In the field of oceanography, researchers have been successful at simulating the ocean’s large-scale behavior but have struggled to capture smaller turbulent motions that play a key role in moving heat through the ocean. A new study co-authored by Brown researcher Baylor Fox-Kemper, professor of Earth, environmental and planetary sciences, shows that these small-scale processes have substantial effects on global model results.
As global temperatures rise, “90% of that extra energy ends up being stored by the warming of the oceans,” Fox-Kemper said, which makes ocean models important for understanding climate change.
The study focuses on the mixed layer, the upper region of the ocean where water interacts directly with the atmosphere and exchanges energy and gases such as carbon dioxide, according to Fox-Kemper.
Within this layer of the ocean are swirling currents called eddies, which are “turbulent motion that transports heat and water from place to place,” Fox-Kemper said. “When they do that they can affect both regional climate, but also collectively affect global climate.”
Kilometer-scale eddies are often too small a movement to be captured by global ocean circulation models, co-author Brandon Reichl, oceanographer at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, wrote in an email to The Herald.
But variation in water flow at the scale of a kilometer affect the ocean’s overall dynamics, so the effects of these small-scale eddies must be approximated and added to the models, Reichl wrote in an email to The Herald
In their models, researchers use parameterizations, simplified mathematical formulas that estimate the average effects of such small-scale processes. “It’s a simplification of a collective group of things to get the average behavior across that large group,” Fox-Kemper said.
The challenge of including small-scale ocean turbulence, like eddies, in global models has possibly been “the most important problem in oceanography and climate prediction that exists since the 1970s,” said Assistant Professor of Earth, Environmental and Planetary Sciences Christopher Horvat, who was not an author on the study.
In the study, researchers compared two such parameterizations: one developed by Fox-Kemper in 2011 and another developed in 2023 by co-author Abigail Bodner ScM’20 PhD’21, assistant professor of Earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology.
To test the two parameterizations, the researchers incorporated them into three different ocean modeling systems and compared the results.
The goal of the study was to “determine which ocean model responses are robust,” or similar across models, as well as “which are sensitive to different choices made when developing the global ocean models,” Reichl wrote, adding that the findings can help advance the accuracy of current climate models.
According to Fox-Kemper, researchers expected both parameterizations to cause the models’ mixed layers to become shallower — a result produced by both the older and the newer parameterization. But while both methods predicted shallower mixed layers overall, there was variation between different areas of the ocean.
“So in some places, one might have a deeper mixed layer, one might have a shallower mixed layer,” Fox-Kemper said.
The 2023 parameterization appears to be more accurate in predicting mixed layer depths, specifically near the equator, which is a “major improvement,” Fox-Kemper said.
The study’s findings suggest that “climate models are highly sensitive to the representation of ocean turbulence,” especially near the mixed layer, Bodner wrote in an email to The Herald.
Bodner noted that “more research is needed to better understand interactions across spatio-temporal scales in the ocean and their effects onto the large-scale climate.”
But “these studies help give us confidence in using these parameterizations in global ocean models and help guide future work to make our models better,” Reichl wrote.
Alice Xie is a section editor for Science and Research from Los Angeles, California. She studies Applied Mathematics and Biology, and enjoys reading gut wrenching literature in her free time.
