Deep-Sea Research I 48 (2001) 189}215
Rates of North Atlantic Deep Water formation calculated
from chloro#uorocarbon inventories
William M. Smethie Jr. *, Rana A. Fine
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA
Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway,
Miami, FL 33149, USA
Received 10 May 1999; received in revised form 14 March 2000; accepted 14 March 2000
Abstract
Chloro#uorocarbon (CFC) inventories provide an independent method for calculating the rate of North
Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11
inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water
(ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland}Scotland Over#ow Water (ISOW), and 5.9 million moles for Denmark Strait Over#ow Water (DSOW).
The inventories directly re#ect the input of newly formed water into the deep Atlantic Ocean from the
Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time
of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970
and 1990, the formation rates estimated by this method represent an average over this time period.
Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for
CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic
water) and 2.4 Sv for DSOW. To our knowledge, this is the "rst calculation for the rate of ULSW formation.
The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of
CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are
12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970}1990 time period. The average formation rate for the
sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which
do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about
17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in
the past, it is clearly an important component that is exported out of the North Atlantic with other NADW
components. 2000 Elsevier Science Ltd. All rights reserved.
Keywords: CFC inventories; North Atlantic Deep Water; Water mass formation rates
* Corresponding author. Fax: 1-914-365-8155.
E-mail address: [email protected] (W.M. Smethie Jr.).
0967-0637/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 3 7 ( 0 0 ) 0 0 0 4 8 - 0
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1. Introduction
The earth's climate is closely linked to the global thermohaline circulation of the ocean.
Formation, sinking and spreading of North Atlantic Deep Water (NADW) is perhaps the most
important component of the global thermohaline circulation, and paleoclimate records (Broecker,
1995) and climate models (Manabe and Stou!er, 1988) suggest that the formation rate of NADW
was less during glacial times than it is today. Thus, it is important to understand and to quantify
the formation of NADW and its components in today's ocean.
In this paper the rate of NADW formation is calculated from its chloro#uorocarbon-11
(CFC-11) inventory. The CFCs are manmade substances that have entered the surface ocean from
the atmosphere during the past few decades. These substances are transported into deep water
during formation processes that transform surface water into deep water masses. The amount of
CFCs present in a deep water mass directly corresponds to the conversion rate of surface and near
surface water to deep water.
2. Background
2.1. Water mass structure
North Atlantic Deep Water is a complex of several water masses that form by di!erent processes.
The four major components form in the northern regions of the North Atlantic Ocean; they are:
Upper Labrador Sea Water (ULSW), Classical Labrador Sea Water (CLSW), Iceland}Scotland
Over#ow Water (ISOW) and Denmark Strait Over#ow Water (DSOW). Subsurface waters
from the Southern Ocean and the Mediterranean Sea are also entrained into the NADW
complex.
The upper portion of NADW consists of ULSW and CLSW, which are formed by open ocean
convection during winter. ULSW has only recently been recognized as being part of the deep
thermohaline #ow in the North Atlantic (Pickart, 1992), and it is the least dense component of
NADW. ULSW appears to form in eddies near the southwest margin of the Labrador Sea, possibly
in the Labrador Current, and then becomes entrained into the DWBC, which rapidly transports it
around the Grand Banks into the subtropical Atlantic Ocean (Pickart et al., 1996, 1997). CLSW
forms in the central Labrador Sea by deep convection that can extend below 2000 m (Lazier, 1995;
Dickson et al., 1996). Its formation has been documented to be variable, and in some years surface
water does not become dense enough to undergo the deep convection (Talley and McCartney,
1982; Lazier, 1995). During the early 1960s, CLSW formation was relatively low. There was
a period of high formation from 1972 to 1976, and starting in the late 1980s there was a strong
increase in the formation of CLSW associated with an increase in the North Atlantic Oscillation
(NAO) index (Dickson et al., 1996; Curry et al., 1998).
The lower portion of NADW is composed primarily of ISOW and DSOW with some contribution from the Southern Ocean. DSOW and ISOW form from Atlantic water that has been
transported into the Greenland/Iceland/Norwegian Seas and the Arctic Ocean, where it is modi"ed
by mixing and winter convection. It recirculates southward to cross the Greenland}Iceland}Scotland Ridge (Swift et al., 1980; Swift, 1984; Mauritzen, 1996), and enters the North
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
191
Atlantic. The composition of ISOW that enters the eastern basin has been estimated from potential
temperature and salinity properties to be 45% pure ISOW, 20% northeast Atlantic water and 35%
CLSW (Smethie et al., 2000). ISOW enters the western basin through the Charlie}Gibbs Fracture
Zone, where it encounters DSOW. DSOW is more dense than ISOW and entrains ISOW as it
#ows downslope, forming a roughly 50 : 50 mixture (Smethie et al., 2000). Both water masses #ow
around the periphery of the Irminger and Labrador Seas in the DWBC, mixing along the #ow path,
and a mixture of these two water masses #ows around the Grand Banks and enters the subtropical
western Atlantic. Core temperature, salinity and density ranges of the major components of
NADW based on the data sets used in this study are presented in Table 2 of Smethie et al.
(2000).
2.2. Input of CFCs into NADW
The CFCs enter the source waters for NADW from the atmosphere by gas exchange. The typical
equilibration time between the atmosphere and the surface ocean is about 1 month. Thus, the
surface ocean CFC concentration tracks the atmospheric concentration as a function of time, and
can be calculated from the atmospheric time history (Fig. 1) (Walker et al., 2000) and the CFC
solubility (Warner and Weiss, 1985). However, in regions where deep convection occurs, vertical
mixing over a deep mixed layer is too rapid for gas exchange to maintain saturation, and the
surface water will become undersaturated with respect to the atmospheric concentration. The
CFC-11 saturation has been measured in the formation regions of ULSW and CLSW to be about
70% (Smethie et al., 2000) and 60% (Wallace and Lazier, 1988; Smethie et al., 2000), respectively.
