The deep waters from the Southern Ocean at the entry to the Argentine Basin

Deep-Sea Research II 46 (1999) 475—499

The deep waters from the Southern Ocean at the entry to the Argentine Basin Michel Arhan *, Karen J. Heywood
, Brian A. King Laboratoire de Physique des Oce& ans, CNRS/IFREMER/UBO, IFREMER/Brest, B.P. 70 Plouzane& , France
School of Environmental Sciences, UEA, Norwich NR4 7TJ, UK James Rennell Division, SOC, Southampton S014 3ZH, UK Received 24 October 1997; received in revised form 24 March 1998

Abstract Hydrographic data from the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) in the region of transition between the Scotia Sea and the Argentine Basin are examined to determine the composition of the deep water from the Southern Ocean that enters the Atlantic, and to describe the pathways of its constituents. The deep current that flows westward against the Falkland Escarpment is formed of several superposed velocity cores that convey waters of different origins: Lower Circumpolar Deep Water (LCDW), Southeast Pacific Deep Water (SPDW), and Weddell Sea Deep Water (WSDW). Different routes followed by the WSDW upstream of, and through, the Georgia Basin, lead to distinctions between the Lower-WSDW (p '46.09) and the Upper-WSDW (46.04(p   (46.09). The Lower-WSDW flows along the South Sandwich Trench, then cyclonically in the main trough of the Georgia Basin. Although a fraction escapes northward to the Argentine Basin, a comparison of the WOCE data with those from previous programmes shows that this component had disappeared from the southwestern Argentine Basin in 1993/1994. This corroborates previous results using SAVE and pre-SAVE data. A part of the Upper-WSDW, recognizable from different h—S characteristics, flows through the Scotia Sea, then in the Georgia Basin along the southern front of the Antarctic Circumpolar Current. Northward leakage at this front is expected to feed the Argentine Basin through the northern Georgia Basin. The SPDW is originally found to the south of the Polar Front (PF) in Drake Passage. The northward veering of this front allows this water to cross the North Scotia Ridge at Shag Rocks Passage. It proceeds northward to the Argentine Basin around the Maurice Ewing Bank. The LCDW at the Falkland Escarpment is itself subdivided in two cores, of which only the denser one eventually underrides the North Atlantic Deep Water (NADW) in the Atlantic Ocean. This fraction is from the poleward side of the PF in Drake Passage. It also crosses the

*Corresponding author. Fax: 00 33 2 98 22 4496; e-mail: [email protected]. 0967-0645/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 0 1 1 0 - 6

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North Scotia Ridge at Shag Rocks Passage, then flows over the Falkland Plateau into the Atlantic. The lighter variety, from the northern side of the PF, is thought to cross the North Scotia Ridge at a passage around 55°W. It enters the Argentine Basin in the density range of the NADW.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The waters of southern origin present beneath the North Atlantic Deep Water (NADW) in the Atlantic Ocean are issued from two distinct components (Reid et al., 1977). The denser one is the Weddell Sea Deep Water (WSDW), thus labeled because of its origin at the mid-depths of this Antarctic basin (Wu¨st, 1933), and the lighter one derives from waters carried eastward through Drake Passage by the Antarctic Circumpolar Current (ACC). Because of this twofold origin and the very complex bathymetry of the Southern Ocean in the Atlantic sector, the pathways of these waters to the Argentine Basin are multiple and not yet fully determined. The Argentine Basin shows three main openings that offer paths to be taken by the deep and bottom southern waters (Fig. 1). The westernmost and shallowest is the Falkland Plateau, which forms a &2500 m-deep saddle between the Falkland Islands and the Maurice Ewing Bank, which culminates at &1200 m. Although the Falkland Ridge and Islas Orcadas Rise form a narrow eastward prolongation of the Falkland Plateau and Maurice Ewing Bank, they are separated from the latter by a wide breach at 35°W—40°W, through which water from the Georgia Basin can enter. We refer to this opening as the Falkland Passage, and to a 5100 m-deep narrow gap in it at &36°W as the Falkland Gap. The third main deep connection (&4500 m) between the Argentine Basin and the Southern Ocean is the region situated to the east of the Islas Orcadas Rise. Whilst the deep circulation is naturally constrained by the bathymetry, several studies have revealed that it is also guided by the northward veering of the ACC in this region (Fig. 1). Peterson and Whitworth (1989) showed that a part of the Circumpolar Water is advected northward over the Falkland Plateau between the Subantarctic Front (SAF) and the Polar Front (PF), but considered that the fraction of it that eventually underrides the NADW in the Atlantic Ocean arrives through the Falkland Passage. Farther east, Orsi et al. (1993, 1995) observed that the equatorward export of WSDW through the Georgia Basin is favoured by an excursion of the cyclonic Weddell Gyre in this basin, a feature depicted in Fig. 1 as a northward loop of the southern boundary of the ACC. Georgi (1981) considered that the Falkland Passage provides the major inflow of WSDW into the Argentine Basin. He also noted a contribution from east of the Islas Orcadas Rise, but suggested that a part of the water arriving along this route recirculates cyclonically and leaves the basin again to the west of the Mid Atlantic Ridge. The inflow from the Georgia Basin turns westward as a deep boundary current along the Falkland Escarpment, and joins with that from the Falkland Plateau. Whitworth et al. (1991) studied this deep slope current at 41°W and confirmed that some water from Drake Passage also flows through Falkland

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Fig. 1. Bathymetric configuration (isobaths 1000 and 3000 m) in the region of the Scotia Sea, Georgia Basin, and southern Argentine Basin, and locations of the hydrographic lines used in the paper (stations of lines A23, S5W and S5E are identified for reference in the discussion of Section 4). Shading marks the regions shallower than 3000 m, and isobath 2000 m is shown only for the visualization of the Maurice Ewing Bank. The fronts and southern boundary of the Antarctic Circumpolar Current (ACC), such as defined by Orsi et al. (1995), are superimposed (Frontal patterns kindly provided by R. Locarnini). SAF stands for Subantarctic Front, PF for Polar Front, SACCF for Southern ACC Front, and the line labeled ‘‘Bndry’’ shows the southern ACC boundary. The following topographic features are indicated by their initials: FI — Falkland Islands; FP — Falkland Passage; EB — Maurice Ewing Bank; SRP — Shag Rocks Passage; SG — South Georgia; NGR — Northeast Georgia Rise; GP — Georgia Passage; The abbreviations used to identify the hydrographic lines are those indicated in Table 1.

