A pair of oceanographers have provided an answer to the riddle of how eddies of circular currents several kilometers wide, known as ocean rings or eddies, remain intact, as detailed the study presented in the latest issue of the journal Geophysical Research Letters.
According to the researchers, these ocean rings are vitally important for transporting heat and nutrients throughout the ocean and can last anywhere from a few months to several years.
Naval Postgraduate School (NPS) Oceanography doctoral student Larry Gulliver and Professor Timour Radko have managed to decipher the main reason why some oceanic rings last up to a decade, while others dissipate within a few months. .
The key is at the bottom of the ocean
The answer lies in the topography of the seafloor. According to scientists, this new understanding of how the ocean floor impacts surface currents will improve the complex numerical models used by the US Navy’s meteorology and oceanography (METOC) community to provide critical information to operational commanders.
“We need to remove systematic biases that numerical models have, and some of them are related to how the models handle small-scale bottom topography,” Radko explained in a statement.
The “sandpaper effect”
Radko and Gulliver call their finding the “sandpaper effect,” a nickname associated with the tiny abrasive particles of sandpaper that can grind down much larger objects. In the same way, the small-scale texture of the seafloor slows currents near the ocean floor, enhancing the stability and longevity of ocean rings near the surface.
Scientists have been trying to figure out what makes large vortices stable and long-lasting for about 50 years, but no one thought to look at the small-scale topography of the ocean floor because it seemed too far away to impact these ocean rings. Theorists typically don’t even consider topographic roughness when looking at surface water activity.
“Now I have doubts (about the current models),” admitted Radko. “If this small-scale topography affects this vortex, it can affect currents, waves and so on. I’m getting skeptical of anything that assumes the bottom is smooth,” he added.
A much more rigorous new model
Regardless of small-scale topography, physics suggests ocean rings should dissipate within a few weeks. This was proven by old documents that did not take background roughness into account in their models.
The NPS researchers realized that the key to the “perfect model” is to make the topography as realistic as possible. To do so, they adopted the statistical representation of bottom roughness provided by real echosounder systems.
Oceanographers may not be able to measure every detail of ocean-wide bottom relief anytime soon, but they have a fairly good understanding of its statistical properties. The bottom roughness model from Gulliver and Radko’s study mathematically represents what an average seafloor looks like.
JU (dpa, nps.edu, agupubs.onlinelibrary.wiley.com)