CFC time series in the deep water masses of the western tropical Atlantic, 1990–1999

Deep-Sea Research I 49 (2002) 281–304

CFC time series in the deep water masses of the western tropical Atlantic, 1990–1999 Chantal Andrie! a,*, Monika Rheinb,c, Se! bastien Freudenthala, Olaf Pl.ahnb a

! Laboratoire d’Oceanographie Dynamique et de Climatologie, CNRS/IRD/Universite! Pierce Marie Curie, 4 Place Jussieu, Case 100, 75252 Paris Cedex, France b Institut fuer Meereskunde, Kiel, Germany c Institut fuer Umweltphysik, Universitaet Bremen, Bremen, Germany Received 25 September 2000; received in revised form 16 May 2001; accepted 30 August 2001

Abstract A unique CFC time series was collected along the 351W meridian in the tropical Atlantic during eight cruises from 1990 to 1999, in order to investigate the large-scale variability of the circulation of the North Atlantic Deep Water (NADW) through transient tracer distributions. Within the upper NADW (UNADW), it appears that CFC distributions, are highly variable; cores of maximal CFC concentrations are closely associated with salinity and are principally caused by the local dynamics of equatorial and extra-equatorial deep jets within the three particular regimes of the 3–11S, 11S–11N and 1–31N bands. CFC distribution within the lower NADW (LNADW) is less heterogeneous and linked to oxygen. A double core has been observed for 1999, possibly due to the recirculation of the deep flow constrained by the circulation of the underlying Antarctic Bottom Water (AABW). The variability of the maximal concentrations presumably results from the seasonal or semi-annual variability of the regional deep circulation. For both layers, the temporal evolution of the mean concentrations has been traced throughout the 1990s. At the UNADW level, the temporal increase of the CFC concentration within the 31S–31N band is mainly due to the transient signal from the atmosphere. This increase is clearly dominant compared with the local dynamics. Similar behavior has been observed for the LNADW in the 31S–11N band, corresponding to the equatorial channel. An attempt has been made to access the transit time for the NADW pathway from the Labrador and Nordic Seas to the tropics. The mean transit time around 25 yr for the UNADW has been compared to other evaluations from hydrographic measurements. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: CFCs; Deep-water masses; Ocean circulation; Temporal distribution; Age of seawater; Tropical Atlantic

1. Introduction Several oceanographic cruises were organized in the tropical Atlantic between 1990 and 1999 in *Corresponding author. Fax: +33-1-4427-3805. E-mail address: [email protected] (C. Andri!e).

order to study the inter-hemispheric exchange of deep waters and to observe the variability of the Deep Western Boundary Current (DWBC). Transient tracers such as CFCs were used to follow the path of the ventilated deep waters originating from convection in the Labrador Sea and the Arctic Seas. The CFC distributions describe the pathways

0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 1 ) 0 0 0 5 3 - X

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of the North Atlantic Deep Water (NADW) along the DWBC from northern to subtropical and tropical latitudes. From the first description of equatorial CFC distribution by Weiss et al. (1985), an eastward bifurcation of deep waters was suggested for the upper NADW (UNADW) near the equator and confirmed, a decade later, during the transatlantic CITHER 1 cruise (Andrie! , 1996). A number of circulation features have been inferred from German and French WOCE experiments in the western area of the tropical ocean since 1990 (Rhein et al., 1995, 1998a; Andrie! et al., 1998, 1999): these cruises have confirmed that the nearshore inter-hemispheric southward transport of the DWBC is complicated by recirculation and by a zonal eastward transport near the equator. Such features have also been described through direct current measurements (Rhein et al., 1995; Fischer and Schott, 1997; Hall et al., 1997; Gouriou et al., 1999; Richardson and Fratantoni, 1999). Recent works have discussed the response of the subpolar and subtropical gyres to the variability associated with the decadal North Atlantic Oscillation (NAO) (Curry et al., 1998; Molinari et al., 1998). When studying global circulation changes, it is essential to examine the time evolution of transient tracer (such as CFC) concentrations at low latitudes (Pickart and Smethie Jr., 1998). This paper attempts to study the variability of tracer distributions within the tropics, where the northern inputs are driven through the meridional overturning circulation (MOC), downstream from the source areas, and to investigate the possibility of long-term, large-scale and low-frequency variability. The study raises several questions: *

*

*

Is a transient signal observed near the equator within the NADW? Is the role of local circulation (advection and mixing) dominant compared to the evolution of global tracer distribution? Are any observed fluctuations associated with the variability discussed for the subtropics?

Rhein et al. (1998a) have described the temporal evolution of tracer signals (tritium and CFCs) of the UNADW within the DWBC from 1990 to

1994. This paper deals with a more complete set of tropical data collected along the 351W meridian between the American Continent near 51S and the Mid-Atlantic Ridge (MAR), during eight cruises from 1990 to 1999, for both the UNADW and LNADW water masses. Fig. 1 shows the different sections sampled and the significant topographic features affecting deep circulation, namely the Ceara Rise and the equatorial channel. In Section 2, we present the analytical procedures and claim that a quantitative comparison of the data sets is possible. In Section 3, we describe the general features inferred from tracer distributions within the NADW. Sections 4 and 5 make a more precise comparison of the tracer distribution changes from 1990 to 1999 within the UNADW and LNADW cores. In Section 6, CFC temporal evolutions within the LNADW and UNADW cores are discussed and compared to atmospheric trends.