Passage. Peterson and Whitworth (1989) pointed out the presence of Southeast Pacific Deep Water (SPDW) in the boundary current, a water mass that Sievers and Nowlin (1984) observed beneath the Circumpolar Water in the southern Drake Passage. Apart from the questions pertaining to the mapping of the pathways and the associated transports, a recent article by Coles et al. (1996) raised the issue of the long-term variability of the influx. A comparison of data from the South Atlantic Ventilation Experiment (SAVE) with previous measurements showed that the volume of the coldest WSDW present in the southwestern Argentine Basin had decreased during the 1980s, an observation that they suggested could only be accounted for by a reduction of the inflow from the south. Although the studies quoted above provide much information, many aspects of the entry of the deep southern waters in the Argentine Basin are still unsettled. Good

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estimates of their volume transport, their variability and breakdown into the different components or pathways eventually will be required for a better evaluation of their impact in the Atlantic Ocean. Prior to such quantification, however, which only dedicated experiments can provide, several more qualitative uncertainties may still be removed using available data. The aim of the present study is to exploit the new hydrographic work carried out in the region in the framework of the World Ocean Circulation Experiment (WOCE) and SAVE. As the new data provide no additional sampling east of the Islas Orcadas Rise, the arrival of southern water through this passage is only occasionally evoked. Data relevant to the two other passages are presented, and combined with previous results in a unified mapping of the pathways of the various components. As the hydrographic lines were not designed to quantify the inflows, the results are mainly qualitative, yet the availability of shipborne Acoustic Doppler Current Profiler (ADCP) measurements on the WOCE lines allowed us to estimate the transports at a few key locations. After a description of the main bathymetric features and the data set in Section 2, we determine isopycnic boundaries for the constituents of the deep southern water at the Falkland Escarpment (Section 3). The pathways of the WSDW in the Georgia Basin are discussed in Section 4, and those of the Drake Passage components in Section 5. Finally, the joint use of the WOCE and SAVE data enables us to investigate a possible continuation of the warming of the deepest WSDW (Section 6).

2. Bathymetry and data set 2.1. Bathymetry The deep waters from Drake Passage bound for the Atlantic Ocean face two major bathymetric obstacles before reaching the Argentine Basin (Fig. 1). They must first find their way out of the Scotia Sea across the deep barrier that surrounds this basin, formed by the North Scotia Ridge to the north, the South Sandwich Arc to the east, and the South Scotia Ridge to the south. They then must cross the line formed by the Falkland Plateau, the Falkland Ridge, and the Islas Orcadas Rise, through one of the passages quoted in the introduction. There are two main gaps of equal depth (&3200 m) out of the Scotia Sea toward the north: The Shag Rocks Passage, at 48°W in the North Scotia Ridge, leads to Malvinas Chasm, a zonally elongated depression connected to the western Georgia Basin (Zenk, 1981), and the Georgia Passage, situated between South Georgia and the South Sandwich Arc, which leads to the southern Georgia Basin (Gordon, 1966; Locarnini et al., 1993). A fraction of the WSDW also flows through the Scotia Sea, which it can enter at the Orkney Passage, a 3500 m-deep gap near 39°W in the South Scotia Ridge (Wu¨st, 1933; Gordon, 1966). The other part flows northward east of the South Sandwich Arc above the South Sandwich Trench, before continuing to the Argentine Basin either through the Georgia Basin and Falkland Passage, or through the passage to the east of the Islas Orcadas Rise (Georgi, 1981).

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2.2. Data set The data set (Fig. 1) is composed of seven segments of hydrographic lines that sampled the region, of which four are from the WOCE Hydrographic Programme (WHP), and three from the SAVE programme. Table 1 lists the origin of the data, and gives abbreviations used in the following to refer to each section. The WHP denominations are used for the WOCE lines, the line from leg 4 of SAVE is referred to as S4, and the two segments from leg 5 of the same programme as S5W and S5E, to indicate their relative positions (west or east).

3. The Deep Southern Waters at the Falkland Escarpment 3.1. Composition of the deep Falkland Current In their description of the water masses at Drake Passage, Sievers and Nowlin (1984) observed that the Circumpolar Water is formed of two varieties: The Upper Circumpolar Deep Water (UCDW), characterized by an oxygen minimum (&160 lmol kg\), is present around 2000 m depth in the northern part of the passage and shallows southward; the Lower Circumpolar Deep Water (LCDW), bounded vertically by stability strata near p "45.79 and p "46.0, is marked by a salinity   maximum S&34.73 at about p "45.87, slightly deeper than 3000 m at the northern  side of the passage. Beneath the LCDW, in the southern part of the passage, are found the SPDW, and some WSDW. The distributions of potential temperature and salinity along SR1 (Fig. 2), located some 500 km to the east of the sections used by Sievers and Nowlin, show the same deep-water mass arrangement. The salinity maximum of the LCDW is also near 3000 m depth in the northern Scotia Sea but shallows southward throughout the passage, with a particularly steep rise at the crossing of the PF. This pronounced change of depth is important with regard to the possible pathways of the LCDW to the Argentine Basin, as the front turns northeastward downstream of SR1 across the North Scotia Ridge and Falkland Plateau (Fig. 1). We come back to this point in Section 5. Beneath the LCDW, the SPDW is not detectable from the distributions of potential temperature and salinity only, but the WSDW, often characterized by h(0.2°C (Whitworth et al., 1991), being nearer to its source than on the sections of Sievers and Nowlin (1984), occupies a larger volume here. The deep circulation in the southwestern corner of the Argentine Basin follows the path of the SAF (Fig. 1). It is dominated by an elongated cyclonic loop of the Falkland Current, which flows northward along the continental slope, retroflects at &40°S on encountering the opposing Brazil Current, and flows back southward offshore of the slope current (Peterson and Whitworth, 1989; Peterson, 1992). The boundary current conveys water from the southern ocean, but Peterson and Whitworth (1989) showed that a mid-depth layer with characteristics influenced by NADW is present in the

The unit (kg m\) of the absolute density is omitted hereafter in the paper.

