Rates of North Atlantic Deep Water formation calculated from chloro#uorocarbon inventories

Transcription

1 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 Elsevier Science Ltd. All rights reserved. Keywords: CFC inventories; North Atlantic Deep Water; Water mass formation rates * Corresponding author. Fax: address: bsmeth@lamont.ldgo.columbia.edu (W.M. Smethie Jr.) /01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 190 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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

3 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) Input of CFCs into NADW W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 for ULSW and CLSW. These values are very close to the values of 2.93C and 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 (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

4 192 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215 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 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

5 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 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 This was done by using the CFC- 11 : 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

6 194 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215 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 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

7 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 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 σ and 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

8 196 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 1986 W. Smethie 1.50}2.10 Smethie (1993) Oce J. Bullister 1.09}1.47 Doney and Bullister (1992) SAVE R. Weiss 1.39 Weiss et al. (1993) W. Smethie SAVE R. Weiss 1.20 Weiss et al. (1993) W. Smethie Smethie et al. (1992) STACS R. Fine 1.17}1.22 Molinari et al. (1992) STACS R. Fine 1.0 Johns et al. (1997) En W. Smethie 1.0 Pickart and Smethie (1993) Pickart et al. (1992) Meteor W. Roether 0.95 Sy et al. (1997) A. Putzka En W. Smethie 0.86}0.95 Pickart et al. (1996) McKee et al. (1995) Meteor M. Rhein 0.70 Rhein et al. (1995) Hesperides W. Smethie 0.70}0.74 Bryden et al. (1996) Trident 1992 R. Fine 0.70}0.74 Fine (pers. comm.) Hudson P. Jones 0.91 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 σ "45.76$0.03 for the Irminger Sea and σ "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 σ "34.35$0.19, and the σ 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 σ "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 σ "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 σ "45.84$0.02.

9 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 σ "34.02$0.22, and the σ "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 σ "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 σ "34.37$0.12 and of the mid-depth CFC minimum was σ "34.75$ 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 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

10 198 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215 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.

11 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} Table 2 CFC-11 inventories in NADW components. Figures refer to the maps that were integrated to calculate the inventories Water mass CFC-11 inventory (million moles) ULSW (Fig. 5) 4.2 CLSW (Fig. 6) Total 14.7 Formation region 2.0 ISOW Total 5.0 Eastern basin (Fig. 7) 2.1 Western subpolar basin (Fig. 7) 2.2 Western subtropical basin (Fig. 9) 0.7 DSOW Total 5.9 Western subpolar basin (Fig. 8) 3.1 Western subtropical basin (Fig. 9) 2.8 Total NADW 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 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

12 200 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215 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 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

13 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 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 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

14 202 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189}215 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

15 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} Fig. 8. Map of CFC-11 water column inventory (mol km ) for DSOW. 4.3 between 1970 and 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 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

16 204 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 ISOW 5.2 DSOW

17 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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) Avg. formation rate (Sv) 1970} na : : : Constant * * na : : : Constant * * 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 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 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

18 206 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 Formation rate (Sv) ULSW 2.2 Total upper NADW"9.6 Sv CLSW 7.4 Total CLSW"9.2 Sv CLSW entrained into ISOW 1.8 ISOW 2.4 Entrained northeast Atlantic water 1.0 Total lower NADW"7.6 Sv DSOW 2.4 Sum 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 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 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

19 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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/S C 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 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%.

20 208 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.

21 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 Subpolar region (42}653N) Subtropical region (20}423N) Tropical Region (203S}203N) CFC-11 inv. (%) CFC-11 inv. (%) CFC-11 inv. (%) (10 mol) (10 mol) (10 mol) ULSW CLSW ISOW DSOW NADW

22 210 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 See text for details.

23 W.M. Smethie Jr., R.A. Fine / Deep-Sea Research I 48 (2001) 189} 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 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 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 27.8σ, which includes both DSOW 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.