Scientists Simulate "Dead Water"
In certain parts of the ocean, huge, slow sub-surface waves travel for miles, rising and falling for hundreds of feet in the ocean’s interior while making barely a ripple at the surface.
These subsurface waves are called internal tides, as they are internal to the ocean and travel at the same frequency as surface tides.
The effects of these internal tides were first reported in the late 1800s, when Norwegian sailors, attempting to navigate a fjord, experienced a strange phenomenon: Even though the water’s surface appeared calm, their ship seemed to strongly resist sailing forward — a phenomenon later dubbed “dead water.”
Internal tides are generated in part by differences in water density, and created along continental shelf breaks, where a shallow seafloor suddenly drops off like a cliff, creating a setting where lighter water meets denser seas.
In such regions, tides on the surface produce oscillating, vertical currents, which in turn generate waves below the surface, at the interface between warmer, shallow water, and colder, deeper water. Internal tides are largely calm in some regions but can become chaotic near shelf breaks, where scientists have been unable to predict their paths.
Now 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 researcher 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.
“Internal tides are a big chunk of energy that’s input to the ocean’s interior from the common [surface] tides,” he explains. “If you know how that energy is dissipated and where it goes, you can provide better predictions and better understand the ocean and climate in general.”
Travelling through “dead water,” much of the energy from a ship's propeller only results in waves and turbulence, leaving a ship capable of traveling at perhaps as little as 20 percent of its normal speed. Lermusiaux explains:
“It would be dead calm in the water, and you’d turn your ship on but it wouldn’t move. Why? Because the ship is generating internal waves because of the density difference between the light water on top and the salty water on the bottom in the fjord. That keeps you in place.”
Over the centuries, 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.
These giant, hidden swells are responsible for alternately drawing warm surface waters down to the deep ocean and pulling marine nutrients up from the abyss.
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.
The research was funded in part by the Office of Naval Research and the National Science Foundation.