Programme

WOCE WOCE WOCE WOCE

SAVE

SAVE

SAVE

Name of cruise

DI199 SR1-93 CITHER-2 JCR10

SAVE-4

SAVE-5

SAVE-5

S5E

S5W

S4

A11 SR1 A17 A23

Abbreviation

Jan./Mar. 1989

Dec. 1988/ Jan. 1989 Jan./Mar. 1989

Jan./Feb. 1993 Nov. 1993 Jan./Mar. 1994 Mar./May 1995

date

R/V Melville

R/V Melville

R/V Melville

RRS Discovery RRS J.C. Ross R/V M.Ewing RRS J.C.Ross

Ship

Table 1 Origin of the hydrographic data used in the paper. The symbols are those used in Fig. 1

SIO/San Diego

SIO/San Diego

IOS/Wormley IOS/Wormley LPO/Brest UEA/Norwich SOC/Southampton SIO/San Diego

Institution

z

z

䊏

# * 䉬 䉲

Symbol

Scripps Institution of Oceanography (1992) Scripps Institution of Oceanography (1992) Scripps Institution of Oceanography (1992)

Groupe CITHER-2 (1995) Heywood and King (1996)

Saunders and King (1995)

Data description

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Fig. 2. Vertical distributions of potential temperature and salinity along SR1 in Drake Passage. The isopycnal limits of the deep southern water components and of LCDW-1 are superimposed (bold dashed lines, p values). The locations of the ACC fronts are reported in (a). 

return southward flow. The deep h—S and h—O scatter plots of the twelve southern most stations of A17 (Fig. 3) illustrate the rapid transition to NADW contamination, as one moves northward from the Falkland Escarpment. Of the stations extending down to the depths of the NADW, only two (stations 9 and 10) are free of this water mass influence at temperatures around 2°C; both are characterized by an oxygen minimum of 170 lmol kg\ at h"2.2°C, close to the properties of UCDW in Drake Passage. The influence of NADW at the other stations is marked by higher oxygen (and salinity) values at 1°C(h(2.5°C. At stations 9 and 10, a kink of the h—S curve at p "45.78, associated with a rapid downward increase of the oxygen values,  suggests a water mass transition. This being the density that Sievers and Nowlin (1984) took as the upper limit of LCDW in Drake Passage, we also consider it the upper limit of the water mass as it enters the Argentine Basin. The vertical density distribution in the vicinity of the Falkland Escarpment (Fig. 4a) reveals that the deep Falkland Current is composed of several westward velocity cores, each recognizable as a southward thickening of isopycnal layers at the approach of the lateral boundary. The sharp vertical density gradient at p "45.98, very  close to the stability stratum that marks the base of the LCDW in Drake Passage (p "46.0 in Sievers and Nowlin, 1984), is chosen as the lower limit of the water mass  here. The LCDW is itself made up of two cores. One centred at p "45.84 (&2500 m) 

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Fig. 3. Deep h—S (a) and h—O (b) diagrams of the southernmost stations of A17, showing the absence of  NADW influence in the Falkland Current (stations 9—10). Isopycnal p "45.78 was chosen as the upper  limit of LCDW.

appears as a deep pycnostad at station 9 (core 1), and a deeper one centred at p "45.90 is visible between 2600 and 3700 m at station 10 (core 2). We take the  separation line at p "45.87 (Figs. 4a and 5), and refer, in the following, to the waters  of cores 1 and 2 as LCDW-1 and LCDW-2, respectively. The isopycnals bounding the two LCDW cores are superimposed in Fig. 4b on the vertical salinity distribution. The NADW influence is marked on this figure by the shading of the domain S'34.74. In the density layer of core 1, the circumpolar water present against the continental slope is replaced, north of 48°S, by diluted NADW, with signs of interleaving at the transition. The isopycnal layer of core 2, on the other hand, is hardly influenced by NADW, its upper limit being close to the isohaline S"34.74. This figure shows that the circumpolar water that eventually underrides the NADW does not encompass the whole density range of LCDW, but LCDW-2 only. It is notable that its upper boundary (p "45.87), which is determined here from the  density and thermohaline profiles at the Falkland Escarpment, is the same as that used by Durrieu de Madron and Weatherly (1994) in the Brazil Basin. The increased presence of NADW above LCDW-2 toward the north does not require a change to their isopycnic boundary. This and other neighbouring choices (e.g., p "45.85 by  Reid, 1989) also show that the LCDW originally situated below the salinity maximum in Drake Passage constitutes the LCDW-2. The steadiness of the arrangement of the LCDW flow in two well-defined velocity cores at the Falkland Escarpment would require several repeated observations to be confirmed. The southern end of A17 is a quasi-repeat of one of the sections used by Peterson and Whitworth (1989), in which a parcel of NADW was present in place of LCDW-1 against the boundary. These authors explained such a setting by occasional water mass exchanges between the adjacent westward and return branches of the SAF.

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Fig. 4. Vertical distributions of density (p , a) and salinity (b) at the southern end of A17. Panel (a) shows  the velocity cores of the deep Falkland Current (shaded). The isopycnic boundaries of the deep southern water components are superimposed on (b). The shaded domain S'34.74 in this figure shows contamination by NADW.

