Antarctic Bottom Water Flux in the Equatorial Western ...€¦ · Antarctic Bottom Water into a...

SEPTEMBER 1997 1903H A L L E T A L .

q 1997 American Meteorological Society

Antarctic Bottom Water Flux in the Equatorial Western Atlantic*

MELINDA M. HALL, MICHAEL MCCARTNEY, AND J. A. WHITEHEAD

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

(Manuscript received 7 October 1996, in final form 18 February 1997)

ABSTRACT

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssalcurrents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from theBrazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv [ 106 m3 s21), comprisingorganized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated bya quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transportsoccur in September–October, while minimum transports occur in February–March. Allowing for this quasi-annual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated meanAABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water(LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speedthrough the array, 9.9 cm s21 into the Brazil Basin. The LNADW records also have a quasi-annual cycle withstrong LNADW flow episodes occurring in April–May. Time series of temperature indicate that the LNADW/AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (upliftedtransition layer during strong AABW transport periods). An observed overall warming trend appears to beaccompanied by a decline in AABW transport.

1. Introduction

The World Ocean Circulation Experiment (WOCE)Deep Basin Experiment (DBE) was conceived as a processstudy to investigate important aspects of deep and bottomwater flow (Hogg et al. 1996). This large internationallycoordinated project was designed to observe and quantifythe deep and bottom water circulation within an abyssalocean basin, including making the measurements neces-sary to provide a bottom water mass balance, distinguish-ing between boundary and interior mixing processes, un-derstanding how passages affect the flow of water throughthem, and studying the means by which deep water crossesthe equator. The Brazil Basin, Fig. 1a, was chosen as thesite for the DBE because the basin geometry and bathym-etry are simple, because transport estimates indicate sub-stantial net northward flow of bottom water and net south-ward flow of deep water through the basin, and becausethe bathymetry particularly constrains the inflows and out-flows of bottom water to small fractions of the basin’sboundary thereby making direct measurement practical.

The shoals of the Rio Grande Rise define the sepa-

* Woods Hole Oceanographic Institution Contribution Number9376.

Corresponding author address: Dr. Melinda M. Hall, Departmentof Physical Oceanography, Woods Hole Oceanographic Institution,Woods Hole, MA 02543.

ration of the Brazil Basin from the immediate source ofAntarctic Bottom Water (AABW) in the Argentine Ba-sin to the south. That rise confines the northward flowof AABW to two pathways: to the west, AABW flowis concentrated at the Vema Channel (Hogg et al. 1982)but also extends over the deeper part of the continentalslope (Speer and Zenk 1993); to the east, northwardtransport through the Hunter Channel has been detected(Speer et al. 1992). The combination of these inflowsis estimated as 6.7 Sv (Sv [ 106 m3 s21) (Speer andZenk 1993). In the north AABW leaves the basin bytwo routes: eastward through the Romanche FractureZone into the eastern Atlantic (Metcalf et al. 1964; Pol-zin et al. 1996; and Mercier et al. 1994) and northwardacross the equator into the western North Atlantic(Whitehead and Worthington 1982; McCartney and Cur-ry 1993). Estimates of these combined outflows rangefrom 3 to 6 Sv. In the DBE each of these inflow/outflowsites has been the focus of concentrated measurements,directed to both specific local issues and to the overallgoal of constraining the mass balance for AABW withinthe Brazil Basin during the DBE.

Our involvement in the DBE centers on a current meterarray and supportive hydrographic measurements in theflow of AABW from the Brazil Basin into the westernNorth Atlantic. At first glance (Fig. 1a), a direct mea-surement of AABW flow at the equator seems a toughdesign problem, for though the Mid-Atlantic Ridge is rel-atively close to the western boundary at the equator, theexit site is by no means a narrow channel. But the AABW

1904 VOLUME 27J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

FIG. 1a. Setting for northward flow of Antarctic Bottom Water through the western basin. The principalbasins and channel connecting them, some blocking rises, and two fracture zones that allow eastwardabyssal flow through the Mid-Atlantic Ridge all are delineated by the shading for depths less than 4000m (bathymetry from the ‘‘Etopo5’’ database, National Geophysical Data Center 1988). The oval indicatesthe confined passage for cross-equatorial flow from the Brazil Basin to the Guiana Basin, with themeridional bar denoting the moored array across that passage; details are shown in Fig. 1c.

is a fairly thin layer at the equator (Fig. 1b), and artifactsof the regional bathymetry confine it to a zonally orientedchannel-like passage of only about 300-km width. Spe-cifically, the Parnaiba Ridge projects eastward from thewestern boundary near 18S, with its crest between 3500and 4000 m, defining the southern channel wall, and thewestward extension from the Mid-Atlantic Ridge of theSt. Paul Fracture Zone defines the north wall, also crestingabove 4000 m (Fig. 1c). The abyssal channel (with depth;4500 m) is aligned along the equator, and AABW fromthe Brazil Basin to the south enters the eastern end of thechannel from eastward of the terminus of the ParnaibaRidge. The AABW is constrained by the channel to flowwestward along the equator and exits northward from thechannel westward of 388–408W, the terminus of the St.

Paul Fracture Zone. Thereafter, it flows over the CearaAbyssal Plain and begins its northward descent into theGuiana Basin at about 48N.

In addition to providing an estimate of the outflowfrom the Brazil Basin, the equatorial moored array pro-vides an estimate of the inflow to the Guiana Basin.Earlier estimates ranged from 1–4 Sv: More accuratequantification of the AABW transport is important forquantifying the meridional overturning cell of the NorthAtlantic, as AABW is a major contributor to the lowerNorth Atlantic Deep Water (LNADW), which dominatesthe cold water export from the North Atlantic by thiscell (Luyten et al. 1993).

The moored array design was based on the propertyfields associated with the 1983 and 1987 sections near


FIG. 1b. Potential temperature sections across the abyssal equatorial passage that channels the cross-equatorial flow ofAntarctic Bottom Water into a westward flow along the equator. From McCartney and Curry (1993), data taken in 1983 (left)and 1989 (right), locations about 50–100 km west of the moored current meter array, Fig. 1c.

378W (Fig. 1b). We took advantage of the narrownessof the equatorial channel and the thinness of the AABWlayer to rather densely instrument the AABW, but ve-locity was also measured in the overlying LNADW toexplore its transport distribution and to measure theshear between LNADW and AABW for comparison togeostrophic shear estimates near and at the equator.Mooring positions are included on Fig. 1c, and Table 1summarizes deployment and recovery statistics and datareturns; the instruments worked well and together pro-vide a roughly 20-month-long sampling of the AABWand adjacent flow elements. In section 2 the mooredmeasurements of velocity and temperature are de-scribed, as well as the hydrographic sections collectedat three times: during deployment, about one month

before recovery, and during recovery. Section 3 presentsthe methodology for calculating time series of AABWtransport through the array and describes the results ofthese calculations. Section 4 discusses the results in thecontext of western basin deep circulation, and also notessome evidence for long-term evolution of the AABWdistribution at the equator. Section 5 summarizes themain results.

2. The experiment: Hydrography and mooredarray results

a. Experimental design and data return

In October 1992, six moorings were deployed fromthe R/V Iselin near 368W on a meridional line from


FIG. 1c. Detailed setting of the equatorial passage (bathymetry from the ‘‘Etopo5’’ database; National Geophysical Data Center 1988),showing the placement of the six moorings of this experiment (Table 1) and locations of other data and earlier experiments. We refer to themoorings with numbers 1 (southern most) through 6 (northernmost).

TABLE 1. Current meter mooring durations and locations. For clarity, the moorings are referred to with numbers 1–6, 1 being thesouthernmost and 6 the northernmost.

Mooringnumber

WHOImooringnumber Date set Date received Latitude Longitude

Bottomdepth(m)

654321

936937938939940941

28 Sep 199228 Sep 199229 Sep 199230 Sep 19921 Oct 19922 Oct 1992

2 Jun 19941 Jun 19941 Jun 1994

31 May 199431 May 19943 Jun 1994

00850.169N00830.109N00800.289S00830.059S00858.809S01819.909S

35854.029W35854.179W35859.469W35854.069W35855.959W36804.949W

448645394536448543964450


1.08N to 1.58S, which was a location about 18 east ofthe sections shown on Fig. 1b. Figure 2 shows themoored instrument placements (Table 1) superimposedon sections of potential temperature acquired during de-ployment and recovery cruises. In the hydrographic dataavailable during the design phase (Fig. 1b), the coldestobserved AABW was twice found right on the equator,and one realization of the temperature distribution ex-hibited an abyssal concavity suggestive of the equatorialgeostrophic expression of a westward flow along thechannel centered on the equator. But rather than tuningthe array design to an equatorially aligned flow, weopted for even sampling across the breadth of the chan-nel, resulting in a mooring spacing of 30 n mi. This wasthought to be an array closely spaced enough to ensuresampling the AABW flow wherever in the channel itmight be found—provided the flow filaments were nottoo narrow. Spanning the full channel turned out to beprudent, for the observed AABW flow was concentratedsouth of the equator: that is, it retains its character asa western boundary current even at these low latitudes.The spacing of 30 n mi turns out to be marginal forresolving the horizontal distribution of AABW flow.

