To date, less than a thousand profiles of turbulence have been acquired in the deep ocean and these are concentrated in only a few geographic regions. As a result of this paucity of direct measurements, the distribution of mixing is inferred largely through proxies and parameterizations that have known limitations. Mixing budgets remain poorly constrained. The purpose of this pilot project is to demonstrate that systematic, repeated sequences of turbulent mixing over full-ocean depths can be obtained in a broad distribution throughout the global ocean. To accomplish this goal, the investigators in this project will deploy chi-pods - "turbulent mixing meters" on standard shipboard CTD/LADCPs package operated by the Global Repeat Hydrography Program. A few thousand turbulence profiles will be collected and analyzed over a three-year period. The chi-pods are self-contained instruments that have been quantifying mixing on equatorial moorings since 2005. They use fast response thermistors and precision accelerometers to measure microscale temperature gradients, from which the dissipation rate of temperature variance, ÷ and the eddy diffusivity of heat and other tracers. Unlike traditional velocity microstructure measurements based on shear probes, temperature microstructure measurements are not sensitive to platform vibration - a necessity for deployment on moorings with a surface expression or from a CTD rosette. The CTD-chi-pod has modifications that allow it to be easily clamped to a standard rosette with minimal impact during repeat hydrography operations, and has been proven successful in several recent pilot experiments in shallower waters.
Intellectual Merit: Diapycnal mixing in the ocean interior has a strong influence on upper ocean heat and nutrient budgets, abyssal circulation, and water mass distributions. An initial attempt to develop and implement parameterizations of diapycnal mixing for large-scale models is underway as part of an NSF-funded Climate Process Team. The largest impediment to that effort is the incredible scarcity of direct turbulence observations, particularly those that cover the full water column depth. This project will demonstrate the feasibility and value of collecting a global dataset of microstructure observations that will be made publicly available as soon as possible after acquisition, following repeat hydrography protocols. The data will allow the PIs, as well as the broader scientific community, to calculate better global and regional averages of diapycnal mixing rates, explore underlying dynamics that control the geographic distribution of ocean turbulence, evaluate gradual changes in basin-averaged mixing rates, assess uncertainty and systematic bias in finescale shear and strain parameterizations of mixing that are becoming widely used, and validate turbulence parameterizations that will be going into the next generation of regional and climate models.
Broader Impacts: The proposed measurements will be the first comprehensive set of direct mixing observations easily available to the broader community. These measurements can be used by a wide range of scientists to constrain everything from regional tracer budgets to biological nutrient fluxes to global energetics. The results will be particularly useful for testing and improving parameterizations of turbulent mixing being developed for global climate models (http://www-pord.ucsd.edu/~jen/cpt/), as diapycnal mixing rates remain one of the most influential tuning parameters in such models. This project will train one graduate student and give other students exposure to open-ocean observational techniques.
