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Sunday 28 May 2017

 Major front gives NZ snow, severe gales and high seas

Saturday 20 May saw a major cold front sweep across New Zealand bringing snow to low levels, and severe gales and high seas that peaked at 11.4m in Cook Strait.

Snow settled as low as 100m above sea level in Dunedin, and NZ Metservice reported it was deep enough to close parts of the main State Highways 1, 85 and 87 to the north, and gave 12cm snow cover at Kingston, south of Queenstown. Much deeper snow fell in the high country in the South Island Alps. On the North Island, there was a dusting of snow on the hills from Wellington to the Bay of Plenty and motorists were trapped by snow and ice on the Desert Road which gets to 1,000m SW of Taupo [Radio NZ]. Icy roads caused problems over much of the country for a few days following the change as temperatures fell well below -5° [Radio NZ]. MetService put together this montage of the country's first major snow event of 2017.

A large swell developed behind the change, driven by severe gales across the huge fetch of the Southern Ocean. A pulse of swells averaging over 6.5m pushed up the east coast of both islands on 21 May. MetService, in its Blog, said "A wave buoy in the Cook Strait has been recording average wave heights of five to six metres, with a peak wave early this morning of 11.4 metres." Not surprisingly, inter-island ferry sailings were cancelled [Radio NZ].

 Monster wave one of largest recorded in southern hemisphere
Updated 31/5/17: substantial information added in paragraphs 4 to 6

The system that brought gales and snow to NZ also produced a monster 19.4m wave at the southern-most wave-measuring buoy in the Southern Ocean, moored in 150m of water 11km S of Campbell Island or 700km S of Invercargill.

The buoy, which was put in service in February by MetOcean Solutions, is designed to collect observations from an area lacking in hard data. MetOcean's Senior Oceanographer Dr Tom Durrant said "During the depths of winter, Southern Ocean waves are enormous, with significant wave heights averaging over 5m, and regularly exceeding 10m. Individual waves can double that size. Accurate measurements of these conditions will help us understand waves and air-sea interactions in these extreme conditions. This, in turn, will lead to improvements in the models used to simulate the waves, providing better forecasts, both for the Southern Ocean and for the wider region."

It's important to note the difference between "highest wave" and "highest significant wave height". Oceanographers describe highest significant wave height as the highest third of waves over a period of time. This is sometimes described as the highest averaged over 15 or 20 passing waves. A highest wave is simply the highest wave of all waves, which may be the highest within the significant wave period or a rogue wave that is far larger than would normally be expected in a normal distribution of waves.

While rogue or freak waves of phenomenal size have been reported for centuries (or not reported, as the crew didn't live to tell the tale), it has only been since two events, in 1995 and 2000, that science has begun to take them seriously. In 1995, what has come to be known as the Draupner Wave was the first rogue wave to be recorded scientifically as it passed the Draupner E oil platform 160km WSW of the southern tip of Norway in the North Sea. The platform was constructed to withstand what was considered to be the 1 in 10,000-year wave of 19.5m and had been fitted with state-of-the-art instrumentation. The isolated Draupner Wave was measured at 25.9m soon after 15.00 on 1 January 1995, fortunately only causing minor damage.

In 2000, the British oceanographic vessel RRS Discovery, fully kitted out with high quality instruments, was battling high seas near Rockall, off the west coast of Scotland, in a severe gale averaging 75km/h. Its wave recorder measured individual waves up to 29.1m from trough to crest with a significant wave height (i.e. highest third) of 18.5m. While not theoretically a rogue wave, it proved that waves existed that were much higher than models at the time predicted based on a normal distribution of wave heights. It also began the rewriting of oceanographic textbooks which up to that time had ignored rogue or freak waves completely. Further evidence of their frequency came in 2004 when the ESA MaxWave project identified over ten giant waves over 25 metres in just three weeks in a limited area of the South Atlantic.

Satellite wave studies now tell us that waves between 20 and 30 metres happen much more often than previously thought. Professor Akhmediev of the Australian National University, one of the world's leading researchers in this field, says there are about 10 rogue waves in the world's oceans at any moment with some researchers believing about 3 of every 10,000 waves could be classified as rogue or freak, with that number rising to 3 in every 1,000 in coastal inlets where energy is focussed. Something to think about next time you go rock fishing.

This Wikipedia article on Rogue waves has riveting reading on the history, current knowledge and research on one of the least known, most dangerous yet not uncommon aspects of the ocean.

Here's the relevant MetOcean Blog and, to appreciate the size and power of the size of waves we're talking about, look at this Royal New Zealand Navy video on Radio New Zealand, and these are relative wimps compared to a 29-metre one. The World Meteorological Organisation maintains a list of highest significant wave heights on record globally - go to the bottom and click the "Phenomena characteristics" for more information on each one. And this BBC article has some information on high individual waves, and that other breeding ground of mighty seas, the North Atlantic.