On all the moorings we placed instruments at threecommon nominal levels, 3900, 4100, and 4300 m (po-tential temperature u ø 1.948, 1.438, and 0.798C, re-spectively), to obtain measurements in the AABW, theoverlying LNADW, and the transition layer in-between.Because mean flow at the middle level is primarily west-ward (and u & 1.58C), we also refer to water at thislevel as the warmer AABW. At the time of the arraydesign, ongoing work suggested that while the AABWflow would be westward in the channel, flow in theLNADW would reverse to eastward, at least in thesouthern part of the channel (McCartney 1993; Fried-richs et al. 1994). The selection of these three levelswas to measure the absolute velocity field of this layeredand vertically sheared flow. Additional instruments wereplaced on the equatorial mooring: near the bottom (4485m, u ø 0.68C), to measure the AABW flow more com-pletely should the velocity maximum be there; and atthree additional levels in the LNADW, 3600, 3300, and3000 m (u ø 2.18, 2.28, and 2.48C), to explore the ve-locity associated with thin layered LNADW structure(McCartney 1993). For additional guidance regardingthe LNADW circulation and geostrophic reference levelissues, the 18N and 18S moorings were instrumented at3300 m (u ø 2.28C). All of these 24 instruments werevector averaging current meters (VACMs) provided bythe subsurface mooring operations group at the WoodsHole Oceanographic Institution. F. Schott of IFM Kieladded to the array four Aanderaa current meters at nom-inal depths 1900, 1700, 1500, and 1300 m (u ø 3.58,3.98, 4.18, and 4.38C) on the equatorial mooring forexploration of the deep equatorial jets of this region(Ponte et al. 1990); these shallow data are described byFischer and Schott (1997).

The moorings were launched from the R/V Iselin in

September–October 1992, accompanied by a conduc-tivity–temperature–depth (CTD) section. Three changesoccurred to planned mooring positions at the time ofdeployment. The equatorial mooring ended up 5 mileswest of the planned site due to a swift surface currentaffecting the deployment. The mooring at 18S wasmoved slightly due to a bump in the sea floor, with thelocation chosen such that the instruments would be atthe same depths as those on the other moorings. Finally,the southernmost mooring was repositioned to be out-side Brazilian waters, as the government of Brazil didnot grant clearance to deploy within their waters. Afterapproximately 20 months, in June 1994, the mooringswere recovered on a cruise aboard the R/V Knorr andanother CTD section was made. For both sections, sta-tions were made at and between mooring locations, fora total of 11 stations each time. In addition, an extensiveDBE hydrographic cruise was made from the R/V Knorrpreceding our recovery cruise, and Bill Smethie in-cluded in that work a third CTD section along the moor-ing line, this one with a full suite of tracer measure-ments, completed about 30 days before our recoverywork. The details of the deployment and recovery sec-tions are summarized in Table 2.

The data return from the VACMs (Table 3) was re-markable: with two exceptions each instrument col-lected over 600 days of continuous time series of u(t),y(t), and T(t). On the 3300-m instrument at 18S the vanestuck after 262 days but, since that is well above thetop of the bottom water, it does not hamper the transportcalculations. The other data loss was a 10-day gap inthe middle of the 4300-m velocities and temperature atthe southernmost mooring. For the purposes of this in-vestigation, we have linearly interpolated the three timeseries through the gap, starting with the low-passed dailysubsampled time series that have been used in all thecalculations. Additional details regarding the data maybe found in Tarbell et al. (1997). Time series plots ofvector velocity and temperature for the three primarysampling levels are shown in Fig. 3. In the vector plots,east is up.

The velocity time series recorded at the 18S mooringat 4300 m differs in character from time series to thenorth and south (Fig. 3c). The mean speed is low com-pared to means at the 4300-m level at 0.58S and 1.58S(Rhein et al. 1995; Rhein et al. 1996) and seeminglyinconsistent with the strong deep shear implied by thethermal wind balance at this longitude during Smethie’shydrographic section, Fig. 2b (at 18S). But the onlyanomaly found in the instrument records is a note thatthe instrument had an overheated electronics odor onopening after recovery. The peculiar character of thespeed record was not noted until after the instrumentwas redeployed for another experiment, so it has notbeen reexamined. Recall, however, that this instrumentwas in the wake of an abyssal shoal: as sited, this moor-ing still was on the edge of this feature, and the bottomcurrent meter is closer to the sea floor than the bottom


FIG. 2. Deep potential temperature for three hydrographic sections near the location line of the moored array. Dots show the mooredinstruments, and triangles along the abscissa show the station locations for the contoured temperature field.

instruments at the adjacent moorings. We have retainedthis record in our analysis and will comment briefly lateron how this record might affect our overall AABWtransport estimate.

b. Mean currents in the abyssal equatorial channel

As a first look at the data, Fig. 4a shows the record-mean current vectors superimposed on a map of thebathymetry around the equatorial array. Moorings willbe referred to by numbers 1–6 with 1 being the south-ernmost and 6 the northernmost. Maps are shown foreach of the three primary sampling levels. The strongflows are confined to the south of the equator, as antic-ipated for the LNADW, but not as anticipated for theAABW. However, the southern intensification patternobserved at the 3900-m and 4300-m levels is similar tothe three Pegasus current profile snapshots 18 east ofour array by Rhein et al. (1995), one taken in late Oc-tober 1992 while our array was in the water. As ex-pected, Fig. 4a shows net westward flow of AABW atthe deepest level and net eastward flow of LNADW atthe shallowest level. At the level in between, which canbe considered the transition layer from AABW toLNADW, there is also a net westward flow, with east-ward mean flow only at the southernmost instrument.

This vertical shear (eastward speed decreasing down-ward or equivalently westward speed increasing down-ward) is consistent with thermal wind and a southwardrise of the transition layer between LNADW and AABWsouth of the equator, as shown in Figs. 2b and 2c. At0.58 and 1.08S, speeds in the 4100-m layer are slightlylarger than at the deepest level, indicative of a slightshear reversal near the bottom of the transition layer.

At the 3900-m LNADW level (Fig. 4a, top panel),the impression is that of a southern boundary layer withmaximum eastward speeds at the southern mooring,which decrease by 45% 30 n mi to the north, then changeto northwestward flow between there and 60 n mi, andto northward flow on the equator at mooring 4. Withthe 3900-m contour of the Parnaiba Ridge falling about24 n mi south of mooring 1, this eastward flowingboundary layer of LNADW appears to be about 80–90 n mi wide. The net transport at 3900 m across the4 northern moorings is westward and about 15% thesize of the eastward transport in the southern boundarylayer. This may reflect a degree of recirculation ofLNADW, which is a partial retroflection of the eastwardflow in the southern boundary layer. This has been in-ferred from hydrographic measurements (McCartney1993, Friedrichs et al. 1994) but was not conclusivelyevident from a few Pegasus measurements (Rhein et al.


FIG. 2. (Continued)

TABLE 2. CTD stations occupied during the deployment andrecovery cruises.

Sta-tion Latitude Longitude

Date1992

Moor-ing

Stationdepth(m)

45678

0851.149N0830.729N0841.049N080.349N0815.849N

35853.869W35854.399W35852.799W35854.249W35854.589W

28 Sep28 Sep29 Sep29 Sep30 Sep

65

4

44634525452345254527

91011121314

0815.039S0830.089S0844.749S0859.829S1814.779S1819.149S

35853.799W35855.139W35854.599W35853.519W35854.429W3684.329W

30 Sep30 Sep30 Sep1 Oct1 Oct1 Oct

3

2

1

451544904463428144244441

1994

12345

1837.289S1819.579S0858.819S0829.919S0814.869S

36802.339W36855.299W35855.969W35853.959W35859.709W

30 May31 May31 May31 May1 Jun

123

42054439438144704494

6789

1011

080.029W0815.299N0829.949N0845.179N0850.269N0844.939S

35855.179W35859.759W35854.179W35859.549W35850.629W35854.069W

1 Jun1 Jun1 Jun2 Jun2 Jun2 Jun

4

5

6

451745194525451944684451

1995). At the 4100-m level (Fig. 4a, middle panel), thereis much weaker eastward flow in the south that givesway to a westward flowing jet centered south of theequator. At the two northern moorings, mean flow isweak. At the AABW level (4300 m; Fig. 4a, lowerpanel) westward flow is found at all three of the moor-ings south of the equator as well as at the mooring onthe equator. Because of the low mean speed at mooring2, the westward flow amplitude varies across the south-ern part of the array. North of the equator, amplitude isvery small.