2. Data analysis and methods The following abbreviations are used below: R/ V METEOR cruises: M14 (October 1990), M16 (May–June 1991), M22 (November 1992), M27 (February–March 1994); CITHER 1 cruise: CIT1 (February–March 1993); ETAMBOT cruises: ET1 (September–October 1995) and ET2 (April–May 1996) and EQUALANT cruise: EQ99 (July– August 1999). During the first seven cruises, similar Neil Brown Mark III CTD probes were used with calibration on water samples for the conductivity sensor. A Seabird CTD was used during 1999. Temperature and salinity accuracy is similar for all the cruises: 0.002–0.0051C and 0.0031C, respectively (for more details about hydrographic measurements see Rhein et al., 1998a; Arhan et al., 1998). A positive shift of about 4 mmol kg
1 has been observed for the CIT1 oxygen measurements along 351W and has been removed from the the CIT1 data. Temperature versus salinity diagrams for bottom waters (below 4200 m) are given in Fig. 2 for 1991 (M16), 1992 (M22), 1993 (CIT1), 1994 (M27), 1996 (ET2) and 1999 (EQ99). The diagram

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

283

10

8

6 Ceara Rise

2

o

Latitude [ N]

4

Cayenne

0

Equatorial

Channel

_2 _4

Atoll das Rocas

_6

Natal BRAZIL

_8 _10 _55

_50

_45

_40

_35

_30

o

Longitude [ E] Fig. 1. Location of the 351W meridian within the ETAMBOT area. The 4000 m isobath identifies some particular topographic features: the equatorial channel, the Ceara rise and the Parnaiba Ridge.

does not show any significant change in the hydrographic characteristics of deep waters from 1991 to 1999. Identical analytical methods have been used for the CFC measurements in the IFM-Kiel and LODYC-Paris groups. Both use gas chromatography and extraction-trapping devices copied from the original Bullister and Weiss (1988) system, using a Porasil packed column and an electron capture detector. Both groups practise similar seawater sampling methods on board, using syringes and Niskin bottles. The major difficulty when comparing the CFC data sets lies in determining contamination levels. For the LODYC group, the detection limit of the method was obtained by closing several bottles at the same depth in an area of nearly CFC-free water

(typically 1000 or 3000 m) (see Andrie! et al., 1998). For the IFM group, the CFC blanks have been checked by analyzing CFC-free water, which was purged by CFC-free gas. Due to unexplained contamination at the beginning of the EQ99 351W section, CFC-12 measurements were rejected for the last cruise. In order to make a comparison with the CFC data for deep waters, it is necessary to check the consistency of the different CFC data sets. Rhein et al. (1998a) describe the difficulties of this task as being principally due to the superposition of the expected CFC temporal increase and vertical and horizontal mixing. Table 1 shows the mean concentrations measured along 351W within Circumpolar Water (CPW) southern water masses at levels of minimal CFC concentrations (Upper

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

284

θ/S _ 35 W o

1.6

45.9 5 45.9

46

1.4 5 45.9

46

o

θ [ C]

1.2 5 46.0

46

1 46.1

5 46.0

0.8 46.1

5 46.0

0.6 46.1

5 46.1

0.4 Ref. Pressure = 4000 dBar

34.74

34.76

34.78

34.8

34.82

34.84

M16 M22 CIT1 M27 ET2 EQ99 34.86

Salinity Fig. 2. Potential temperature versus salinity diagrams relative to bottom waters (below 4200 m) for five cruises in 1991 (circles for M16), 1992 (asterisks for M22), 1993 (dots for CIT1), 1994 (crosses for M27), 1996 (oblique crosses for ET2) and 1999 (stars for EQ99). The s4 isopycnals are superimposed.

Table 1 Mean CFC values and corresponding standard deviation values in the temperature range 4–51C at the level of UCPW (around 1000– 1200 m depth)a Cruise

Sampling date

Mean CFC-11

s11 (detection limit)

Mean CFC-12

s12 (detection limit)

M14 M16 M22 CIT1 M27 ET1 ET2 EQ99 (1000 m) EQ99 (3000 m)

October 1990 May–June 1991 November 1992 Feb.–March 1993 Feb.–March 1994 Sept.–Oct. 1995 April–May 1996 July–August 1999 July–August 1999

15 24 23 15 18 19 9 24 9

6 25 11 8 7 11 9 21 6

8 13 15 7 9 12 10 nd nd

5 9 8 6 5 6 8 nd nd

a For comparison, the values at the 3000 m level (not polluted by mixing with surrounding waters such as AAIW or UNADW) are given for EQ99. The CFC unit is: fmol kg
1 (10
15mol kg
1 or 10
3 pmol kg
1).

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Circumpolar Water (UCPW) around 1000 m and CPW between 2500 and 3500 m for EQ99). The standard deviation of each mean value can be considered as the detection limit of the corresponding cruise. The performances of each analytical method are very similar, close to the standard deviation of the global mean CFC values over the eight cruises. Despite limited CFC enrichment due to mixing with neighboring waters (AAIW or UNADW), there is no detectable temporal evolution of the CFC concentrations within the UCPW. Moreover, the homogeneity of the CFC distributions at the UCPW depth validate the blank contamination levels determined for each cruise and allow for comparison of the data sets. However, it must be borne in mind that low concentrations and especially F11=F12 ratios have to be considered with caution in the vicinity of the detection limits of the method. For each cruise, absolute calibrations were carried out with gas standards provided by R. Weiss, Scripps Institution of Oceanography. The data are reported on the SIO1993 scale.

3. General features of NADW water masses and circulation 3.1. CFC-11 and LADCP distributions for EQ99 As shown in Fig. 3a, at around 1600 m, the CFC cores correspond to the UNADW level and result from recently ventilated water coming from convection processes in the Labrador Sea (Weiss et al., 1985). The CFC-11 maxima are associated with salinity maxima resulting from mixing which occurs south of 351N between the recently ventilated upper Labrador Sea Water and Mediterranean Water, at a depth of around 1600– 1700 m (Reid, 1994). The UNADW lies below the UCPW originating from the southern hemisphere and located at a depth of about 1000 m at those latitudes (Oudot et al., 1999). For the EQ99 cruise, we observe a heterogeneous structure of the UNADW layer with five principal cores centered at 31S, the equator, 21N, 41N and 71N (Fig. 3a). The CFC-11 distributions generally agree with the circulation scheme in-

