During the past decades, a variety of anthropogenic trace substances have been delivered to the ocean surface at relatively well-known rates. These transient tracers are used to study deep-water formation processes and rates, and to estimate mean residence times of waters found in the thermocline. Long-term observations of transient tracers indicate that they are useful parameters for studying ocean variability. On the basis of a review of the main elements of the tracer methodology and tracer time series from the Greenland/Norwegian seas and the Deep Western Boundary Current along the east coast of North America we discuss the potential of transient tracers as a tool for studying variability of ocean circulation.
Prediction of the potential impact of greenhouse gases on our present climate requires understanding of the major elements of the climate system, including the ocean. Of particular concern is the understanding of the natural variability of the climate system, which has to be subtracted from any observed climatic trend before its significance can be evaluated.
There are numerous tools that can be used to study variability in the ocean, such as long-term observations of the classical parameters, temperature and salinity, or of currents. A more recently developed tool is the measurement of trace substances of natural or anthropogenic origin that carry time information. The time information can be derived from radioactive decay (radioactive clock) or from time-dependent delivery of the tracers to the surface waters from which they spread into the deep basins (dye tracers). In principle, both steady-state tracers and transient tracers can be used to detect variability in the ocean. A basic requirement for the application of the tracer method to detection of ocean variability is that multiple observations be made over a period of time of a certain tracer in a specific water mass or at a fixed location. Unfortunately, few such tracer time series exist, and the existing observations cover a rather short time span.
However, the available data clearly demonstrate the potential of tracers for studies of ocean variability (e.g., Rhein, 1991; Schlosser et al., 1991a). In this contribution we review the principles of the transient-tracer methodology and discuss its potential for studies of ocean variability on the basis of two selected examples.