In summary, the mean fields show the southward in-tensified eastward flow of LNADW in a southern bound-ary layer, with a footprint extending down into the tran-sition layer at the southernmost mooring. There is atransition to distinct westward flow north of the southernboundary layer in both the LNADW and the transitionlayers. The westward AABW flow appears to be broad,but mostly south of the equator. At all three depths,mean flows at the two northern moorings are weak.

c. Time variability of the currents in the abyssalequatorial channel

Next we consider aspects of the time variability inthe moored records. How well defined are the meanvelocity vectors and the temperatures? Figure 5 shows

progressive vector diagrams for each of the three pri-mary sampling levels, plotted to maximize the displace-ment scale. At locations where the magnitude of themean velocity exceeds 4 cm s21, mean kinetic energy(KM) exceeds eddy kinetic energy (KE) (Table 3), andthe corresponding progressive vector diagrams show aquasi-steady situation where displacement accumulatesrather steadily in one general direction with small veer-ing. In fact, this is the case for roughly half the instru-ments, even where KE . KM. In the northern half of thearray, however, mean flows are weak and are dominatedby eddies (moorings 5 and 6 in Fig. 5).

Like most moored records around the world, our datahave considerable variability over a broad range of pe-riods (Figs. 3a–c). When low-pass filtered (40 days) tosuppress the higher frequencies, a quasi-annual signalis the strongest signal, with strong AABW speeds inSeptember–October and weak AABW speeds in Feb-ruary–March. The LNADW flow in the south tends to-ward the reverse cycle, but with its strongest eastwardflow peaking somewhat later than the minimum west-ward AABW flow. Figures 4b,c,d show the mean vec-tors at these times during strong westward AABW flow(at the beginning of the experiment and a year later);during weak westward AABW flow (about six monthsinto the experiment and a year later), and during strongeastward LNADW flow (about seven months into theexperiment and a year later). Here we describe the fieldof current vectors during these three distinct flow re-gimes. The implications of the quasi-annual signals forAABW transport are discussed in section 3.


TABLE 3. Statistics of the current meter records. The southernmost mooring is number 1 and the northernmost is 6. Overbars denote arecord-length mean. KM is mean kinetic energy, (u2 1 2)/2; KE is eddy kinetic energy, (u92 1 y92)/2.y

Mooringnumber

Depth(m)

u(cm s21)

y(cm s21)

T(8C)

u92

(cm2 s22)y92

(cm2 s22)(T92)1/2

(8C)u9y9

(cm s21)KM

(cm2 s22)KE

(cm2 s22)

6 3297389640964297

21.140.75

20.120.35

20.860.46

20.050.07

2.5472.2781.8121.135

10.608.844.142.65

11.693.771.390.71

0.01860.02820.12660.0446

2.862.75

20.7320.04

1.010.390.010.06

11.146.302.771.68

5 390041004300

20.4020.13

0.02

0.8920.11

0.14

2.2801.7691.116

12.568.926.65

7.964.033.23

0.02320.13440.0390

6.012.162.70

0.480.010.01

10.266.484.94

4 2993329335933892

20.9322.0222.1220.22

20.310.150.272.41

2.7242.5572.4322.292

12.269.409.344.69

9.656.422.745.89

0.02750.02230.02450.0257

0.6020.6820.11

1.96

0.482.052.272.93

10.967.916.045.29

409342924485

21.7820.6421.31

0.460.040.46

1.8001.1260.929

6.055.493.16

3.372.531.99

0.13560.03630.0128

21.7421.5020.29

1.680.210.96

4.714.012.57

3 388540854285

21.5425.5324.22

1.560.720.32

2.3031.8511.147

9.2824.8610.97

7.304.542.73

0.01870.12680.0409

1.400.17

21.16

2.3915.55

8.96

8.2914.69

6.84

2 3307390741074307

1.335.39

21.9121.35

0.280.890.980.02

2.5352.2781.6691.124

4.0225.4913.74

6.92

2.487.76

12.148.50

0.01860.02350.10290.0353

0.816.304.834.41

0.9214.92

2.300.91

3.2516.6312.94

7.71

1 389940994300

9.901.48

24.88

20.650.86

20.42

2.2471.6851.192

19.758.43

12.70

6.686.842.50

0.04550.11150.0367

1.972.732.38

49.171.46

12.00

13.227.637.60

During the two periods of strong westward AABWtransport, the vectors at moorings 1, 2, and 3 that dom-inate the flow at 4300 m are 20% to 50% greater thanthe record means (Figs. 3c and 4b, bottom panel). Atthe transitional level of 4100 m, the flow is more di-vergent during the strong westward AABW transportperiods than on average (Fig. 4b, middle panel). In par-ticular, westward velocities at moorings 2 and 3 haveaccelerated by about 70%, but at mooring 4, on theequator, the velocity is nearly northward, rather than itsrecord-average direction of nearly due westward, thoughwith little change of speed. This northward flow appearsto turn east at mooring 5, north of the equator, wherea modest eastward flow develops. At mooring 1, southof the westward jet, the eastward flow observed in themean is intensified during the two periods of strongwestward AABW transport (Fig. 4b, middle panel). Incontrast, the eastward flow of the southern boundarylayer in the overlying LNADW at 3900 m is substan-tially reduced at moorings 1 and 2 (Fig. 4b, top panel),while net westward transport across moorings 3–6 in-creases and is 45% as large as the southern eastwardflow. In summary, during the two periods of strong west-ward AABW transport, the westward flow of both colderAABW (4300 m) and warmer AABW in the transitionlayer (4100 m) is greater; but at 4100 m, the eastwardrecirculation in the south has actually strengthened, andweak eastward flow has also developed north of theequator. The eastward flow of LNADW in the southernboundary layer (moorings 1 and 2) is slower when the

westward transport of AABW is strong, but north of theboundary layer the northwestward and northward flowsof LNADW are little changed.

The two periods of weak westward AABW flow showa greatly diminished flow amplitude at 4300 m (Fig. 4c,bottom panel), in the colder part of the AABW. At 4100m, in the warmer part of the AABW (Fig. 4c, middlepanel), the westward jet present at moorings 2, 3, and4 in the mean field has diminished greatly. At the sametime, the eastward flow in the southern boundary layerhas broadened to mooring 2 and has strengthened rel-ative to the mean situation. This southern recirculationof warmer AABW is twice as large the weakened west-ward flow at moorings 3 and 4. At 3900 m during thisperiod of weaker AABW flow at depth, the eastwardflow of LNADW in the southern boundary layer hasaccelerated (Fig. 4c, top panel), and the net flow ofLNADW offshore reverses to weakly eastward, withdistinct northward LNADW flow at moorings 3, 4, and 5.

Shortly after the two periods of weakest westwardAABW flow are two periods of strongest eastward flowof LNADW (Figs. 3a,b,c). Accompanying the pro-nounced acceleration of eastward speeds at 3900 m (Fig.4d, top panel) at the southern two moorings, there is anenhancement at the next two moorings to the northwardflow that exists to some degree in all these 3900-mvector plots. However, net transport is again very weak-ly eastward. The two periods of strong LNADW trans-port in the southern boundary layer at 3900 m exhibitpattern shifts in the flows at the other two levels. At


4100 m (Fig. 4d, middle panel), the warmer AABW inthe transition layer still shows a jetlike structure, butthe jet’s center is displaced northward, from mooring 3to mooring 4 on the equator. Curiously, the eastwardrecirculation of warmer AABW (4100 m) at the south-ernmost mooring is weaker in the strong LNADW trans-port periods than in the record mean and the other pe-riods illustrated in Fig. 4. At 4300 m (Fig. 4d, lowerpanel), the westward flow of AABW slows down some-what at moorings 1, 2, and 3, and expands northwardto include moorings 4 and 5 at and north of the equator.Thus, the two periods of strong eastward flow ofLNADW in the southern boundary layer are character-ized by an enhanced northward LNADW flow outsidethat boundary layer, a northward displacement of thecentroid of westward flow of AABW at 4100 and 4300m, and a near disappearance of warmer AABW recir-culation southward of the displaced AABW jet at 4100 m.

d. The deep temperature field

On both the deployment and recovery cruises, CTDsections were occupied across the array, with stationsnominally at and between mooring locations, for a totalof 11 stations in each section. An additional section wasmade a few weeks before recovery. Twenty-four watersamples were collected at each station and analyzed forsalinity to calibrate the conductivity sensor. The esti-mated accuracy of the data is 6.0018C for temperatureand 6.003 psu for salinity.

