Entry Date:
December 22, 2005

Internal Waves

Principal Investigator Thomas Peacock

The core topic of Professor Peacocks’s research on stratified fluids is internal waves. These waves, which are propagating disturbances of density stratifications, are prevalent in the ocean, where they are primarily generated by flow over topography and surface forcing by storms. Their subsequent dissipation a effects ocean mixing, which influences the Earth's climate. From an engineering perspective, internal waves impact the performance of marine technology such as submarines and pipelines. A detailed understanding of internal wave generation, evolution and dissipation is thus environmentally and industrially important. Peacock gave a plenary lecture at the APS DFD 2014 meeting summarizing Peacock’s group's extensive research on internal waves over the past decade.

Experimental methods: Professor Peacock developed a state-of-the-art experimental facility to study internal waves using funding from the Office of Naval Research (ONR) and a National Science Foundation (NSF) CAREER award. The facility is centered around a 5.5m-long wave tank with a computer-controlled filling system to establish nonlinear density stratifications. Stereo Particle Image Velocimetry (SPIV) and Synthetic Schlieren (SS) are used to mea- sure velocity and density-gradient fields, respectively. A standout feature of the facility is the extensive novel internal wave generator technology; we have large scale vertical and horizontal generators that enable excitation of high-quality wave fields with sufficiently large wavelengths that viscosity has little influence, which is key when attempting to model processes relevant to the ocean. We are at the forefront of visualization methods, having performed the first 3D SPIV visualization of internal waves. Peacock has also authored two book chapters on experimental methods.

Internal tides: When internal waves are generated by tidal flow past topography, they are also of tidal frequency and are referred to as the internal tide. Peacock’s group has developed the foremost analytical models that are used by the community to predict internal tides and internal tide attractors, recently removing the requirement for the WKB approximation. The principal model, based on Green's function analysis, has been validated against laboratory experiments and numerical models, and is part of the contribution to the ONR IWISE (Internal Waves in Straits Experiment) program. Other contribution is an ambitious series of experiments performed at the Coriolis platform in Grenoble.

Topographic scattering: Peacock’s group received NSF funding to collaborate with the NSF EXITS (Experimental Study of Internal Tide Scattering) program. For this, the Green Function model was adapted to handle internal tide scattering, the process by which energy is transferred from large to short wavelengths via interactions with topography, making it more prone to mix the ocean. Studies confirmed that small amplitude sea floor topography scatters 5-10% of the internal tide energy ux. However, studies newly revealed that individual tall features, such as ridges and seamounts, can be highly effective at scattering, with efficiencies of up to 50%. These new results suggest that, contrary to a previously held belief, topographic scattering may be a globally important process for dissipation of the internal tide. Results are being used to guide parameterizations of internal-tide driven mixing in global climate models via participation in an NSF Climate Processes Team.

Surface excitation and propagation: A leading research issue over the next few years is the increased level of internal wave activity in the Arctic Ocean due to forcing by storms during increasingly ice-free summer months. We have developed an analytical model for studying the excitation and downward propagation of internal waves through complex stratifications characteristic of the Arctic Ocean. This model predicts the downward internal wave energy flux into the deep ocean and can identify conditions for which the upper ocean is prone to instability and mixing. In 2014, we obtained new funding from the NSF Physical Oceanography program to study this process; model is being used as a decision-making tool for field studies in summer 2015, in which we will participate. The model has been tested against laboratory experiments, and this research is being performed with collaborators at ENS de Lyon, with whom we have obtained MIT-France and ANR PICS funding. Finally, through using the method to study internal wave beam propagation we identified an elegant parallel with the scenario of an optical Fabry-Perot optical interferometer. Lee waves: Lee waves are internal waves generated by quasi-steady flow past topography. We have obtained new 5-year ONR funding to perform field studies of lee wave generation by the Mariana Ridge in the southwest Pacific Ocean. The experiments will advance the capabilities of Pressure Inverted Echo Sounders (PIES) technology that will be used to obtain year-long in-situ records of lee wave activity.

Atmospheric internal waves: Peacock been fortunate to twice visit northern Australia and experience gliding on the Morning Glory cloud, which is an atmospheric internal solitary wave. The last trip, in 2012, obtained preliminary data that can be used as the basis for a proposal to perform a more substantial field study.