the boundary currents in the western argentine basin

Deep-Sea Research, Vol. 39. No. 3/4, pp. 623~544, I992. 01984)149/92 $5.00 + 0,00 Printed in Great Britain. © 1992 Pergamon Press plc

The boundary currents in the western Argentine Basin

RAY G. PETERSON*

(Received 13 December 1990; in revised form 10 May 1991; accepted 21 May 1991)

Abstract - -A quasi-synoptic hydrographic data set enclosing the Brazil-Falkland (Malvinas) Confluence Zone is used to investigate the absolute geostrophic volume transports of the western boundary currents in the region. Water mass characteristics provide a basis for adjusting geostrophic shears near the continental margin at 38°S, and the depth-integrated southward transport there, comprised of the Brazil Current and deep boundary flow, is est imated to be 68 Sv ( 1 Sv = 106 m 3 s - 1), Of this, 26 SV are thermocline water, 18 Sv are Antarctic Intermediate Water , and 24 Sv are deep waters of circumpolar and North Atlantic origin. Assuming the bot tom flow is parallel to the steep bathymetry at the western boundary of the Argentine Basin, a geostrophic transport of 143 Sv is found to move seaward across the 4600-m isobath between the latitudes of 38 ° and 46°S. Top-to-bottom northward transports in the region of the Falkland Current are then solved as residual quantities from mass balances for enclosed areas. The likelihood of significant northward bot tom velocities in deep western boundary currents there, combined with there being no apparent reversals within the water column in the direction of flow, makes such a procedure necessary. The resulting est imates for the depth-integrated northward transports in the Falkland Current region are 75 Sv at 42°S and 88 Sv at 46°S. Approximately 60 and 70 Sv, respectively are contained in the upper 2000 m as a direct extension of the northern Antarctic Circumpolar Current , while 34 and 40 Sv are contained in the density range of surface and intermediate waters. The northward transports in layers beneath the 2000-m level belong to the deep western boundary currents of the southern Argentine Basin. These numbers for the Falkland Current region are much larger than the 10-20 Sv values typically found in the literature, but they are consistent with other information such as the volume transport in the upper 2000 m of the northern Antarctic Circumpolar Current in Drake Passage, velocities of surface drifters in the Falkland Current , and the full-depth circulation in the interior of the Argentine Basin.

1. I N T R O D U C T I O N

THE upper-level circulation in the western Argentine Basin is marked by the opposing flows of the Brazil and Falkland (Malvinas) currents, each of which executes its own form of retroflection after having come into contact with one another (Fig. 1). Arising from the confluence of these two currents is an intense eddy field where tropical and subtropical waters meet and mix with water masses from the northern and middle portions of the Antarctic Circumpolar Current (ACC). The resulting upper-level discontinuities have been documented with shipboard m e a s u r e m e n t s (GORDON, 1981; GREENGROVE, 1986; GORDON and GREENGROVE, 1986a; GORDON, 1989; PIOLA and GORDON, 1989; PETERSON

* Institut ffir Meereskunde an der Universitfit Kiel, Dfisternbrooker Weg 20, D-2300 Kiel 1, Germany. Present address: Scripps Institution of Oceanography, Physical Oceanography Research Division, MS 0230, University of California, San Diego, La Jolla, C A 92093, U.S.A.

623

624 R.G. PETERSON

~ o ~ , ; ~N ~ ~ 0 35~

45

Fig. 1. Schematic representation of the large-scale geostrophic currents and fronts in the southwestern South Atlantic Ocean. The 1000-. 3000-, and 5000-m isobaths are shown; depths less

than 3000 m are shaded.

and WHITWORTH, 1989) and satellite infra-red imagery (LEGECKIS and GORDON, 1982; OLSON et al., 1988; GARZOL1 and GARRAFFO, 1989).

The confluence of the Brazil and Falkland currents marks not only a rapid transition between upper-level regimes; sharp changes in the boundary currents exist at depth as well. Aside from the confluence zone itself, the greatest degree of complexity in the vertical structure of velocity is north of the confluence near the continental margin where one or more reversals in flow direction often exist with increasing depth. Well north of the confluence zone, in the Brazil Basin, the southward-flowing Brazil Current is a shallow and weak feature confined to the upper few hundred meters next to or over the continental shelf, and its volume transport there is generally well under 20 Sv (1 Sv = 106 m 3 s - l ; PETERSON and STRAMMA, 1991). Beneath this is a core of Antarctic Intermediate Water (AAIW) streaming north at the edge of the shelf (EVANS and SIGNORINI, 1985), but away

Boundary currents in the Argentine Basin 625

from the western boundary the AAIW appears to move south in the same direction as the surface waters of the wind-driven anticyclonic gyre (BuscAGLIA, 1971; REID et al., 1977; REID, 1989). At greater depth along the continental margin, beneath the northward flow of AAIW and the underlying upper branch of Circumpolar Deep Water (CDW), is the southward-flowing North Atlantic Deep Water (NADW), which in turn overlies the northward-moving lower branch of CDW (often referred to generically as Antarctic Bottom Water). Weddell Sea Deep Water (WSDW), found at abyssal depths beneath the lower CDW in the Argentine Basin, does not enter the Brazil Basin in any appreciable amounts (REID et al. , 1977).