The CFC input to these water masses can be estimated by multiplying the equilibrium concentration by the saturation. For DSOW, which is strongly in#uenced by winter convection in the
Greenland and Iceland Seas, Smethie et al. (2000) have estimated the CFC-11 saturation from
measurements south of Denmark Strait to be 60}75%. Measurements of CFC-11 in bottom water
over the Denmark Strait sill (Tanhua, 1997) indicate a saturation of 70%. CFC-11 input functions
for ULSW, CLSW and DSOW were calculated using saturations of 70, 60, and 70%, respectively,
and also 100% (Fig. 2). The temperature and salinity of the source waters used to calculate the
equilibrium CFC-11 concentration are 3.03C and 34.80 for ULSW and CLSW. These values are
very close to the values of 2.93C and 34.78 reported by Pickart et al. (1996) for ULSW at its
formation site and at the low end of the range of 2.83}3.63C and 34.83}34.90 reported by Dickson et
al. (1996) for CLSW at its site of formation. The temperature and salinity used for DSOW are
!0.53C and 34.80 (Swift et al., 1980).
The input of CFCs into ISOW is more complicated since it involves three di!erent water masses
(see above discussion on water mass structure). The CFC input function for the mixture of
ISOW/northeastern Atlantic water/CLSW was calculated by combining the input functions for the
three components in the proportion they occur in the mixture (45 : 20 : 35) (Smethie et al., 2000).
Pure ISOW originates from about 900 m in the Norwegian Sea and has a relatively low CFC
concentration. Two mechanisms that have been proposed for its formation are lateral mixing along
isopycnal surfaces that approach the surface in the Greenland gyre (Smethie, 1993) and in#ow of
water from the Arctic Ocean that has been isolated from the surface for a number of years
(Mauritzen, 1996). Depending upon the formation process, the shape of the CFC-11 input will
probably be di!erent. However, both processes will produce a low CFC-11 source water. Here the
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Fig. 1. CFC-11 versus time for the northern hemisphere troposphere (Walker et al., 2000).
Fig. 2. CFC-11 concentration versus time for the source waters of ULSW, CLSW, ISOW, and DSOW. See text for how
these concentrations were calculated.
simulated CFC-11 concentration from Smethie's (1993) box model is used for CFC-11 input for
pure ISOW. Northeast Atlantic water undergoes convection to about 900 m each winter (Harvey,
1982; Robinson et al., 1980), and Smethie (1993) has estimated the CFC-11 saturation to be about
85%, which agrees with estimates from CFC-11 measurements made in this region by Tanhua
(1997) in 1994. The CFC-11 concentration in the northeast Atlantic water component was taken to
be 85% saturation. A saturation of 60% was used for CLSW as reported above, but there is a time
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193
lag between its formation in the Labrador Sea and mixing with water #owing over the Iceland}Scotland Ridge in the eastern basin. Sy et al. (1997) estimated a transit time of about 5 years
for CLSW to reach the northeastern basin, so the CFC input function for the CLSW component
was lagged by 5 years before combining it with the other input functions for the mixture. The
CFC-11 input function for the ISOW mixture is shown in Fig. 2.
2.3. Distribution of CFC-11 in the North Atlantic
The CFC distribution in the North Atlantic (Fig. 3) re#ects the water mass structure, circulation
and input processes discussed above. In the subpolar region there is a deep mixed layer of relatively
high CFC concentration corresponding to CLSW. Beneath there is a mid-depth layer of relatively
low CFC concentration and a bottom layer of relatively high concentration. The high concentration at the bottom re#ects ISOW east of and DSOW west of the Reykjanes Ridge. In the
subtropics, most of the CFC signal is in the western basin. There are two distinct subsurface
maxima. The upper maximum has a core potential temperature of about 4.53C and is thought to be
derived mainly from ULSW (e.g., Fine and Molinari, 1988; Smethie, 1993; Smethie et al., 2000).
However, CLSW may contribute some CFCs to the region between the upper maximum and the
underlying minimum. The lower maximum is a mixture of DSOW and ISOW, but about 80% of
the CFC signal is from DSOW (Smethie, 1993; Smethie et al., 2000). Smethie et al. (2000) provide
a detailed discussion of the water mass structure of NADW and the CFC input and distribution
within NADW based on the same data sets we use here.
3. Methods
3.1. Data used in this study
During the 1980s and early 1990s there were a number of CFC surveys in the North Atlantic, but
not a total synoptic survey. To obtain a quasi-synoptic data set, data are combined from 13 cruises
(Table 1). Since the CFC concentrations continually increased with time from their initial production in the 1930s until the early 1990s (Elkins et al., 1993; Cunnold et al., 1994), the water column
inventories were normalized to a common time of 1990. This was done by using the CFC11 : CFC-12 ratio [which increased with time until the late 1970s and has been constant since then
(Walker et al., 2000)], to estimate the year of formation (Weiss et al., 1985; Smethie, 1993). Then the
annual percent change in the atmospheric CFC concentration was estimated for the year of
formation from the atmospheric time history (Smethie, 1993; Warner et al., 1996). Although the
year of formation could not be accurately estimated for waters formed since the late 1970s, the rate
of CFC increase was nearly constant from the mid-1970s until the early 1990s at about 5% per
year, so the annual percent change could be estimated for water formed after the late 1970s. The
annual percent change was then multiplied by the time di!erence in years between the date of
observation and 1990, and the inventory was either increased (for observations prior to 1990) or
decreased (for observations after 1990) by this amount.
To produce as nearly a synoptic map as possible and to minimize the normalization factor
(reported in Table 1), only data collected within 2 years of 1990 were used, except for the WBEX
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Fig. 3. Vertical pro"les of CFC-11 with water mass boundaries used for integrating the CFC-11 station inventories for (a)
the northeastern basin, (b) the Irminger Sea, (c) the Labrador Sea, (d) the Newfoundland Basin, (e) the subtropical western
North Atlantic at 433N, 553W and (f) the subtropical western North Atlantic at 343N, 753W.
data collected in 1986. A more detailed discussion of the normalization procedure as well as maps
of CFC ratio ages and dilution factors derived from this same data set are presented in Smethie et
al. (2000). Some of the WBEX stations overlapped stations taken on the En 214 cruise in 1990, and
a comparison of the normalized WBEX inventories to the En 214 inventories agreed to within
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
195
Fig. 3. (continued ).
10}15%. Since the WBEX data had to be adjusted for a 4-year period, compared to a maximum of
2 years for the rest of the data, this represents an upper limit for the error associated with
normalization of the data to 1990.
3.2. Water column CFC inventories
The water column inventory of a CFC is the amount of the CFC per unit area, and is calculated
by integrating a vertical pro"le of the CFC concentration. In this study vertical pro"les were
segmented into the various components of NADW (ULSW, CLSW, ISOW, DSOW) and the
pro"le within each segment integrated with respect to depth using the trapezoidal rule, which
assumes a linear change in concentration with depth between the data points. Water column CFC
inventories are reported in mol/km. There were six basic types of vertical pro"les, which are
presented in Fig. 3 along with the boundaries between the di!erent NADW components. A discussion of how these boundaries were chosen follows.
