JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. C6, PAGES 14,29%14,323,JUNE 15, 2000
Tracing the flow of North Atlantic Deep Water
using chlorofiuorocarbons
WilliamM. Smethie
Jr.,• RanaA. Fine,:AlfredPutzka,
• andE. PeterJones
n
Abstract. Chlorofluorocarbon
(CFC) andhydrographic
datacollectedin the North Atlanticin the
late 1980sandearly 1990sareusedto confirmandaddto earlierworkon the large-scalecirculation
pathwaysandtimescales
for the spreading
of NorthAtlanticDeep Water (NADW) components
and
howthesecomponents
relateto the hydrographic
structure.Throughoutthe westernNorth Atlantic,
highCFC concentrations
arecoincidentwith newlyformedNADW components
of UpperLabrador
Sea Water (ULSW), ClassicalLabradorSea Water (CLSW), and Overflow Waters (OW). ULSW is
markedby a CFC maximumthroughout
the westernsubtropical
andtropicalAtlantic,andCLSW is
markedby a CFC maximumnorthof 38øN in datacollectedin 1990-1992. Iceland-Scotland
OverflowWater (ISOW) splitsinto two branchesin the easternbasin,with onebranchenteringthe
westernbasinwhereit mixes with Denmark Strait Overflow Water (DSOW) and the densestbranch
flowssouthward
alongthebottomin theeasternbasin. DSOW contributes
the largestportionof the
CFC signalin OW. It is estimatedthattheseNADW components
are at 60-75% equilibriumwith
the CFC concentration
in the atmosphere
at the time of formation.The large-scaledatasetconfirms
thatNADW spreadssouthwardby complexpathwaysinvolvingadvectionin the Deep Western
BoundaryCurrent(DWBC), recirculationin deepgyres,andmixing.Maps of the CFC distribution
showthatpropertieswithinthe gyresarerelativelyhomogeneous,
particularlyfor OW, andthereis
a profoundchangeat the gyreboundaries.
The densityof thecoreof ULSW increases
in the
equatorwarddirectionbecauseof entrainmentby overlyingnorthwardflowingUpper Circumpolar
Water andat the equator,ULSW hasthe samedensityasCLSW in the subtropics
but is warmerand
saltier. The densityof OW decreases
betweenthe subpolarregionandthe subtropics.
This is
causedby theleastdensepartof OW exitingthe subpolarregionin the DWBC, while the densest
component
recirculates
in the subpolarbasins.Somevariabilityis observedin OW densityin the
subtropics
andtropicsbecauseof variabilityin mixingwith AntarcticBottomWater andchangesin
the subtropics
thatareprobablyrelatedto thetransportof differentvintagesof DSOW. Ages
derivedfromCFC ratiosshowthattheNADW components
of northernoriginspreadthroughoutthe
western
NorthAtlantic
within
25-30years.Thiscorresponds
toa spreading
rateof 1-2cms'• andis
comparableto the time a climateanomalyintroducedinto the subpolarNorth Atlanticwill take to
penetratethe entirewesternNorth AtlanticOcean.
1. Introduction
Global thermohalinecirculationis an importantcomponentof
the Earth'sclimate system. This circulationoccursby numerous
pathwaysthat transportwarm watersto high latitudeswhere they
becomemore dense,sink, and spreadthroughoutthe ocean [e.g.,
Broecker,1991; Gordon, 1986]. One of the primarypathwaysby
whichthis occursis the formationand spreading
of North Atlantic
DeepWater(NADW), the properties
of whichhavebeenobserved
throughout
muchof the deepoceanandextendas far as the North
Pacific[Reidand Lynn, 1971]. The flow of warm shallowwaterto
the northern North Atlantic to feed NADW
formation results in
Europe having an anomalouslywarm climate for its latitude.
Fluctuations
betweenwarmandcoldclimatesin thepastarethought
to havebeen causedby changesin the formationof NADW [e.g.,
Broecker,1995; Manabe and Stouffer,1988].
Duringthe pasthalf centurya numberof manmadesubstances
have been
introduced
into
the environment.
Some
of
these
substancesenter the surface ocean on a global scale and are
•Lamont-DohertyEarthObservatory
of Columbia
University,
Palisades,New York.
2Rosenstiel
School
ofMarineandAtmospheric
Science,
Universityof Miami,Miami, Florida.
3Institut
farUmweltphyik,
University
ofBremen,
Bremen,
Germany.
aBedford
Institute
of Oceanography,
Dartmouth,
NovaScotia,
Canada.