Nearer the confluence zone in the Argentine Basin, the Brazil Current strengthens under the influence of a recirculation cell (REID et al., 1977; GORDON and GREENGROVE, 1986a; STRAMMA, 1989) before separating from the western boundary at latitudes of 33 °- 38% (OLSON et al. , 1988). Inspection of figures given by REID (1989) suggests that in locations poleward of the separation point the structure of flow in the Brazil Current region simplifies to one where all layers from the surface down through the NADW move in the same general southward direction before a reversal is met in the abyssal layers, consistent with observations by PETERSON and WHITWORTH (1989). The total southward transport of the non-abyssal layers at 37°S has been estimated by MCCARTNEY and ZEMBA (1988) and ZEMBA and MCCARTNEY (1988) as 76 Sv, a figure much larger than any previous observational ones for the Brazil Current region.

This superpositioning of the upper wind-driven flow with the deeper thermohaline circulation, all being in the same southward direction down through the NADW in the region of the poleward extension of the Brazil Current, introduces a vagueness in the meaning of the term Brazil Current. It seems most appropriate to consider the Brazil Current as just the wind-driven part of the total southward flow, a loose definition that can be applied to the length of the current. A strict definition may, however, be elusive considering that along much of the continental margin the AAIW moves as a thermohaline boundary current, while in regions farther offshore it circulates with the wind-driven flow.

A similar problem of definition exists for the Falkland Current as well, because south of the confluence zone along the western boundary all the flow appears to be toward the north (PETERSON and WHITWORTH, 1989; REID, 1989). The shallower portions of this flow, nominally in the upper 2000 m, are direct extensions of the northern ACC from Drake Passage over the Falkland Plateau, and might thus be most suitably regarded as constitut- ing the Falkland Current. At greater depth along the Argentine margin, the flow belongs to the deep thermohaline circulation.

Lately it has been demonstrated by PETERSON and WHITWORTH (1989) that the northernmost of the two principal fronts within the ACC, the Subantarctic Front (SAF), turns sharply northward east of Drake Passage to become a part of the Falkland Current (Fig. 1). Earlier studies of the current structure in Drake Passage have shown that the SAF is associated with enhanced vertical shears of zonal velocity from the sea surface down to the bottom (NowLIN and KLINCK, 1986). This vertical coherency likely persists as the SAF turns north and traverses the Falkland Plateau before entering the southwestern Argen- tine Basin, whereupon it comes to override abyssal waters also moving north.

The constancy with depth in direction of flow along the Argentine margin combines with the Falkland Current being tightly pinned to the continental slope to render its absolute northward volume transport a poorly known quantity. Using historical density fields in a multilevel diagnostic model, ZYRYANOV and SEVEROV (1979) obtained depth-integrated

626 R . G . PETERSON

northward transports of 32 Sv at 45°S during the austral summer and 40 Sv in winter. GORDON and GREENGROVE (1986a) assumed a reference level at 1400 m and estimated the northward geostrophic transport above this depth as 10 Sv at 46°S. But their northward geostrophic speeds at the surface (17 cm s -1) were only half as large as those from a pair of surface drifters which passed through the area in 1979, leading them to conclude that non- zero bottom velocities probably exist and that their transport estimate represented a lower limit. PIOLA and BIANCHI (1990) used a number of hydrographic sections made in the years since 1980 and found the Falkland Current transport to range from just 10 to 12 Sv relative to 1000 m; the density field within the current therefore can be regarded as being highly stable. Most recently, GARZOL! et al. (1990) have used hydrographic data obtained from the Confluence Project to portray schematically the time-averaged transport of the Falkland Current as being about 15 Sv near 45°S and 10 Sv near 40°S, each being above and relative to 1000m.

Given the great complexity of the overall circulation in the Argentine Basin, it is obvious that in the absence of correlated direct measurements across each boundary current the accurate assessment of absolute volume transports is not a simple matter. North of the confluence zone one may determine with some degree of confidence a suitable level of no motion on the basis of water mass properties, but south of the confluence a level of no motion probably does not exist along the continental margin; and by selecting one, particularly a shallow or mid-depth one, serious underestimates can be made. The abyssal circulation must be taken into account and the geostrophic shears adjusted accordingly.

The abyssal circulation was considered by REID (1989) when he assumed bottom velocities along the western boundary that would bring his geostrophic computations of the total flow in the South Atlantic into mass balance. An estimate of 20 Sv or so for the total northward transport in the region of the Falkland Current can be extracted from his final figure, an estimate larger than most others based on hydrographic measurements. But when regional mass balances are required from quasi-synoptic data, as in the following, much larger numbers still are exacted.

In this paper, the absolute geostrophic circulation in the western Argentine Basin is addressed through the use of a quasi-synoptic hydrographic data set in conjunction with patterns of the mean bottom circulation as inferred from the small number of current meter records presently available and from observations of bottom sediments. Because the Falkland Current is a branch of the ACC, one that is associated with the SAF, the portion of the ACC transport in the northern Drake Passage which most likely enters the Falkland Current is first considered. The full-depth circulation in the western Argentine Basin is then estimated under the assumption that mass is conserved in enclosed regions and in layers defined by surfaces of potential density. Preliminary results of this analysis were reported by PETERSON (1990), and the end outcome is that the total northward flow in and beneath the Falkland Current is significantly more robust than previously discussed in the literature. Although the information which can be used to assess exact magnitudes is sparse, it nonetheless provides good reason for us to re-evaluate our conceptual image of this current.

