June 2009 - Issue #6
A modern-day oracle on the ocean waves
Story by Julian Cribb
View articles in related topics: Sustainability & The Environment
In the secret life of ocean waves are prognostications of the human future.
For more than 30 years a squadron of polar-orbiting satellites and a vast fleet of buoys have monitored the oceans, trying to get a fix on the size, frequency and power of its waves because, just as a thermometer reveals the patient’s state of health, waves offer insights into the Earth’s climatic temperament.
Like gypsies read tea leaves to foresee the future, researchers can read something of our future in the winds and waves.
A deep knowledge of waves can tell us how to design safer, stronger, more efficient marine structures – or it may reveal aspects of how our world is changing, indecipherable by any other means, says the Vice-Chancellor of Swinburne University of Technology, Professor Ian Young.
Like the augurs of old, Professor Young hopes we can read something of our future in the winds and waves by amassing and analysing all the available data collected on them over a generation. With the help of an Australian Research Council Linkage Grant and in partnership with Perth-based company RPS MetOcean, he is attempting to assemble the world’s first complete picture of ocean wave activity.
“In the sizes of waves you can see the fingerprints of the El Niño that is causing the present huge drought in south-eastern Australia,” Professor Young explains. “You can see how the low pressure systems change latitude.”
The size, frequency and location of waves – great and small – are another way to study how the Earth’s atmosphere and oceans interact; a way to discern on a global scale how vigorously they work together as the planet warms, like a pot of water being heated on a stove.
The task is gargantuan. Seven US and European satellites have each been recording the height of the waves passing in a swathe beneath their radar altimeters every second of the day for more than quarter of a century. From 200 kilometres or more in space they can estimate the height of a wave within 40 to 50 millimetres, and from this much can be deduced about the winds, weather systems, currents and other conditions that produced it. Together they have generated a massive 12 terabytes of information.
However, each of the satellites has a slightly different instrument and their data have been collected and stored using different programs.
The data from each must be laboriously compared with the others and ground-truthed for accuracy against a fleet of wave-riding buoys. Then they must be harmonised into a single set that scientists and engineers can interrogate to find out what is afoot with the world’s waves.
“Waves have a great advantage over other indicators of the planet’s condition, such as temperature or wind speed,” Professor Young says. “These vary widely and rapidly, so when you look at them you inevitably get a lot of ‘noise’, which makes it hard to interpret what is going on. Waves smooth all that out. They enable us to read the average wind speed at the sea surface from space over a longish period of time, without recording all the gusts. This tells us a lot more about the overall conditions that produced the waves. We’ll be able to see more subtle changes that may have previously been masked by the amount of noise in the measurements.
“If, for instance, the winds are changing as a result of global warming, these changes will be reflected in the waves that they generate. We can also look at changes in the extremes.”
Though each satellite monitors only a narrow swathe of ocean as it passes overhead, eventually, as the Earth rotates, these swathes cover the entire planet and a detailed global picture of wind and wave conditions over years can be assembled, he explains.
Besides helping to identify global changes before they come on us unawares, Swinburne’s work has a supremely practical application: a better knowledge of wave heights – and hence of the frequencies of severe storms – can be used to design safer ships, offshore platforms, harbours, jetties and other marine structures.
“To design a platform you need to know the typical wave height in a one-in-100-year storm, so the top of the platform will be above the waves. Every extra metre you don’t need to build can save you $10 million,” Professor Young says. Likewise marine architects can study the conditions along the sea routes their new vessels will sail based on decades of information, and shape the design accordingly. Planners of coastal cities and ports can better adapt their seafront to the extremes expected under climate change.
Industry partner RPS MetOcean hopes to obtain precise information about ocean conditions in specific locations out of the new dataset to improve the design quality and efficiency of their projects.
“As physical oceanographic consultants working mainly with the offshore oil and gas industry, our job is to quantify the marine environment their structures are likely to have to withstand over the coming 50 to 60 years,” explains MetOcean’s Steve Buchan. “We need to be able to predict the future wave climate in particular, because waves are usually the greatest determinant of the survival of offshore structures, typically exerting more force than winds or currents.”
Optimising the design of these steel leviathans requires a deep insight into likely wave conditions now and in the future. For the first time, the Swinburne project offers that information planet-wide, Mr Buchan says. “We will use the data to improve and calibrate the models we use to predict ocean conditions so platform developers can design to a less than one-in-10,000 chance of structural failure.”
RPS MetOcean is one of a handful of companies worldwide with this sort of capability, and early access to the consolidated global wave data will give Australia a clear competitive edge in the field, he adds.