Copyright2000by theAmericanGeophysical
Union.
Papernumber1999JC900274.
0148-0227/00/1999JC900274509.00
chemicallystablein seawater. These substances
are excellent
tracersof deepwaterformationand circulationsincethey become
incorporatedinto the deep water when it is renewedfrom the
surface. The substancesthat have been most widely used in
oceanographic
studiesare bombtritiumandradiocarbon
that were
injectedinto the atmosphere
by nuclearweaponstesting,mostlyin
theearly1960s,andanthropogenic
chlorofluorocarbons
(CFCs)that
havebeenenteringthe atmosphere
sincethe 1930s.
This paperis a synthesis
of availableCFC datafor the North
AtlanticOceanup to 1992. We review the formationprocesses
of
NADW, show how CFCs becomeincorporatedin NADW, map
spreadingpathwaysof NADW within the Atlantic Ocean using
CFCs, and estimatethe timescalefor NADW to flow along these
pathways.
14,297
14,298
SMETHIE
ET AL.' TRACING
THE FLOW OF NORTH ATLANTIC
40
2. Background Information on CFCs
35
CFCs are primarily anthropogenic
substances
that have been
usedextensivelyin our society. The two CFCs thathavebeenmost
widely measuredin the oceansare CFC-11 (CCI3F) and CFC-12
(CC12F2),and they have no naturalsources. These substances
are
chemically stable in the tropospherebut do decomposein the
stratospherewhere their decompositionproducts cause the
destruction
of ozone[Andersonet al., 1991]. Becauseof thisglobal
environmentalproblem,the concentrations
of CFC-11 and CFC-12
havebeencloselymonitoredin the atmosphere
sincethe mid-1970s
[Elkins et al., 1993; Cunnold et al., 1994, 1997]. Prior to that time
the atmosphericconcentrationcan be reconstructed
from industrial
productiondata [Fisheret al., 1994] and the atmospheric
lifetimes
that have been determinedfrom the atmosphericmeasurements
referenced
above.
DEEP WATER
The concentrations of CFC-11
and CFC-12
continuouslyincreasedwith time from theirinitial productionin the
1930s until the early 1990s (Figure 1). The rate of increasewas
differentfor thetwo CFCs (Figure2), resultingin theCFC-11:CFC12 ratio changingwith time (Figure 1) until the late 1970s,after
which the rate of increase for both CFCs was about the same until
the 1990s. The rate of increasedropped in the 1990s as a
consequenceof the efforts of countries complying with the
provisionsof the MontrealProtocolto phaseout CFCs. By 1995 the
concentrationof CFC-11 was beginning to decrease in the
atmosphere,
but CFC-12 wasstill increasing
slowly[Montzkaet al.,
1996; Walker et al., this issue].
CFCs enter the surfaceocean from the atmosphereby gas
exchange,and the averageequilibrationtime betweenthe surface
mixedlayerandthe atmosphere
is ~1 month[Broeckeret al., 1980].
Most of the surfacewaters,exceptin regionsof deep convection,
are within a few percentof equilibrium[Warner, 1988], and the
expectedconcentration
at a given point in time can be calculated
from the atmospheric
concentration
and the solubility[Warnerand
Weiss,1985]. Both CFC-11 and CFC-12 are chemicallystablein
seawater,but CFC-11 does decomposeunder anoxic conditions
[Bullisterand Lee, 1995; Shapiro et al., 1997], which are rarely
foundin theopenocean.
CFCs can be used to estimateages of water parcels. The
simplestway is to convertthe measuredconcentration
in waterto an
atmosphericconcentration(using the solubilities [Warner and
Weiss,1985], assumingthe water was saturatedwhen it was at the
surface)and thencomparingthis to the time historyof CFCs in the
atmosphere(Figure 1). This age, subsequently
referredto as the
oo
o
• 30
o CFC-11
• 25
o
o
o
•, 20
•. 15
O0
•: •o
<1:
5
'5
1 •50
0 000000
eeeee oo
Oooo
eeeeeeeeeeeeeøOOo
CFC-12 eeeeeøe•/•ee
,
I
I
I
I
1960
1970
1980
1990
,
2000
Year
Figure 2. Annualpercentchangein CFC-11 and CFC-12 versus
time for the NorthernHemispheretroposphere
calculatedfrom the
dataplottedin Figure1.
pCFC age [Fine et al., 1988;Doneyand Bullister,1992; Warneret
al., 1996], is valid if no mixing occursafter the water parcelleaves
the surface,which is usually not the case. The CFC-1 l:CFC-12
ratio changeswith time prior to the late 1970s,and it can be used
for age estimationfrom the 1950sto that time [e.g., Weisset al.,
1985]. The ratio ageis not affectedby mixingwith CFC free water.