2. NORTHERN ANTARCTIC CIRCUMPOLAR CURRENT

The northernmost front within the A C C , the SAF, turns sharply northward just east of Drake Passage to enter the Falkland Current (PETERSON and WHITWORTH, 1989). This


front approximately follows the 2000-m isobath to around 40°S where it retroflects sharply back toward the south before continuing east with the ACC just north of the Falkland Plateau. Two types of upper-level water from the ACC are thus carried equatorward by the Falkland Current: those from the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ). The former can reach to as far north as 36°S at the surface (BoLTOVSKY, 1970; fig. 11 in GARZOLI and GAR~FFO, 1989), whereas the latter is limited to around 40°S in the narrow cyclonic trough described by the retroflecting SAF. The cyclonic trough of PFZ water in the western Argentine Basin appears to be continuous with its source in Drake Passage, implying that most or all of the upper-level flow in Drake Passage (i.e. that which is shallower than the 2000-m sill depth of the part of the North Scotia Ridge traversed by the SAF) from the northern PFZ northward to the continental shelf can enter the Falkland Current. One therefore might expect this part of the ACC transport through Drake Passage to represent a nominal value for the Falkland Current transport in the upper 2000 m south of where the SAF makes its poleward return.

Several hydrographic surveys have been made across Drake Passage, with the highest quality and most closely spaced station data presently available coming from cruises of the International Southern Ocean Studies (ISOS) program from the years 1975 to 1980. Large amounts of information regarding the ACC's structure have been revealed by those surveys, not the least important being that the enhanced zonal flow associated with the SAF extends from the ocean surface to the sea floor (NOWLIN and KLINCK, 1986). NOWLIN and CLIFFORD (1982) have estimated that the flow associated with the narrow SAF alone accounts for an average of 32% of the 87-Sv baroclinic transport of the ACC through Drake Passage (above and relative to 2500-m depth).

The most salient question here concerns the total geostrophic transport of the ACC within the northern Drake Passage that can plausibly enter the Falkland Current. This would be the flow contained, approximately, in the upper 2000 m from the continental shelf off Cape Horn southwards to the northern PFZ. The location of the SAF is determined here by two methods: on the basis of water mass properties in the usual way, that is, by where the salinity minimum and oxygen maximum of AAIW make their rapid northward descents from the near surface in the PFZ to greater depth in the SAZ; and by where the largest geostrophic vertical shears and eastward velocities occur. The two methods consistently yield the same frontal locations.

Of all the hydrographic sections made across Drake Passage during the ISOS program, there are seven in which at least one station was made within the PFZ (Fig. 2). Sources for these observations are data reports by NOWLIN et al. (1977), WHITWORTH et al. (1978), WORLEY and NOWLIN (1978, 1979) and WORLEY (1982). Eastward geostrophic transports for each of the sections are computed relative to the deepest common depths between station pairs, with the resulting transports being on the whole somewhat larger than those obtained by previous investigators who have used the same data with non-bottom reference depths (e.g. 2500 m by NOWLIN and CLIFFORD, 1982). Table 1 summarizes the transports computed here, both for the entire water column and for the upper 2000 m.

As can be seen in Fig. 2 and Table 1, three of the depicted sections poorly resolve the flow field near the continental shelf. The Melville 1975 section in the east (Stas 43--46) had its northernmost station, which did not extend to the bottom, in a depth of more than 2000 m. The other two sections (Melville 1977 Stas 1-3 and Thompson 1976 Stas 1-4) had their northernmost stations at 370- and 680-m depths whereas the next stations off the shelf, also not reaching the bottom, were in depths of more than 3000 m; large triangular

628 R.G. PETERSON

67°W 63 ° 55°8

~ . ~ ! ~"~-~ ; ' " M E L V I L L E 1975{: ~ " i. ~,~ y r/ ~ / ~ ( .... • ~ THOMPSON 1976

L ~ 0 4 6 Jr MELVILLE 1977 ~ ,,' ~/- ...... • YELCHO 1979

I .... Horn .... ./ /~: I ~ -"

l l ( , 0 4 4 " ',

58 °

%, "\ -,,: 50 L • -

J j I \~ p", l K ___L___ ±

Fig. 2. Locations of hydrographic stations in the northern Drake Passage along seven sections (each terminated in the south with a station immediately south of the Subantarctic Front) used to estimate the volume transport potentially available to the Falkland Current. An additional section (Yelcho 1979) is included in which the Subantarctic and Polar fronts were in close proximity and not

individually resolved. Stations are from the indicated cruises.

regions next to the shelf were thus left unsampled and the actual transports underesti- mated. This is particularly evident in view of current meter measurements showing that through-passage speeds of 60 cm s -1 or more can occur in the northern passage near depths of 500 m (PILLSBURY et al., 1981). Also in these latter two sections, large vertical shears next to the shelf, as evidenced by surface geostrophic speeds of 20 and 29 cm s- t (Table 1), further indicate that sizeable transports have been missed with these calculations. Conversely, a fourth section, made by the Yelcho in 1979, provides an overestimate; the SAF and PF were very near one another when this section was made and the zonal jets associated with the two fronts were not clearly resolved as distinct features.