285

ferred from LADCP measurements (Fig. 3b). For example, the maximal velocity observed within the DWBC at around 31S (Fig. 3b) corresponds to the maximal CFC signal at 1700 m (Fig. 3a); at the level of the stacked equatorial jets between 11300 S and 11300 N (see below), CFC maxima correspond to maximal eastward velocities; at around 21N, the CFC core near 1600 m corresponds to an eastward flow. In some cases, however, CFC cores correspond to recirculated westward flows: the westward flow observed between 11S and 21S in the 1400–2400 m depth range (Fig. 3b) corresponds to the CFC-enriched (Fig. 3a) recirculated branch of the DWBC (McCartney, 1993). Correspondence between the northern CFC-11 maxima and the velocity field around 41N and 71N is less clear and will be discussed in Section 4. The second well-known deep-water CFC-11 maximum, observed at a depth of around 3800 m, corresponds to the lower NADW (LNADW), fed by the ventilated waters flowing from the Denmark Strait Overflow and originating mainly from convection in the Norwegian and Greenland Seas (Fine and Molinari, 1988). The CFC core has totally invaded the equatorial channel, restricted by the topography (Figs. 1 and 3). The LADCP section (Fig. 3b) shows an eastward velocity maximum at around 3600 m associated with a CFC-11 core between 11S and 11N. A second CFC-11 core has been observed south of 21S, associated with a westward current and thus probably recirculated from the northern current. Within the middle NADW (MNADW), a significant CFC-11 signal has been observed south of the equator, between the UNADW and LNADW tongues (Fig. 3a). This CFC corresponds to the vertical continuity of the eastwardflowing DWBC from the north (Fig. 3b). Below the LNADW lies the Antarctic Bottom Water (AABW), composed mainly of Lower Circumpolar Water at these latitudes (Hall et al., 1997; Rhein et al., 1998b). There is no noticeable CFC-11 signal at 351W in this water mass, mainly formed from ‘‘old’’ Lower Circumpolar Water (Fig. 3a) flowing westward along the bottom of the equatorial channel (Fig. 3b).

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Fig. 3. (a) Vertical distribution of CFC-11 (in pmol kg
1) below 1000 m during the EQ99 cruise along 351W. (b) Vertical distribution of zonal velocity in cm s
1 during the EQ99 cruise along 351W (LADCP measurements).

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

3.2. Equatorial deep jets

zonal velocity U (cm/s) -20

Evidence of equatorial deep jets (EDJs) has been found across the equatorial Atlantic (Ponte et al., . 1990; Boning and Schott, 1993; Gouriou et al., 1999, 2001). The EDJs are characterized by stacked structures with a thin vertical scale around 200–300 m (Gouriou et al., 1999, 2001), as observed in the Pacific (Firing et al., 1998). Fig. 3b shows the EDJs flowing alternately eastward and westward between 11300 S and 11300 N, in the 1000– 2500 m depth range. Surrounding the EDJs, vertically continuous eastward extra-equatorial jets (EEJs) are observed, centered at 3–21S and 21N (Fig. 3b). On a long temporal scale, Richardson and Fratantoni (1999) have described mean zonal velocities east of 401W, in 11 latitude bands, from SOFAR float trajectories tracked acoustically for 3.7 yr at a nominal depth of 1800 m. The mean flow in the equatorial band 11S–11N is described as a weak westward flow along the equator corresponding to the EDJs, and as eastward jets off the equator, within the 1–31N and 1–31S bands, corresponding to the EEJs. Fig. 4 compares the CFC-11 and zonal LADCP velocity profiles observed at the equator-351W during ET2. CFC-11 maxima at 1500 and 2000 m correspond to the maxima of eastward velocities (eastward EDJs). Conversely, a CFC-11 minimum has been observed around 1700 m, corresponding to a westward EDJ. Such split CFC-11 cores, previously observed at the UNADW level in the equatorial band (Andrie! et al., 1998, 1999; Messias et al., 1999) are probably also associated with deep jet reversing structures. Very similar CFC-11/ velocity correlated structures are described by Gouriou et al. (2001) for EQ99 at 351W, 231W and 101W along the equator. As was found during ET2, a CFC minimum was observed during EQ99 at around 1700 m. This indicates that EDJs seem to have a dominant structure with eastward EDJs at around 2000 and 1500 m, a westward jet in between maintaining the CFC minimum. The CFC-enriched cores observed at around 21S and 21N (Fig. 3a) are linked to the extra-equatorial eastward currents EEJs (Fig. 3b).

287

-15

-10

-5

0

5

10

1000

U

2000

CFC-11 depth (m) 0

0.1

0.2

0.3

CFC-11 (pmol/kg) Fig. 4. Superimposed equatorial profiles of CFC-11 (in pmol kg
1) and zonal LADCP velocity (in cm s
1) observed during ET2 at 351W.

4. Variability of CFC-11 distributions at the UNADW level, 1990–1999 4.1. Spatial CFC-11 and salinity distributions and equatorial dynamics The METEOR, CITHER and ETAMBOT distributions of CFC-11 along 351W have been described by Rhein et al. (1995, 1998a) and Andrie! et al. (1998, 1999). Six successive sections from the 1990–1999 data set along the 351W meridian are given in Fig. 5 in order to compare CFC and salinity distributions and to describe the progressive CFC-11 enrichment of the UNADW core between the western boundary (41S) and 1400 km (71N) offshore. The 34.42 and 34.70 s1:5 isopycnals specify the limits of this water mass for each cruise (Rhein et al., 1995).

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Fig. 5. Vertical sections of CFC-11 (in pmol kg
1) for the M22, CIT1, M27, ET1, ET2 and EQ99 cruises along the 351W meridian. The abscissa is the distance from the American continent, in kilometers. (a) M22, (b) CIT1, (c) M27, (d) ET1, (e) ET2 and (f) EQ99. Sections (g), (h), (i), (j), (k) and (l) are the same except for salinity.

