ANN ARBOR—University of Michigan researchers have used a U.S. Navy ocean forecasting model to predict where internal tides occur in the ocean in order to bring ocean patterns important to weather forecasting and shipping into clearer focus.
In particular, scientists are interested in observing ocean patterns called small-scale oceanic eddies—small swirls of water that break off larger eddies created by the Gulf Stream—because they are instrumental in transporting heat and carbon in the ocean. Understanding them can also help the Navy predict weather forecast models for fleet operations, according to U-M oceanographer Brian Arbic .
In 2022, NASA and the French space agency CNES launched a satellite called SWOT, or the Surface Water and Ocean Topography mission, with the primary goal of observing small-scale eddies. SWOT can track ocean water movements on a minute scale, on the order of 5-to-10 kilometers rather than around 100 kilometers. However, internal tides—water that moves vertically up and down an ocean column—can mask images of the eddies observed by the satellite.
Now, the U-M research team has devised a way to use the Navy's Hybrid Coordinate Ocean Model, or HYCOM, to predict where and when these internal tides take place and then remove them from SWOT data. The new team's approach, developed by former U-M postdoctoral fellow Yadidya Badarvada , now an oceanographer at Florida State University, can account for 60% more of the internal tide signal seen in SWOT, relative to current approaches. This improvement yields a much clearer picture of small-scale ocean eddies.
"SWOT is already giving us all kinds of new information about what's happening in the physics of the ocean, tides, currents and waves. Our work is important because it shows that we can use Navy ocean forecast models to map these tides at high accuracy, which is important for this satellite mission because it allows SWOT to get a clearer image of other ocean features," said Arbic, co-author of the study and a professor in the U-M Department of Earth and Environmental Sciences.
"Additionally, our findings have climate relevance not just because the ocean interacts with the atmosphere right at the sea surface, but also because those internal waves break down below the sea surface, and the mixing from that wave breaking affects ocean circulation and biology."
The study, funded by the Office of Naval Research, the Naval Research Laboratory, NASA and CNES, is published in Science Advances.
A seething ocean
Internal tides are generated when large-scale tides—the tides caused by the gravitational fields of the sun and moon across Earth—pull water across ocean floor features such as mountains and ridges. Because seawater is stratified by temperature and salinity, these tidal motions cause water along interfaces of different densities to move up and down within the column. Although these vertical motions are largest at great depths, they can still leave a signature on the ocean's surface.
"When we are trying to observe these small-scale eddies from satellites, these signals from internal tides become a noise for them," said lead author Badarvada. "You need to accurately account for them and then remove them if you really want to see those small eddies clearly."
The researchers used HYCOM to predict internal tide sea-surface height signatures and generate an estimate of the world's internal tides and how they change and evolve over time. Then, they compared their estimate of internal tides against SWOT observations of sea surface height. This allowed the researchers to see how much of SWOT's sea surface height data could be accounted for by the internal tides that the researchers estimated using HYCOM.
"The cool thing about this is that we are using the Navy forecast model to improve the observations that NASA is measuring, and then the Navy in turn is going to use those observations to make their predictions even better," Badarvada said. "This is a cycle that could go on and help both agencies do their job in a better way."
Arbic says the team's findings build on decades of research invested at both the U.S. Navy and NASA in order to study the physics of ocean surface water—and underscores the necessity of science funding for long-term goals.
"The work Yadi has done wouldn't be possible without the background work of many people over the years putting tides in the Navy model," Arbic said. "Lots of work by lots of people over decades has gone into building both HYCOM and SWOT."