The result is that only four sections appear to provide realistic estimates of the transport in the region from the northern PFZ to the continental shelf: they give an average of 53 Sv for the entire water column and 47 Sv in the upper 2000 m. But then, these values must be regarded as being on the low side of the actual transports because no bottom speeds have


Table 1. Eastward volume transports (total and above 2000-m depth) and surface speeds relative to deepest common depths between ISOS stations for the region of Drake Passage from the northern Polar Frontal Zone to the

South American shelf

Transport (Sv) Surface speed

Ship andyear Stations Depth (m) Total <2000 m (cm s -1) Remarks

Melville 1975 18-20 2970 8.1 7.6 14.5 20--23 3790 31.2 26.5 33.0 23-24 3890 13.5 10.9 18.1 24-26 2770 -0.3 0.3 -0.7 26-27 820 1.4 1.4 15.5

Sum 53.9 46.7

Melville 1975 43-44 3460 25.8 22.4 15.1 44--45 3710 -1 .4 -1 .2 -0.6 45-46 1640 8.3 8.3 17.6

Sum 32.7 29.5

Melville 1975 47-48 2860 -0.2 -0 .8 -2 .6 48-49 3450 48.1 42.3 33.8 49-50 3360 3.4 4.0 6.5

Sum 51.3 45.5

Melville 1977 1-2 370 1.4 1.4 20.4 2-3 2560 24.6 23.7 24.3

Sum 26.0 25.1

Thompson 1976 1-2 680 1.9 1.9 28.9 2-3 2290 23.4 23.3 46.5 3-4 2290 5.9 5.7 10.3

Sum 3l .2 30.9

Thompson 1976 26-27 540 -0.3 -0.3 -4.4 27-28 2920 10.8 10.2 17.1 28-29 2920 8.2 7.7 19.1 29--30 4140 20.5 17.0 29.6 30--32 3160 16.3 15.1 29.7

Sum 55.5 49.7

Yelcho 1979 1-2 570 0.6 0.6 18.1 2-3 1480 -0 .2 -0.2 -0.5 3-4 2560 3.7 3.8 11.5 4-5 3640 12.9 -11.2 18.8 5--6 3540 16.1 13.8 38.3 6-7 3480 31.7 27,7 45.7

Sum 64.8 56.9

Atlantis H 1980 23-24 44(I 0.3 0.3 5.6 24-25 1370 -1.3 -1.3 13.9 25-26 2620 13.5 13.3 43.0 26-27 3610 21.0 18.1 28.2 27-28 3360 17.7 15.3 29.0

Sum 51.2 45.9

Poor resolution near continental shelf; cold eddy from Polar Frontal Zone north of Subantarctic Front

Poor resolution near continental shelf; stations not to full depth

Poor resolution near continental shelf; stations not to full depth

Station 6 is within Subantarctic Front, Sta. 7 is within Polar Front

630 R.G. PETERSON

been considered. Year-long records from deep current meter moorings deployed during ISOS reveal near-bottom speeds averaging 8-9 cm s -1 through the northern Drake Passage at depths of up to 2600 m (PILLSBURY et al., 1981). If the above transports in the upper 2000 m are adjusted by this "barotropic" component over a shelf width of just 50 km, an increase of 5--6 Sv is realized. But at the average location of the SAF in Drake Passage, another 100 km offshore near the ISOS ML-2 mooring, an average speed of 13 cm s-I obtains at 2700-m depth (PILLSBURY et al., 1981), meaning that substantially more than just 5 or 6 Sv should be added to the 47 Sv obtained above, though exactly how much is difficult to say. This in itself, however, is not of primary importance. What is most important is that the ACC can easily supply much more water to the Falkland Current than has generally been believed. This amount may well be as large as 60 Sv in the upper 2000 m, and as I will show in the following, this quantity is consistent with other pertinent information presently available.

3. WESTERN ARGENTINE BASIN

3.1. A byssal circulation

Before making any computations of transport in this region, it is first necessary to establish the probable mean circulation at abyssal depths; it is simply too ad hoc to select arbitrarily a level-of-no-motion, particularly near the western boundary where important barotropic components presumably exist. Unfortunately though, only a small number of deep current meter records are presently available from the western Argentine Basin, and some of these span time periods of only 2 or 3 weeks; additional information is thus required.

In the Argentine Basin there are waves of fine-grain sediments (mud waves) that require tens of thousands of years to develop and which have heights ranging from less than 10 m to more than 100 m, wavelengths of less than 1 km to about 10 km, and crests identifiable over distances much greater than their wavelengths (FLOOD, 1988). They are set up by the bottom currents, and because they take so long to develop they are reflective of the very long-term mean circulation at the bottom. Acoustic measurements of these features, together with photographs of sediments which indicate only the most recent bottom current activity, have been used by FLOOD and SHOR (1988) to give a summary of the bottom circulation in the Argentine Basin. Their patterns are shown in Fig. 3, along with the mean near-bottom velocities obtained from current meter observations spanning nearly a year by WEATHERLY (1991) and a month or less from the I N D O M E D 13 expedition of 1978 (J. L. REID, 1990, personal communication). Features to note are an anticyclonic circulation around the Zapiola Ridge, including a mean speed of nearly 10 cm s l at the northern end of the ridge, and consistent cyclonic flow around the boundary of the basin. These patterns will be used in the following.

3.2. Western boundary currents

The aim now is to use the distributions of water properties in the region of the Brazil Current to infer probable directions of flow, and to use the resulting transport rates together with those obtained from the alongshelf pressure gradients to solve for the total northward transport in the Falkland Current region as a residual quantity. In order to do


60°W 50 40

f ' ,

~ j,' ",

• / ' 7 j p i o l RiO'go

W E A T H E R L Y 11990) \ , " - . . .