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In Fig. 5a–f, spatial heterogeneity of the CFC11 distributions is very noticeable for each cruise. The location of the cores is not really permanent but corresponds more or less to the location of the five principal cores observed during EQ99 (Fig. 3a). A striking feature is the similar spatial heterogeneity observed in the salinity and CFC distributions (Fig. 5). The salinity patterns mirror the CFC patterns, with the exception of the transient behavior of CFCs, which does not exist for salinity. For each cruise, the 34.97 isohaline (dark blue isoline in Fig. 5g–l) closely matches the 34.70 s1:5 isopycnal. Because of the transient behavior of CFCs, different temporal evolution has been observed for CFC-11 distributions: since 1995 (ET1), the CFC signal crosses the 34.70 s1:5 isopycnal down to the level of the ‘‘classical’’ Labrador Sea Water (LSW). Spatial distributions in the 51S–51N tropical band are principally due to zonal circulation near the equator. The latitudinal extension of the CFC and salinity signals within the UNADW core is not limited to the 51S–51N band (Fig. 5) because of recirculation and extra-equatorial currents, and because the UNADW is not topographically guided (Rhein et al., 1998a; Richardson and Fratantoni, 1999). For example, the CFC-enriched cores observed around 4–51N during the ET2 and EQ99 at a depth of about 1600 m are related to a salinity maximum (Fig. 5e, f, k and l). The tracer concentrations and salinity observed in this area are higher than in the equatorial band, but silicate concentrations are lower (Andrie! et al., 1999). Characteristics associated with the salinity cores observed at 1600 m during ET2 along 351W (61200 N) and along 71300 N (421W) are given for comparison in Table 2. For similar salinity values, the CFC-11 concentrations decrease eastward as silicate concentrations increase, suggesting that the tracer is not fed by equatorial flows at those latitudes, but rather directly from the DWBC. At the time of the ET2 cruise (May 1996) and EQ99 cruise (July 1999), the direct LADCP velocity measurements (Gouriou et al., 1999, 2001) do not show any strong eastward current associated with the tracer maxima. Those CFC cores are probably the remnants of an eastward flow during the

289

Table 2 Water characteristics of CFC-11 enriched samples during ET2 cruise 1600 m

71300 N-421W st. 25

61200 N-351W st. 39

F11 (pmol kg
1) F12 (pmol kg
1) Salinity Silicate (mmol kg
1)

0.316 0.128 34.994 15.07

0.245 0.107 34.996 16.02

previous winter. This assumption is inferred from the results of the DYNAMO model comparative . experiment (Boning et al., 1998). They describe a seasonal variability of the deep current field with the existence, during winter, of an eastward zonal current located south of 101N, fed directly from the DWBC. This flow is not reproduced by the OGCMs during summer. Similar structures had already been observed at 41N during ET1 and during CIT1 (Fig. 5d, j and b, h). Salinity and CFC-11 distributions during M27 also show increasing values to the north of 41N (Fig. 5i and c). Such features were not sampled during the other METEOR cruises. On the other hand, according to hydrographic data, Talley and Johnson (1994) have previously described deep, zonal subequatorial currents in both the Pacific and Atlantic Oceans centered at 5–81N as permanent westward flows, as shown in the Richardson and Fratantoni (1999)’s float trajectories. So, the question concerning the origin of the CFC signal associated with such westward flows is still open. 4.2. Temporal evolution of mean CFC-11 concentrations within the 31S–31N latitudinal band The variability of zonal equatorial currents observed through direct current measurements (Thierry, 2000; Gouriou et al., 1999, 2001) and float trajectories (Richardson and Fratantoni, 1999) is responsible for the highly variable spatial distribution of the tracer maxima (salinity and CFC) for time-scales of o1 yr (seasonal or semiannual). In the 31S–31N band, the spatial heterogeneity of the CFC-11 and salinity distributions

290

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observed in Fig. 5 is mainly due to local dynamics (see the EQ99 example in Fig. 3a and b). The meridional distributions are very variable from one cruise to another. However, for each cruise, we notice considerable similarity between the CFC-11 and salinity structures: tracer cores can be superimposed on salinity cores. In order to distinguish the effects of local dynamics from the transient behavior of the CFC-11 distributions in the 1990–1999 series, we have examined the correlation of CFC-11 and salinity for each cruise. The CFC-11 versus salinity diagram for the 31S–31N band for EQ99 data in the 1450–1550 m depth range presents different correlation coefficients compared to the 1550– 1850 m band (Fig. 6a), probably because of a greater influence of vertical mixing with UCPW and a lesser influence of Mediterranean Water (MW) than in the underlying water. Consequently, in order to compare tracer distributions within the UNADW core from 1990 to 1999, Fig. 6b shows salinity versus CFC-11 for each cruise, in the 1550–1850 m range and over the 31S–31N band. Coefficients for the CFC-11/salinity correlations are given in Table 3. In this approach, we consider that, for each cruise, the CFC-11/salinity correlation is the result of equatorial dynamics as observed in the meridional distributions in Fig. 5. The more or less steady increase of the correlation curve from the M14 to EQ99 reflects the temporal variability due to the transient nature of the CFCs. This large-scale temporal variability greatly exceeds the spatial variability as observed through salinity heterogeneity. In the following observations, we therefore consider that the temporal variability of the CFC-11 concentrations within the UNADW tongue in the 1990–1999 interval is mainly due to its transient behavior. Despite the complex equatorial dynamics, we assume that a general temporal variability for the mean CFC concentration of the UNADW can be computed from mean values over the 351W meridional section, regularly sampled throughout the 1990s with a relatively precise resolution. We have calculated, for each station and each cruise, the mean CFC-11 concentration in the 1550– 1850 m depth range (s1:5 ranging between 34.64 and 34.68) and within the 31S–31N latitudinal

