wide margin. The good coverage of most regions
of the world ocean, together with the possibility of
integrating the tracer data into a rich oceanographic data set of unprecedented proportions
for many types of observations (e.g. hydrographic
measurements, Eulerian and Lagrangian current
measurements, satellite observations, etc.) and
models, offers the possibility for tracer studies of
many oceanographic problems at a new level of
detail.
In this contribution, we briefly describe the
existing WOCE data set and demonstrate the type
of results that can be expected from its interpretation on the basis of selected examples. These
examples include: (1) the ventilation of the upper
waters in the Pacific Ocean (see Price, Chapter
5.3); (2) the penetration of near-surface signals
into the deep ocean via the deep western boundary
current in the Atlantic (see Hogg, Chapter 4.5); (3)
the renewal of Antarctic Intermediate Water (see
Rintoul et al., Chapter 4.6; Hanawa and Talley,
Chapter 5.4); (4) the deep water formation around
Antarctica; (5) the spreading of intermediate water
in the Pacific Ocean; (6) the bottom water circulation in the Pacific Ocean; (7) the addition of
glacial meltwater around Antarctica; and (8) the
circulation of the Indian Ocean and the exchange
with the Pacific Ocean (see Gordon, Chapter 4.7).
The preliminary results presented here demonstrate the potential of the WOCE tracer data set
for obtaining insights into the oceanic circulation
that were not possible on the basis of pre-WOCE
data sets.
5.8.2 Tracer methodology and techniques
In this section we briefly discuss the underlying
principles of tracer applications in oceanographic
studies. Detailed accounts can be found, for
example, in Broecker and Peng (1982), Roether
and Rhein (1989) or Schlosser and Smethie (1995).
There are several ways in which tracers can be
used in oceanographic (and other environmental)
studies:
1 As analogues to dyes. The time-dependent
delivery of
3
H (Dreisigacker and Roether, 1978;
Doney et al., 1992), CFCs (Warner and Weiss,
1985; Bu and Warner, 1995; Bullister and
Wisegarver, 1998; Walker et al., 2000), and
bomb
14 C (Druffel and Linick, 1978; Druffel,
1980) to the ocean is relatively well known. It
occurs mostly from the atmosphere over large
spatial (frequently global) scales. The resulting
signal allows us to follow, and estimate time
scales for, the penetration of these tracers from
their entry point into the interior of the ocean.
This is close to performing a gigantic dye experiment. Also, the inventory of these tracers in
subsurface water masses is directly dependent
on the rate of formation of the water masses
during the time interval of the tracer input
(roughly the last 40 years). This has been
demonstrated with CFC-11 inventories (Orsi
et al., 1999; Rhein et al., 2000; Smethie and
Fine, 2001).
2 As radioactive ‘clocks’. Radioactive tracers such
as
14 C or
3 H decay at a known rate. If we can
determine the concentrations of the tracer at the
entry point into a specific water body (e.g. a
deep ocean basin), and compare it with the
mean concentration within this reservoir, then
we can derive a mean residence time of the
water from the concentration difference due to
radioactive decay in the reservoir (e.g.
14 C).
Alternatively, in some cases we can measure
both the radioactive mother and daughter products (e.g.
3
H and
3
He) and can calculate an
apparent age or an isolation time of specific
water masses.
3 As markers of water masses with special
sources. Some tracers have distinct sources in
certain areas or at certain depths in the ocean.
Examples are stable isotopes of water (H 2
18
O/
H 2
16
O,
2 HH
16 O/H 2
16
O), or mantle helium with
a
3 He/
4 He ratio that is significantly higher
(about one order of magnitude) than that of
atmospheric ‘background’ helium in the ocean.
The former is typically added in high latitudes
where fractionation of stable isotopes due to
preferential removal of the light isotope during
precipitation leads to precipitation and runoff
that is significantly lighter in ␦
18
O (of the order
of 910 to 920‰) compared with the bulk of
the ocean water (␦
18 O ϷO‰) (e.g. Craig and
Gordon, 1965). The latter is injected along
the spreading centres of the Mid-Ocean Ridges
or at Hot Spots. The injection depths range
from roughly 1000 m to roughly 2500 m depth,
but there can be injections into waters as shallow as 130 m depth (see below). The ␦
3
He values of helium plumes close to their injection
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
432
of the world ocean, together with the possibility of
integrating the tracer data into a rich oceanographic data set of unprecedented proportions
for many types of observations (e.g. hydrographic
measurements, Eulerian and Lagrangian current
measurements, satellite observations, etc.) and
models, offers the possibility for tracer studies of
many oceanographic problems at a new level of
detail.