/ N D O M E D 13, 1978 , .

10 c m s -1 ~ . , . , , ~ ,

t , / , I , < J Fig. 3. Abyssal circulation in the western Argentine Basin. Heavy arrows indicate only the direction of the mean flow as inferred from bottom sediments. Thin arrows indicate mean near-

bottom velocity vectors from the indicated sources.

this, the Brazil-Falkland Confluence Zone must be fully enclosed by full-depth stations, and they should be as synoptic as possible owing to the great temporal variability of circulation in the area. These requirements severely limit the amount of data which can be used; in fact, there is just one hydrographic survey in the historical database that is even marginally well-suited: Atlantis H cruise 107, leg III, made in the austral summer of 1979- 80 (Fig. 4) (GvERRERO et al., 1982). The hydrographic observations made from this cruise have previously been used by GORDON (1981), GREENGROVE (1986), GORDON and GREEN- GROVE (1986a), PIOLA and GORDON (1989) and PETERSON and WHITWORTH (1989) to discuss various aspects of mixing and circulation in the region.

Each of the seven principal water masses in the southwestern Atlantic can be identified in the oxygen field (refer to fig. 3 in PETERSON and WHITWORTH, 1989), and the oxygen distributions with depth for this At lan t i s / / cru i se have been shown by GREENGROVE (1986). It is worthwhile, however, to show them once again here (Fig. 5). In the north (38°S), there is an upper-level oxygen maximum near the western boundary associated with the AAIW. Farther north, in the region of the continental shelf near 23°S, EVANS and SIGNORINI (1985) used a Pegasus profiler to observe a northward flow of AAIW directly beneath the shallow, southward-flowing Brazil Current. But away from the western boundary the AAIW appears to move in the same direction as does the wind-driven geostrophic circulation at the surface (BuSCAGLIA, 1971; REID et al., 1977; REID, 1989). Where this

632 R.G. PETERSON

60°W

9 40os - -

45 ° [ J~S I ~ / V I / I I I I I l I T I

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o

o •

o 35 30 24 ~ I o 0 o 0 o ~ • o • o o o • o •

o o o ~eTO

o o o o o o

o o o o o

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4°° I I Fig. 4. Hydrographic stations occupied by Atlantis I1 during the austral summer of 1979-80. Solid dots represent full-depth stations, open dots represent stations made to 1000-1500 m depth

( G U E R R E R O e t al., 1982).

transition takes place is not particularly clear, but once the Brazil Current separates from the continental shelf south of the Vema Channel, where a recirculation cell fuels an intensification of the current, the AAIW beneath the Brazil Current as well as layers down through the N A D W all appear to move in the same southward direction. This is not inconsistent with the oxygen section at 38°S (Fig. 5); near the South American shelf is where the highest values (nearly 7 ml 1-1) within the AAIW are found, suggesting a southern source. But these high values do not continue seaward across the Brazil Current, indicating that the western boundary current of the northward-flowing AAIW is approximately limited to the east by the Brazil Current. At greater depth in this section, the highest values of oxygen associated with the NA D W are found near the western boundary, in turn suggesting that the NADW is flowing south as a deep western boundary current. However, at abyssal depths along the western margin of the Argentine Basin, the mean flow of the WSDW is toward the north (Fig. 3; also REID et al., 1977; G~ORaI, 1981; REID, 1989). For geostrophic computations along this section near the western margin, the shear profiles are adjusted so that the different layers each move as one would expect them to.

At 42%, the Brazil Current lies about 130 km farther offshore of the 500 m isobath than at 38°S, and the well-oxygenated core of NAD W is similarly displaced to the east by the appearance of a thick layer of CDW near the western boundary. The low oxygen values (<4 ml 1-1) near the shelf here, as well as at 46°S, have their source with the ACC farther south; it is expected that inshore of the cyclonic trough formed by the Falkland Current and its return all of the water is moving north, which is supported by observations that the SAF in Drake Passage is a bottom-reaching feature and by the abyssal flow patterns generally accepted for this region. Northward bottom flow must therefore be taken into

B o u n d a r y cur ren ts in the A r g e n t i n e Bas in 633

86 ~t , , , 6a ¥ , i , 10 . . . . 15 . . . . 20

~ 4 5 . ~ . + Z b • : • .

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- 3 8 o s i +1 : _ - o

48 , , , 44 , , 40 V , , 35 . . . . 30 . . . . . 24

3 \ - 4 . 8 +',' : - - --~;;'" - +" + - ~ + - : - ~ J + - " " - : . . . . . . . . . . . . . "' + + + : - . . . . . . . . . _ _ _ + . . . . . .

s 4 2 ° S ~ ~ ~ 2. \ . • - - - - - -

•

51~F ~AF 9 6 Y 9 0 Y 8 5 8 0 7 4

- 2

0 k m 5 0 0 1 0 0 0 ,I 1 ~ , I 1 , I , I I [ 1, ~ ,I [ , I , i 1 I ' i , ,i I , i I ' ~ ' ' I

§ O ° W 5 5 5 0 4 5

Fig. 5. Ver t ica l sec t ions of d issolved oxygen (ml l t) a long the zona l l ines of s ta t ions shown in Fig. 4. Pos i t ions of the Subanta rc t ic F ron t (SAF) and Brazi l Cu r r en t (BC) are accord ing to

PETERSON and WH+TWORTIJ (1989).