In the northeastern basin only two of the NADW components were present, CLSW and ISOW
(Fig. 3a). The upper and lower boundaries for CLSW were taken to be the p
34.62 and 34.69
density surfaces, which cover the density range for CLSW formed between 1962 and 1995 (Dickson
et al., 1996). The underlying ISOW had a CFC maximum at the bottom, and its boundaries were
taken as the lower boundary of CLSW and the ocean bottom.
In the Irminger and Labrador Seas three components were present, CLSW, ISOW and DSOW
(Fig. 3b, c). CLSW was a thick layer of high, nearly homogeneous CFC concentration, with the
thickest layers in the Labrador Sea. The same CLSW density boundaries were used as for the
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W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
Table 1
Data sources and normalization factors for the inventory maps presented in Figs. 5}9. The measured water column
inventory at each station was multiplied by the appropriate normalization factor to produce these maps. A range for the
normalization factor indicates that it varied between stations or was di!erent for the upper and lower NADW
components. These normalization factors are reported in greater detail in Table 1 of Smethie et al. (2000)
Cruise
Year
Principle investigator
Normalization factor
Reference
WBEX
Oce 202
SAVE 1
1986
1988
1988
1.50}2.10
1.09}1.47
1.39
Smethie (1993)
Doney and Bullister (1992)
Weiss et al. (1993)
SAVE 2
1989
1.20
STACS 3
STACS 4
En 214
1989
1990
1990
W. Smethie
J. Bullister
R. Weiss
W. Smethie
R. Weiss
W. Smethie
R. Fine
R. Fine
W. Smethie
Meteor 18
1991
0.95
En 223
1991
W. Roether
A. Putzka
W. Smethie
Weiss et al. (1993)
Smethie et al. (1992)
Molinari et al. (1992)
Johns et al. (1997)
Pickart and Smethie (1993)
Pickart et al. (1992)
Sy et al. (1997)
0.86}0.95
Meteor 22
Hesperides 06
Trident
Hudson 92014
1992
1992
1992
1992
M. Rhein
W. Smethie
R. Fine
P. Jones
0.70
0.70}0.74
0.70}0.74
0.91
1.17}1.22
1.0
1.0
Pickart et al. (1996)
McKee et al. (1995)
Rhein et al. (1995)
Bryden et al. (1996)
Fine (pers. comm.)
Smethie et al. (2000)
northeastern Atlantic. There was a sharp CFC concentration gradient between CLSW and
underlying ISOW, which occurred throughout these basins as a layer of low CFC concentration.
The upper boundary of ISOW was taken to be the lower boundary of CLSW. ISOW was underlain
by DSOW, which had a maximum CFC concentration at the ocean bottom. The boundary
between ISOW and DSOW was taken to be the minimum in CFC concentration between the two
water masses at each station. The mean density of this boundary was p "45.76$0.03 for the
Irminger Sea and p "45.82$0.03 for the Labrador Sea.
All four of the NADW components were present in the Newfoundland Basin (Fig. 3d). ULSW
and CLSW were usually two distinct maxima in CFC concentration. The upper boundary of
ULSW was taken to be the overlying CFC minimum at each station, which had a mean density of
p "34.35$0.19, and the 34.62 p density surface was taken as the boundary between USLW
and CLSW. The distinct change in the slope of the vertical pro"le at the base of the CLSW layer
slightly denser than p "34.69 was taken as the lower boundary of CLSW. This denser lower
boundary was chosen because downstream of the formation region the CFC signal in CLSW had
mixed vertically beyond its formation density. The mean density of this boundary was
p "34.71$0.01. The lowest CFC concentration was in the ISOW layer, and there was
a bottom maximum in underlying DSOW. The boundaries for these layers were the same as in the
Irminger and Labrador seas, and the mean density of the ISOW/DSOW boundary was
p "45.84$0.02.
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197
As discussed previously, in the subtropical western Atlantic south of the Grand Banks there were
generally only two CFC maxima, an upper maximum associated with ULSW and CLSW and
a deep maximum, which is from a mixture of ISOW and DSOW. For ULSW the boundaries for
integration were taken to be the shallow CFC minimum, which had a mean density of
p "34.02$0.22, and the p "34.62 density surface. The boundaries for CLSW were the
lower boundary for ULSW and the mid-depth CFC minimum (Fig. 3f), which had a mean density
of p "34.74$0.02. A signi"cant amount of the CFC signal in the CLSW density horizon may
have entered by diapycnal mixing from ULSW, and CFC observations taken in the mid-1980s
suggested little recently formed CLSW was present at that time (Smethie, 1993). CLSW was present
along the western boundary just south of the Grand Banks in the 1991 data set (En 223) used in this
study (Pickart and Smethie, 1998), but it had only recently entered this region as the result of
intense formation of CLSW that began in the late 1980s. For these stations, two upper maxima
were usually present (Fig. 3e).
The densest component of DSOW in the subpolar basins does not #ow around the Grand Banks
into the subtropical basin. Instead a mixture of ISOW and DSOW enters the subtropical North
Atlantic. The CFC inventory was determined for the mixture, taking the boundaries of the mixture
to be the mid-depth CFC minimum at each station and the ocean bottom (Fig. 3e, f ). Then the
inventory of the mixture was separated into 80% DSOW and 20% ISOW (Smethie, 1993; Smethie
et al., 2000) as discussed previously.
The vertical CFC distribution in the tropics was the same as in the subtropics, and the same
water mass boundaries were used. The mean density of the upper CFC minimum was
p "34.37$0.12 and of the mid-depth CFC minimum was p "34.75$0.03.
3.3. Water mass CFC inventories
CFC-11 inventories for water masses were calculated by plotting the water column inventories
on a Lambert equal area projection map, manually contouring the data and integrating the maps.
Areas between isopleths were measured using a planimeter with a precision of better than 1%.
Since the maps were equal area projections, the calibration factor to convert the planimeter
measured map area to geographical area was constant with location on the map. Geographical
areas between isopleths were multiplied by the average CFC-11 water column inventory between
the isopleths to obtain the CFC inventory between the isopleths, and these were summed to
calculate the total CFC inventory for a water mass.