band. The UNADW core described above lies in the lower part of the conventional limits of the UNADW of 34.42os1:5 o34.70, according to Rhein et al. (1995). In the chosen depth range, CFC-11 concentrations lie within 75% of the maximal value. Generally, at least 3 samples per station have been sampled in this layer. The temporal evolution of the mean CFC-11 concentrations within the 31S–31N latitudinal band is shown in Fig. 7, from 1990 to 1999. It reflects the temporal increase of the correlation curve from 1990 to 1999 as a result of the impact of the atmospheric increase in the Labrador Sea. A deviation from a regular linear increase has been observed for ET2 (May 1996). This ‘‘anomaly’’ will be discussed below. 4.3. Seasonal and inter-annual variability The latitudinal distribution of the mean CFC-11 concentrations in the 1550–1850 m range from 1990 to 1999 is shown in Fig. 8. Mean CFC-11 concentrations for the EQ99 have increased considerably since the ET cruises. The greatest variability has been observed in the northern latitudes (north of 2–31N). As discussed in Section 4.1, a particularly strong CFC-11 signal has been observed for EQ99 at around 41N and further north. In addition, a minimum has been observed at around 5–71N when a maximum is noticeable for ET2 (Fig. 8). Moreover, we observed maxima at 21300 N and 41300 N during the ET1, when there were minima during EQ99 with a maximum in between. For the ET2, the only location where CFC-11 concentrations are significantly higher than for the ET1 is the CFC core in the north (located between 51N and 61200 N). This extraequatorial variability can be associated with the seasonal variability of deep zonal currents as . reported by Boning et al. (1998). It is poorly documented elsewhere through direct current measurements. Several studies have been made of the variability of currents within the 31S–31N latitudinal band. From 1-yr mean float trajectories at 1800 m, Richardson and Fratantoni (1999) describe seasonal variability in the northern and equatorial bands of the deep tropical Atlantic, more notice-

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

291

CFC_11/S_ UNADW_ 3oS/3oN _ 35oW 0.35

(a)

_1

CFC_ 11 [pmol.kg ]

0.3

0.25

0.2

0.15

0.1

0.05

0

34.97

34.975

34.98

34.985

34.99

Salinity 0.35

(b)

_1

CFC_ 11 [pmol.kg ]

0.3

EQ99

0.25

0.2

ET1 ET2

0.15

M27 CIT1

0.1

M22

0.05

M14 0 34.97

34.975

34.98

34.985

34.99

34.995

Salinity
1

Fig. 6. (a) CFC-11 (in pmol kg ) versus salinity diagram during the EQ99 in the 1450–1550 m depth range (white circles) and in the 1550–1850 m depth range (elsewhere). (b) CFC-11 (in pmol kg
1) versus salinity diagram for the M22, CIT1, M27, ET1, ET2 and EQ99 cruises in the 1550–1850 m depth range.

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292

Table 3 Correlation coefficients (r) of linear regressions CFC-11 ¼ aS þ b for the 1990–1999 cruises Cruise

r

a

b

M14 M16 M22 CIT1 M27 ET1 ET2 EQ99

0.45 (3 points only) 0.63 0.53 0.67 0.87 0.81 0.72

1.9 5.17 2.9 2.9 5.4 8.0 7.15 12.1


67.8
180.82
102.6
102
190
281.2
250
423.5

able in the northern band, with only poor evidence in the southern band (their Fig. 13): over 4 yr of data, the distribution of monthly mean velocity shows maximal eastward velocity in the 1–31N band from October to February, for which a minimal westward velocity has been observed along the equator in the 11S–11N band. Conversely, minimal eastward velocity has been observed in the 1–31N band from March to September, when maximum westward velocity has been observed in the equatorial band. The northern and equatorial series vary in phase

0.3

EQ99

0.2

-1

CFC_11 [pmol.kg ]

0.25

ET1 0.15

UNADW CIT1

0.1

M16

0.05

M22

ET2

M27

M14 0 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year Fig. 7. Temporal distribution of the mean CFC-11 concentrations (in pmol kg
1) at the UNADW level (1550–1850 m) between 31S and 31N. Error bars are the standard deviations over each mean.

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293

0.35

0.3 EQ99 7/99

CFC-11 [pmol/kg]

0.25

0.2 ET2 5/96 ET1 9/95 M27 2/94

0.15

0.1

0.05

M16 6/91

M22 11/92 CIT1 2/93

M14 10/90 0

_4

_2

0

2

4

6

8

o

Latitude [ N] Fig. 8. Latitudinal distribution of the mean CFC-11 concentrations (in pmol kg
1) in the 1550–1850 m depth range.

(maximum eastward anomaly in boreal winter, maximum westward anomaly in boreal summer), in agreement with the seasonal pulsing described at 441W from current-meter measurements (Fischer and Schott, 1997). Similarly, Thierry (2000) has described a 1992– 1994 time series from current-meter measurements obtained on deep moorings located within the Romanche Fracture Zone (near 01400 N–141450 W), 201 to the east of our study area. At the UNADW level (1700 and 2000 m), they identified a timeaveraged zonal flow of nearly zero and lowfrequency (quasi-annual) energetic current with maximal eastward velocity in November. Although the time resolution of our data set is clearly not sufficient to resolve seasonal variability from the 1990–1999 series with any great accuracy, we have attempted to determine from extreme situations whether the CFC concentration could

correspond to features observed through velocity fields. In this approach, we have compared the temporal evolution of the CFC concentrations at around 1800 m with the temporal distribution of mean float trajectories at the same depth (Richardson and Fratantoni, 1999). Fig. 9 shows the temporal evolution of the maximal CFC-11 value within the 11S–11N, 1–31N and 3–11S bands, in the 1750–1850 m depth range of the UNADW level. A relative CFC-11 maximum in the 1–31N band (bold line) has been observed for ET1 (October 1995) corresponding to the expected stronger eastward flows in November. The close similarity observed (after 1993) between the temporal evolutions of the maximal CFC-11 concentrations within the southern 3–11S band (thin line) and the equatorial 11S–11N band (dashed line) indicates that the equatorial band is fed either directly from the DWBC, or from the southern band, with

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

294 0.35

EQ99 o

o

1 N-3 N

0.3

o

o

o

o

1 S-1 N

CFC_11 [pmol/kg]

0.25

3 S-1 S

ET1

0.2

M27 0.15

M22

0.1

ET2

M14

0.05

CIT1 M16 0 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year Fig. 9. Temporal evolution of the mean CFC-11 concentrations at a depth of 1800 m (in pmol kg
1) in 3 latitudinal bands for the UNADW; dots and full bold line for 1–31N, circles and dashed line for 11S–11N and crosses and light line for 3–11S. The error bars reported on the 1–31N curve are the detection limits computed in Table 1 for each cruise.

similar variability. The behavior of the 1–31N band is very different: in particular, for the EQ99 (July 99), considerable deviation between southern and northern maximal CFC concentrations can be observed, which indicates that there is no direct connection between the two bands, at least during this boreal summer situation.