634 R.G. PETERSON

account when evaluating the transport of the Falkland Current. But in the absence of direct measurements, this cannot be done in a straightforward manner.

A useful feature of this Atlantis H data set is that it has bottom-reaching stations made at similar depths near the continental shelf along the three zonal lines (Fig. 6a). Because the bottom geostrophic flow must be parallel to the bathymetry in regions of large depth changes, the volume transports normal to the continental slope can be easily obtained by assuming reference levels at the bottom for station pairs oriented parallel to the bathymetry. Four such station pairs can be formed here (Fig. 6a), and when the transport calculations are made, they reveal an extremely vigorous flow toward the interior of the basin between 38 ° and 42°S (Fig. 6b): there is a total of 130 Sv crossing the 4600-m isobath between these two latitudes. Add another 13 Sv moving away from the shelf between 42 ° and 46°S, and the sum comes to 143 Sv, 20 Sv larger than the mean transport of the ACC (123 Sv) through the upper 2500 m of Drake Passage (WmTWORTH and PETERSON, 1985). This transport is much larger than would be expected from a review of the literature, and if it stands up it casts the importance of the Brazil-Falkland Confluence Zone into a new light with respect to the general circulation.

As described in Section 2, surface waters of subantarctic origin flow northward along the continental shelf to as far as 36°S, and as also pointed out earlier, the oxygen distributions at 38°S (Fig. 5) suggest that the well-oxygenated layer of AAIW inshore of the Brazil Current is similarly moving north. At greater depth along the slope, the high-oxygen core of NADW is most likely moving south, whereas at abyssal depths near the boundary the flow should again be toward the north. These inferred directions of flow are used for adjusting the profiles of geostrophic shear near the western boundary along this section. It should be noted that the most serious problem is encountered with Sta, 6, made just inshore of the core of the Brazil Current at a depth exceeding 3000 m but reaching a depth of just 1500 m. To fill this gap, the temperature and salinity sections (not shown) are contoured from the interior all the way to the slope, such as the oxygen section for 38°S in Fig. 5, and dynamic height information derived from them. Also, the inferred bottom velocities are used to estimate transports in the small triangular regions beneath the deepest common depths between stations over the strongly sloping bottom.

At 38°S, a southward transport of approximately 59 Sv is found to enter the northern enclosed region (Fig. 6b). As will be seen below, there is additional southward transport just offshore of Sta. 11 that brings the total poleward transport in the region of the Brazil Current to 68 Sv. Potential density surfaces lying between the relative vertical extrema in the oxygen field are used to define the vertical extents of the various water mass layers (Table 2). The contributions to the total southward transport are: 26 Sv from the surface (therrnocline) layer, 18 Sv from the AAIW, and 24 Sv from the deep waters of circumpolar and North Atlantic origin. From the same data, GORDON and GREEN6ROVE (1986a, their Fig. 3a) obtained 26 Sv relative to 1400 m, and this includes the thermocline and AAIW. But the total transport calculated here is comparable to the 76 Sv obtained by MCCARTNEY and ZEMBA (1988) at 37°S using a newer data set and reference levels similarly chosen on the basis of water mass characteristics.

The top-to-bottom transport in the region of the Falkland Current can now be estimated as a mass balance residual between 38 ° and 42°S inshore of the 4600-m isobath. The northward transport crossing 42°S comes up to be nearly 75 Sv, and by continuing this process southwards for depths less than 4600 m the total northward transport is nearly 88 Sv at 46°S (Fig. 6b). These are very large numbers when compared with estimates of the


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Fig. 6. (a) Full-depth stations made near the western boundary of the Argentine Basin (refer to Fig. 4). Heavy lines connect stations made in nearly equal water depths (shown in parentheses). (b) Total geostrophic transports (Sv) obtained as described in text. (c) Geostrophic velocities at the ocean bottom required to attain the transports shown in (b). (d) Geostrophic velocities at the sea

surface required to attain the transports shown in (b).


Table 2. Potential density surfaces used to define the water mass layers AAIW, Antarctic Intermediate Water; CDW, Circumpolar Deep Water; NADW, North Atlantic Deep Water; WSDW, Weddell

Sea Deep Water.

L a y e r U p p e r i sopycnal L o w e r i sopycna l

Sur face - - % = 27.0 kg m -3

A A I W o~ = 27.0 kg m - ; o 0 = 27.35 kg m -3

U p p e r C D W % = 27.35 kg m -3 o 2 = 36.9 kg m -3

N A D W / C D W 02 = 36.9 kg m -3 o 4 = 45.85 kg m -3

L o w e r C D W 04 = 45,85 kg m - 3 04 = 46.03 kg m -3

W S D W c~ 4 = 46.03 kg m 3 __

Falkland Current available in the literature, but they do have credibility when compared with what is known of the flow in the northern Drake Passage. As detailed earlier, the ACC in the northern Drake Passage should be able to supply something on the order of 60 Sv to the Falkland Current in the upper 2000 m. The transport estimates obtained here for the upper 2000 m of the Falkland Current are 60 Sv at 42°S and 70 Sv at 46°S, the latter probably indicating that a recirculation cell exists within the cyclonic trough between the Falkland Current and its return. The magnitudes expected and observed are very similar, and furthermore, supporting evidence for such a vigorous Falkland Current can be found elsewhere, albeit neither as abundant nor convincing as one would like.