3.4. Water mass formation rates
The total CFC inventory of a subsurface water mass is directly related to the rate of formation of
the water mass,
I"RC *t,
(1)
where I is the total CFC-11 inventory, R is the rate of water mass formation, C is the CFC-11
concentration in the source water as a function of time, and *t is the time step. The summation is
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carried out over the time period of CFC input, 1945}1990. Using this equation and the input
functions presented in Fig. 2, graphs of CFC-11 inventory versus time were constructed for the
various water masses for di!erent rates of formation. The formation rate that matched the observed
CFC-11 inventory was interpolated from these graphs. As an example, the graphs for ULSW are
presented in Fig. 4.
The source water regions for DSOW, ISOW and ULSW are located at the geographical
boundaries of these water masses, and CFC-11 inventories within the source waters are not
included in the water mass CFC inventories (Table 2). However, for CLSW the source waters are
within the geographical boundaries and thus included in the CFC-11 inventory. The CFC-11
saturation in the CLSW formation region (observed to be 60%) is maintained by three processes:
(1) downward mixing of preconditioned surface and near surface water formed since the previous
Fig. 4. CFC-11 inventory (million moles) versus time for di!erent formation rates of ULSW assuming the source water to
be at (a) 100% saturation and (b) 70% saturation.
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
199
Table 2
CFC-11 inventories in NADW components. Figures refer to the maps that were integrated to calculate the inventories
Water mass
ULSW (Fig. 5)
CLSW (Fig. 6)
Total
Formation region
ISOW
Total
Eastern basin (Fig. 7)
Western subpolar basin (Fig. 7)
Western subtropical basin (Fig. 9)
DSOW
Total
Western subpolar basin (Fig. 8)
Western subtropical basin (Fig. 9)
Total NADW
CFC-11 inventory (million moles)
4.2
14.7
2.0
5.0
2.1
2.2
0.7
5.9
3.1
2.8
29.8
In the western subtropical Atlantic there is a single deep CFC-11 maximum which is a mixture of DSOW and ISOW.
The CFC-11 inventory of this mixture south of 423N (Fig. 9) is 3.47 million moles. About 80% of the CFC-11 inventory
comes from DSOW and 20% from ISOW (see text for details) and the CFC-11 inventory determined for the
DSOW/ISOW mixture has been partitioned accordingly.
winter, (2) the rate of deep convection during winter, and (3) the gas exchange rate during winter.
The rate of CLSW formation calculated from the CFC-11 inventory is for the export of this water
from the formation region. Thus, similar to other water masses, the CFC-11 inventory in the
formation region should be excluded from the calculation. The size of the formation region is
estimated to be 500 km;600 km in the central Labrador Sea (Lilly et al., 1999). This corresponds
to the region of maximum water column CFC inventory in the central Labrador Sea and the
CFC-11 inventory for this region is 2.0 million moles. This was subtracted from the total CLSW
inventory of 14.7 million moles yielding an inventory of 12.7 million moles that was used for
calculating the formation rate of CLSW.
4. Results
4.1. Inventory maps
Maps of water column CFC-11 inventory were prepared for ULSW (Fig. 5), CLSW (Fig. 6),
ISOW (Fig. 7), DSOW (Fig. 8), and the DSOW/ISOW mixture (Fig. 9). The ULSW and CLSW
maps cover the extent of penetration of CFC-11 in these water masses as of 1990. The northern
boundary for ULSW was taken to be 463N, which is about 33 south of where its formation was
observed near the Grand Banks (Pickart et al., 1996). ULSW is rapidly transported to the
subtropics in the DWBC as discussed previously, and there is no evidence that it enters the eastern
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Fig. 5. Map of CFC-11 water column inventory (mol km\) for ULSW.
basin at high latitude. Its CFC-11 inventory was assumed to decrease linearly from its easternmost
measured value at 443N to zero at the mid-Atlantic Ridge. The southward extent of relatively high
inventories south of the Grand Banks is supported by CFC-11 measurements along 523W made in
1983 (Smethie, 1993) and 1997 (Smethie, 1999). The CFC-11 inventory for CLSW is present
throughout the subpolar North Atlantic and has a similar distribution to the ULSW inventory in
the subtropical and tropical Atlantic. It is assumed not to extend south of 413N in the eastern basin,
except for the equatorial plume, because a strong front at this latitude separates CLSW and water
of Mediterranean Sea origin (Talley and McCartney, 1982; Doney and Bullister, 1992).
The ISOW and DSOW maps extend only to the southern tip of the Grand Banks because of the
di$culty in distinguishing separate ISOW and DSOW signals south of the Grand Banks. The
combined DSOW/ISOW map covers the entire extent of penetration of the CFC signal in over#ow
waters as of 1990. It was prepared by summing the DSOW and ISOW water column inventories in
the subpolar regions and combining this with DSOW/ISOW inventories calculated for the
subtropical Atlantic. As was the case for ULSW and CLSW, the southern extent of relatively high
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
201
Fig. 6. Map of CFC-11 water column inventory (mol km\) for CLSW.
CFC-11 inventories south of the Grand Banks is supported by the 1983 and 1997 data sets
referenced above.
4.2. Water mass inventories
CFC-11 inventories for the NADW components are summarized in Table 2. The largest CFC-11
inventory, 14.7 million moles, is found in CLSW. ULSW has an inventory of 4.2 million moles. The
inventories in ISOW and DSOW are 5.0 and 5.9 million moles, respectively.
4.3. Water mass formation rates
The formation rates for NADW components estimated from their CFC-11 inventories are
summarized in Tables 3 and 4. The formation rates calculated here are strongly dependent on the
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Fig. 7. Map of CFC-11 water column inventory (mol km\) for ISOW.
extent to which the formation waters are in equilibrium with the atmospheric CFC concentrations.
There is some uncertainty in this, and the saturations probably vary with time, particularly during
the exponential phase of the CFC increase in the atmosphere. However, during the linear phase of
increase in the 1970s and 1980s, which provided most of the CFC signal to the ocean up to 1990,
the saturations are thought to have been fairly constant. For instance for CLSW; the saturation
was 60% in 1986 (Wallace and Lazier, 1988) and 62% in 1992 (Smethie et al., 2000). Formation
rates have been calculated using 100% saturation, and using a saturation that has been determined
from observations; the formation rates based on observed saturations are considered the most
accurate.
