For the first time, ocean engineers and scientists from MIT, the University of Minnesota at Duluth (UMD), and the Woods Hole Oceanographic Institution (WHOI) have accurately simulated the motion of internal tides along a shelf break called the Middle Atlantic Bight—a region off the coast of the eastern U.S. that stretches from Cape Cod in Massachusetts to Cape Hatteras in North Carolina.
They found that the tides’ chaotic patterns there could be explained by two oceanic “structures”: the ocean front at the shelf break itself, and the Gulf Stream—a powerful Atlantic current that flows some 250 miles south of the shelf break.
From the simulations, the team observed that both the shelf break and the Gulf Stream can act as massive oceanic walls, between which internal tides ricochet at angles and speeds that the scientists can now predict. The team includes Samuel Kelly, an assistant professor at UMD who was a postdoc at MIT for this research; Pierre Lermusiaux, an associate professor of mechanical engineering and ocean science and engineering at MIT; Tim Duda, a senior scientist at WHOI; and Patrick Haley, a research scientist at MIT.
Lermusiaux says the team’s simulations of internal tides could help to improve sonar communications and predict ecosystems and fishery populations, as well as protect offshore oil rigs and provide a better understanding of the ocean’s role in a changing climate.
Scientists have found that surface tides, just like internal tides, are generated by the cyclical, gravitational pull of the sun and the moon, and travel between density-varying mediums. Surface waves travel at the boundary between the ocean and the air, while internal waves and internal tides flow between water layers of varying density.
In the summer of 2006, oceanographers embarked on a large-scale scientific cruise, named “Shallow Water ’06,” to generate a detailed picture of how sound waves travel through complex coastal waters, specifically along part of the Middle Atlantic Bight region. The experiment confirmed that internal tides stemmed from the region’s shelf break at predictable intervals. Puzzlingly, the experiment also showed that internal tides arrived back at the shelf break at unpredictable times and locations.
The researchers sought to find mathematical equations that would describe the underlying fluid dynamics that they observed in their simulations. To do this, they started with an existing equation that characterizes the behavior of internal tides but involves an idealized scenario, with limited interactions with other features. The team added new “interaction terms,” or factors, into the equations that described the dynamics of the Gulf Stream and the shelf break front, which they derived from their data-driven simulations.
The match between their simulations and equations indicated to the researchers that the Gulf Stream and the shelf break front were indeed influencing the behavior of the internal tides. With this knowledge, they were able to accurately predict the speed and arrival times of internal waves at the shelf break, by first predicting the strength and position of the Gulf Stream over time. They also showed that the strength of the shelf break front alters the speed and arrival times of internal tides.
The team is currently applying their simulations to oceanic regions around Martha’s Vineyard, the Pacific Islands, and Australia, where internal tides are highly variable and their behavior can have a large role in shaping marine ecosystems and mediating the effects of climate change.
Source: PHYS