The 380 m-thick thermostad observed below the LCDW on Fig. 5 has properties (p "46.02, h"0.32°C, O "213 lmol kg\) similar to those of the SPDW observed   by Sievers and Nowlin (1984) in Drake Passage. Peterson and Whitworth (1989) recognized this water at the Falkland Escarpment from a silicate maximum of 131 lM l\, a remnant of its most prominent signature in Drake Passage. No maximum could be detected at station 10 of A17 because the water mass was only sampled by one measurement (at the bottom), yet the value thus obtained (126 lmol kg\) is very close to that reported by these authors. In Drake Passage, Sievers and Nowlin (1984) ascribed the density range 46.0(p (46.04 to the SPDW,  a layer centred at 4000 m against the escarpment on A17, in which there are indications of another (weak) velocity core. Following our previous choice of a lower limit for the LCDW, we take here 45.98(p (46.04 as the SPDW density range.  The water denser than p "46.04, finally, is WSDW. It occupies a 1500 m-thick layer  at stations 12 and 13 (Fig. 4a), and the southward thickening of layer 46.04(p (46.08 south of station 13 suggests an abyssal core of the slope current  there. 3.2. Transport of the Deep Falkland Current The barotropic character of the flow in the Argentine Basin makes it difficult to estimate volume transports from hydrographic data only (Peterson, 1992; Saunders and King, 1995). We show in Fig. 6 the profiles of absolute cross-track velocities at the station pairs over the Falkland Escarpment, deduced from an adjustment of the geostrophic shear onto the ADCP velocities. The Falkland Current stands out in the

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Fig. 5. Deep vertical profiles of h, S, and O at station 10 of A17, for the determination of the isopycnal  limits of LCDW-1, LCDW-2 and SPDW.

upper &2500 m at station pairs 8—9 to 10—11, with a magnitude of 0.25 m s\ at the former, decreasing to 0.08 m s\ at the latter. The current reversal north of station 11 over most of the water column is consistent with the property change and interleaving observed at this station at the NADW level (Fig. 4b). Below 2000 m, the velocity signatures of the slope current cores described above are visible: Core 1 of LCDW is associated with velocities O(0.2 m s\) and core 2 of the same water mass with velocities O(0.1 m s\); The velocity core of SPDW, though well defined, is only O(0.01 m s\), and the westward flow of WSDW is O(0.05 m s\). The latter is observed seaward of the shallower components, below their return flow. Table 2 gives a breakdown of the westward volume transport with respect to the various components of the Falkland Current. A total of approximately 61 Sv (1 Sv"10 m s\) is obtained for the Falkland Current, a magnitude that matches other recent estimates at neighbouring locations. Peterson (1992) deduced a value of 88 Sv at 46°S from a local inverse box model, and Saunders and King (1995) found 50 Sv at 45°S from ADCP-referenced geostrophic

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Fig. 6. Absolute cross-track velocity profiles at station pairs 8—9 to 12—13 of A17, in the deep Falkland Current and its return flow. The absolute velocities were obtained by adjusting the geostrophic shear to the shipborne ADCP velocities.

computations. The LCDW-2, SPDW, and WSDW transports add up to 7.5 Sv of water denser than the NADW. Core 1 of LCDW, though thinner than core 2 (Fig. 4), is associated with a higher transport, because of velocities more than twice as large. In order to test the sensitivity of the volume transport computations to uncertainties in the ADCP velocities, the computations were made again with increased, then decreased, values of the latter by 3 cm s\. This caused the transport of the Falkland Current to vary from 78 to 46 Sv. The lower value is unlikely, however, as it would imply a quasi-vanishing WSDW transport, and an eastward flow of SPDW.

4. The WSDW in the Georgia Basin region In Section 2 we noted the two possible routes of the WSDW to the southern Georgia Basin, either through the Scotia Sea, or along the South Sandwich Trench (Fig. 1). In the Georgia Basin proper, the water mass may proceed northward to the

p (45.78 

50.5

Layer

Volume Transport Sv

3.5

45.78(p (45.87  LCDW-1 2.5

45.87(p (45.98  LCDW-2 1

45.98(p (46.04  SPDW

4.0

46.04(p  WSDW

61.5

Total

Table 2 Volume transport of the Falkland Current (positive westward), showing the contributions of the deep components. The value reported in the first column is integrated from the southernmost station of A17 (Station 3, in 220 m of water depth) to station 11 (see Fig. 4a). The others are integrated northward from the escarpment to stations 11 (LCDW-1, LCDW-2), 13 (SPDW) and 14 (WSDW), north of which the current direction reverses

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east of the Northeast Georgia Rise, or enter the western part of the basin through the saddle between South Georgia and the Northeast Georgia Rise. The set of hydrographic lines formed of A23, S5W and S5E provides a sampling of several key passages along these paths. South of South Georgia, the northward deepening of isopycnals along A23 (Fig. 7) shows the southern part of the ACC (Fig. 1), with the southern ACC front (SACCF; Orsi et al., 1995). As the section is positioned at only a short-distance upstream of Georgia Passage, it confirms that the WSDW present in the eastern Scotia Sea leaves it through this breach. The passage being &3200 m deep, the highest density involved in this route should be, from Fig. 7, around p "46.09. North of South Georgia, the  reversal of the isopycnal slopes at stations 55—60 marks the anticyclonic loop of the SACCF around the island. The presence of densities higher than p "46.04 at these  stations shows that WSDW can overflow the col between the island and the Northeast Georgia Rise, and contribute to the anticyclonic flow. The bottom density at the sill of the passage (Station 58) is p "46.091 (h"!0.254°C), close to the highest  density at Georgia Passage. The other possible entry of WSDW in the western Georgia Basin is to the north of the Northeast Georgia Rise. Stations 67—71, which sampled this route, show densities as high as p "46.126 (Fig. 7), corresponding to bottom potential temperatures lower  than !0.5°C, in agreement with a map of this parameter in Whitworth et al. (1991).

Fig. 7. Vertical distribution of density (p ) on the A23 segment 58°S—47°S. Shading shows the density  ranges of UWSDW (light) and LWSDW (dark). The arrow marks the location of the sill between South Georgia and Northeast Georgia Rise, and the initials ‘‘FG’’ that of Falkland Gap.

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As p "46.09 is the highest density exiting the Scotia Sea through Georgia Passage,  any denser water in the Georgia Basin is necessarily from the South Sandwich Trench route: We name this water the Lower WSDW (LWSDW) and refer to the lighter WSDW (46.04(p (46.09) as Upper WSDW (UWSDW). Gordon (1966) already  stated that the coldest water of the western Georgia Basin (here the LWSDW) derives from the South Sandwich Trench route. Fig. 8a and b showing the deep h—S diagrams of selected stations of A23 reveals that property changes occur in the UWSDW in the course of its northward spreading. A salinity difference of &0.003 along isopycnals already exists (Fig. 8a) between the UWSDW of the northern Weddell Sea (stations 29—30 near 62°S—31°W) and that of the eastern Scotia Sea (stations 47—48). Farther north, stations 58—59 (Fig. 8b) sampled the throughflow between South Georgia and the Northeast Georgia Rise; their

Fig. 8. Deep h—S diagrams at selected stations of A23 and SAVE-5, showing the different properties of the UWSDW from the Scotia Sea, relative to those in the northern Weddell Sea (a), northern Georgia Basin (b, c), and Argentine Basin (to the north of the Islas Orcadas Rise (c), and at the Falkland Escarpment (d). Fig. 1 shows the station locations.