These formation rates represent averages over the time of input of CFC-11 to the ocean,
1945}1990. However, this average is heavily weighted toward the 1970}1990 time period. Examination of the CFC-11 atmospheric time history (Fig. 1) and NADW CFC-11 input functions (Fig. 2)
shows that the atmosphere and source water CFC-11 concentrations increased by a factor of about
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
203
Fig. 8. Map of CFC-11 water column inventory (mol km\) for DSOW.
4.3 between 1970 and 1990. The cumulative CFC-11 inventory in the ocean, assuming a constant
deep water formation rate, increased by a factor of about 10 (see Fig. 4) indicating about 90% of the
CFC-11 inventory in the ocean entered during the 1970}1990 time period. Figs. 1 and 2 also show
that although the increase in atmospheric and source water CFC-11 concentrations were nearly
exponential up to the 1970s, the increase was roughly linear between 1970 and 1990. Thus, the
formation rates presented here closely approximate average formation rates for the 1970}1990 time
period.
As discussed previously, the formation rate of CLSW has varied dramatically in the past, with
high rates of formation from 1972 to 1976 and from 1988 until the mid-1990s. Therefore, the
formation rate was allowed to vary using Eq. (1) to simulate the measured CFC inventory for
CLSW. Four cases were run: (1) CLSW formation occurred only during the 1972}1976 and
1988}1990 periods, (2) the ratio of high to low rate was 10 : 1 and (3) 5 : 1 with the high rates
occurring during the 1972}1976 and 1988}1990 periods and (4) the ratio of the high to low rate was
204
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
Fig. 9. Map of CFC-11 water column inventory (mol km\) for DSOW#ISOW.
Table 3
Formation rates of components of NADW estimated from CFC-11 inventories
Water mass
Source water saturation (%)
Formation rate (Sv)
ULSW
100
70
1.6
2.2
5.2
1.7
2.4
ISOW
DSOW
100
70
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
205
Table 4
Formation rates of CLSW estimated from CFC inventories
Source water
saturation (%)
Ratio of high : low
formation rate
High formation
rate (Sv)
Low formation
rate (Sv)
100
na
10 : 1
5:1
10 : 1
Constant
na
10 : 1
5:1
10 : 1
Constant
13.5
11.3
9.7
7.5
*
22.5
18.7
16.2
12.5
*
0
1.1
2.0
0.8
*
0
1.9
3.3
1.3
*
60
Avg. formation
rate (Sv)
1970}1990
5.4
5.2
5.1
4.5
4.6
9.0
8.6
8.4
7.4
7.6
Scaled to CLSW thickness, see text for details.
10 : 1 but linearly scaled to the thickness of CLSW reported by Curry et al. (1998). The maximum
thickness for the 1970}1990 time period was 2000 m in 1990 and the minimum thickness was
1200 m in 1982. A run was also made assuming a constant rate for the entire period. The results are
summarized in Table 4.
Also discussed previously, ISOW is a mixture of three components, pure ISOW, northeast
Atlantic water and CLSW. The formation rate for this mixture is estimated to be 5.2 Sv. This can be
broken down into formation rates for each component by multiplying 5.2 Sv by the component
fractions, 45% pure ISOW, 35% CLSW and 20% northeast Atlantic water, which yields 2.4 Sv for
the pure ISOW fraction, 1.8 Sv for the CLSW fraction, and 1.0 Sv for the northeast Atlantic water
fraction. These rates have been combined with the estimates presented in Tables 3 and 4 based on
observed CFC-11 saturations for the source waters to yield the best estimates for formation rates
(Table 5). For CLSW the average rate for the 10 : 1 ratio of high to low formation rate scaled to the
CLSW thickness was used.
4.4. Error analysis
The errors in determining water mass formation rates from CFC inventories in this paper fall
into three groups: the error on the total CFC inventory of a given water mass, the error on the
source water CFC concentration, and the error that arises from assuming that the water mass
formation rate is constant during the time of the CFC input.
There are several errors that a!ect the accuracy of the total CFC inventory of a water mass. The
most basic error is the measurement error on the individual water samples, which is generally
1}2% and insigni"cant relative to other errors. There is an error on the water column CFC
206
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
Table 5
Best estimates of formation rates of components of NADW based on CFC-11 inventories. See text for details
Water mass
ULSW
CLSW
CLSW entrained into ISOW
ISOW
Entrained northeast Atlantic water
DSOW
Sum
Formation rate (Sv)
2.2 ,
7.4
,
1.8
2.4
1.0
2.4
Total upper NADW"9.6 Sv
Total CLSW"9.2 Sv
Total lower NADW"7.6 Sv
17.2
inventory for each station from the vertical integration of the CFC pro"le and from the adjustment
made in the water column inventory to the common date of 1990. Most stations used in this study
have good vertical coverage (Fig. 3) and the error in the vertical integration is small, about 2%
from propagating the errors on the individual measurements. As previously mentioned, the
maximum error due to adjustment to a common date of 1990 is 15%. Ten percent is a more realistic
error because most of the data were collected within 2 years of 1990. If this is a random error,
propagation of it through the lateral integration results in an error of 2}3% for the total inventory.
This error may be systematic for a given cruise, but it is highly unlikely that it will be systematic in
the same way for the 13 di!erent cruises used in this study. We expect some adjustments to
overestimate the 1990 water column inventory and some to underestimate it and hence the error is
approximately random. There are two other sources of error for the total inventory, contouring the
water column CFC inventory data to produce the maps and measuring the area between contours
using a planimeter. The planimeter area determination has an error of about 1%. Thus, the errors
on the total CFC inventory, excluding the contouring error, propagate to about 3}4%.
By far the largest and dominant error on the total inventory is in the placement of the contours
in constructing the map. This is complicated by lack of data in a large region in the northwestern
subtropical Atlantic. To estimate this error, three maps of the ULSW CFC-11 inventory were
constructed, the contour map used to determine the inventory (Fig. 5), a map which was contoured
to produce the smallest inventory possible and a map contoured to produce the largest inventory
possible. For the latter two maps, contour lines were shifted to extreme positions without violating
the data. Also the eastward extent of the inventory at the northern boundary was allowed to extend
to only 403W for the low inventory map and to extend fully to the mid-Atlantic Ridge for the high
inventory map. The CFC-11 inventories for these two maps were 3.3 and 4.9 million moles
compared to 4.2 million moles for Fig. 5 which corresponds to an average error of $19%. This is
also representative of the other maps, which have similar sized regions of sparse data. Combining
this error with 3}4% for other errors on the total inventory yields a total error of 19.4% on the
total CFC-11 inventory.