In this contribution, we briefly describe the
existing WOCE data set and demonstrate the type
of results that can be expected from its interpretation on the basis of selected examples. These
examples include: (1) the ventilation of the upper
waters in the Pacific Ocean (see Price, Chapter
5.3); (2) the penetration of near-surface signals
into the deep ocean via the deep western boundary
current in the Atlantic (see Hogg, Chapter 4.5); (3)
the renewal of Antarctic Intermediate Water (see
Rintoul et al., Chapter 4.6; Hanawa and Talley,
Chapter 5.4); (4) the deep water formation around
Antarctica; (5) the spreading of intermediate water
in the Pacific Ocean; (6) the bottom water circulation in the Pacific Ocean; (7) the addition of
glacial meltwater around Antarctica; and (8) the
circulation of the Indian Ocean and the exchange
with the Pacific Ocean (see Gordon, Chapter 4.7).
The preliminary results presented here demonstrate the potential of the WOCE tracer data set
for obtaining insights into the oceanic circulation
that were not possible on the basis of pre-WOCE
data sets.
5.8.2 Tracer methodology and techniques
In this section we briefly discuss the underlying
principles of tracer applications in oceanographic
studies. Detailed accounts can be found, for
example, in Broecker and Peng (1982), Roether
and Rhein (1989) or Schlosser and Smethie (1995).
There are several ways in which tracers can be
used in oceanographic (and other environmental)
studies:
1 As analogues to dyes. The time-dependent
delivery of
3
H (Dreisigacker and Roether, 1978;
Doney et al., 1992), CFCs (Warner and Weiss,
1985; Bu and Warner, 1995; Bullister and
Wisegarver, 1998; Walker et al., 2000), and
bomb
14 C (Druffel and Linick, 1978; Druffel,
1980) to the ocean is relatively well known. It
occurs mostly from the atmosphere over large
spatial (frequently global) scales. The resulting
signal allows us to follow, and estimate time
scales for, the penetration of these tracers from
their entry point into the interior of the ocean.
This is close to performing a gigantic dye experiment. Also, the inventory of these tracers in
subsurface water masses is directly dependent
on the rate of formation of the water masses
during the time interval of the tracer input
(roughly the last 40 years). This has been
demonstrated with CFC-11 inventories (Orsi
et al., 1999; Rhein et al., 2000; Smethie and
Fine, 2001).
2 As radioactive ‘clocks’. Radioactive tracers such
as
14 C or
3 H decay at a known rate. If we can
determine the concentrations of the tracer at the
entry point into a specific water body (e.g. a
deep ocean basin), and compare it with the
mean concentration within this reservoir, then
we can derive a mean residence time of the
water from the concentration difference due to
radioactive decay in the reservoir (e.g.
14 C).
Alternatively, in some cases we can measure
both the radioactive mother and daughter products (e.g.
3
H and
3
He) and can calculate an
apparent age or an isolation time of specific
water masses.
3 As markers of water masses with special
sources. Some tracers have distinct sources in
certain areas or at certain depths in the ocean.
Examples are stable isotopes of water (H 2
18
O/
H 2
16
O,
2 HH
16 O/H 2
16
O), or mantle helium with
a
3 He/
4 He ratio that is significantly higher
(about one order of magnitude) than that of
atmospheric ‘background’ helium in the ocean.
The former is typically added in high latitudes
where fractionation of stable isotopes due to
preferential removal of the light isotope during
precipitation leads to precipitation and runoff
that is significantly lighter in ␦
18
O (of the order
of 910 to 920‰) compared with the bulk of
the ocean water (␦
18 O ϷO‰) (e.g. Craig and
Gordon, 1965). The latter is injected along
the spreading centres of the Mid-Ocean Ridges
or at Hot Spots. The injection depths range
from roughly 1000 m to roughly 2500 m depth,
but there can be injections into waters as shallow as 130 m depth (see below). The ␦
3
He values of helium plumes close to their injection
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
432