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h—S diagrams are characteristic of the eastern Scotia Sea, with only a slight perturbation toward the signature of the South Sandwich Trench route at the deepest levels. This perturbation may be an indication that a thin bottom layer from the latter is flowing beneath that from the Scotia Sea, which constituted the bulk of the overflow at the time of A23. On the same figure, stations 68—69 situated north of the Northeast Georgia Rise, show LWSDW with characteristics very close to the original ones, but UWSDW saltier than in the Weddell Sea by &0.003. In order to check the constancy of the difference revealed by Fig. 8b, we show in Fig. 8c equivalent diagrams of stations from S5E also located on either side of the Northeast Georgia Rise. The difference also exists at temperatures higher than !0.1°C, but not in the lower part of the UWSDW. The SAVE data therefore confirm that the water from the Scotia Sea route is fresher, but they also suggest that there was, at the time of SAVE-5, more UWSDW from the South Sandwich Trench route overflowing at the col between South Georgia and the Northeast Georgia Rise, than during A23. The deep h—S diagram of station 268 of S5E (Fig. 8c), which sampled the arrival of WSDW from the region east of the Islas Orcadas Rise, has h—S properties similar to those in the northern Georgia Basin. For an interpretation of the h—S disparities evidenced by Fig. 8 we refer to the deep circulation as inferred from the ACC boundary and SACCF (Fig. 1), and call for influences of Weddell Sea Shelf Water and LCDW on the UWSDW. Orsi et al. (1993) and Whitworth et al. (1994) showed that the fresh shelf waters from the northwestern Weddell Sea are entrained eastward along the southern boundary of the ACC in a region that includes the eastern Scotia Sea. In Orsi et al. (1993), a map of potential temperature on the isopycnal surface p "46.08 reveals that the shelf water influence  can be detected downward to this density. It is most likely, therefore, that the lower salinities of the UWSDW in the Scotia Sea result from this effect. The similarity of the deep h—S curves at stations 47, 48 and 58, 59 of A23 reflects the connection of both locations through the anticyclonic path of the SACCF around South Georgia. Finally, the highest salinities observed to the equatorward side of the Northeast Georgia Rise should be attributed to mixing with the overlying LCDW of the ACC. Several isopycnic tracer distributions in Orsi et al. (1993) give evidence of this mechanism, which, according to these authors, should be particularly active along the northward loop of the Weddell Gyre into the Georgia Basin. The fate of the UWSDW from the Scotia Sea route in the western Georgia Basin is uncertain. The SACCF pattern in Fig. 1 (and a similar one in Orsi et al., 1995) suggests that, after flowing through the col to the south of the Northeast Georgia Rise, the water proceeds westward to the entry of Malvinas Chasm (station 281 of S5W), then flows back eastward round the Northeast Georgia Rise and out of the Georgia Basin again. The domains where fresher UWSDW was observed along S5W, A23 and S5E corroborate this view. It was only present at station 281 of S5W (Fig. 8d), and the northern crossings of the two other lines by the SACCF also proved to be the limit of the fresher Scotia Sea influence. The only exception to this was station 270 of S5E which, though located some 200 km to the north of the front, also showed the fresher influence. There are further indications, from the bottom velocities across A23 (ADCP/Geostrophy estimates), that the UWSDW from the Scotia Sea follows the

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cyclonic loop of the SACCF. Westward velocities ranging from 0.04 to 0.13 m s\, associated with a transport of 4.9 Sv of WSDW, were found at the passage between South Georgia and the Northeast Georgia Rise, and an eastward value of 0.09 m s\, with a transport of 4 Sv, was obtained at the northernmost crossing of the SACCF, at station pair 60—61. From these observations we infer a basic cyclonic flow of UWSDW along the SACCF; yet station 270 of S5E suggests that northward shedding of water may take place at the front. The presence of recently ventilated WSDW in the northern Georgia Basin also was pointed out by Whitworth et al. (1991) from dichlorofluoromethane (F-12) data. The eastward continuation of the flow along the SACCF prevents any additional feeding of the northern and western parts of the Georgia Basin by UWSDW from the South Sandwich Trench route. Also, the UWSDW being deeper than 2500 m in the ACC band bounded by the PF and the SACCF (Figs. 2 and 7) cannot flow over the North Scotia Ridge into the Georgia Basin, because of a shallower bathymetry east of 47°W. The water occupying the density layer 46.04(p (46.09 in the northern  Georgia Basin should be viewed, therefore, as renewed only by the exchanges across the SACCF. This may explain their higher salinity, through a longer contact with the overlying LCDW. A part of this water probably flows southeastward between the PF and the SACCF out of the basin again, but Whitworth et al. (1991) also recognized it, from its F-12 signature, just to the north of the Falkland Passage. The northward spreading of LWSDW to the northern Georgia Basin is best evidenced by the distribution of bottom temperature shown in previous studies (e.g., Whitworth et al., 1991). It occurs over the South Sandwich Trench and the eastern trough of the basin, along a route that the topographic constraints force to intersect the southern boundary of the ACC and the SACCF (Fig. 1). Whitworth et al. (1991) observed an important downward increase in F-12 concentrations at !0.2°C at several stations on either side of Falkland Passage, indicative of a more recent ventilation of the colder waters. As this temperature approximately marks the transition from Upper to Lower WSDW, an (at least partial) explanation of the F-12 vertical step may reside in the more direct advective path of the LWSDW to the northern Georgia Basin, in contrast to the diffusive, and probably less efficient, exchanges of UWSDW across the SACCF. A part of the LWSDW that spreads in the Georgia Basin further proceeds through Falkland Gap. Whitworth et al. (1991) described this transfer (and that of the overlying UWSDW) as highly variable, and associated with the meandering of the ACC fronts. They provided evidence of a westward continuation of the throughflow along the Falkland Escarpment, as also illustrated in Fig. 9a by a 400—1000 m-thick layer of LWSDW along S5W. During SAVE-5, the amount of this water at the northern side of the Falkland Ridge to the east of the Gap (S5E, not shown) was much lower, in comparison with a layer less than 200 m-thick only present at a single station. Along A23, a westward bottom velocity (0.14 m s\) into the Argentine Basin was measured at station pair 70—71, right at the Falkland Gap. At the same location, the sudden northward deepening of isopycnal p "46.04 (Fig. 7) illustrates the  coupling of the LWSDW circulation to the PF. The thickness of the LWSDW decreases from &800 m in the northern Georgia Basin, to less than 200 m to the north of the front. Like the SACCF for the UWSDW, the PF appears as a hindrance

