The rise of winds and waves
The Australian National University Science Alert 28 Mar 11;
Oceanic wind speeds and wave heights have increased significantly over the last quarter of a century according to a major new study undertaken by ANU Vice-Chancellor Professor Ian Young.
Published in Science, the study is the most comprehensive of its kind ever undertaken.
Other authors include Swinburne University oceanographers Professor Alex Babanin and Dr Stefan Zieger.
“Careful analysis of satellite data shows that extreme oceanic wind speeds and ocean wave heights have increased dramatically over the last 23 years,” Professor Young said.
“Off the southern coast of Australia, the highest one per cent of waves have increased in height from approximately five metres to almost six metres over the last 20 years.
“Extreme conditions are where we are seeing the largest increases, but mean conditions are also going up.
“Extreme wind speeds have increased over most of the globe by approximately 10 per cent over the last 20 years, or 0.5 per cent every year.
“Extreme wave heights have increased by an average of seven per cent over the last 20 years, or 0.25 per cent a year in equatorial regions and 0.5 per cent a year in higher latitudes.
“The results have potential impact on the design of coastal buildings and other structures as well as shipping. They could also have a profound effect on the transfer of energy (heat) between the sea and the atmosphere – one of the great unknowns of climate change.
“Using recently developed satellite data allowed us to investigate trends on a global scale for the first time. This has really given us a much clearer picture of what is happening in the world’s oceans.”
The study looked at satellite data over 23 years from 1985 to 2008, and was funded under an Australian Research Council Linkage Grant, with sponsorship from MetOcean Engineers.
Scientists find waves are getting bigger
Bridie Smith Sydney Morning Herald 25 Mar 11;
Ocean wind speeds and wave heights around the world have increased significantly over the past quarter of a century, according to Australian research that has given scientists their first global glimpse of the world's rising winds and waves.
Published in the journal Science today, the research – the most comprehensive of its kind ever undertaken – used satellite data collected from 1985 to 2008.
It shows the extreme wave height off the coast of south-west Australia today is six metres on average, more than a metre higher than in 1985.
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"That has all sorts of implications for coastal engineering, navigation and erosion processes," said Alex Babanin, an oceanographer at Swinburne University of Technology, Melbourne, and co-author of the paper.
However, there are greater uses for the data compiled by Professor Babanin, his Swinburne colleague Stefan Zieger and the Australian National University vice-chancellor, Ian Young.
To date scientists have largely focused on temperature as an indicator of climate change. But climate is about much more than temperature, as winds and waves control the flux of energy from the atmosphere to the ocean.
"Scientifically, this is another set of environmental properties which can be used as indicators of what is happening to the climate," Professor Babanin said. "Temperature changes the global patterns of the pressure, pressure defines the winds, winds define the waves. It's all connected."
The trio established that between 1985 and 2008, global increases in wave height were most significant for extreme waves – large spontaneous waves. They increased in height by an average of 7 per cent in the past 20 years. In equatorial regions the rise was 0.25 per cent a year, while in higher latitudes the rise was up to 1 per cent a year. The mean wave height also increased, but to a lesser degree.
When analysing extreme wind speed data over the world's oceans, the researchers found they increased by 10 per cent in the past two decades, or by 0.5 per cent a year.
Professor Babanin said waves were generated by wind. However, the data show the lift in wind speed was greater than wave height increase.
He said he doubted the 23 years of data could be immediately used to forecast future wind and wave conditions.
"These are the environmental properties which can be used as indicators for the climate behaviour along with the other properties, such as temperature and precipitation, and extrapolations have to be made with caution," he said.
World's wind and waves have been rising for decades
Wendy Zukerman New Scientist 24 Mar 11;
Wind speeds and wave heights over the world's oceans have been rising for the past quarter-century. It's unclear if this is a short-term trend, or a symptom of longer-term climatic change. Either way, more frequent hurricanes and cyclones could be on the horizon.
Ian Young at the Australian National University in Canberra and colleagues analysed satellite data from 1985 to 2008 to calculate wave heights and wind speeds over the world's oceans. They found that winds had strengthened – speeding up over most of the world's oceans by 0.25 to 0.5 per cent, on average, each year. Overall, wind speeds were 5 to 10 per cent faster than they had been 20 years earlier.
The trend was most pronounced for the strongest winds. For instance, the very fastest 1 per cent of winds were getting stronger by 0.75 per cent per year, says Young.
Average wave height was also on the rise, but less so; and the highest waves showed the strongest trend.
The results were compared against conventional measurements taken from deep-water buoys and numerical modelling. "There is variability, but the same general features are observed," Young says.
From space to sea
Previous attempts to investigate these phenomena used observations from ships and buoys, but these could generally provide only a regional picture. Using altimeter data from satellites allowed the team to detect decadal trends on a global scale for the first time.
Satellite altimeters use radar to measure the height of points on the Earth's surface, and can measure wave height very precisely. Measuring the amount of backscattering from the radar signals, meanwhile, can help calculate wind speed.
The global view afforded by the satellites reveals stronger trends in some areas than in others. For example, both wave height and wind speed have been increasing more rapidly in the oceans of the southern hemisphere than in the north.
Wave driver
Young can only speculate on what is causing the increases. "If we have oceans that are warming, that energy could feed storms, which increase wind speeds and wave heights," he says. But with a data series that covers just two decades, it's too early to tell whether there's a long-term trend at work. "We don't know the driving force."
Considering there are so many regional forces influencing waves and wind, "it's surprising that there is such a uniform trend", says Mark Hemer, a wave researcher at the Centre for Australian Weather and Climate Research in Hobart, Tasmania. Variability in winds and waves associated with weather systems such as El Niño and La Niña, the North Atlantic Oscillation and the Southern Annular Mode could all help to explain it, he says.
In either case, if winds continue to strengthen and waves to rise – even if only for a few years – it suggests more intense storms, hurricanes and cyclones are on the horizon, says Young.
However, Tom Baldock, a coastal engineer at the University of Queensland in St Lucia, Australia, says that although there is no reason to doubt the analysis, it doesn't mean more coastal natural disasters will necessarily ensue. "Tornados, hurricanes and cyclones occur through complicated regional weather conditions, and are not just related to wind speed and wave height," he says. For example, there are higher wind speeds at high latitudes, but most cyclones hit around the equator.
The new study may be more relevant to the burgeoning offshore gas, wind, wave and tidal power industries, Baldock thinks. "Larger waves are a hazard for any offshore construction."
Journal reference: Science, DOI: 10.1126/science.1197219
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