Research Highlight: Waves Bring the Heat

Scientists at Scripps Institution of Oceanography at UC San Diego who study the oceanography of the nearshore environment have pioneered several aspects in recent decades—from developing surf forecasting tools to understanding how pollution moves along the beach.

Now, Scripps coastal oceanographers have reached another milestone with an unexpected discovery about breaking waves and their importance in the beach environment, well beyond their recreational attraction for tourists and surfers.

Backed by measurements captured with an array of equipment, Scripps researchers Falk Feddersen and Greg Sinnett found that energy from breaking waves generates much more heat in the surf zone than anticipated, surprisingly comparable to a quarter of the daily radiation that comes from the sun. Surf zone heating was previously thought to primarily come from the sun and a portion from the air. The new data show that internal friction generated by the energy of breaking waves produces far more heat than anyone expected.

“Nobody has calculated this before, but now we’ve been able to develop the first surf-zone heat budget,” said Feddersen, a Scripps alumnus and now an acting professor of coastal physical oceanography at Scripps.

Feddersen said the results are significant for shoreline ecosystems, particularly for organisms along the shoreline and tidepools whose habitats are influenced by breaking waves and corresponding temperature fluctuations.

The measurements behind the findings were recorded over 47 days in the summer of 2013 with equipment deployed off the Ellen Browning Scripps Memorial Pier. The array included temperature sensors to measure heat content and instrumentation provided by Scripps climate researcher Dan Cayan to measure incoming solar radiation. This information was cross-referenced with wave energy information from the Coastal Data Information Program. (CDIP is a program developed at Scripps that transmits valuable coastline information from a network of buoys to a variety of end users.) The researchers then tested the relative solar heating to wave heating to calculate temperature variations in the surf zone.

“The evidence (in the new study) is compelling,” said Sinnett, a third-year Scripps graduate student.

The results reflecting the magnitude of breaking wave heat have led the scientists to dig deeper into the inner details of surf zone heat dynamics. Current work focuses on areas such as how much light and radiation is reflected by breaking waves, otherwise known as measuring “albedo.”

“Addressing additional questions will not only help refine the surf zone heat budget, but will also help our understanding of coastal mixing, diffusion, and stratification—all important factors when considering the health of our coastal oceans,” said Sinnett.

While the data backing the new study, which appeared in the journal Geophysical Research Letters, were extracted during Southern California summer days, with seasonally typical small waves and plenty of sunlight, Feddersen says a more telling story of breaking waves and the heat they impart could be revealed in locations such as the Pacific Northwest. Up north, the pounding energy of big and powerful breaking waves and cloudy conditions in winter could mean that surf zone heat could play an even larger role.

The study was funded by the National Science Foundation and the Scripps Graduate Teaching Fellows in K-12 Education (GK-12) program. Kent Smith, Rob Grenzeback, Brian Woodward, Dennis Darnell, and Kai Hally-Rosendahl collected the field data for the study.

-- Mario C. Aguilera

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