Project: Measuring Diapycnal Mixing in the Upper Ocean therMocline using Noble Gas Supersaturation

NSF Award Abstract:
In this project, researchers at the University of Washington School of Oceanography will develop a new method of constraining the rate of diapycnal (cross-isopycnal) mixing in the ocean using the natural distributions of dissolved noble gases. They will apply this method to determine the diapycnal mixing rate in the ventilated thermocline of the subtropical oceans where there is long-standing uncertainty about the physical mechanisms supplying nutrients to the euphotic zone. Noble gases are not affected by biology, so their distribution in the ocean is determined purely by physical processes. Because the equilibrium concentrations of these gases are non-linear functions of temperature, mixing between waters equilibrated with the atmosphere at different temperatures induces a supersaturation in the gases. Advances in analytical methodology have recently made it possible to measure this mixing signal, and a theoretical basis for understanding it has also just been developed. The theory indicates that noble gas supersaturation accumulates over the time since the water parcel left the surface and that it is most sensitive to diapycnal mixing in the ventilated thermocline of the ocean. Thus, this tracer records the effect of diapycnal mixing over time scales of decades and compliments purposeful tracer release experiments that last months to a year and whole-ocean analyses of thermocline mixing that represent hundreds of years.

The project will combined analytical and theoretical research. The research team will measure the concentrations of Ne, Ar, Kr and Xe in transects through three sections of the world's ventilated thermocline. Two meridional sections through the central North Pacific and eastern South Pacific and a zonal transect across the southern North Atlantic cross contrasting regions where we expect the noble gas tracers to reveal different degrees of supersaturation due to diapycnal mixing. The theoretical/modeling aspect of the proposal focuses on using a series of ocean global circulation model runs to help separate the different physical processes causing noble gas supersaturation. The model will then be used to determine the effect of the diapycnal mixing rates deduced from the inert gas tracers on the transport of nutrients to the euphotic zone in the subtropical oceans. Using this interdisciplinary approach the team will evaluate the utility of noble supersaturation as a tracer of diapycnal mixing in the ocean thermocline and advance our understanding of a classic problem in oceanography.

The project is expected to have a number of broader impacts. By developing a new method of quantifying diapycnal mixing rates in the ocean's thermocline, this project should help to solve the many issues that depend on this fundamental quantity, from determining biological productivity and its controls to understanding the driving forces behind the overturning circulation. Better constraints over mixing rates and wide dissemination of the observational dataset for other data/model comparisons will lead to improved predictions for anthropogenic CO2 uptake by the ocean and for changes in biological productivity caused by global warming, both topics of clear interest to society. The project will also promote education by involving a graduate student that will be jointly advised by the principle investigators and will enhance international scientific collaboration by establishing joint field and analytical research with Japanese and Canadian colleagues.