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Fig. 9. (a) Vertical density distribution (p ) along S5W at 41°W to the north of Maurice Ewing Bank,  showing the velocity core of SPDW. (b) Deep h—S diagrams of SAVE-5 stations at the eastern opening of Malvinas Chasm and in the deep Falkland Current at 41°W.

to the northward spreading of LWSDW, a part of this water mass being therefore expected to flow southeastward out of the Georgia Basin again, to the south of the PF. There are indications, on a map of salinity at p "46.14 in Orsi et al. (1993), of this  water returning from the Georgia Basin to the Weddell Sea abyssal plain.

5. Pathways of the Drake Passage Components 5.1. Pathway of the SPDW to the Argentine Basin The superposition of the isopycnic water mass boundaries on the potential temperature distribution along SR1 in Fig. 2a defines approximately the temperature range of the SPDW as 0.2°C(h(0.6°C. The bottom imprint of these waters (located at 57°S—58°S on SR1) allowed Gordon (1966) to trace back their origin to the Southeastern Pacific, and to further describe them as ‘‘compressed into a very rapid bottom current which leaves the Scotia Sea and enters the South Atlantic through a passage west of Shag Rocks’’. The role of Shag Rocks Passage for the exit of SPDW from the Scotia Sea was further demonstrated by Zenk (1981) from direct near-bottom current measurements in the gap, which showed that, on the average, ‘‘the outflow is characterized by west-northwesterly currents with speeds of 38.2$8.8 cm s\ and associated temperatures of 0.47$0.11°C’’. This temperature range indeed falls within that of the SPDW. When only considering the depth of isopycnal p "45.98 at the northern end of  SR1 (&4000 m; Fig. 2a), one would consider it very doubtful that the SPDW, of

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higher density, can overflow the 3200 m-deep Shag Rocks Passage. On A23, however, some 1500 km to the east of SR1, the same isopycnal is found at only 2300 m against the southern flank of North Scotia Ridge (Fig. 7). This important eastward shallowing of the deep isopycnals along the ridge has a magnitude comparable to that of their southward shallowing across Drake Passage (Fig. 2). It has its origin in the latter, and in the northward veering of the PF in the Scotia Sea (Fig. 1). We noted, in Section 3, that the southward shallowing of the isopycnals in Drake Passage occurs principally at the crossing of the PF: There, the isopycnal p "45.98 rises from 3500 to 2500 m.  Assuming that this vertical displacement is conserved downstream of SR1, a throughflow of SPDW at Shag Rocks Passage requires that the front intersects the North Scotia Ridge at, or west of 48°W, the longitude of the passage. Although the intersection appears slightly to the east of this location (Fig. 1), it is most likely that the finite frontal width and weak zonal motions of the front permit a variable influx of SPDW into Malvinas Chasm. Zenk’s (1981) current measurements in the passage indeed showed that the flow occurs as occasional major events, rather than continuously. One expects from the preceding observations that the depth range of the SPDW that has just entered Malvinas Chasm is equivalent to that observed south of the PF in Drake Passage, that is approximately 2500—3000 m. As the sill depth of the Falkland Plateau is 2500 m, the water mass can only continue toward the Argentine Basin by first flowing eastward into Georgia Basin, then northward around Maurice Ewing Bank. The stations of S5W located at the eastern opening of Malvinas Chasm and across the Falkland Escarpment north of Maurice Ewing Bank (Fig. 1) provide information about the flow there. There are indications, from the vertical density distribution along that part of S5W, that a core of eastward velocity exists within the SPDW at station 282 against the southeastern side of Maurice Ewing Bank. Because this feature was only poorly sampled, due to insufficient lateral resolution, it is not shown here. We present, instead, the deep h—S diagrams of stations 281 and 282 (Fig. 9b), which illustrate the property differences in the SPDW density range at both stations. We pointed out above that station 281 sampled deep water that has flowed around South Georgia to enter the western Georgia Basin. On the other hand, station 282, located in the slope current, is expected to have sampled SPDW from Shag Rocks Passage. Both diagrams are indeed different, the latter being saltier by &0.005. The diagram of station 285, shown on the same figure, is identical to that of station 282, because of the continuity of the flow around Maurice Ewing Bank. The slope current of SPDW is better resolved by the SAVE stations on the northern side of Maurice Ewing Bank (Fig. 9a). A well-defined core of the water mass stands out there, of width &15 km, comparable to that of the Falkland Escarpment, and a thickness of &1250 m around a median depth of 2750 m. A comparison with the core of SPDW observed 800 km downstream at 55°W during A17 (Fig. 4), reveals several major differences. The A17 feature is wider (&25 km), and thinner (&400 m), than the SAVE one. The steeper isopycnals reveal that the latter is more energetic: The maximum shear velocity was found to be &0.07 m s\ at station pair 285—286 of S5W, whereas it was only &0.01 m s\ at station pair 10—11 of A17 (Fig. 6). Another striking difference is the disparity of the cores’ mean depths, around 2700 m at 41°W, and 4000 m at 55°W.

