Chlorofluorocarbon Tracer Program

John L. Bullister, PMEL  
David P. Wisegarver, PMEL;  Frederick A. Menzia, JISAO 

To document the transient invasion of Chlorofluorocarbons (CFCs) into the thermocline and deep waters of the the world ocean. These tracer data are used to estimate the rates and pathways of ocean circulation and mixing processes, and as a means of testing and evaluating numerical models of ocean circulation.

The concentrations of CFC-11, CFC-12, CFC-113 and carbon tetrachloride in the atmosphere have increased rapidly during the past decades. These compounds dissolve in surface seawater and are carried into the interior of the ocean.  The PMEL CFC Tracer Group has developed and improved techniques for the analysis of trace levels of CFCs in air and seawater samples.  These techniques have extended the range in which these compounds can be detected in the deep ocean.   Each of these compounds has a unique history of increase in the  atmosphere, and measurements of the concentrations and ratios of  these compounds in seawater can be used to estimate the rates of circulation processes on annual-to-decadal time scales.


Field Work: 

With Support from NOAA's Climate and Global Change Program, the PMEL CFC Tracer Group has participated in a number of World Ocean Circulation Experiment (WOCE) Hydrographic Program Cruises, including Sections P12, P13, P14S, P15S, P16C, P16N, P18 and, P21W in the Pacific Ocean (see map below) as well as sections in the Indian, Atlantic and Southern Oceans. 

  The CFC-11 section along 24oN  collected in 1998 (left) shows deep CFC maxima along the western boundary in the NorthAtlantic, associated with dense outflows of recently ventilated waters from the Labrador and Greenland-Iceland-Norwegian Seas.  As part of the 'global conveyor belt', these dense flows provide means of transporting surface-acquired properties (e.g.. heat, carbon dioxide, CFCs) into the interior of the ocean. 
The CFC-11 section along P15S (CGC96) in the Pacific (left) highlights the ventilation processes occurring in upper and intermediate waters in the Southern Ocean, as well as the transport of CFCs in northward flowing Antarctic Bottom Waters.  At the time of this survey, a detectable CFC signal in this abyssal flow extended almost to the equator in the western Pacific. 

Decadal scale variability in deep water renewal in the Greenland and Norwegian Sea

In collaboration with colleagues at Brookhaven National Laboratory, Lamont-Doherty Earth Observatory and the Institute of Marine Research (Bergen,Norway), the PMEL CFC Tracer group has participated in annual hydrographic and transient tracer surveys in the Greenland Sea since 1992.  Together with earlier data, these studies have detected significant decadal-scale changes in the strength of deep convective mixing processes in the Greenland Sea, and in the properties of deep water in this region. CFC-11 and CFC-12 concentrations in the deep (> 2500 dB) Greenland Sea (below) have increased by less than 20% since the early 1980's, indicating a much reduced level of deep convective renewal of these waters compared to the two decades prior to 1980. Following a cold period in the late 1960's to the early 1980's, the deep water in the Greenland Sea (bottom) has undergone a gradual warming, indicative of a reduction of the rates of renewal by wintertime deep convective mixing.

Model-Observation Studies:

One of the key goals of the PMEL CFC Tracer Program is to use the CFC observations as a means of testing and evaluating large-scale numerical models of the ocean.  The data have been utilized in a number of model simulations.

Comparisons of temperature, salinity and CFC-11 observations along 170oW in the Pacific with output from a 1 degree Parallel Ocean Program Global Model at NCAR  are shown below: 

Comparisons of CFC observations have been made with model CFC sections generated from a coarse resolution coupled atmosphere-ocean model at GFDL (below and right).

These comparisons can highlight strengths and weaknesses in a model, and suggest areas where models need to be improved if we are to have confidence in their ability to predict change in the coming decades.

 Future Directions:

*Work closely with the the CTD and CO2 groups at PMEL and AOML to analyze existing data sets. Comparisons of repeat hydrographic sections, such at the section along 20W in the North Atlantic occupied in 1988 and 1993 (see figure at left), indicate large changes in the CFC content of the water column on 5 year time-scales. This time dependent information, combined with hydrographic and CO2 observations, is being used to improve estimates of the rate of invasion of anthropogenic carbon dioxide into the ocean.

* Continue to document the invasion of CFCs, CO2 and other compounds into the ocean and detect decadal-scale changes in water mass propreties, by means of repeat hydrographic sections in key regions.* Continue the time-series study in the Greenland Sea, to determine the variability of deep convective processes in this region, the sensitivity of these processes to changes in surface forcing, and evaluate possible links to other regions in the North Atlantic.

* Work closely with numerical modelers, with the goal of using the CFC observations to help test and improve model simulations.

*Take the lead in analyzing the WOCE Pacific Basin CFC data set (see map at right), and work with other groups as part of the global WOCEsynthesis effort.

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                    Funding support provided by NOAA Office of Global Programs
           1998 PMEL Science Review