Transient tracer data have contributed substantially to our
understanding of the lower limb of the thermohaline circulation.
During 1991-94 major advances have been made in deriving time scale
information from tracers. Tritium
He dating has been extended
by correcting for the primordial component through its correlation
with silica concentrations. The use of new tracers (F113, CCl
)
has extended the time scales of CFC dating in both directions.
These new tracers are becoming increasingly important, because the
usefulness of the F11/F12 ratio is diminishing with the phase out
of these compounds. It is necessary to improve the analytical
systems used to measure all four halocarbons, so that the sample
analysis time for F113 and CCl
can be reduced to that for F11 and
F12, and without employing additional technicians at sea.
Transient tracer data have had a significant impact on
identifying the recently ventilated components of the high velocity
DWBC, as compared with recirculation gyres that have high
velocities yet low tracer concentrations. A new high tracer
component of the DWBC identified as the SLSW probably forms in the
southern Labrador Sea. Only the deep part of the SLSW makes it
south of the Gulf Stream crossover. The structure and continuity
of the DWBC were established from the Denmark Strait into the South
Atlantic to 19
S. At the equator both tracer maxima cores of
the DWBC follow three paths: recirculation back northwards in the
Guiana Basin gyre, eastward flow along the equator, and southward
flow into the South Atlantic. The branching of the SLSW appears to
be time dependent, while there are major changes in the LNADW
between the equator and 5
S. Tracer gradients and inventories
show considerable variations in the equatorward and eastward spread
of the NADW on decadal time scales. The transient tracers are
observed eastward throughout the subpolar region, west of the mid-
Atlantic Ridge in the subtropics, and within a few hundred km of
the boundary in the tropics. Exchange between the DWBC and
recirculation gyres is an effective means of ventilating the
interior. Future observations providing residence times and a
better understanding of the dynamics of deep gyres including the
role of topography, will help to model how the thermohaline
circulation is coupled to climate variability.
Finally, there is a growing consensus that tracer derived spreading rates reflect the importance of mixing and circulation in deep gyres, as part of the equatorward transport of water masses from their source regions. The tracer derived ``effective'' spreading rate is a meaningful quantity for estimating the ocean's role in meridional heat transport, monitoring the ocean's response to climate anomalies introduced at the high latitude source regions, and for evaluating the capacity of the ocean to take up atmospheric compounds.
Acknowledgments. The author acknowledges the careful review of an anonymous referee. This work is supported by the National Oceanic and Atmospheric Administration/Atlantic Climate Change Program under Contract NA90-RAH-00075.