5. Variability of CFC distributions at the LNADW level 5.1. Spatial CFC and oxygen distributions and local dynamics From 1990 to 1999, a LNADW core has always been observed (Fig. 10a–f) in the middle of the equatorial channel between 200 km (31S)

and 600 km (11N) offshore. As previously described (Pl.ahn and Rhein, 1998; Andrie! et al., 1998, 1999), this corresponds to the water mass flowing eastward through the equatorial channel (Figs. 1 and 3). Maximal CFC concentrations have been observed between 21S and the equator, as previously described in Rhein et al. (1995), guided by the Parnaiba Ridge to the north of 11300 S. Fig. 10 gives the compared CFC-11 and oxygen distributions for the LNADW layer. Apart from the transient behavior of CFCs, both distributions are very similar. For the more recent EQ99 cruise, two tracer cores appear, in the center of the deeper bowl of the equatorial channel (around 11S–01N) and around 3–21S. The two separated CFC cores at around 3700 m (Fig. 3a) correspond to an eastward flow centered around 11S and to a

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

295

Fig. 10. CFC-11 (a, b, c, d, e, f) and oxygen (g, h, i, j) distributions along the meridional 351W section within the LNADW core (45.83os4 o45.90), from 1990 to 1999. Superimposed is the bathymetry for 351W. The CFC-11 concentrations are in pmol kg
1. Oxygen concentrations are in mmol kg
1. The spatial resolution for oxygen measurements during the METEOR cruises was not sufficient to be considered in this comparison.

296

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

westward flow centered around 21S (Fig. 3b). So the CFC core seems to be split into two cores within the equatorial channel: a northern core (centered at 01300 S) associated with a permanent eastward current, and a southern core (centered at 21S) associated with a westward flow, permanent or transient. Compared to mooring or LADCP current measurements along the channel (Rhein et al., 1995; Fischer and Schott, 1997; Gouriou et al., 1999, 2001), the northern part of the CFC core is linked to the permanent direct eastward LNADW flow. The origin of the southern core is less obvious, because of the coarse data resolution around 21S. Rhein et al. (1995) have clearly described a westward current core between 21300 S and 21S for the M14, and southern limits for the direct eastward LNADW flows north of

11300 S for the M16 and M22. As shown in Fig. 11, the very steep bathymetry observed to the west of 351W along the zonal seamounts in continuation of the Parnaiba Ridge could block any direct southeastward flow from the DWBC south of 21S: the southern CFC core could be due to recirculation of the northern core. 5.2. Temporal variability of the LNADW core In order to compare the CFC distributions in the 1990–1999 time interval, we have calculated, for each station and each cruise, the mean concentrations of the LNADW core in the 31S– 11N latitudinal band within the equatorial channel in the 3550–3950 m depth range. The CFC and oxygen cores (Fig. 10) lie in the lower part of the

Fig. 11. Contours of bathymetry at 351W just to the east of the sampled area 35–381W (Etopo5 database).

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

conventional limits of the overflow lower NADW (OLNADW) 45.83os4 o45.90, according to Rhein et al. (1995). The chosen depth range corresponds to the observed maximal concentrations. Generally, four samples are taken within this depth range (Fig. 3a). A comparison of latitudinal CFC distributions over the 1990–1999 period (Fig. 12) points out a variability of the mean LNADW flow. In the northern and deeper part of the channel (north of 11200 S), the pattern of the CFC-11 distributions is very similar from one cruise to the other, despite the transient behavior of the CFCs, with relatively wide extensions off the 11200 S–01300 N band. It corresponds to the quasi-permanent eastward flow of the LNADW in the center of the equatorial channel. More significant variability concerns the

297

distributions south of 11200 S, within the shallower part of the channel. The double maximum observed during the EQ99 was apparently not present during previous cruises, perhaps partly because of the inadequate spatial resolution. The minimum observed between the two EQ99 cores at around 11300 S (where the shear between eastward and westward currents occurs, see Fig. 3b) corresponds to maxima for the ET1 and CIT1. Seasonal variability of the LNADW and AABW flows has been observed through direct current-meter measurements along the equator near 361W for the 1992–1994 period (Hall et al., 1997): it appears that both transports vary with a quasi-annual cycle, in phase opposition. At the 11200 S and 11S moorings from February to June (1993–1994), they describe maximal eastward

Fig. 12. Latitudinal CFC-11 distributions in the equatorial channel for the M22, CIT1, M27, ET1, ET2 and EQ99 cruises in the depth range 3550–3950 m along 351W. CFC-11 concentrations are in pmol kg
1.

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velocity of the LNADW flow at a depth of 3900 m (and, simultaneously, weak AABW transport at a depth of 4300 m) and, from September to January, maximal westward velocity for the AABW at a depth of 4300 m. The CFC-11 maxima observed south of 11200 S north of the Parnaiba Ridge for EQ99 could be the result of the LNADW recirculation induced by the westward underlying AABW flow, slightly to the south. However, because of the poor time and space sampling resolution, it is impossible to conclude that there is any similarity between the CFC-11 variability and the seasonal variability of deep currents described from current measurements. In order to follow the temporal evolution of the mean CFC-11 concentrations at the LNADW level, mean CFC-11 concentrations have been calculated in the 3550–3950 m depth range over the 31S–11N band, which corresponds to the latitudinal band covering the equatorial channel (Fig. 13a). Error bars are the standard deviations over the calculated means. The temporal evolution of the LNADW mean concentrations over the 1990–1999 period is compared in Fig. 13b to the UNADW temporal evolution in the 31S–31N band in the 1550–1850 m depth range. Both general trends seem very similar with regard to the respective standard deviations over the means (Figs. 7 and 13a).