Figure 2 shows u below 3000 m for all three sections.The 1.88C potential isotherm is at the top of the stronglystratified transition layer between cold AABW andsomewhat warmer LNADW, both of which are moreweakly stratified. This layer of stronger vertical densitygradient is extensive in the tropical and subtropicalNorth and South Atlantic. At extra-equatorial locations,it is the tilt of this transition layer that dominates thegeostrophic shear between the two water masses and,with suitable reference level inference, was used byWright (1970) to produce the first comprehensive treat-ment of the western basin AABW transport distribution.While direct usage of geostrophy within 1.58 latitude ofthe equator is a delicate matter, which will not be ex-plored here, note that the current vector plots of Figs.3 and 4 do show considerable shear across this layer,especially at the southern mooring between the pro-nounced eastward LNADW flow at 3900 m and thepronounced westward AABW flow at 4300 m. In the604-day mean there is nearly a 15 cm s21 differenceacross 400 m. Vertical movement of this transition layerleads to stronger temperature variability at 4100 m,where vertical temperature gradients are relatively high,compared to the variability at 3900 and 4300 m (Table3). This is illustrated in Fig. 6, which shows time seriesof T at the seven depths on the equatorial mooring.(From deep pressure measurements, we estimate thatthe effect of mooring motion on temperature measure-

ments is more than an order of magnitude smaller thanthe natural variability.) This figure also suggests thatthere is a long-term warming at 4100 m over the20-month deployment as well as the same quasi-annualcycle observed in the velocities. In fact, though notdiscernible by eye in Fig. 6, similar (but weaker) fea-tures also occur at 4300 m. Both trends occur at theselevels throughout the array. Thermistors at and belowthe 4100-m level have a 10-yr calibration history, withaverage drifts of 0.1 to 0.7 millidegrees per year. Incontrast, least squares linear fits to the warming trends,after removing the seasonal signal, yield warming rateson the order of 0.0988C/yr at 4100 m, and 0.0168C/yrat 4300 m, about two orders of magnitude greater thanthe sensor drift rates. The difference in magnitude ofthe trends at the two depths (about 6 to 1) is the sameorder as the difference in strength of the vertical tem-perature gradient at the two levels (about 4 to 1); hence,the warming may also be interpreted as a deepening ofthe interface between LNADW and AABW. From thebeginning to the end of the experiment, this deepeningwould be about 40 m at 4100 m and 25 m at 4300 m.

Differences in the deployment and recovery hydro-graphic sections are consistent with the observed tem-perature variability measured by the VACMs. The re-covery section (Fig. 2c) was made during the warmphase of the seasonal cycle: notice that the pool of watercolder than 0.68C is smaller in May 1994 than in October1992. Long-term variability is discussed in greater detailin section 4. The coldest measured potential temperaturealso warmed from 0.5828C at 08159S in 1992 to 0.5958Con the equator in 1994. Figure 7 shows the averagevertical u profiles for 1992 and 1994. In 1994, the ver-tical potential temperature profile below 28C has de-scended about 80 m and also flattened out from thesomewhat curved nature of the 1992 profile (Fig. 7).The descent is larger than that estimated for the long-term trend alone because the peak-to-peak amplitude ofthe seasonal cycle is roughly the same size as the trend.In order to facilitate the transport calculations forAABW in section 3, the average temperature gradientsdu/dz have been calculated from least squares linear fitsto the high gradient portions of the u(z) curves in Fig.7. These slopes are 24.03 3 10238C m21 in 1992 (eval-uated over a depth range of 3950 to 4200 m) and 23.563 10238C m21 in 1994 (over 3950 to 4275 m). We haveconverted the temperature time series to potential tem-perature using the mean pressure of each instrument andthe CTD data as a guide to the departure of the likelyactual salinity from a mean value. The method is de-scribed in appendix A.

3. Transport calculation

As noted in the introduction, a primary purpose ofthe current meter deployment was to obtain an accuratemeasurement of AABW flow leaving the Brazil Basinthrough this passage. First we had to choose a definition


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FIG. 5. Progressive vector diagrams for currents at (a) 3900 m; (b) 4100 m; (c) 4300 m and4485 m (circles), from mooring 4. Asterisks mark the mooring locations; crosses are placed at100-day increments.

for this water mass. One could simply use a depth suchas 4000 m, which is roughly in the transition layer be-tween AABW and NADW, but most investigators haveused a potential isotherm to identify the top of the bot-tom water. For this transport calculation, the potentialisotherm u 5 1.88C has been used to define the top ofthe AABW layer. This isotherm is near the top of thestrong vertical temperature gradient where water prop-erties change from NADW properties to AABW prop-erties. This choice is also consistent with recent choicesby others (Molinari et al. 1992; Friedrichs and Hall1993), although it does differ from the choice of 1.98Cby Whitehead and Worthington (1982). Hence, alter-native definitions are also presented. It will be shownthat transport is not overly sensitive to the definitionused. Without a continuous representation of either theflow field or the temperature field, the use of the 1.88Cisotherms involves, in essence, a double interpolationamongst the moored instruments. Details of the trans-port calculation are presented in appendix B.

Table 4 lists mean transports and standard deviations

for four definitions of AABW: u 5 1.88C; z 5 4000 m;u 5 1.78C; and u 5 1.98C. Those cases labeled A utilizethe full array dataset. Cases labeled B use only the foursouthernmost moorings, as they have the preponderanceof flow in the array. Summing all transport below 1.88Cthrough the channel yields an estimate for mean AABWtransport of 22.0 Sv (Table 4, 1A). This is the sum ofan organized westward flow of 2.24 Sv and a recircu-lating (eastward) flow of 0.24 Sv, which occurs at the4100-m level at the southernmost mooring. Case 2 rec-ognizes that the interpolation for 1.88C is a bit moresophisticated than that possible for velocity and uses thealternative simpler AABW definition as water below4000 m. This results in a mean westward transport of2.18 Sv, the sum of organized westward flow of 2.31Sv, and a weaker eastward recirculation of 0.13 Sv.Mean transport below u 5 1.78C is 22.06 Sv (westwardflow of 2.18 Sv and eastward flow of 0.12 Sv); for u5 1.98C, the westward flow is similar (2.22 Sv), butthe recirculation is much stronger (0.39 Sv), so the netis only 1.83 Sv westward. The 1.88C isotherm lies above


FIG. 6. Time series of in situ temperature at seven levels from the equatorial mooring. For depths ofmeasurements see Table 3.

4000 m for most of the experiment at the southern twomoorings (where the eastward flow of the southernboundary layer is found at 3900 m and intermittentlyat 4100 m). Thus, relatively more of this eastward flowis mapped onto the AABW layer for cases 1 and 4 (u5 1.88C, 1.98C) than for cases 2 and 3 (z 5 4000 m;u 5 1.78C). Transports for the four cases are all within10%–15% of each other because two-thirds of the trans-port occurs below 4200 m where u , 18C.

As noted in section 2, the strong eastward flows ob-served at the southern end of the array are associatedwith the flow of LNADW from the Guiana Basin to theBrazil Basin, the crossing of the equator by the deepwestern boundary current. This eastward flow includeswater colder than 1.88C from entrainment of AABWinto the overlying jet. An estimated 1.24 Sv flows be-tween 3800 and 4000 m, based on the record meanspeeds at the southern two moorings, with correspond-ing temperature range of 1.88–2.08C. Rhein et al. (1995)use several Pegasus velocity sections along 358W toestimate a transport of ‘‘old’’ NADW (;1.88–2.38C) as4.6 6 2.6 Sv. Friedrichs et al. (1994) estimate about 7Sv transequatorial LNADW transport between 1.88 and2.48C with 3 Sv of that colder than 2.08C. Our arraydata represent only a small piece of this LNADW flow,which for the most part passes above the array, as well

as partly south of it, according to the Rhein et al. (1995)velocity and CFC-11 sections at 358W.

Both the marginal spatial resolution of the array andthe strong time variability of the records introduce alevel of uncertainty in our transport estimates. To assessthe effect of horizontal resolution, consider first the rel-atively low velocity measured at 4300 m near 18S, moor-ing 2. Suppose that low speed is in error: using insteada zonal speed of 24.5 cm s21 (the rough average of the24.88 and 24.22 cm s21 recorded at moorings 1 and3, respectively) would increase the case 1 estimate inTable 4 by 15% from 2.0 to 2.3 Sv. One could go a stepfurther and suppose that instead the 4300-m velocitydistribution might be jetlike, with a maximum at thislocation. The three realizations of Pegasus sections byRhein et al. (1995) and a fourth reported in Rhein et al.(1996) suggest such an AABW jet and show its west-ward speed maximum comparable to the eastward speedmaximum of the overlying LNADW boundary layereastward flow. The eastward LNADW maximum speedin the array is about 9.9 cm s21, and imagining anAABW jet with this speed westward at the problematicinstrument would increase the case 1 transports by 40%from 2 to 2.8 Sv. While these uncertainty estimates fo-cus on the problematic 4300-m instrument on mooring2, they also provide some general indication of the ef-


FIG. 7. Comparison of section-averaged vertical profiles u(z) at de-ployment (solid) and at recovery (dashed).