Because isopycnal layers in the water column are lost as the slope is ascended, it is possible to assume some small bottom velocities at the greatest depths beneath the Falkland Current, stipulate that mass within isopycnal layers be conserved, and then proceed up the slope to obtain estimates of how much the individual profiles of geostrophic shear need to be adjusted to satisfy the transports inferred above. If the adjusted bottom velocities are additionally constrained to increase smoothly up the slope, and have a target value somewhere along the way, then the process of shifting the shear profiles becomes straightforward. The target value used here is taken from Fig. 3: INDOMED 13 current meter number 1 was situated at 91 m above the bottom in the far southwestern Argentine Basin where the water depth was 1524 m; the mean current speed there over a 10-day period was 17 cm s -~ in a nearly westward direction parallel to the bathymetry. The mooring was placed at a location where the SAF has been observed to traverse (PETERSO~ and WmxwoRxu, 1989), and the detided current velocities were essentially steady (J. L. REID, 1990, personal communication). Following the course of the SAF, it is expected that a high degree of continuity exists in the bottom speeds along particular isobaths, and this expectation is central in arriving at the adjusted bottom speeds at 42 ° and 46°S in Fig. 6c. At each of these latitudes, it is now seen that 45% of the total transport (34 and 40 Sv, respectively) is contained within the densities corresponding to the AAIW and surface layers given in Table 2.

Although the bottom velocity assumptions used here are based on an extremely limited amount of information, a positive aspect of the procedure is that it yields surface speeds (Fig. 6d) consistent with the (also meager) information about those. Figure 7 shows the trajectories of three undrogued, low-profile surface drifters deployed during the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE). These trajectories have also been discussed by GORDON and GREENGROVE (1986a). Here the


40"S

60"W

,+

5 0

s° I Fig. 7. Trajectories of low-profile, undrogued drifting buoys which passed through the Brazil- Falkland Confluence Zone during the FGGE period in 1979. The trajectories are smoothed with a 40-h low-pass filter. Time marks are at 4-day intervals. Bathymetry is at intervals of 1000 m, except

for the dotted line which is the 200-m isobath.

tracks have been fitted with a cubic spline and then smoothed with a 40-h low-pass filter to remove inertial and short-period tidal variability. In a study of trajectories from similar undrogued drifters through a current meter array at Drake Passage, PEa'ERSON (1985) estimated the steady nongeostrophic surface drift due to local winds (a combination of Ekman and Stokes drifts) as being on the order of 3.4% of the 10-m wind velocity deflected by - 2 5 ° cum sol to the wind. The wind-induced surface drifts for the present buoys are estimated by first interpolating the concurrent fields of surface wind stress from the F G G E atmospheric data set (twice daily on a 1.875 ° lat. × 1.875 ° long. grid) to the buoy positions. These stresses are then converted to 10-m wind velocity vectors using the standard stress- drag relation in which the drag coefficient is the neutral form given by SMITH (1980). Finally, the wind-induced surface drifts are obtained by attenuating and rotating the 10-m wind vectors according to the estimates given above. This procedure is the same as that described in greater detail by PETERSON and WHn'WORTn (1989), and as they pointed out, the drift estimates are crude in that errors in the stress fields, unsteadiness, boundary layer stability and atmospheric baroclinicity are not considered; however, these estimates do

638 R.G. PETERSON

indicate whether the local winds or the geostrophic currents, or both, are important in the observed motions of individual drifting buoys.

The lower row of panels in Fig, 8 show the estimated "geostrophic" speeds of the buoys, i.e. the speeds at which they would have traveled in the absence of wind-induced drift. For the two buoys that traveled along the core of the Falkland Current (17622 and 17649), their eastward motions were due, to a very large extent, to local winds. But the northward motions of these buoys were not; their "uncorrected" northward speeds, while in the core of the Falkland Current average about 40 cm s -~, and their "corrected" speeds remain similarly high. The largest adjusted surface speeds in the Falkland Current are 40-45 cm s- 1 (Fig. 6d), at roughly the 1000-2000 m isobaths, in good qualitative agreement with the buoy speeds. It might also be noted that within Drake Passage the maximum surface geostrophic speeds (relative to 2500 m) associated with the SAF approach 50 cm s 1 (fig. 14c in NOWLIN and CLIFFORD. 1982).

3.3. Interior circulation

Another check that can be made with regards to the credibility of the high transport rates obtained for the across-slope flow is whether they fit in with any realistic circulation pattern in the interior of the Argentine Basin. Here again, instead of choosing arbitrary reference levels, geostrophic profiles are adjusted on the basis of the abyssal flow patterns depicted in Fig. 3. In specifying an initial field of bottom velocities, it was desired that it be as simple as possible and yet retain the essential characteristics. The initial field resulted in surprisingly close mass balances in each of six isopycnal layers (Table 2) within the enclosed regions offshore of those which have been dealt with in the preceding, and with minor adjustments the field of bottom velocities in Fig. 9A was arrived at. The three vectors at 46°S between 52 ° and 54°W are required to obtain the proper anticyclonic circulation in an abyssal eddy reported on by GORDOn and GREENGROVE (1986b).