6. Discussion 6.1. Apparent ages and transit times from the CFCs ratio method From the mean CFC-11 and CFC-12 concentrations measured at 351W, the time evolution of the atmospheric CFC concentrations and the hydrographic characteristics of the water mass in the area of formation, the traditional CFC ratio aging method allows the ‘‘apparent’’ age and dilution factor of a water mass to be determined (Fine et al., 1988). This method is based on the crude assumption that the CFC-11=CFC-12 ratio of a water mass remains constant from the time it is formed until its sampling date. The atmospheric time trend from Walker et al. (2000) and the

solubility function of Warner and Weiss (1985) are used for the calculation. Given that little is known of the formation conditions of water masses in the regions of both the Labrador Sea and Arctic Seas, no deviation from the solubility equilibrium (undersaturation of surface waters during convection) has been considered in the calculation. Hydrographic characteristics of the sources have been taken as 41C in temperature and 35 in salinity for UNADW and as 21C in temperature and 34.91 in salinity for the LNADW (Vaughan and Molinari, 1997). Table 4 reports the CFC-11=CFC-12 mean ratio for each cruise in the 1550–1850 m (31S–31N) range for the UNADW and in the 3550–3950 m (31S–11N) range for the LNADW. For each layer, the mean CFC ratio increases steadily, except for ET2, corresponding to dates of formation ranging from 1965 to 1972. During this period, the use of the CFC-11=CFC-12 ratio is not limited by the constancy of the ratio that occurred after 1975. In Fig. 13b, we can see a steady and almost linear increase of the temporal evolutions for the UNADW and LNADW from 1990 to 1995 (ET1), apart from an ‘‘anomaly’’ for ET2 in the UNADW. Because of the lack of sampling between 1996 and 1999, we have considered the first half of the series (1990–1995) to calculate mean apparent ages and dilution factors. The resulting mean apparent ages are 2571 yr for the UNADW and 26.571.5 yr for the LNADW (Table 4). The corresponding dilution factors are 12 and 14 (72) for the UNADW and LNADW. Using these mean parameters, we have reconstructed the theoretical CFC-11 concentrations in the source areas (the Labrador Sea for the UNADW and the Arctic Seas for the LNADW) expected from the 1990– 1999 series measured at 351W. A striking feature is that the reconstructed series for the EQ99 (not considered in the mean calculations) match the source input functions, corresponding to the 1966– 1975 time interval (Fig. 14). Nevertheless, this approach has limitations. Uncertainties of 71–1.5 yr for apparent ages and 72 for dilution factors stem from the respective standard deviations over the mean values. But they are probably underestimated because of the possible bias of the method, due to mixing and recirculation during the NADW journey from its

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

-1 CFC_11 [pmol.kg ]

0.2

299

(a)

EQ99

0.15

ET2 M27

0.1

LNADW

M22 M16 ET1

0.05

CIT1

M14 0 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year

0.2

(b)

-1

CFC_ 11 [pmol.kg ]

UNADW 0.15

LNADW

0.1

0.05

0 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year Fig. 13. (a) Temporal distribution of the mean CFC-11 concentrations (in pmol kg
1) at the LNADW level (3550–3950 m) between 31S and 11N. Error bars are the standard deviations over each means. (b) Temporal evolution of the LNADW mean CFC-11 concentrations between 31S and 11N and in the 3550–3950 m depth range (dots and full line) compared to the UNADW mean concentrations between 31S and 31N and in the 1550–1850 m depth range along 351W (circles and dashed line). CFC-11 concentrations are expressed in pmol kg
1.

sources to the equator (Pickart et al., 1989; Rhein, 1994). In addition, the degree of undersaturation (60–90%) of newly formed LSW and Arctic Sea

surface waters (Smethie Jr. et al., 1986; Rhein, 1996; Wallace and Lazier, 1988; Smethie Jr. and . Fine, 2001) and their variabilities (Bonisch et al.,

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300

Table 4 CFC-11=CFC-12 ratio and apparent ages corresponding to the different cruises 1990–1996 Cruise

Sampling date

UNADW CFC-11/ CFC-12 mean ratio

UNADW apparent age (yr)

LNADW CFC-11/ CFC-12 mean ratio

LNADW apparent age (yr)

M14 M22 CIT1 M27 ET1

1990.8 1992.9 1993.2 1994.2 1995.8

1.6470.15 1.6970.13 1.8270.13 1.8470.16 1.9670.16

25.3 25.9 24.2 24.8 23.8

1.6070.21 1.6570.21 1.7270.21 1.8370.16 1.8170.3

26.3 27.9 26.7 24.8 27.3

Mean apparent age (for 1990–1995) ET2

24.8 1996.4

1.8670.16

26.3

26.6 1.7570.17

29.3

be explained by changes in subsurface mixing properties during the water transit.

UNADW and LNADW CFC-11 concentrations compared to the northern sources 4 3.5

6.2. Comparison with hydrographic series results

F11 (pmol/kg)

3 2.5 2 1.5 1 0.5 0

1965

1970

1975

1980

Fig. 14. Temporal evolution of CFC-11 surface concentrations (in pmol kg
1) for the Labrador Sea (full line) and the Arctic Seas (dotted line). Superimposed are the mean concentrations measured at 351W (black circles for UNADW and open circles for the LNADW), shifted by apparent ages of 25 and 26.5 yr and dilution factors of 12 and 14 for the UNADW and LNADW, respectively.

1997; Schlosser et al., 1991) are not taken into consideration here. Taking into account a 60% saturation value for Labrador Sea surface waters, the apparent age is decreased by around 4 yr within the 1965–1980 range. Using CFC=tritium ratio, Rhein et al. (1998a) have estimated a 20-yr transit time from Labrador Sea to 51S. Consequently, the present 25-yr transit time for the LSW to reach the tropics using the CFCs ratio method has to be considered as an upper limit. In addition, we cannot exlude that part of the variablity in the tropical CFC distributions could

Our estimations are consistent with the results obtained from hydrographic series in the Labrador Sea. Particular features have been observed in the hydrographic characteristics of the Labrador Sea during the last two decades. A break in convection was observed around 1970, associated with salinity and temperature increases, and minima in Labrador Sea core thickness around 1971 (Lazier, 1988; Vaughan and Molinari, 1997; Curry et al., 1998, Molinari et al., 1998). The ET2 CFC-11 gap observed in 1996 in the tropics (Fig. 14) could possibly be linked to the situation of low convection in the Labrador Sea in the 1970s. The observed deviation from the general source function may result from a change in solubility equilibrium conditions at the sea surface, which have not been taken into account in the mean calculation of the Labrador Sea source function in Fig. 14. Another break in the LSW convection, which occurred around 1980, might also explain the 1996 anomaly. But the resulting dilution factor of 35, calculated from the CFC-11 concentration expected from the temporal evolution curve in 1980 (3.5 pmol kg
1) divided by the measured CFC-11 concentration at 351W during 1996 (0.1 pmol kg
1), is not a realistic value for UNADW. Thus, the coherence between the F11=F12 ratio method and the independent approach comparing

