The wind and the tides input the overwhelming majority of the energy into the internal-wave field, whose breaking is the primary driver of turbulent mixing in the ocean. It has recently become clear that much of the input energy takes the form of low-mode internal waves, which can propagate far (> 1000 km). It follows that, in principle, the global magnitude and distribution of turbulent mixing depends primarily on the sources and subsequent evolution of the propagating, low-mode internal waves. New interpretation of historical moorings, new observations from the Hawaii Ocean Mixing Experiment, and new numerical modeling results suggest that long-range wave propagation is strongly modulated and, at times, completely disrupted, by interactions with other internal waves, topography, and the mesoscale background. This project seeks to better understand the processes that govern long-range internal-wave propagation, with an ultimate long-range goal of determining the global distribution, magnitude and time-dependence of internal-wave driven mixing.
The project will integrate three principal activities: o Observational Study of Long-range Propagation. A line of six Moored Profilers extending northward from Hawaii, spanning 26N-37N, will measure velocity and density from 100-2600 m down to 2-m scales, for 50 days. Ship-based sensors will measure upper-ocean shear and density along this track, obtaining 2 broad snapshots and three spring-tide 5-day time series at three latitudes. Energy, energy flux, and dissipation rate (from overturning scales) will be computed at all measurement locations. This experiment will enable a coherent picture of the long-range propagation of the tide over about 1200 km. o Numerical simulations. A series of hypothesis-driven, controlled numerical experiments will be conducted to optimally design the observations, and to investigate dynamical processes that can drain energy from a propagating internal tide. o Historical Data Analysis of moored records and upper-ocean shear from ship-based instruments will be done to characterize the spatial dependence of internal-wave shear, ray slopes, energy and flux, and to provide large-scale observational context for the above simulations.
Broader Impacts: This work has four vital implications for the broader community. First, a dynamical understanding of the resultant geography of mixing is required for accurate understanding and modeling of the large-scale circulation in past, present and future climates. The combination of observations and process-oriented modeling will contribute to the knowledge required to construct physically motivated mixing parameterizations that are dynamically coupled to the large-scale circulation. Second, satellite altimetry has been increasingly seen as an important community resource for monitoring global internal-wave fields. Careful comparison of altimetric results and subsurface observations will help interpretation of altimetric data. Third, the 50-day, 1200-km array is a prototype observing system for long-term monitoring not only of the internal tide and associated turbulence, but of many other mesoscale phenomena. Fourth, the proposed work will contribute to education by providing support and mentoring for two post-docs, and at-sea experience for several graduate students. Results will be shared at national and international workshops and conferences.