TABLE 4. AABW transport (106 m3 s21), calculated as described intext (westward transport is negative). Cases 1A, B integrate transportbelow the 1.88C isotherm. Cases 2A, B integrate below nominally4000 m (actually below midpoint of current meters at nominal 3900-and 4100-m depths). For both 1 and 2, A refers to through channeltransport; B integrates only over the southern four moorings (i.e.,south of ;08159N). Cases 3 and 4 integrate transport below the 1.78and 1.98C isotherms, respectively.

Case Mean Std dev Max MinTop ofAABW

1A1B2A2B34

21.9922.0122.1822.1922.0621.83

1.581.391.611.421.541.63

2.332.012.301.822.192.55

28.6827.2628.5827.1928.3428.94

u 5 1.88Cu 5 1.88C4000 m4000 mu 5 1.78Cu 5 1.98C

fects of inadequate sampling resolution for flows ofsmall horizontal scale. However, since smaller horizon-tal scales tend to be associated with higher frequencies,such errors should tend to average out over a long time.Thus, we expect that the most significant error due tohorizontal resolution is that associated with the anom-alous instrument record described above.

It is similarly difficult to estimate uncertainties ofvertical resolution—harder than for an extra-equatorialarray where geostrophy provides a natural shear esti-mate for comparison with the current meter shears. Inour array, only mooring 1 in the south shows a simplemean vertical shear of the zonal currents: the othersshow varying shears and/or meridional components offlow between the three levels. It is thus possible thatsampling at the three primary levels of our array isinadequate for characterizing the depth-integratedAABW zonal speed. In addition, the equatorial mooringprovides our only measurement of flow below 4300 m(Fig. 2; Table 3). Its average westward velocity of 1.3cm s21 at 4485 m is actually stronger than the averageat 4285 m (0.6 cm s21 westward) on the same mooring.However, the 4300-m measurements of the primary ar-ray dominate the AABW transport estimate at the othermoorings, where we have no deeper speed measure-ments. Thus, the array may have failed to sample theAABW flow nearer the bottom, and the AABW trans-port may be underestimated by not allowing for shear

between 4300 m and the sea floor. Fortunately, unde-tected near-bottom shear is not likely to greatly enhancethe AABW transport since an extra 0.7 cm s21 speed inthe bottom 150 m across the entire width of the arraywould boost the transport by only 0.4 Sv. The presenceof significant northward flow components at somedepths should not introduce additional uncertainty:through channel transport depends only on the velocitycomponent normal to the channel cross section, that is,zonal velocity.

The time series of daily average through channeltransport of water colder than 1.88C is shown in Fig. 8,along with the 40-day running mean; time series for allother cases except case 3 would be nearly indistinguish-able from this. In an event near the start of the dailyaverages, maximum westward transport exceeds 8 Sv,and at times the flow reverses to eastward with totaltransport as high as 2 Sv. Although standard deviationsfor the different transport time series are 65%–80% ofthe record means, the integral timescale for transport isabout 10 days, suggesting about 60 degrees of freedomfor the 604-day-long record. Using N 5 60 yields stan-dard errors on the order of 10% of the means. In additionto the obvious high-frequency content of the time series,there is a seasonal cycle visible in the 40-day filteredcurve: this results from the above-described periods ofstrong and weak AABW velocity vectors. The recordis not long enough to constrain a seasonal cycle, nor toisolate it as a spectral peak, but the appearance of aquasi-annual cycle is striking none the less: the recordbegins with strong AABW transport and has a secondstrong period about a year into the record. In addition,there are weak AABW transport periods about one-halfyear into the record and again about one and a half yearsinto the record. Notable peaks in the power spectrumfor the transport time series occur at 120 days and atperiods of 50–60 days, as well as in the band from 25to 30 days. Whitehead and Worthington (1982, near48N) and Johns et al. (1993, 88N) found a prominent60-day periodicity in the bottom water velocities, butthe 120-day period has not been observed before, as faras we know. The AABW transport minima in Fig. 8 are


FIG. 8. For the 604-day experiment the daily average transport of AABW defined as water with u , 1.88C,estimated as described in the text from the daily average westward flow speeds and potential temperatures.Superimposed is the running 40-day (boxcar) filtered AABW transport.

FIG. 9. The running yearly (365-day boxcar) averaged transport ofAABW defined as the water with u , 1.88C. The filtered time seriesbegins at day 183 of the daily average transport time series of Fig.8, and the last day is day 422 of the daily series.

rather sharply peaked in time due to the interaction ofthe 120-day and seasonal cycles. As noted in the dis-cussion of Fig. 4, the LNADW also has a seasonal cyclewith a strong eastward flow phase in the south that lags

the weak AABW phase by about a month. This delayis particularly visible in the time series of vector ve-locity, Figs. 3a–c.

To assess the effect of the lower-frequency variabilityon the calculations of mean transport, the time serieswas fitted with a model that included a seasonal har-monic and a 122-day cycle (one-third of a year). It wasalso necessary to include a term that could describe thelong-term trend observed in the transport: in particular,the 365-day running mean persistently declined from2.2 to 1.9 Sv westward (Fig. 9). Details of the fits aregiven in appendix C. In summary, using a linear termin the model leads to a mean transport of exactly 2 Svif we extrapolate to a full two years. On the other hand,the transport is not expected to decrease indefinitely.Modeling the ‘‘trend’’ instead as part of a two-year-longcycle leads to a mean transport of 2.14 Sv westwardover a full two years, an increase of about 7%.

Naturally it is impossible to determine variability ofthe current for times longer than or equal to the 604-day sampling period, but we can assess the stability ofour transport estimate by computing the mean over sub-sets of the time series. Table 5 shows the mean transportover 604 days, and means over sequential intervals of302, 151, and alternating 75 and 76 days. The 302-dayvalues differ from the 604-day value by just 3%, but


TABLE 5. AABW volume fluxes averaged over assorted time in-tervals, from top to bottom: 604 days; 302 days; 151 days; 75/76days. Flux is westward and units are Sverdrups.

FIG. 10. The 40-day filtered AABW transport estimate for AABWdefined as water with u , 1.88C, from Fig. 8, compared with severalbulk averages: the record mean, and means for successive recordhalves, quarters, and eighths.

the similarity of these numbers is deceptive for it reflectsthat the array deployment coincided with the maximumtransport phase. A running 302-day mean yields valuesranging from 21.86 to 22.34 Sv, which differs fromthe 604-day value by up to 15%. Figure 10 shows therunning 40-day mean volume flux from Fig. 8 as wellas the values from Table 5. In addition to the seasonalcycle with annual minima in late February/early March,the intermediate minima near 280 days and 400 daysare evidence of the 120-day signal noted above. Thisappears to reinforce the seasonal minima at 160 daysand 520 days and may account for the sharpness of theannual minima, and the flatness of the annual maxima.The 151-day mean values (triangles in Fig. 10) beginto define the annual signal and the 75/76 day averages(3’s in Fig. 10) show it even more clearly. The variationof these transport estimates over subsets of the datashow that shorter current meter records could have beenmisleading.

4. Discussion

For the 604 days of the current meter array velocitydata we estimate that between 2.0 and 2.2 Sv of AABWflowed through the array (Table 4), and thus from theBrazil Basin of the South Atlantic into the Guiana Basinof the North Atlantic. This measured AABW flow wassouthern intensified within the equatorial channel, andrecorded almost completely by the four moorings thatwere placed between 1.38S and the equator, with neg-ligible flow recorded at two moorings north of the equa-tor. Therefore, even though the array is in the extremenorthwestern ‘‘corner’’ of the Brazil Basin, the crossingof the equator by the AABW is achieved west of thearray. Given the restriction of the AABW to depthsgreater than 4000 m, the bathymetry of Fig. 1c showsthis crossing must occur between the longitude of ourarray, 35.98W, and 398–408W, a zonal width of 350 to450 km, about twice the meridional width of the west-ward flow through our array. The space available, andthe shape of the bathymetry, are such that this crossingcan occur at about a 308 angle from westward withoutrequiring a narrowing of the current as it crosses. Atthe colder sampled level of 4300 m, Fig. 4, the distanceto the western boundary from the array is only about

320 km, requiring a larger turning angle for the coldAABW unless it rises as it turns across the equator. Themean velocity vectors at this level of the array showonly about a 58 veering of the flow to the north ofwestward, Fig. 4a, and thus the veering of the 4300-mflow from westward to flow northward across the equa-tor must be quite abrupt as it has barely begun at ourarray.

