Inferring Ocean Mixing Rates from Hydrography and Turbulent Energy Sources

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Overview: Ocean mixing occurs at the smallest ocean scales down to the molecular level, yet in aggregate, it exerts influence over the large scale ocean circulation, including the overturning circulation, the timescale of response to atmospheric changes, and the uptake of heat and carbon. The goal of the project is to map ocean mixing rates globally in a way that synthesizes direct observations of mixing, hydrographic observations, ventilation rates, and turbulent energy sources. The approach of the project is to combine techniques for inverting hydrography with the inclusion of age tracers and a small-scale mixing parameterization in order to estimate global mixing rates and to reduce its remaining uncertainty. An hydrographic inversion technique that diagnoses ocean mixing rates robustly, the inclusion of constraints from small-scale processes such as tidal mixing and the sources of energy for mixing, and application of the inversion method to already-collected observations from around the world ocean will be developed. This project is relevant to understanding how the ocean transports tracer quantities, and the interaction of physical, biological, and chemical processes as captured in the age tracer, radiocarbon. Intellectual Merit: Ocean mixing rates have been previously estimated from at least four different sources of information: 1) direct observation of micro- and fine-scale structure, 2) parameterizations of mixing based upon knowledge of ocean processes such as tides, internal waves, and turbulence, 3) inversion of large-scale hydrographic observations and geostrophic balance, and 4) consideration of age tracers such as radiocarbon and their record of advective versus diffusive transports. Significant uncertainties are inherent to each of these estimates, and an original method will be developed to estimate mixing rates that takes into account information from these four sources simultaneously with a mathematically-rigorous inverse technique. Too few in-situ measurements will be made in the near future to determine the spatial variability of ocean mixing, so the best hope to break this impasse is to synthesize the many forms of already-collected data in a modeling and analysis framework. The resulting global three-dimensional maps of diffusivity will be related to specific bathymetric features and ocean processes, which are envisioned to lead to process studies that focus on particular regions. The global spatial map will also serve as a present-day benchmark of the best current maps of diffusivity, and will serve as a guide by which to plan new in-situ measurement campaigns. Broader Impacts: Improved estimates of ocean mixing rates are especially important to reconstruct and predict the ocean uptake of heat and carbon, and the determination of how actively the ocean participates in climate variability. The spatial distribution of ocean mixing influences just how well the ocean circulation acts to isolate the abyss, with relevance to the fate of carbon that has already been sequestered but can potentially re-emerge in the next few hundred years. The resulting global mixing rates can be prescribed in climate simulations and may reduce a major uncertainty in ocean component of these models. A more realistic simulated ocean is key on the long timescales inherent to many climate processes. The output of the project will be exported to a complementary NSF-funded project of the KeckCAVES team at UC Davis, where collaboration with computer scientists will help visualize the results in a way that will broaden participation in science and broadly disseminate climate science to the general public. In addition, this project would primarily support a young investigator in the beginning stages of building a research group.