The error on the source water concentration arises from three sources, uncertainty in the
atmospheric time history of CFCs, uncertainty in the temperature and salinity of the source water
which determines the CFC solubility, and uncertainty in the extent to which the source water has
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
207
equilibrated with the atmosphere. Atmospheric concentrations of CFCs have been carefully
monitored since the late 1970s and the uncertainty in the atmospheric time history for CFC-11 is
generally less than 1% since the late 1970s and 2}4% prior to that time (Walker et al., 2000). The
temperature and salinity of the source waters do vary with time but there is no evidence for
variability greater than about $0.53C and $0.5 psu, which would result in a 3% uncertainty in
the solubility of CFC-11 (Warner and Weiss, 1985). These two uncertainties combine to yield
an uncertainty of about 4% on the equilibrium CFC-11 concentration of source water, C .
Uncertainty in the extent of equilibration is much larger than this. As discussed previously the
source waters are not in equilibrium with the atmosphere because of deep convection. Also the
percent saturation may not be constant with time. We believe that the percent saturations that
have been measured and used in this paper are accurate to within 20%. Also, as discussed
previously, the percent saturation of CLSW source water was observed twice between the
mid-1980s and the early 1990s to be about 60%.
To combine the e!ects of the uncertainties in source water concentration and total CFC
inventory, Eq. (1) can be rewritten as
R"I/SC *t,
(2)
where S is percent saturation and the other terms are as previously de"ned. Propagation of the
19% error on I, the 20% error on S and the 4% error on C *t yields an error of 28% on R,
assuming R is constant with time.
A sensitivity study was carried out to estimate the error introduced by assuming the formation
rate was constant. CFC-11 inventories were calculated using Eq. (1) for the 1970}1990 time period
with the CFC input function (Walker et al., 2000) for that time period. In one series of runs the
formation rate was increased abruptly by factors of 2 and 10 in 1973, 1975, 1977, 1979, 1981, 1983,
1985, 1987, and 1989. In another series of runs the formation rate was decreased by factors of 2 and
10 in the same years. For each individual run, the average formation rate for the 20-yr period was
also used in Eq. (1) to calculate the CFC-11 inventory. The percent di!erence between the CFC-11
inventories determined for the variable and constant formation rates were calculated and are
plotted in Fig. 10 against the fraction of time with the high formation rate. For the 10 : 1 change in
formation rate, the maximum di!erence between the CFC-11 inventories generated with constant
and variable formation rates is 32%, and this is the maximum error that would result from
assuming a constant formation rate during this time period. For the 2 : 1 change, the maximum
error is 10%.
For the over#ow waters (DSOW and ISOW) it is possible that the input into the Atlantic has
varied by a factor of 2 over the past four decades (Bacon, 1998), but no more than that. There is also
no evidence that the ULSW formation rate has varied by more than a factor of 2. Thus, the
error for these water masses is a combination of the 28% error for a constant formation rate and
the 10% error that could result from a factor of two variation in the formation rate. If the errors
combine as (E
)#(E
)"(E ), the total error is 30%. For CLSW, as discussed
previously, there is evidence of variability much greater than a factor of two. Since it is known that
the formation rate was much higher during the 1972}1976 and 1988}1990 time periods, a variable
formation rate was used in this calculation and thus the total error in formation rate is expected to
be about 28%.
208
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
Fig. 10. Error in the CFC inventory derived water mass formation rate resulting from the assumption that the rate is
constant with time. See text for explanation.
5. Discussion
5.1. Inventory maps
The water column inventory maps reveal the large-scale circulation patterns for the NADW
components. The ULSW inventory map (Fig. 5) is similar to the CFC-11 concentration map for the
upper CFC maximum reported in Smethie et al. (2000). Relatively high inventories are found south
of the Grand Banks just downstream of the source region and along the western margin re#ecting
transport of recently formed ULSW in the Deep Western Boundary Current (DWBC). Relatively
high inventories extend into the interior north of 303N latitude re#ecting recirculation of ULSW
into the interior via recirculation gyres. Near the equator it is apparent that the #ow of ULSW
splits into two branches, one extending along the equator and the other extending along the
western boundary into the South Atlantic Ocean as "rst observed by Weiss et al. (1985).
For CLSW the highest inventories are found in the formation region in the central Labrador Sea,
which is expected (Fig. 6). Three branches of relatively high CFC-11 inventory extend from this
region, one into the Irminger Sea, one into the eastern basin, and one along the western boundary
around the Grand Banks, which is similar to the pattern Talley and McCartney (1982) described
using salinity and potential vorticity. The distribution in the subtropical and tropical Atlantic has
the same pattern as for ULSW. Some of the CFC signal in the CLSW density horizon may be
derived from vertical mixing with overlying ULSW which would produce the same distribution
pattern. The extent to which this has happened, is not clear and Smethie et al. (2000) have suggested
that the CFC signal in the CLCW density horizon for most of the subtropics and tropics is from
ULSW.
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
209
The ISOW map (Fig. 7) indicates that recently formed ISOW splits with some #owing through
the Charlie}Gibbs Fracture Zone into the western Atlantic and some continuing southward along
the eastern #ank of the mid-Atlantic Ridge. The CFC-11 signal extends to about 233N in the
eastern basin. In the western basin, highest CFC-11 inventories in ISOW are observed in the
Irminger Sea, consistent with #ow of ISOW northward along the western #ank of the Reykjanes
Ridge, and along the western margin of the southern Labrador Sea and the Newfoundland Basin
consistent with #ow of the DWBC. The contours suggest westward #ow of ISOW from the
Charlie}Gibbs Fracture Zone to the southern Labrador Sea, but the data are too sparse to con"rm
if such a #ow pattern exists.
The DSOW map (Fig. 8) reveals high inventories along the western margin of the subpolar
Atlantic extending from just south of Denmark Strait into the Newfoundland Basin. Highest
inventories are found at the western margin of the southern Labrador Sea. South of the Grand
Banks, the ISOW/DSOW inventory map (Fig. 9) is similar to the ULSW and CLSW maps (Figs.
5 and 6) re#ecting a similar circulation pattern. The main di!erence in the two maps is that the
equatorial plume for ISOW/DSOW does not extend as far eastward because it is blocked by the
mid-Atlantic Ridge.