It was a simple matter to attain mass balances for the full-depth flow (Fig. 9B), and of course there are any number of possible solutions which would do the same. There are also any number of possible solutions that would yield a surface layer transport field (Fig. 9C) consistent with patterns previously obtained from this data set (i.e. GORDON, 1981; GORDOn and GREEN~ROVE, 1986a; PETERSON and WmTWORTI-t, 1989; STRA~MA and PETER- SON, 1990). (Note that the total southward transport in the region of the combined Brazil and Falkland Return currents across 42°S is 110 Sv in Fig. 9B.) But it is gratifying that each of the six layers balance out to within 3 Sv in each of the enclosed regions, and that the bottom velocities required for this are very much in line with what is known about the mean abyssal circulation in the Argentine Basin. The large transport rates obtained for the across-slope flow, and the strengths of the western boundary currents required to support them, are thus plausible in terms of the circulation in the interior of the basin.

4. DISCUSSION

In this paper a case is built around the idea that the western boundary currents in the Argentine Basin, particularly the Falkland Current, are significantly more robust than has been generally recognized. The total southward geostrophic flow in the region of the southern Brazil Current has recently been shown to be as large as 76 Sv by MCCARTNEY and ZEMBA (1988), and the estimate of 68 Sv obtained here is consistent with their findings.

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Net imbalances are shown in the centers of the enclosed regions.

These numbers come from adjusting vertical profiles of geostrophic shear on the basis of water mass characteristics which suggest that reversals with depth in the direction of flow occur in the region. The Brazil Current is highly baroclinic.

Unlike the Brazil Current, the Falkland Current is strongly barotropic; there is much less vertical shear in its velocity structure, therefore making the absolute transport of this current far more difficult to assess. But when one looks at the amount of water flowing seaward from the continental slope in the Brazil-Falkland Confluence Zone, 143 Sv altogether across the 4600-m isobath between 38 ° and 46°S, it becomes immediately obvious that our longstanding concepts about the Falkland Current being relatively weak need to be reappraised. Essential elements pointing toward it being one of the more vigorous currents in the world are the transport of the portion of the ACC in the northern Drake Passage which feeds the Falkland Current, the strong likelihood that significant bot tom flow exists in the current along a narrow continental slope, and surface velocities measured with drifting buoys that are more than twice as large as what geostrophic shears alone relative to the bottom can reveal.

But just how robust is the Falkland Current? The numbers I get, as residuals in balancing mass transports, indicate that a total northward flow of 75 Sv crosses 42°S, while 88 Sv


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crosses 46°S. For the upper 2000 m, which might be regarded as being the Falkland Current proper , the transports are 60 and 70 Sv, respectively. These values are far larger than the estimates of 10-20 Sv typically found in the literature, but they are nonetheless in concordance with the small amount of available information that can be used to test them.

A potential source for error are the across-slope transports upon which my estimates for the Falkland Current so heavily depend. The distances between stations parallel to the bathymetry, from which the across-slope transports are obtained, range from 460 to 560 km, so an error of just 1 cm s 1 for the station pairs along the 4600-m isobath can mean a total transport error of up to 25 Sv. But this is probably not as important as it might at first seem. For the station pair with the greatest separating distance and total transport (11,41), there is no vertical shear in the deepest 200 m of the water column, and a 1 cm s- i geostrophic speed relative to the bot tom is not attained until another 1200 m are ascended. The upper levels are where nearly all the vertical shear is found. The assumption of zero bot tom flow normal to the bathymetry is likely a very good one, subject to errors quite a bit smaller than the signal being sought.

Another potential source for error is non-synopticity of the data set. The time interval between Stas 11 and 41, for example, was about 8 days, a period long enough for the spatial flow structure to have changed; this is of course a problem with any hydrographic data set.

642 R.G. PETERSON

But at any one par t icular t ime in the s u m m e r season, one could be almost certain that a s tat ion with similar t empera tu re and salinity profiles as n u m b e r 11 could be made at about

4500-m depth in the region of the sou thern Brazil Cur ren t while ano the r stat ion very similar to n u m b e r 41 could be made at the same depth within the no r the rn end of the cyclonic t rough descr ibed by the retroflecting SAF. The resul t ing t ranspor t rates would thus be about the same as I have ob ta ined here. The alongshelf pressure gradients must be t aken into account if any t ranspor t est imates in the region are to be justified.

A discomfort ing aspect about this study is that there is so little i n d e p e n d e n t in format ion available by which to verify, or disclaim, these new t ranspor t est imates of the Falk land Curren t . This paper should thus be seen as having as much to do with raising quest ions as it does with answer ing them. The absolute t ranspor t of the Fa lk land Cur ren t may well be smaller than what appears with this analysis, but there are very good reasons for th inking that this cur ren t is quite a lot more robust than has been general ly thought , which in turn raises impor t an t quest ions regarding the role of this current in the general circulation. Specifically, how much water ( in te rmedia te and deep) is lost by the Antarc t ic Ci rcumpolar Cur ren t along the western bounda ry of the South At lant ic , and what does this mean for the heat and salt ba lances of the basin? In order to say, we first need to know with quant i ta t ive certainty what the ful l-depth, absolute circulation in the southwestern South At lant ic looks like.

Acknowledgements--This work has been supported by the Deutsche Forschungegemeinschaft Grant Si 111/34-1. I would like to thank Lothar Stramma for his assistance with data sets in the early phase of this study, and Joe Reid and Georges Weatherly for kindly providing me with their deep current meter measurements.

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