May 9, 2001 A new study by scientists at Scripps Institution of Oceanography at the University of California, San Diego, has dramatically elevated the importance and influence of oceanic whitecaps on global climates.
Whitecaps, the bright, wind-driven result of breaking wave crests, have been mostly ignored by climate models.
In a paper that appeared in the April 15 edition of Geophysical Research Letters, Robert Frouin and Sam Iacobellis of Scripps, along with Pierre-Yves Deschamps of the Laboratoire d'Optique Atmospherique in France, for the first time clearly define the amount whitecaps impact climate by reflecting sunlight, or solar radiation. The presence of whitecaps results in less radiation reaching the surface of the ocean--especially in cloudless skies-- changing the way this radiation energy impacts the ocean, the atmosphere, and entire climate system.
"Our estimate of the global radiative forcing by oceanic whitecaps is small, yet not negligible compared with the direct forcing by some greenhouse gases and even anthropogenic aerosols," said Frouin. "In contrast to well-mixed greenhouse gases, whitecaps are more localized, and their forcing may be much larger on regional and seasonal scales, with definite effects on climate."
Basing their new research on Frouin's earlier findings that identified the spectral dependence of whitecap reflectance, the researchers used satellite data and other measurements to calculate how much solar radiation whitecaps reflect away from the surface. They found a global average of .03 watts per meter squared. However in certain regions, such as parts of the Indian Ocean, this average jumped significantly--in some cases up to .7 watts per meter squared.
This was particularly true in the case of the Arabian Sea, which can exhibit cloudless skies and great wind speed--two important factors in increasing the role of whitecaps.
Previously, white caps were largely disregarded in climate models. Historically, they played a stagnant and incomplete role in these models. Most models pegged their importance on a broad brush figure, rather than a calculated global and regional influence factoring wind speed and cloud cover, as Frouin, Iacobellis, and Deschamps have done.
"We've demonstrated that in certain cases these whitecaps might be important players in evaluating how regions respond to climate change," said Iacobellis. "Hopefully we've shown that whitecaps should be included in climate models."
The authors note that compared with carbon dioxide, whitecaps have a relatively small influence on climate. With other gases, however, such as nitrous oxide, whitecaps can have a comparable effect.
The authors also note that their findings play into the tangled equation of changes due to greenhouse gases. Greenhouse warming may change wind speed, thus altering the amount of whitecaps, and as a result changing the amount of radiation white caps reflect, ultimately changing heat content and temperatures.
"Many competing effects and feed-backs may be involved, and are difficult to untangle," said Frouin.
The research was supported by the National Aeronautics and Space Administration, the Department of Energy, the California Space Institute, the Centre National d'Etudes Spatiales, and the Centre National de la Recherche Scientifique.
Note: Images available at http://scrippsnews.ucsd.edu/releases2001/frouin_whitecaps.html
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