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The difference in intensity could be a consequence of time variability, as Peterson and Whitworth (1989) suggested that the presence of SPDW against the Falkland Escarpment is not continuous. We note, however, that the situation shown in Fig. 9a at 41°W matches a 14 month-averaged zonal velocity distribution at the same longitude in Whitworth et al. (1991, their Fig. 3), which shows an intense ('0.3 m s\ maximum) core of westward current intensified at 2500 m against the escarpment. Despite the variability, which these authors also point out, an intense slope current of SPDW seems, therefore, to be the normal state at 41°W. At 55°W, on the other hand, we have two samplings of the SPDW core that both reveal a weaker feature: the one presented in Peterson and Whitworth (1989) is only detected from the silicate maximum, whilst that of A17 (Fig. 4) has a weak density signature. This hints at a real downstream weakening of the SPDW slope current along the Falkland Escarpment. The sinking of the SPDW core, from 2700 m at 41°W to 4000 m at 55°W, also seems real. At the former longitude, we pointed out the close agreement of our observations with the 14 month-average of Whitworth et al. (1991). At the latter, both available realizations show the same core depth. Another sampling of the vein of SPDW at 47°W, in Georgi (1981), shows a core centred at 3800 m, thus revealing that most of the sinking takes place between 41°W and 47°W. This longitude band is where the PF crosses the escarpment (Fig. 1). The important westward sinking of the SPDW core at this location should therefore be viewed as reflecting the steep isopycnal slopes associated with the front. 5.2. Pathway of the LCDW-2 The crest line of the North Scotia Ridge is shallower than 2000 m everywhere between the northern end of SR1 and Shag Rocks Passage. We observe in Fig. 2 that the isopycnal p "45.87, which marks the upper limit of LCDW-2, is deeper  than 2500 m north of the PF in Drake Passage, but rises to &1500 m on crossing the front. As for the SPDW, therefore, the westernmost location where LCDW-2 can cross the North Scotia Ridge is Shag Rocks Passage. It should do so in the depth range 1500—2500 m, which is that of the isopycnic layer 45.87(p (45.98 south of  the PF in Drake Passage. It is unlikely that the LCDW-2 (and SPDW) sink in Malvinas Chasm, as the deepest levels of this bathymetric depression are fed with WSDW from the east (Locarnini et al., 1993; Cunningham and Barker, 1996). As the sill depth of the Falkland Plateau is 2500 m, there is therefore no reason why the LCDW-2 should follow the route around Maurice Ewing Bank to enter the Argentine Basin. A flow of LCDW-2 over the Falkland Plateau is confirmed from the property profiles at station 10 of A17, located at the Falkland Escarpment (Fig. 5). Sharp vertical gradients at density p "45.98 mark the lower limit of the Circumpolar Deep  Water at this station. The mid-gradient temperature and salinity values of 0.7°C and 34.71, respectively, are very close, not only to the values at the lower limit of the water mass in Drake Passage (Fig. 2), but also to the bottom values at the saddle of the Falkland Plateau, according to Gordon and Greengrove (1986). These authors refer

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to historical data that show that water as cold as 0.71°C, with salinity of 34.71, overflows at the plateau. These observations show that the whole temperature (or density) range of Circumpolar Deep Water from Drake Passage can overflow at the Falkland Plateau. This includes the density range 45.87(p (45.98 of LCDW-2.  Farther downstream, LCDW-2 is observed against the Falkland Escarpment in the depth range 2700—3700 m (Fig. 4a), revealing an average sinking of &700 m between the saddle and the southern end of A17. Like the SPDW, we observe that the LCDW-2 is poleward of the PF on entering Malvinas Chasm, and beneath the SAF at the southern end of A17. The intervening sinking, here also, reflects the depth variation of isopycnals across the PF.

6. The warming trend Coles et al. (1996) observed a warming of the densest WSDW present in the southwestern Argentine Basin during the 1980s. This alteration was corroborated by results of Hogg and Zenk (1997) in the Vema Channel, the deep passage connecting the Argentine and Brazil basins, near 31°S. There the temperature of the throughflow, which had remained nearly constant at !0.18°C since 1973, rose abruptly to !0.15°C in 1992, a value that has remained constant until at least 1996. The availability of the WOCE data allows us to complement these observations. Fig. 10 displays the deep temperature distributions along four lines of the southwestern Argentine Basin from the period 1980 to 1994. Panel (a) shows the distribution along a track surveyed during cruise 107 of RV Atlantis II in 1980 that is quasi-identical to that of A17. Fig. 10b and c is the western ends of S4 (1988 sampling) and A11 (1993 sampling), respectively, and Fig. 10d is the southern end of A17 (1994 sampling). The four segments are considered sufficiently close to one another (Fig. 1) to justify a direct comparison. The bottom layer h(!0.2°C had a thickness reaching 400 m in 1980, but had become thinner and confined to the deepest part of the basin by the time of S4: This alteration, from 1980 to 1988, is the one documented in Coles et al. (1996). The two WOCE lines (Fig. 10c and d) show that the process continued during the early 1990s, leading to the disappearance of layer h(!0.2°C. As a consequence of the warming of the bottom waters, the LWSDW (p '46.09) is also no longer observed in  the southwestern corner of the Argentine Basin. In 1980, a layer of it as thick as 400 m was observed at the foot of the Falkland Escarpment by Peterson and Whitworth (1989; their Fig. 6c). A maximum density of p "46.085 along the S4 segment shown  in Fig. 10b revealed that it had already disappeared in 1988—1989, although a thin layer of water colder than !0.2°C was still present. As, in this region, a density p "46.09 corresponds to a potential temperature of about !0.22°C, the continua tion of the warming, since SAVE, has prevented any reappearance of the LWSDW. Coles et al. (their Fig. 8) estimated the change of volume in 0.1°C temperature classes over the area of the Argentine Basin. Their results reveal a reduction of about 40% in the temperature class !0.3°C(h(!0.2°C, but no significant volume decrease of the warmer classes. The net diminution of the volume of WSDW (h(0.2°C) suggests a reduction of the inflow from the Weddell Sea. We present in

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Fig. 10. Deep vertical distributions of potential temperature along four lines in the southwest Argentine Basin, showing the disappearance of layer h(!0.2°C from 1980 to 1994 (Fig. 1 shows the locations of the lines). (a) (Reproduced from Peterson and Whitworth, 1989) along a line quasi-identical to the southern end of A17, surveyed by Cruise 107 of R/V Atlantis II in 1980. (b) Along the western end of S4 (1988). (c) Along the western end of A11 (1993). (d) Along the southern end of A17 (1994).

