Over the last three decades, ocean winds and waves have grown stronger and taller — as much as 8% and 5% respectively — in most regions around the world, with the greatest increases occurring in the waters surrounding Antarctica, according to a new study.
Using global satellite measurements of wind speed and wave height spanning more than thirty years, from 1985 to 2018, the results, published in the journal Science on April 26, are helping to refine the understanding of if and how critical oceanic and atmospheric processes are impacted by reoccurring climate patterns like the El Nino-Southern Oscillation or by long-term climate warming caused by human activity.
Ocean wind and wave patterns play a central role in Earth's climate system and the potentially damaging effects of extreme weather events. Not only do these processes greatly contribute to the ability of oceans to pull carbon dioxide from the atmosphere and store it away for centuries, changes in wind speeds and wave heights are largely responsible for increased storm surge levels, coastal flooding and erosion during storms.
The ability to make accurate projections of ocean winds and waves, particularly under extreme weather conditions, relies on understanding if and how wind velocity and wave heights have changed over the past few decades and what forces are driving such changes, according to the authors.
However, measuring small changes in global wind and wave trends like these is challenging and requires carefully calibrated long-term global data sets, said lead author Ian Young, a researcher at the University of Melbourne, Australia.
Records of wind speeds and wave heights collected from oceanographic buoys scattered throughout the oceans are generally not suitable for long-term trend analysis, since differences in buoy types and instrumentation limit their validity. Furthermore, despite providing comprehensive global coverage for the last 33 years, concerns about the consistency of data collected from satellites orbiting high above have not yet been fully addressed.
"Changes in wind speed and wave height year-to-year are small, but become important over long periods of time," said Young. "Accurately measuring these differences is important as small errors in calibration of the satellites can give rise to spurious trend measurements."
Satellite measurements of wind speeds and wave height on the surface of Earth are collected using several independent instruments, which each work in a different way; altimeters measure wave height and wind speed, radiometers measure wind speed and scatterometers measure wind speed and direction. Additionally, each instrument collects and calibrates measurements differently across satellite platforms, which can result in inconsistent and incomparable data that bias survey results.
To overcome these challenges, Young and co-author Agustinus Ribal of Hasanuddin University in Indonesia carefully calibrated 33 years of global satellite measurements against a global network of nearly 80 oceanographic buoys to provide a reliable multi-platform dataset of nearly 4 billion individual observations of wind speeds and wave heights.
Young and Ribal's analysis revealed significant increases in global wind speed and wave heights, particularly during storms, which have grown more severe over the last 33 years. What's more, the authors found strong regional variations in the trends — winds and waves in the Southern Ocean appear to have increased more than those in the equatorial Pacific and North Atlantic.
The larger increases in the Southern Ocean are important because it is where many long-distance ocean swells dominate the wave conditions for the Indian Ocean, the Southern Atlantic and much of the Pacific. It's these wave patterns that determine the rates of erosion for much of the Southern Hemisphere, the authors suggest.
However, across all the oceans, increases in wave heights do not seem to be keeping pace with increases in wind speeds, which was a surprise according to Young.
"To generate large waves, you need both a strong wind and it must be sustained for a sufficient period," said Young. "Although the mean wind speeds are increasing it appears that their duration has not also increased. As a result, the wave heights have not increased as rapidly as the winds."
Young hopes the study helps inform the upcoming 2022 International Panel on Climate Change report and assists coastal planners who are developing strategies to combat flooding resulting from storms and sea-level rise.
"The offshore wind industry will also be interested in the results," suggested Young. "Planning such offshore facilities requires knowledge of the future wind resources."