The water column inventory maps reveal that NADW formed within the last four decades is
nearly entirely contained within the North Atlantic Ocean. The regional distribution of the
CFC-11 inventory is summarized in Table 6. The ULSW CFC-11 inventory is found predominantly in the subtropical region and the CLSW inventory predominantly in the subpolar region.
The amount of CLSW CFC-11 inventory in the subtropics and tropics is an upper limit since some
of it may have been derived from ULSW as discussed previously. The CFC-11 inventories in the
over#ow waters are highest in the subpolar region and are also found in the subtropical region with
a small amount in the tropics. Overall a little less than two-thirds of the NADW CFC-11 inventory
is found in the subpolar region, about one-third in the subtropical region and about 3% in the
tropics. These data provide strong constraints for time integration of climate models. They also
have implications for the physical transport of an atmospheric constituent delivered to the high
latitude North Atlantic, such as CO . Such a constituent will similarly be stored mostly in the
subpolar and subtropical North Atlantic on a 20}30 yr time scale. On longer time scales the
constituent will be transported into the tropics and the South Atlantic.
Table 6
Distributions of CFC-11 inventories in 1990
Water mass
ULSW
CLSW
ISOW
DSOW
NADW
Subpolar region
(42}653N)
Subtropical region
(20}423N)
Tropical Region
(203S}203N)
CFC-11 inv.
(10 mol)
(%)
CFC-11 inv.
(10 mol)
(%)
CFC-11 inv.
(10 mol)
(%)
0.10
12.40
3.82
3.11
19.43
2.4
84.4
77.2
52.9
65.4
3.8
2.0
1.06
2.48
9.34
90.5
13.6
21.4
42.2
31.4
0.3
0.3
0.07
0.29
0.96
7.1
2.0
1.4
4.9
3.2
210
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
5.2. Water mass formation rates
The rates of NADW formation calculated using CFC-11 inventories are independent of methods
previously used, such as direct current measurements and geostrophic transports. The inventory
rates represent an average rate over several decades (1970}1990 for this study), which has not been
possible to determine by other methods. It has been possible to use this method because the CFC
signal thus far has been contained essentially in the North Atlantic Ocean, where it can be mapped
in its entirety, and the CFC signal in NADW has not been altered by mixing with CFC bearing
waters of southern origin.
The rate of formation for ULSW, 2.2 Sv, is the "rst estimate to our knowledge for this water
mass. The 2.2 Sv can be considered a lower bound on the formation rate for the 1970}1990 period,
because some of the CFC signal may have mixed downward into the underlying CLSW density
horizon. If the CFC inventory in the CLSW density horizon is added to the ULSW inventory for
the subtropics and tropics, a formation rate of 3.1 Sv is obtained, which is the upper limit for
formation rate of ULSW. Both estimates are considerably less than the formation rate for CLSW.
As discussed previously there is strong evidence that the formation rate of CLSW has varied
during the past several decades. CLSW formation rates were estimated using several scenarios of
variable formation rates (Table 4, Fig. 11), and these di!erent scenarios result in a wide range of
high and low rates of formation. The high rates, using 60% saturation for the source water, ranged
from 12.5 to 22.5 Sv and the low rates ranged from 0 to 3.3 Sv. However, the average rates for the
1970}1990 period have a much smaller range, 7.4}9.0 Sv. When a constant formation rate is
assumed for the entire period the rate is 7.6 Sv. The sensitivity analysis presented previously
showed that the discrepancy between average formation rates calculated with constant and factor
of 10 variable formation rates could be as great as 32% (Fig. 10). Such di!erences were not
Fig. 11. Formation rate of CLSW versus time for the four scenarios reported in Table 4 when the CFC saturation of the
source water is 60%. The highest rates are for the scenario of formation only during the 1972}1976 and 1988}1990 time
periods. All scenarios yield the same CFC-11 inventory in 1990. See text for details.
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
211
observed in the scenarios used here, because the variability was spread over the 1970}1990 time
period rather than occurring as a single step function as was the case for Fig. 10.
Although it is not possible to know if any of these scenarios represent reality, we feel that
a scaling of the rate to the thickness of the CLSW with a 10 : 1 ratio of high to low rates is the most
realistic. Smith and Dobson (1984) determined that heat #ux varied by a factor of 10 at OWS Bravo
in the central Labrador Sea between 1946 and 1974. Also the thickness of CLSW must re#ect the
formation rate during the recent past, although the annual formation rate is unlikely to scale
linearly as a function of annual thickness. The maximum and minimum rates for this scenario are
12.5 and 1.3 Sv, and the average for the 1970}1990 period is 7.4 Sv.
Our average value of 7.4 Sv compares well with the estimate of 7.0 Sv reported by McCartney
(1992), which is based on a heat and mass budget calculation using data collected mainly in the
1950s and 1960s, but is higher than Marsh's (1999) estimate based on more recent data. Marsh
estimates the formation rate, using observed surface heat and freshwater #uxes, to range from close
to zero in 1980 to a maximum of about 10 Sv in 1990, with an average of 3.4 Sv for the 1980}1997
time period. Our best estimate of the maximum formation rate in 1990 is 12.5 Sv, also higher than
Marsh's estimate. Our rate of 12.5 Sv implies that a layer about 1200 m thick in the
500 km;600 km formation region would have to be replaced annually, which may be unrealistic.
However, this may not be so unreasonable if the region of CLSW formation extends beyond the
Labrador Sea during the extreme winters that result in high formation rates. Thick layers of CLSW
have been observed in the Irminger Sea (Fig. 3b), and there is recent evidence that CLSW forms
there (Pickart, 1999).
The in#ow of ISOW into the North Atlantic has been measured in the Faeroe-Bank Channel
from CTD and ADCP measurements and a year-long current meter array from 1987 to 1988 to be
1.9$0.4 Sv (Saunders, 1990). ISOW also #ows over the ridge between Iceland and the Faeroe
Islands, and Meincke (1983) has estimated this #ow to be about 1 Sv based on hydrographic
observations and current measurements from the 1970s. This yields a total of about 2.9 Sv, which is
about 20% higher than our estimate of 2.4 Sv. Doney and Jenkins (1994) estimated the ISOW and
DSOW input by simulating the tritium inventory in the deep North Atlantic in the early 1980s
using a model for the DWBC that included exchange with the interior. Their input of 2.0 Sv for
ISOW is about 20% less than our estimate.