C. Andrie! et al. / Deep-Sea Research I 49 (2002) 281–304

the hydrological 1971 event in the Labrador Sea to the 1996 anomaly at 351W in the tropics lends weight to the mean age evaluations made. Adopting another totally independent approach, Koltermann et al. (1999) determined a transit time of 15–20 yr for the LSW to arrive in the DWBC at 24.51N, from hydrographic series occurring over the last four decades (including the LSW 1970 warming event). This evaluation is in agreement with ours as it integrates the hydrographic modifications of the LSW all along its southward path. Similarly, from a 1989–1994 hydrographic time series measurements, Stramma and Rhein (2001) have estimated between 13 and 17 yr, the LSW transit time between Labrador Sea and 441W (DWBC). Our 25-yr evaluation seems again to be an upper limit for the UNADW transit time. 6.3. Comparison of reconstructed UNADW and LNADW time evolutions at 351W It can be observed from Figs. 13b and 14 that, despite the global similarity of both general time trends, the UNADW and LNADW temporal evolutions downstream from the DWBC seem out of phase. For example, an anomaly similar to the 1972 UNADW anomaly has not been observed in the LNADW during the decade. This difference was to be expected for several reasons: *

*

*

Labrador Sea and Nordic Sea do not respond similarly to low frequency variability such as the NAO (Curry et al., 1998). Different wind regimes, ice cover effects,... can lead to different CFC undersaturation values (Smethie Jr. et al., 1986; Rhein, 1996). The LNADW and UNADW flow dynamics are different: the LNADW resides several years before overflowing the sills, is strongly constrained by sea bottom topography and presents a quasi-permanent pathway; conversely, UNADW CFC distribution presents great spatial variability, as do deep circulation dynamics, which are not so constrained at that level. However, downstream from the DWBC, it is difficult to distinguish the variability of

301

water mass characteristics at their Nordic sources from the variability of the MOC and its associated transport and to determine their link with NAO (Bersch et al., 1999; Koltermann et al., 1999; Mauritzen and H.akkinen, 1999; Joyce et al., 1999). 6.4. Comparison with direct velocity measurements and concluding remarks The ‘‘apparent’’ velocities inferred from the CFC ratio method are generally well below the directly measured values (see i.e. Andrie! et al., 1998). This is also the case here. Taking a transit time of 25 yr for the UNADW coming from the Labrador Sea, we have estimated a mean velocity of around 1.4 cm s
1. Direct current-meter measurements (Hall et al., 1997, Thierry, 2000) give maximal values of around 20 cm s
1 (eastward or westward), similar to the maximal LADCP zonal velocities (Gouriou et al., 1999, 2001). On the other hand, mean velocities obtained from buoy drifts over a 2-yr period are relatively low (
1 to 5 cm s
1; Richardson and Fratantoni, 1999), close to the 1–3 cm s
1 mean value obtained from the Romanche and Chain moorings (Thierry, 2000). A large discrepancy is observed between transit times evaluated by indirect methods (CFCs or other transient tracers, hydrographyy) and those measured by direct velocity measurements (buoys, moorings, ADCPy). Generally, indirect methods use average values or data on relatively coarse resolution. For example, the CFCs ratio dating method averages over spatial and temporal variabilities of different scales (Rhein, 1994), leading to an overestimation of the apparent transit time compared to direct measurements. A similar remark comes from the transit time overestimation from average current measurements (2 cm s
1) compared to instantaneous measurements (20 cm s
1). Recent methods tracking the late 1980s LSW ‘‘vintages’’ using tracer measurements (Sy et al., 1997; Curry et al., 1998), give the LSW transit times in good agreement with direct measurements and well below the mean methods: for the subtropical region, Molinari et al. (1998) estimate a 8-yr LSW transit time. The apparent contradiction between mean approaches and local

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measurements is principally linked to more and more precise measurements with finer and finer spatial resolution which allow to ‘‘catch’’ rapid water cell characteristics. But due to the heterogeneity of the tracer distributions and the high temporal variability within the DWBC, such high velocities cannot be attributed to the global water mass. To conclude, our results confirm several important general implications in addition to providing further knowledge of deep tropical water dynamics. Despite dilution and inter-annual variability, the atmospheric CFC input signal at high latitudes is clearly transmitted to the tropical Atlantic. On a global scale, the variability of mid- and low-latitude circulation at the western boundary does not have a great effect on the arrival of the transient signal. Mean apparent ages of around 25 yr for deep-water masses at 351W are coherent over the entire 1990–1999 series. Our estimate is in the upper limit of published transit times from some other approaches (Rhein et al., 1998a; Stramma and Rhein, 2001). The method used here is a global approach, relative to a large spatial and temporal scale, which integrates highly variable signals over a large latitudinal band in the tropics (31S–31N) and leads to a spreading rate instead of a real transit time (Doney and Jenkins, 1994). It allows access to mean water mass flows in thermohaline circulation rather than to instantaneous velocity values. These results also confirm the potential use of tracers such as CFCs to follow water mass ‘‘vintages’’: particular anomalous events can be ‘‘tracked’’ along the entire pathway of the deep branch of global thermohaline circulation.

Acknowledgements This work represents German and French contributions to the international WOCE Program. French cruises on Research Vessels from GENAVIR/IFREMER were financed by the Institut de Recherche pour le De! veloppement (IRD) as part of the Programme National d’Etude de la Dynamique du Climat (PNEDC) for WOCE and CLIVAR. The German Federal Ministry for

Education, Science, Research and Technology supported German cruises on R.V. METEOR. We thank Y. Gouriou for providing the ET2 and EQ99 LADCP data along 351W.

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