Hidden Tides

By Alice Waugh

The effects of ordinary ocean waves on ships, beaches, and shoreline structures are well known, but there’s another kind of huge, slow, undersea wave that’s almost undetectable at the surface.

Associate professor of mechanical engineering Pierre Lermusiaux and his MSEAS group (Multidisciplinary Simulation, Estimation, and Assimilations Systems), along with researchers at the Woods Hole Oceanographic Institution (WHOI), are part of a team that has modeled the motion and interactions of these internal tides. As reported by MIT News, the team has successfully predicted the speed and direction of movement of internal tides near the US Atlantic Coast as they are generated off the shelf break—a seabed feature where the ocean bottom suddenly drops off—and interact with the Gulf Stream and other ocean features.

These internal tides may be hidden, but their effects are not. The applications of the research go deep:

Sonar accuracy: When a ship emits sonar waves, the echoes aid navigation by revealing contours of solid objects and the sea bottom— but internal tides are one factor that can distort the echoes. “If you know how those [undersea] waves mostly evolve, and you know something about their uncertainties and probabilities, you can get better sonar performance,” Lermusiaux says. MSEAS is also working on a related project using real-time forecasting, simulations, and analyses of underwater sound speed variability to develop underwater GPS.

Deep-sea oil rig stability: The large, slow waves can interact with currents at the same frequency and create resonance, which exacerbates a wave’s potentially destructive effects. Even in the absence of storms, internal wave and current action of varying strengths and directions can cause damage to rig structures and underwater pipes.

Insight into marine ecosystems: The waves in internal tides, which can be hundreds of feet high, are also instrumental in moving water from the depths of the ocean to areas closer to the surface. That movement delivers deep-sea nutrients for phytoplankton and zooplankton that in turn attract fish that feed on them. More quantitative understanding of this dynamic of ocean life could be particularly useful to fisheries, says Lermusiaux.

Climate science: Because some internal tides can travel a long way without dissipating much, they carry energy over long distances. “As those waves get created, where they interact, break, and ultimately dissipate is important for climate because that’s where their energy goes,” Lermusiaux explains.