being towed by horses suddenly stopped, releasing a wave from its bow. To his amazement, the wave continued on, neither slowing nor changing shape, for around 15km. Intrigued, Russell built his own wave tank and began studying the phenomenon, which seemed to defy conventional physics. Theorists later showed how such waves - known as solitons - can maintain their speed and height long after conventional waves fade away. The explanation lies in so-called non-linear effects - put simply, effects where the whole is greater than the sum of the parts. Theorists believe these might explain the occurrence of rogue waves, where two waves come together, spawning a single wave much greater than twice the height of its parents.

The same can happen when waves formed by strong winds and currents combine with the long, rolling swell of the sea surface - which is a common occurrence in the part of the Pacific where the Suwa-Maru No 58 was lost. Among oceanographers, the area is renowned for the so-called Kuroshio Extension, one of the most powerful currents in the Pacific which transports around 10 trillion tonnes of warm water through the area each day. The colossal amounts of heat flowing through the sea and air makes the area prone to large waves and storm-force winds.

But did they generate lethal rogue waves on that June day last year? To find out, Dr Hitoshi Tamura and colleagues at the Japan Agency for Marine-Earth Science and Technology in Yokohama used weather records from around the time of the sinking to examine the pattern of waves and swell in the area, and see how they worked together. At first sight, the records show nothing unusual: conditions were rough, but hardly exceptional, with wind speeds of around 40kph, and waves around 3m in height. But the team noticed that about a day before the sinking, the wind-speed suddenly picked up, making the local waves both taller and more steep-sided. At the same time, the waves began to feed energy into a long, rolling swell that had come in from the Japanese coast. Then, around four hours before the sinking, the wind direction began to change - spelling disaster for the Suwa-Maru. Theoretical work has shown that if wind-generated waves cut across a swell at the right angle and undulate at a similar rate, the result can be rogue waves. And according to the computer simulation run by Tamura and his colleagues, that is precisely what happened in the morning of June 23: the wind and swell conspired to work together, with disastrous consequences for the crew of the Suwa-Maru. An investigation by the Japanese Coastguard suggests that the fishing vessel encountered two sets of rogue waves, and sank in just 10 minutes.

The analysis of the incident, published last month in Geophysical Research Letters, is more than a mathematical post-mortem. It has provided the first real-life confirmation of theories about how rogue waves are formed. Better still, by focusing attention on how wind-driven waves interact with sea swell, forecasters may be able to warn of rogue waves long before they form. If so, the sinking of the Suwa-Maru may mark the end of the reign of terror of these monsters of the sea.

Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England