Knowing the age of water masses is important in estimating anthropogenic CO2 uptake by the ocean. The age of a water mass can be characterized by a probability distribution function, or transit time distribution (TTD), and transient tracers such as CFCs and SF6 are used to constrain the TTD. However, one of the largest sources of uncertainty in constraining TTDs is lack of knowledge about surface tracer saturations. Knowledge of the surface saturation is also needed to investigate ocean mixing and circulation using tracers in ocean circulation models and to calculate water mass formation rates from tracer inventories.
To reduce the uncertainty in CFC and SF6 saturations, researchers at the University of Hawaii at Manoa and at the Lamont-Dougherty Earth Observatory of Columbia University will construct an automated analysis system for continuous measurements of CFCs and SF6 from the surface ocean and the atmosphere. Even though the measurement of CFCs and SF6 in seawater is fairly routine, having an automated system operated by one person will permit saturation data to be acquired from a variety of ships with little manpower. The new analysis system will be deployed on CLIVAR/Repeat Hydrography cruises in a subpolar/polar region (e.g., S4P) and a subtropical region (e.g., A20/A22/A16), with the eventual goal of deploying the system on future CLIVAR/Repeat Hydrography cruises, and on ships of opportunity to regions during winter when deep water formation occurs and very few measurements have been made. By documenting variations in tracer saturation levels and environmental variables (e.g., temperature, salinity, wind speeds, mixed layer depth), the research team expects to provide the community with much needed measurements to constrain tracer-based estimates of anthropogenic CO2 uptake, provide better boundary conditions for model calculations using CFCs and SF6, and begin to gain some insights into processes controlling the saturations of these gases in the surface ocean.
Broader Impacts The study of water mass ages using transient tracers is an important aspect of the overall study of the ocean circulation and of the ability of the ocean to take up the large amounts of CO2 that are being released to the environment by the burning of fossil fuels. The uptake of CO2 by the ocean, in turn, will moderate changes in the Earth's climate due to human perturbations. Better knowledge of SF6 and CFC saturations will reduce errors in determining water mass formation rates from CFC and SF6 inventories, reduce errors in circulation time scales determined from SF6 and CFC data, provide tighter constraints on circulation models that utilize these tracers, and reduce errors in determination of oceanic uptake of anthropogenic CO2 based on CFC and SF6 water mass ages.
As an educational component, undergraduate students will be hired to work during the second and third summers on the project to calculate CFC saturations from previous cruises to regions where data will have been acquired with the underway system. The student will compare these data to the underway data to determine if CFC saturations have changed with time. In the process, these students will learn about oceanography and gain research experience working on one of the most pressing global environmental problems of our time.