Did we measure the total AABW flow that passesfrom the Brazil Basin to the Guiana Basin with ourarray? There are two sorts of uncertainties. The first setcomprises issues of the horizontal and vertical sampling,as discussed already. The second is the possibility ofother pathways for cross-equatorial flow outside the do-main sampled by the array. The latter issue depends onthe accuracy of a variety of bathymetric datasets, in-cluding the contoured one in Fig. 4, which portray theParnaiba Ridge as a continuous southern wall to theequatorial channel westward of our array; these datawere also used as a guide in selecting a longitude suf-ficiently far east that the north channel wall is a con-tinuous barrier to northward AABW flow east of ourarray. Overall, bathymetric data densities are muchgreater for the Parnaiba Ridge than for the north wall,and thus the possibility of a leak somewhere east of thearray is of greater concern than for leaks through theParnaiba Ridge west of the array. We know of no par-ticular evidence of such a leak, but simply do not haveenough bathymetric data to rule out the possibility al-together.

The three temperature sections (Fig. 2) present a bitof a paradox, for they show the coldest AABW, u #0.68C, north of 0.48S, in spite of the bulk of the esti-mated transport of AABW being south of there. Northof the equator the 4300-m instruments indicated neg-ligible flow. Perhaps the coldest water there migratednorthward beneath the relatively motionless 4300-m


TABLE 6. Estimates of AABW transport in the South Atlantic (Brazil Basin), North Atlantic (Guiana Basin), and near the equator. ‘‘RL’’means reference level (level of no motion).

Reference LocationTransport

(Sv) Comments

Brazil BasinWright (1970) 88S 2.8 RL 5 top of LNADW to AABW transitionMcCartney and Curry (1993) 118S 5.5 RL 5 1.98C; water below 1.98CSpeer and Zenk (1993) 118S 3.0 RL 5 2.18C; same data as McCartney and Curry; different

bottom triangle treatmentDeMadron and Weatherly (1994) 158S 4.4 RL 5 1.9–2.08C (s4 5 45.87 kg m23)

Guiana BasinWright (1970) 88N 2.7 RL 5 top of LNADW to AABW transition; inappropriate

RL for GB.Whitehead and Worthington

(1982; ‘‘WW82’’)48N 1 Current meters; flow over Ceara Rise only

WW82 48N 2 (2.3) RL 5 1.98C; total flow, Ceara Rise to Mid-Atlantic Ridge;computational error raises estimate to 2.3 (Luyten et al.1993).

McCartney and Curry (1993) 48N 4.0 Used WW82 data; RL 5 1.98C; different bottom triangletreatment

Luyten et al. (1993) 48N 3.2–4.0 Water below u 5 1.98C; varying RLMcCartney et al. (1991) 118N 2.2 Water colder than 28C; flow is through Vema Fracture

Zone into eastern basinFriedrichs and Hall (1993) 118N 2 Net for entire transect; negligible net flow in Guiana Basin

EquatorMcCartney and Curry (1993) 378W 4.3 RL 5 4000 dbar (u ø 1.938C); equatorial geostrophyRhein et al. (1995) 358W 0.6, 2.6, 4.6 Pegasus sectionsRhein et al. (1996) 358W 1.8 Pegasus section

level north of the equator. Such flow is hinted at by themean current measured at the deepest instrument on theequatorial mooring, which is oriented 208 north of west.Again, however, these considerations presumably in-volve relatively small transports of a few tenths of aSverdrup.

How does our AABW transport compare with pre-vious estimates of Atlantic AABW circulation? The ear-liest attempt to estimate AABW transport through thewestern basin used the Meteor data from the 1920s(Wust 1957) but was not successful due to coarse res-olution. Since then many estimates have been made inboth the Guiana Basin of the North Atlantic and theBrazil Basin of the South Atlantic. Some of these appearin Table 6. Note that AABW transport from the BrazilBasin supplies not only the northward flow into theGuiana Basin that we attempted to measure, but alsothe eastward flow through the Romanche and ChainFracture Zone into the eastern basin. From Table 6,roughly 3 to 5 Sv of AABW is approaching the equatorfrom midlatitudes of the Brazil Basin (no estimates ofthe effect of time variability were made). The amountis smaller than the 7 Sv estimated entering the BrazilBasin from the south (Speer and Zenk 1993) presumablybecause of upwelling within the basin. Our array esti-mate of somewhat more than 2 Sv continuing into theGuiana Basin implies that between 1 and 3 Sv is avail-able for the combination of the eastward flow throughthe Romanche Fracture Zone and the upwelling fromAABW into the LNADW in the Brazil Basin.

In the Guiana Basin, there is a strong recirculatinggyre in the deep-water and AABW circulation (Fried-richs and Hall 1993), and a deep level of no motion isinappropriate. At a minimum, a level of no motion isrequired above the deep water. Thus, Wright’s calcu-lations in the Guiana Basin, though fortuitously closeto ours, are flawed; indeed, they simultaneously give anet deep-water flow that is northward rather than south-ward. Four estimates are presented for flow of AABWnear 48N, all based on the current meter and/or hydro-graphic data presented in Whitehead and Worthington(1982). These all use a reference level of 1.98C, but theestimates range from 2 to 4 Sv for the total flow betweenthe Ceara Rise and the Mid-Atlantic Ridge. The veryrough topography at the 48N section makes the estimatesof near-bottom shear and transport speculative sincethere is no clear evidence that the gaps connect to thenorth. Moreover, the time variability of the current isknown to be significant, and no single geostrophic cal-culation can estimate the mean flow.

Another recalculation of this 48N transport is givenin Luyten et al. (1993) showing the dependence of thetransport colder than 1.98C on an assumed level of nomotion that ignores the current meter measurementssupporting 1.98C. In such a case, there is a gyre aspectto the shear and flow east of the Ceara Rise, and raisingthe level of no motion primarily increases the gyre in-tensity while the net northward transport of AABWstays in the range 3.2–4.0 Sv. In particular, using areference level of 3.28C, deduced as optimal by Fried-


richs and Hall (1993) for a location farther north, Luytenet al. (1993) show a transport of 3.6 Sv. This is largerthan our transequatorial estimate, which could reflectthe inclusion of some northward recirculating LNADWin the 48N estimate, through the use of 1.98C as the topof the AABW, and the exclusion of the southward flow-ing waters between 1.78 and 1.98C over and west of theCeara Rise. In addition, there could be entrainment ofLNADW into the AABW as the mechanism of pro-gressively warming the AABW as it flows northward.Thus, although the use of 1.98 as a reference level astested by current meters by Whitehead and Worthington(1982) may be correct, the northward modification ofAABW through mixing complicates comparisons oftransport.

Farther north in the Guiana Basin, McCartney et al.(1991) estimated a flow of about 2.2 Sv colder than 28Cdebouching from the Vema Fracture Zone into the east-ern North Atlantic. The immediate source of that wateris the northward flow of AABW in the Guiana Basin,part of which is diverted to the eastern basin by the flowthrough the Vema. At the point where this transportestimate applies, well eastward along the Vema, thecoldest water is 1.588C, considerably warmer than thewater entering the Vema from the northward flowingAABW in the Guiana basin, where bottom temperaturesare about 1.278C. How much the entrainment of over-lying deep water has increased the transport in the pro-cess of raising the temperature as the water passesthrough the Vema is of course unknown, so it is notpossible to estimate what fraction of the AABW (andentrained LNADW) flowing northward near 108N in theGuiana Basin is diverted eastward into the Vema, otherthan an expectation that it is less than 2 Sv.

Friedrichs and Hall (1993) examined a full North At-lantic transect that falls near 118N. The use of a fullocean section has the advantage of allowing rigorousapplication of overall mass balance. The section is com-pletely north of the Vema Fracture Zone through theMid-Atlantic Ridge. East of the ridge they found a netnorthward transport of AABW amounting to 2 Sv, butnegligible net northward transport of AABW west ofthe ridge. This most likely reflects inadequate samplingalong the western flank of the Mid-Atlantic Ridge,where the colder classes (and strong flow) are not re-solved due to a large bottom triangle at the critical sta-tion pair where the northward flow is concentrated. Withthe northward flow underestimated because of this dif-ficulty, the recirculating southward flow of AABW inthe deep western boundary current essentially cancelsthe northward flow.

Some estimates of AABW transport right at the equa-tor also exist (Table 6). McCartney and Curry’s ratherhigh estimate of 4.3 Sv falls within the rms of the dailytransport estimates, but is outside the range of the40-day filtered transport estimates (Fig. 8). But the 1983section (Fig. 1c) was in the area where there must be asignificant northward flow (see above). Rhein et al.