Fig. 11 a similar analysis at the four available intersections between the SAVE and WOCE lines. The plots, which show the thicknesses of the layers characterised by the temperature classes, confirm the vanishing of layer h(!0.2°C, but also indicate that the evolution reached the class !0.2°C(h(!0.1°C at three comparison sites (A, B, D) out of four. The continuation of the change at the deepest levels, and its upward propagation, would be consistent with a decrease of the inflow rate limited to the densest water. The comparison at site D shows that the change had reached the northern part of the Argentine Basin before 1994, in accordance with the results in the Vema Channel. The fourth comparison site (C) also shows a diminution of the volume of layer h(!0.2°C, but not of layer !0.2°C(h(!0.1°C. There is, however, an intense deep lateral variability on S5E near this site, probably associated with the neighbourhood of the SAF (Fig. 1), which suggests that the differences at this site should not necessarily be interpreted as long-term variations.

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Fig. 11. Comparison of the WOCE and SAVE deep potential temperature profiles at the four sites shown on the map. The plots show the thickness of layers corresponding to 0.1°C temperature classes for the WOCE data (continuous) and the SAVE data (dashed), and the difference ‘‘WOCE minus SAVE’’ (stippled). Note the site-dependent scale of the thickness axis.

7. Summary The descriptions in Sections 4 and 5 provide the elements for a tentative schematic synthesis of the pathways of the deep southern waters across the bathymetric barriers that separate the Southern Ocean from the Argentine Basin (Fig. 12). We pointed out that only the SPDW and LCDW-2 present at, or south of, the PF in Drake Passage are shallow enough to flow through Shag Rocks Passage. We therefore start the two lines representing the routes of these components near 58°S—56°W, the approximate location of the front along SR1. After flowing through Shag Rocks Passage, the LCDW-2 proceeds directly to the Falkland Escarpment over the Falkland Plateau, while the SPDW skirts Maurice Ewing Bank. In the UWSDW, the slightly different thermohaline properties of the Scotia Sea variety were observed to the south of the SACCF. As high velocities were also observed in this water mass beneath the SACCF, this front should probably be considered the main path of the UWSDW from the Scotia Sea. Fig. 12 shows the inferred route, with a possible weak contribution from the east of the South Sandwich Trench. In agreement with previous descriptions (Orsi et al., 1993), this circulation pattern suggests that most of the UWSDW that intrudes into the Georgia Basin leaves it to the south of the Islas Orcadas Rise. Our only

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Fig. 12. Schematic circulation patterns of the deep southern water components across the bathymetric obstacles to the south of the Argentine Basin. Red line: LCDW-2; Brown line: SPDW; Blue line: UWSDW; Green line: LWSDW.

evidence of a northward leakage of UWSDW at the SACCF was from a single SAVE station. As other proofs were given by Whitworth et al. (1991), we schematically represent this equatorward export from the SACCF by two dashed routes with extensions to the Falkland Escarpment across Falkland Passage. These should only be regarded as averaged flow directions, as Whitworth et al. (1991) also pointed out the high variability of the exchanges at the passage. The shrinking of the layer of UWSDW at the escarpment during S5W (Fig. 9a) indeed suggests a weak, if not reversed, westward velocity in this water at the time of the cruise. The cyclonic intrusion of LWSDW in the Georgia Basin is fed through the South Sandwich Trench route and, if compared with that of the UWSDW, occurs more to the north along the main trough of the basin. The PF acts as a northern boundary near the Falkland Passage and, probably, as a guide for the southeastward return flow of most of this water to the Weddell Sea abyssal plain. The SAVE-5 data on either side of Falkland Gap suggest that most of the equatorward export at this location (Whitworth et al., 1991) turns west along the Falkland Escarpment. The gathering of the Drake Passage and Weddell Sea components at the escarpment takes the form of distinct stacked velocity cores. The LCDW was itself subdivided in two varieties when sampled during A17, of which only the denser one, formed of waters from below the LCDW salinity maximum in Drake Passage, eventually underrides the NADW in the Argentine Basin. The splitting of the LCDW should probably be ascribed to different pathways across the North Scotia Ridge. We pointed out the necessity, for the denser variety (LCDW-2), to flow along the PF at Shag Rocks Passage. LCDW-1 could be formed of water located to the north of the

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PF in Drake Passage, for which a 2000 m-deep gap is available at &55°W. The LCDW-2 and SPDW both experience a drastic sinking on entering the Argentine Basin. The necessity for these waters, originally located at the poleward side of the PF, to adjust to the SAF environment at the Falkland Escarpment, explains the sinking. We observed, finally, that there was no LWSDW present at 55°W at the foot of the escarpment during A17. A part of the westward boundary flow of this water probably deviates toward the basin interior before reaching this longitude. A comparison of the WOCE and SAVE data, however, showed that the warming of the densest WSDW of the Argentine Basin, revealed by Coles et al. (1996), had continued during the first half of the 1990s, leading to a total disappearance of the LWSDW from the southwestern corner of the Argentine Basin.

Acknowledgements The authors are pleased to present these WOCE South Atlantic results in the framework of the DSR-II volume dedicated to G. Siedler, who greatly contributed to the realization of WOCE in this part of the ocean. M.A. was supported by Grant 210161 of IFREMER, in the framework of the French Programme National d’Etude de la Dynamique du Climat (PNEDC) and its WOCE/France subprogramme. WOCE sections A11, SR1 and A23 were supported through the NERC UK WOCE programme. K.J.H. acknowledges the support of UK WOCE Special Topic grant GST/02/575. The authors are thankful to A. Piola and M. McCartney for the availability of the SAVE-4 and SAVE-5 data. We also warmly thank R. Locarnini for providing the frontal patterns reported in Fig. 1 and for many helpful comments. The detailed remarks of an anonymous reviewer also greatly contributed to improve the paper. Useful discussions with several other colleagues, and the aid of P. Le Bot and J. Le Gall for the preparation of the manuscript, are acknowledged.

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