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Stanford scientists say waves hitting the equatorial seafloor create centimetre-scale turbulence which is essential in driving ocean circulation on a global scale. The new findings published in the journal Geophysics Research Letters could eventually lead to improved future climate forecasts. “Climate models don’t do a great job of simulating global ocean circulation because they can’t simulate the small scales that are important for deep ocean mixing,” said study co-author Ryan Holmes in a statement, a graduate student at Stanford’s School of Earth, Energy & Environmental Sciences. The meridional overturning circulation (MOC) is a global conveyor belt where cooled surface waters in high latitudes flow toward the equatorial regions. These waters eventually mix with warmer, less dense water and rise to the surface. They then flow toward the higher latitudes to complete the cycle. One such cycle takes hundreds to thousands of years to complete. Until now, scientists didn’t exactly know where in the tropical oceans this mixing of currents took place. They believed that intense deep ocean mixing required water to flow over rugged terrain. James Moum, a professor of physical oceanography at Oregon State University, and Holmes, who had been investigating equatorial mixing in ocean models for his PhD, went on a five-week research cruise in the equatorial Pacific to better understand mixing in tropical oceans. The team encountered strong turbulence along a 700-meter vertical stretch of water close to the ocean floor. This turbulence was unexpected as this part of the ocean floor was relatively smooth. “This was the first time that anyone had observed turbulence over smooth topography that was as strong as that found over rough topography,” said Holmes. Using computer models Leif Thomas, an expert in the physics of ocean circulation at Stanford and Holmes created a model simulating how winds blowing across the ocean surface create ‘internal waves’. However, their model did not produce the turbulence necessary for abyssal mixing. Instead the internal waves bounced between two vertical bands of water and the smooth sea floor without breaking. It wasn’t until Thomas incorporated the horizontal components of Earth’s spin into their simulation that everything began to fall into place.“It occurred to me that internal waves at the equator, where the effects of the horizontal component of Earth’s spin are most pronounced, could experience an analogous behavior when the waves reflect off the seafloor,” said Thomas. Holmes says, after including this horizontal component they found it changed the physics of waves in the deep equatorial oceans. This component likely drove them to break and cause turbulence and mixing. Thomas says the new findings point out the important role the deep equatorial waters play for Earth’s climate system. “Scientists have long known that the equatorial upper ocean is critical for the physics of internal variations in climate such as El Niño,” he said. “Our new study suggests the abyssal ocean at the equator could impact the climate system on much longer timescales.”
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