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Storm surges in the Southern Ocean have grown an average of 30 centimeters since 1985.
iStock.com/Bobbushphoto
By Colin Barras
The icy southern ocean is well known for its brutal storms, which can sink ships and trigger coastal flooding on remote tropical islands. Now, a new study suggests that the biggest waves – already the biggest ones in the world – are getting bigger, thanks to the faster winds attributed to climate change.
Peter Ruggiero, a geophysicist from Oregon State University in Corvallis who did not participate in the study, described this increase as "substantial" and is particularly concerned by the evidence that the waves were higher are gaining the fastest speed. "Yes [those waves hit] at high tide it could be catastrophic.
For 33 years, global satellites have been collecting data on ocean waves and the winds that drive them. By bouncing the energy pulses on the wave peaks and measuring the time required for these pulses, instruments called satellite altimeters can measure the height of the waves: the higher the waves, the more the signal comes back. quickly. Other satellite instruments monitor the changes in the reflectivity of the ocean surface, which is reduced by the ripples generated by the wind, to estimate the speed of ocean winds. But interpreting the data is difficult: different satellites, for example, can give different estimates of wind speed.
To minimize these discrepancies, physical oceanographer Ian Young of the University of Melbourne in Australia and mathematician Agustinus Ribal of Hasanuddin University of Makassar (Indonesia) compared the information provided by different satellites and calibrated their data against a set of independent data collected by a global network of buoys floating in the ocean. At the end of this work, two trends emerged: since 1985, the average speed of ocean winds in most countries of the world has increased by 1 to 2 centimeters per second per year, which has led to an increase in the height of waves in many places.
In the Southern Ocean, trends are particularly strong. For example, although the average wind speed has increased by 2 centimeters per second every year, the speed of the 10% fastest winds has increased by 5 centimeters per second per year. And although the average wave height has only increased by 0.3 centimeters per year, the highest 10% has increased on average by 1 centimeter per year, a 30-centimeter increase since 1985, according to the published report. aujourd & # 39; hui. Science.
The trends could be bad news for coastal communities, who face serious risks associated with rising sea levels and extreme storms, said Young. If the ocean winds are stronger and the waves higher, the storms could be much more damaging.
Young and Ribal did a good job of cross-checking and re-checking the data of the three types of satellite instruments, says Ole Johan Aarnes of the University of Bergen in Norway. But, he adds, it may be "optimistic" to think that the data now contains no errors. Confirming trends will probably require more work, he says.
The new document does not say clearly why wave height and wind speed are changing, although Young suspects a connection to climate change. Ruggiero thinks that makes sense: he points out that a recent study on Nature Communications suggests that the higher global temperatures associated with climate change are causing an expansion of the tropics – and an increase in wind speed there. "These are the side effects of climate change, not the most obvious ones like sea level rise," Young says. "That's where you put a lot of emphasis on research."
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