(1995) estimated AABW transports just east of ourmoored array site using three Pegasus sections (Table6). Their reported AABW transport of 2.6 Sv 6 2 Svis the average and standard deviation of these three es-timates, one of which they describe as ‘‘not well re-solved’’ due to station spacing. The third section wasmade about 50 days into our moored experiment, whenthe 40-day filtered time series shows 2.4–2.6 Sv ofAABW flow, agreeing well with the 2.6 Sv they mea-sured. On the other hand, the daily average transporttime series, Fig. 8, shows much larger fluctuations, fromgreater than 5 Sv westward to essentially zero westwardtransport. Indeed, the plot of daily average array trans-port is not particularly encouraging for the use of a smallnumber of velocity snapshots to define the mean flow.A fourth Pegasus section (Rhein et al. 1996) was madefrom a cruise in February–March 1994, about 75 daysbefore the end of our moored experiment. Its estimatedwestward transport lies in the range of daily transports(2 Sv westward to 2 Sv eastward), while the 40-dayfiltered transports, Fig. 8, show this period as the secondtransport minimum of the record.

As mentioned in section 2d, the moored array dataand the supporting hydrography both indicate a warmingtrend over the duration of the experiment. The coldAABW at 4300 m warmed at a rate of 0.0168C/yr; thiscan be compared to a rate of warming found by Zenkand Hogg (1996) for the coldest AABW entering theBrazil Basin through the Vema Channel of 0.0158C/yrover a two year stretch (1991–92). They noted that spo-radic data at this site back through 1973 indicates thatcoldest temperatures were stationary in the range20.178 to 20.198C, with the warming from that rangeto 20.138C during the experiment therefore lookinghighly anomalous. Coles et al. (1996) have documenteda shift in the distribution of coldest levels of AABW inthe Argentine Basin in the late 1980s, whose subsequentadvection (with mixing) into the Vema Channel mightbe related to this warming signal. At our site on theequator, the overall history is somewhat different. Datawithin 10–100 km of the array in the equatorial channelin 1972, 1983, and 1989 all show coldest AABW tem-peratures quite close to each other (0.6118, 0.6178, and0.6128C), and all warmer than our observations (0.5828–0.5958C) by about 0.038C. Thus, the warming trend dur-ing the experiment may be a recovery phase from somecooler period between the earlier warmer period and theexperiment. At the Vema Channel the warming perioddescribed by Zenk and Hogg (1996) may be a responseto an earlier warming event of the upstream source forthe Vema waters. At the equator we can speculate on asomewhat different scenario.

As the AABW flows from the Brazil Basin to theGuiana Basin it warms considerably. Basically this re-flects vertical exchange between the AABW flowing onedirection beneath the LNADW flowing in the other di-rection (although by no means do the cores of currentlie over each other). The near-bottom temperature has


a large gradient northward from the site of our exper-iment into the Guiana basin. For example, Whiteheadand Worthington found the coldest AABW at 48N in1977–78 to be 0.998C, and based on the historical datajust described, at that time the coldest AABW at ourarray location was 0.618–0.628C. The likely connectingpathway between the two locations is about 780 km, sothe bottom temperature gradient is about .058C/100 km.Thus, the 604-day warming trend can be interpreted asa shift of between 20 and 30 km of the bottom isothermstoward the Brazil Basin during the experiment. The oth-er change during the experiment is the descent by order25–40 m (after removing the seasonal effect) of thetransition layer between the LNADW and the AABW,giving a distinct warming trend at the 4100-m level ofour array and a weaker warming trend at the 4300-mlevel. There is little indication of change at the 3900-mlevel. The temperature sections, Figs. 2 and 7, confirmthe warming trend at depth, but also show the trendpassing from warming through zero to a slight coolingtrend above 3900 m that fades away near 3000 m. Thus,by the end of the experiment, the LNADW has thick-ened, the transition layer has moved downward, accom-modating the thickened LNADW, and the AABW layerhas thinned with the coldest AABW getting warmer;the latter warming may be interpreted, as just men-tioned, as a shift toward the Brazil Basin of the bottomtemperature gradient that delineates the erosion andwarming of the AABW as it flows into the Guiana Basin.

The counterflow arrangement of the LNADW andAABW, and the quasi-stationary mixing between them,are the fundamental basis for interpreting the distribu-tion and variation of these water masses. In particular,the LNADW in the deep western boundary current inthe southern Guiana Basin has a much thicker character,with its low vertical stratification extending to colderthan 1.758C, while in Fig. 2 the higher gradients of thetransition layer reach warmer than 1.858C, and at 118Sin the Brazil Basin the higher gradients reach warmerthan 2.08C (see, e.g., McCartney 1993). This progres-sion, as well as the opposing progression of warmingof the AABW layer toward the north, reflects the ad-vective diffusive balance between these opposing flows.We thus are led to interpret the above summarized trendsof thickening LNADW, thinning AABW, deepening ofthe transition layer, and warming of the coldest AABWas indicative of a shift in this balance toward a strongerLNADW influence and a weaker AABW influence. Anatural candidate for such a shift is that the period ofour experiment corresponds to one where the LNADWtransport is stronger than the preceding period and/orthe AABW transport is weaker; a geographical shift ofthe thermal structure gives all of those trends. Indeed,as discussed in section 3 and appendix C, there is ev-idence that AABW transport weakens over the 604-dayobservation period. However, while the fits are usefulfor eliminating the seasonal signal to estimate meanAABW transport, the transport record is too short to

unambiguously resolve and quantify the overall trend.It is interesting that Zenk and Hogg (1996) found thatthe two-year warming of the coldest AABW in the VemaChannel in 1991–92 was accompanied by decreasedgeostrophic shear and a decline in net AABW transportbetween the Rio Grande Rise and South America fromabout 6 to 4–5 Sv.

The 4100-m temperature records across the array, Fig.3e, lend credence to the idea that the movement up anddown of the transition layer does, in general, track onthe waxing and waning of AABW transport. Recallingthat the AABW transport, Fig. 8, is greatest in Septem-ber–October and smallest in February–March, followedby greatest LNADW transport in April–May, we seethat superimposed on the overall warming trend at 4100m are relatively cold events (uplifted transition layer)during the strong AABW periods (a shift of theLNADW/AABW balance toward the Guiana Basin re-flecting stronger AABW influence all across our array)and relatively warm events (depressed transition layer)during the weak AABW and strong LNADW periods(a shift of the LNADW/AABW balance toward the Bra-zil basin reflecting enhanced LNADW and diminishedAABW influence all across the array). Since the sea-sonality of AABW transport is tracked by the verticalexcursion of the transition layer, it seems plausible thatthe overall descent of the transition layer during our604-day experiment may indicate declining AABWtransport.

5. Summary

As part of the Deep Basin Experiment of the inter-national efforts of the World Ocean Circulation Exper-iment, an array of six moorings was deployed for 604days at abyssal levels in the western Atlantic Ocean.The goal was to measure the net flow of AABW fromthe Brazil Basin of the western South Atlantic into theGuiana basin of the western North Atlantic. This overallnorthward flow locally becomes westward along theequator because of an abyssal channel: northward flowof AABW enters the eastern end of the channel eastwardof 328W, is constrained to flow westward along the equa-tor to 388W, and debouches northward onto the CearaAbyssal Plain of the Guiana Basin westward of 388W.The moored array at 368W spanned this channel andthus sampled the total AABW flow.

Several estimates are made of the average AABWtransport for the 604-day record. The first uses an es-timate of the depth of the u 5 1.88C isotherm at eachmooring and yields 2.0 Sv below that temperature. Thesecond uses 4000 m as the bounding surface for theAABW and yields 2.2 Sv below that surface. Transportis also estimated for bounding isotherms of u 5 1.78Cand u 5 1.98C. In all four transport estimates, organizedwestward flow of 2.2–2.3 Sv is partially compensatedby eastward recirculation of 0.1–0.4 Sv, which occursat 4100 m at the southern end of the array. Above this,


the strongest average speed of the whole array was re-corded: an eastward flow of nearly 10 cm s21, repre-senting the northern edge of a southern boundary currentthat conveys LNADW into this northwestern corner ofthe Brazil Basin.

The directions of the significant currents sampled bythe array are persistent, but there is considerable low-frequency variability of the individual speeds at fre-quencies from daily through seasonal, and these fluc-tuations carry over into the AABW transport estimates.Using the integral timescale of 10 days, we estimatestandard errors of about 10% for the means. Sparse sam-pling in the horizontal and vertical most likely leads tounderestimating the transport. The largest such error isprobably that associated with the 4300-m instrument onmooring 2 at 18S, where flow was relatively weak asthe mooring was in the wake of a topographic feature.This error is estimated to be 0.3–0.8 Sv (15%–40%)depending on the details of the flow structure. An annualcycle is tantalizing, though indeterminate given the 604-day duration of the experiment. The AABW transport,low-pass filtered at 40 days to suppress higher-frequencyfluctuations, begins the record (September–October1992) with strong westward transport greater than 3 Sv,but declines to a minimum of only about 0.3 Sv byFebruary–March 1993. The transport then recovers tothe same high value above 3 Sv in September–October1993, then repeats the decline to a minimum of 0.1 Svin February–March 1994. The recovery toward hightransports after this second minimum is cut short by thearray recovery since the record is shorter than two years.If the transports are indeed annual, then extrapolationfor the additional 126 days to complete a full two cycleswould elevate the above transport estimates by order7%. The simplest description of the interhemisphericAABW flow is that it is southern intensified within theequatorial channel at an amplitude of 2.0–2.2 Sv, withan annual cycle about that mean that waxes to a max-imum transport of 3.1 Sv in September–October andwanes to a nearly vanishing transport of 0.2 Sv in Feb-ruary–March.

