This paper reviews the contributions of transient tracer
research to our understanding of the lower limb of the thermohaline
circulation in the North Atlantic Ocean. The review is limited
geographically to the North Atlantic because of its importance for
decadal scale climate variability. The tracers reviewed are the
transient tracers of anthropogenic origin that give decadal time
scale information, the chlorofluorocarbons (CFCs) and
tritium/
He. The emphasis is on work by United States (US)
investigators from 1991-94. Also included are some non-US and
earlier contributions as they are linked to the recent work and
have not been reviewed in this publication before.
The Geochemical Ocean Sections (GEOSECS) [ Bainbridge, 1981] and Transient Tracers in the Ocean (TTO) [ Williams, 1986] programs of the past two decades provided data for mapping the large scale tracer distributions. Contributions of the pioneering tracer oceanographers, including W.S. Broecker, H. Craig, H.G. Östlund and C. Rooth, were original and significant for demonstrating the potential these new tracers held for oceanography [e.g., Craig, 1957; Broecker and Olson, 1960; Fonselius and Östlund, 1959; Östlund et al., 1969; Rooth and Östlund, 1972]. In recent years, major advances in our knowledge of the thermohaline circulation can be attributed to information derived from tracer data, particularly for two reasons. First has been the development of analytical techniques so that oceanographers can easily produce large quantities of high quality data. These include the chlorofluorocarbons, which are measured at sea. Recently the shipboard extraction of helium isotopes [Lott and Jenkins, in preparation] has substantially decreased the time over which these data become available. Additionally, tracer oceanographers have benefited greatly from multi-investigator programs that include active participation by physical oceanographers measuring hydrographic properties, and direct velocities (via current moorings, acoustic Doppler profilers, floats, etc.). Two such programs are the World Ocean Circulation Experiment (WOCE) and the Atlantic Climate Change Program (ACCP). Data from these programs help to provide the physical context in which to more fully utilize the information from tracers.
This review is divided into three main sections. The first section introduces the concept of the thermohaline circulation and its relationship to climate is introduced. The second section is the review; the first part includes the anthropogenic tracer source functions and how they are used for estimating ages. This is followed by a review of the source components of North Atlantic Deep Water. The third part is a comprehensive review of tracer observations in the Deep Western Boundary Current (DWBC), including large scale studies, time series, and a discussion of the interpretation of tracer ages. In the third section future directions are explored, with the emphasis on application and implementation of the scientific results derived from these and future work.