When Old Dominion University Ocean & Earth Sciences (OES) Professor Tal Ezer agreed to help a graduate student of biological oceanography on a research project, he didn’t know it would lead him to challenge a century-old oceanography theory.
Ezer, a member of the University’s Center for Coastal Physical Oceanography (CCPO), helped graduate student Huangqing Huang set up a computer model of ocean mixing that could be used to study distribution of biological particles in the ocean, a part of Huang’s research with OES Professor Alexander Bochdanski. However, Ezer, whose main research focus is climate change, sea level rise and flood prediction, had the idea of a side project to use the same model to test the validity of a 120-year-old oceanography theory he had been teaching for years in his Physical Oceanography classes.
The story behind the original theory starts back in the late 1800s in the Arctic. From 1893 to 1896, Norwegian explorer Fridtjof Nansen led the Fram voyage across the Arctic Ocean. While he was unsuccessful in reaching the North Pole, he made many new oceanic discoveries, including the observation that sea ice is drifting to the right of the wind direction.
In 1905, the Swedish oceanographer Vagn Walfrid Ekman was able to explain Nansen’s observations and published the classical Ekman theory (Ekman, 1905) on how wind-driven ocean currents turn with depth (to the right in the northern hemisphere). The theory shows the importance of Earth rotation (the Coriolis effect) and water turbulence in wind-driven ocean currents. Today, the terms “Ekman Transport,” “Ekman Layer,” “Ekman Spiral” and “Ekman Upwelling” appear in most oceanographic textbooks.
Ezer did not intend to challenge the Ekman theory.
“I was using the Ekman theory ever since I've been teaching physical oceanography for quite some time,” he said. “And it's a very important theory for many processes in physical and biological oceanography such as upwelling that increases biological productivity in many regions.”
The Ekman theory assumed a constant wind, a constant (unknown) turbulence and constant density – conditions that are rarely found in the real ocean. “Therefore, I set up experiments to test the Ekman theory by comparing the theory against a computer ocean model that can simulate more realistic oceanic conditions with wind- and wave-driven turbulent mixing,” Ezer explained. The results were published last month in Springer-Nature’s journal Ocean Dynamics.
“I conducted more than 20 different model experiments with different winds and different density stratifications and demonstrated the conditions in which the classical theory works better or when it is less accurate,” Ezer said.
An important finding of the study was an empirical formula that allows scientists to estimate the turbulent mixing from observed wind and mixed-layer depth, making the original Ekman theoretical calculation more applicable to realistic oceanic conditions.
“This finding can help oceanographers study many important oceanic processes, such as the projects conducted in Professor Bochdansky’s biological lab,” Ezer said.
When the Ekman theory was developed “it was elegant and simple, but very limited observations were available at the time to validate the theory, and computer models like those I use did not exist at all. Therefore, I decided to test how we can use the computer ocean model to verify the old theory and to find out how to adjust the old model so it can be more practical for real ocean conditions,” he said. “It is always interesting to check old theories with more modern tools that we have today.”
Following this project, interdisciplinary research on physical-biological interactions of particles in the ocean is being developed by Ezer and Bochdansky’s groups. Huang plans to present preliminary results at the Ocean Sciences Meeting next year in New Orleans.