The in#ow of DSOW has also been measured directly by current meters. Ross (1984) observed
a #ow of 2.9 Sv from a 5 week mooring in Denmark Strait in 1973. Dickson and Brown (1994) made
extensive measurements of #ow downstream of Denmark Strait from 1986 to 1991 and "nding no
seasonal variability, concluded that Ross' measurement was representative of #ow of pure DSOW.
Similar to ISOW, the directly measured DSOW transport is about 20% higher than our estimate of
2.4 Sv. The Doney and Jenkins' (1994) estimate of the formation rate of DSOW is 2.5 Sv, in
excellent agreement with ours. Recent observations suggest there are changes in DSOW characteristics (Dickson et al., 1999). Bacon (1998) has shown from analysis of 22 hydrographic sections from
the southern Irminger Basin that over#ow transports below 27.8p , which includes both DSOW
F
and ISOW, were relatively weak in 1955}1967 and 1991}1997, and strong in 1978}1990. The later
weak phase averages 4.3 Sv, while the strong phase averages 7.7 Sv (McCartney et al., 1998) and
corresponds best in time to the inventory results presented here. The 7.7 Sv is in excellent
agreement with the sum for ISOW, entrained CLSW, entrained northeast Atlantic water and
DSOW of 7.6 Sv presented in Table 5.
212
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
The sum of the individual water mass formation rates gives the overall NADW formation rate.
The NADW formation rate has been estimated over a range of time scales and from a variety of
methods including radiocarbon (Broecker, 1991), hydrography (e.g., Reid, 1994; Roemmich, 1980;
Rintoul, 1991), and current meter moorings at various locations along the western boundary. The
estimates based on hydrography and current measurements range from 7 to 31 Sv and in some
cases include recirculating components, which clearly are not included in our results. From an
extensive review of NADW formation rates based on hydrography and current measurements
taken primarily from the 1960s through the 1980s, Schmitz and McCartney (1993) estimate the
strength of the North Atlantic thermohaline circulation to be 13 Sv for water colder than 43C; this
includes CLSW and the over#ow waters, but not ULSW. The best estimate based on CFC-11 for
the formation rate of CLSW plus the over#ow waters is 15.0 Sv, which is similar to (within our
error) their estimate. Broecker (1991) estimates the NADW production rate for water below 2000 m
from a radiocarbon budget to be 20 Sv with a 25% error bar. This represents an average over the
#ushing time for the deep Atlantic, which is several hundred years. Broecker's estimate does not
include ULSW, and is about 30% higher than the CFC estimate for over#ow waters plus CLSW.
Although not included in earlier work, ULSW, accounts for 13}18% of the NADW formation rate
and should be included in evaluating the overall strength of the NADW thermohaline circulation.
The best estimate based on CFC-11 for the total formation rate of NADW from 1970 to 1990 is
17.2 Sv with an error of about 30%. This value represents the conversion of surface and near
surface water in the subpolar North Atlantic into NADW plus the in#ow of intermediate waters
into the North Atlantic across the Greenland}Iceland}Scotland Ridge. The two largest components of the error in this study were uncertainty in the total inventories caused by scarcity of data in
some regions and uncertainty in the source water concentrations resulting from few observations in
the formation regions. Both of these uncertainties can be reduced substantially by more observations, such as achieved in the WOCE program, and an overall error of 15% for future CFC
inventory based estimates of water mass formation rates should be achievable.
6. Summary and conclusions
Inventories of CFC-11 calculated from data collected between 1986 and 1992 for the major
components of NADW are: 4.2 million moles for ULSW, 14.7 million moles for CLSW, 5.0 million
moles for ISOW and 5.9 million moles for DSOW.
As of 1990 about 90% of the CFC-11 inventory for ULSW and 85% for CLSW are found,
respectively, in the subtropical and subpolar North Atlantic. The CFC-11 inventories for ISOW
and DSOW are found mainly in the subpolar and subtropical regions, with higher inventories in
the subpolar region. Only a small fraction of the inventory has reached the tropics. For the overall
NADW CFC-11 inventory, a little less than two-thirds is found in the subpolar region, about
one-third in the subtropical region and about 3% in the tropics.
Rates of formation for the NADW components were calculated from the CFC-11 inventories.
Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990 and
its increase in input was nearly linear during this time, the formation rates estimated by this method
represent an average over this time period. The formation rates, based on best estimates of source
water CFC-11 saturations, are 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure
W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215
213
ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for
DSOW, with an estimated error of about $30%. To our knowledge this is the "rst calculation of
the rate of ULSW formation.
The formation rate of CLSW was calculated assuming that the formation rate varied by a factor
of 10 and scaled linearly to the CLSW thickness measured in the central Labrador Sea. The best
estimate of the maximum, minimum, and average rates are 12.5, 1.3, and 7.4 Sv, respectively, for the
1970}1990 time period.
The rate for CLSW#ISOW#DSOW of 15.0 Sv is similar to previous estimates based on
hydrography and current measurements.
When ULSW is considered, the total NADW formation rate is 17.2 Sv. The ULSW contributes
13}18% of the NADW production, but it has not been previously considered as part of the North
Atlantic thermohaline circulation. However, it is clearly an important component that is exported
to the South Atlantic with the other components of NADW.
The CFC inventories directly re#ect the input of newly formed water from the surface to the deep
North Atlantic during the time of input of CFC-11 to the ocean, and provide an independent
method of estimating water mass formation rates. An advantage to this method is that it integrates
over time. The method has implications for estimating the oceanic storage of atmospheric
constituents such as CO and provides data to test time integration of climate models. Thus, tracer
inventories provide an integrated yet sensitive method for monitoring the strength of the thermohaline circulation and changes in its partitioning.
Acknowledgements
We thank Debbie Willey and Frank Zheng for assembling the combined data set used for this
study, Frank Zheng, Manfred Mensch and Hoyle Lee for calculating the CFC inventories and
preparing the inventory maps and Patty Catanzario for drafting the "gures. We also thank two
anonymous reviewers for thoughtful and helpful comments. This work was supported by the
NOAA Climate Change Program, NOAA grants NA26GP0231 and NA46GP0168 to WMS and
NA67RJ1049 to RAF, and by NSF grants OCE 89-17801 and OCE 90-19690 to WMS and OCE
94-13222 and OCE 98-11535 to RAF. L-DEO Contribution no. 6011.
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