Being able to discern the strength of breaking waves in the open ocean is crucial for those forecasting stormy seas and for scientists wrestling with questions of how the Earth's oceans absorb and release greenhouse gases and heat.
A new remote infrared imaging technique has given scientists a promising way to better understand breaking waves, according to a report in this week's issue of Nature.
The technique reveals the area of the wake following individual breaking waves in the open ocean -- something scientists have not been able to measure before, according to Andrew Jessup, lead author and a senior oceanographer at the Applied Physics Laboratory of the University of Washington. The new technique also has made it possible to remotely measure the energy dissipated by breaking waves, something that has long eluded oceanographers.
In this case, Jessup and his collaborators from the University of Washington and University of Toronto are taking advantage of infrared imagery's ability to detect temperature changes in the "skin-layer" of the ocean, the top millimeter. This layer is generally a few tenths of a degree cooler than the water below. When the skin is disturbed by a breaking wave, the slightly warmer water mixed up to the surface can be detected by infrared technology. Until the skin-layer re-establishes itself, differences in temperature provide information about the strength of the breaking wave and the area of the ocean surface that is being disturbed.
"Scientists have studied waves going back to the time of Ben Franklin trying to predict how rough the sea will be in stormy weather" Jessup says. Those involved in shipping and transportation, including the U.S. Navy, already use measurements from satellites -- primarily radar data -- to estimate wind speed and the height of waves.
The new information provided by infrared imagery should help reveal how much turbulent mixing happens at the same time. It should help to quantify how much of the wind energy is actually transferred to the water.
For those working on global-wave models, this new information could help them close the loop on predicting what kind of seas to expect when the wind blows at a given speed for a given amount of time.
The data gathered using the new technique also could help those interested in the heat transfer between the ocean and the atmosphere, which impacts weather prediction. It also could help those studying gas transfer, which impacts global climate-change models.
Most intriguing of the possibilities, Jessup says, is the chance to study the elusive small-scale processes taking place at the ocean surface.
"White caps are the largest and most obvious kind of wave breaking," Jessup says. "But the oceans also have very small-scale breaking, what's called micro-scale breaking. Waves down to the scale of centimeters also disrupt the ocean surface and allow a conduit for the exchange of heat and gases. We'd like to accurately measure these processes and take them into account in order to improve existing models.
"I think that throughout science, and not just oceanography, you make significant advances when you have a new technique that allows you to look at the details of a process that has been known and speculated about but has never been measured."
Jessup and his colleagues are one of only a handful of groups exploring the potential of infrared techniques for air-sea interaction studies and are the first to publish information about this particular application in a major scholarly journal.
Co-authors are Chris Zappa, a University of Washington doctoral candidate in civil engineering, Professor Mark Loewen of the University of Toronto and engineer Vahid Hesany of the UW's Applied Physics Laboratory.
The work described in Nature was funded by the Office of Naval Research, NASA and the Applied Physics Laboratory. Loewen's part of the project was funded by the Natural Sciences and Engineering Research Council of Canada. (Jessup also receives funding from the National Science Foundation.)