The field of abyssal temperatures across the channel,as sampled by the moored instruments and by hydro-graphic section measurements near the times of arraydeployment and recovery, show a progressive warmingcorresponding to a descent of the isotherms that definethe transition between the AABW and the overlyingLNADW amounting to 50–80 m. About half is due tothe seasonal cycle and half to a longer term trend. At4100 and 4300 m the warming rates were 0.0988C/yrand 0.0168C/yr. The coldest observed AABW temper-atures in the hydrographic measurements warmed from0.5828C at deployment to 0.5958C at recovery, but it isimpossible to say how much of this warming is due toa trend and how much to the seasonal cycle. In fact, the604-day warming may actually be a recovery from somecooler phase before the experiment, as measurementswithin 10–100 km of this longitude in the channel in

1972, 1983, and 1989 were warmer by 0.02–0.038C. Ingeneral, during the experiment the quasi-annual waxingand waning of AABW transport is reflected by theAABW transition layer rising and falling, the AABWthickening and thinning, and the LNADW thinning andthickening. We thus speculate that the overall trend inthe experiment of a deepening transition layer meansthat the experiment overall is in a period of waningAABW transport.

Acknowledgments. We are extremely grateful to DickLimeburner who was responsible for acquisition of theCTD data, Scott Worrilow who conducted the currentmeter operations, and Susan Tarbell who analyzed thecurrent meter data. In all these cases the results areexceptionally good and it has been a pleasure to workwith them. We also thank our Brazilian colleagues AnnaMaria Scofano and Friedrich W. Herms and his studentsfor assistance at sea. We received fine cooperation fromMaster Michael Dick of R/V Iselin and his crew andMaster Carl Swanson of R/V Knorr and his crew, andwe thank them all. Comments from referees greatly im-proved the final version of this paper. Thanks to DeborahTaylor for typing the manuscript. This research was sup-ported by the Division of Ocean Sciences, National Sci-ence Foundation under Grant OCE91-05834.

APPENDIX A

Calculating u from T Time Series

First we calculate section-averaged profiles of u andS for both 1992 and 1994, which we denote , S92,u92

, S94. For each T(t) measurement we use the Fofonoffu94

and Millard (1983) formulas to compute u(t) 5 u(T(t),P0, S0), where P0 is the (fixed) pressure at the mooringdepth and S0 is a time-weighted average of the salinityvalues from the CTD data at pressure P0. Using thisvalue of u, we then ‘‘look up’’ more exact salinity valuesfrom each of the two tables of , Sn (n 5 1992, 1994),un

which we will denote S1 and S2. Next, two new valuesof u are evaluated from ui(t) 5 u(T(t), P0, Si), i 5 1, 2(corresponding to 1992 and 1994). The process is re-peated until it converges, which is usually after the firstiteration. Finally, we linearly interpolate in time be-tween the two u values corresponding to 1992 and 1994,to obtain the final value u(t) for that time and location.

APPENDIX B

Transport Calculation from T, u Time Series

The first step in the calculation was to estimate thedepth z0 of the 1.88 isotherm at each of the mooringlocations. As noted at the end of the last section, po-tential temperature time series have been calculated forall moorings, so these were used in conjunction withthe known instrument depths to create a time series ofz0 for each location. Although u 5 1.88C nearly always


TABLE C1. Parameter values for least squares fits to transporttime series. Phase shifts are relative to a start date of 3 Oct 1992.

Functional

a b c dFit

h f c

C.1 22.54 1.47 3 1023 20.8260.294

0.6664.46

C.2, n 5 2 22.14 0.3382.69

20.9690.271

0.6424.45

C.2, n 5 3 22.28 0.4493.75

20.9070.311

0.6554.46

C.2, n 5 4 22.48 0.6384.33

20.8890.325

0.6594.47

FIG. C1. Forty-day low-passed AABW transport (heavy solidcurve), and four fits discussed in text: C.1 (heavy dashed curve); C.2,n 5 2 (light solid curve); C.2, n 5 3 (light dashed curve); and C.2,n 5 4 (dotted curve).

fell between the instruments at 4100 and 3900 m, it isinappropriate to obtain z0 by linear interpolation be-tween their daily measured temperatures. Because u3900

usually lay above the strong gradient region, linear in-terpolation would predict a z0 that is too shallow.

Recognizing the inappropriateness of a linear tem-perature interpolation we instead used the linear fits foru(z) calculated from the CTD data (see section 2e) asfollows: Note that the calculated slopes are a good rep-resentation of the strong gradient region over the rangeu ø 1.08–1.858C in both 1992 and 1994. At 4100 mpotential temperature u always lay within this range,while at 3900 m temperatures u , 1.858C occurred onlytwice at mooring 6, four times at mooring 2, and lessthan 5% of the time at mooring 1 (mostly at the begin-ning). Thus, we allowed for two cases when evaluatingz0. 1) When u3900 . 1.858C, which was most of the time,we extrapolated upward from the 4100-m measurementto obtain z0 (1.88C):

z0 5 z4100 1 slope 3 (1.8 2 u4100),

where ‘‘slope’’ is the time-interpolated value of dz/duusing the average 1992 and 1994 slopes at that u fromthe CTD data. Here z3900 and z4100 are the actual ratherthan nominal depths of the current meters. 2) In the fewcases when u3900 # 1.858C, both temperatures were inthe strong linear gradient region, and it was more sen-sible to interpolate between them:

1.8 2 u4100z 5 z 1 (z 2 z ).0 4100 3900 4100u 2 u3900 4100

Applying this method to all six locations, we obtainedtime series z0 (t; 1.88C) at each one. Jumps in z0 fromcase 1 to 2 occurred almost exclusively at mooring 1.Though they caused high-frequency transport changesas large as 1 Sv, the effect on the mean and low-fre-quency total transport was negligible.

It was then relatively straightforward to integrate iny and z to obtain transport time series. Integration wastrapezoidal in both the horizontal and vertical. Thus,areas associated with the velocities at each of the sixinstruments extend halfway to the next instrument (orthe channel edges) horizontally, and halfway to the nextinstrument (or all the way to the bottom) vertically,except at 3900 m where the upward extension was takento be 100 m. (Since all the AABW lies below 3900 m,this choice has no effect whatsoever on the bottom watertransports.) Eastward velocities are multiplied by theseareas to obtain transports associated with each instru-ment, and then percentages of the transports at 3900 or4100 m lying above and below the 1.88C isotherm werecalculated using z0(t) from above.

APPENDIX C

Trend Fitting of the Transport Time Series

Visual inspection of the 40-day low-passed time se-ries of transport suggests fitting it with both a seasonal

component and one that has a 122-day period. (Attemptsto include a 60-day period as well were unsatisfactory.)The longer-term trend revealed by the annual runningmean transport (Fig. 10) is accommodated by either alinear trend, or a periodic component of low frequency.The fits are of the form

a 1 bt 1 c cos(vt 1 f) 1 d cos(3vt 1 c) (C1)

or

va 1 b cos 1 h 1 c cos(vt 1 f)1 2n

1 d cos(3vt 1 c), (C2)

where v 5 2p/1 yr and n 5 2, 3, 4. Variables a, b, c,d, f, c, and h are free parameters. Least squares fits tothe daily time series give the results in Table C1. Thefour fits and the 40-day running mean are shown in Fig.C1: for the first 604 days, the fits are nearly indistin-guishable. They then diverge rapidly with increasing


FIG. C2. Annual running means calculated for the periodic fits inC.2: n 5 2 (solid), n 5 3 (dashed), n 5 4 (dotted), and the data(heavy solid curve).

time. As v/n decreases, the total range of the functionalfit (C2) increases, leading to ever larger (westward)long-term means for the transport (term ‘‘a’’ in TableC1). Figure C2 illustrates this effect by showing annualrunning means for the periodic fits: they track the run-ning mean transport from the data initially, then divergeover their longer timescales. The ‘‘goodness of fit,’’ asmeasured by the rms difference between the measuredand fitted transports, is the same within 1.5% for allfour fits.

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