This is the casefor the leadingedgeof the CFC signalas it enters
the deep ocean in boundarycurrents. However, Pickart et al.
[1989] and Rhein [1994] have shown that after adjacentwater
beginsto accumulateCFCs by mixing, the ratio is underestimated
and the age is overestimated
for the core of the boundarycurrent
becauseof thismixing.
3. Observationsof AnthropogenicTransient
Tracers
in NADW
It has beenknown for sometime that NADW is producedat
high latitudesin the North Atlantic Oceanfrom a combinationof
deepconvectionof densewaterthat formsat the surfaceandinflow
of densewater from behindthe Greenland-Iceland-Scotland
Ridge.
Measurement of transient tracers in the Noah Atlantic has enhanced
600
f
i
i
!
0.6
.
CFC-11
:CFC-12
nd]c•qaøoo:•c•nq:30o,._eeee
500
Ratio _•
400
•00•
o
• •0
CFC-I
•
2
o
300
200••
eee••
and bomb radiocarbon.
A zonal section of both substances in the
western Noah Atlantic was measured in 1972 on the Geochemical
0.5
OceanSectionsStudy (GEOSECS)expeditionand clearly showed
the input of recentlyventilatedwater to the deep Atlantic at high
ß
oø
•
øø
oo
•
ß
latitudes [Stuiver and Ostlund, 1980].
•••
o•ø
o• ooO•
oß ooø
o
_oo
o*•ooø
0.1
_coo"_•
1S•O
......
1S70
Year
0.0
1S80
our knowledgeof the pathwaysby which newly formed NADW
entersthe AtlanticOceanand the rate at whichnewly formedwater
spreadsalong thesepathways(for review see Fine, [1995]). The
first transienttracersto be measuredon a large-scalewere tritium
• SSO
2000
Tritium and bomb
radiocarbonwere observedthroughoutthe water column noah of
45øN with a maximumat the bottomextendingnorthwardto the
Greenland-Iceland
Ridge [Ostlundand Rooth, 1990] wheredense
waterentersthe AtlanticOceanthroughDenmarkStrait. This same
zonal structurewas observedagain in the early 1980s on the
TransientTracers in the Ocean ('I'TO) expedition,but the deep
tritium and bomb radiocarbon
concentrations had increased and
extended farther south [Ostlund and Rooth, 1990].
Tritium
measurements
made at the Blake-BahamaOuter Ridge in 1977
revealed high levels in the Deep Western Boundary Current
Figure 1. CFC- 11, CFC- 12, and the CFC- 11:CFC-12 ratio versus (DWBC) and demonstratedfor the first time that NADW no older
time for the NorthernHemispheretroposphere.The data for these than 15 yearswas transportedfrom the northernformationregions
plotswereprovidedby Walkeret al., [thisissue].
to the subtropics[Jenkinsand Rhines, 1980]. The presenceof
SMETHIE
ET AL.: TRACING
THE FLOW OF NORTH
recentlyformed NADW in the subtropicalAtlantic DWBC was
confirmedby tritium[Olsonet al., 1986;Doneyand Jenkins,1994]
and CFC [Fine and Molinari, 1988; Smethie,1993] measurements
madein the early and mid-1980s,and theseresultswill be discussed
in detaillaterin the paper.The presence
of recentlyformedNADW
in thetropicalAtlanticwasshownby Weisset al., [ 1985], Molinari
et al., [1992], Rhein et al., [1995], andAndtie et al., [1998], and in
theSouthAtlanticwasshownby Wallaceet al., [ 1994].
ATLANTIC
DEEP WATER
14,299
depthsas greatas 2000 m in the centralLabradorSea [Wallaceand
Lazier, 1988; Lazier, 1995; Dickson et al., 1996]. This water mass
is characterizedby a relatively low salinity acquiredfrom inflow
from the Arctic Oceanand a low potentialvorticityresultingfrom
the breakdownof verticalstratificationcausedby deepconvection
[Talley and McCartney, 1982]. The characteristicpotential
temperature,
salinity,
anddensity
are3.5øC,
34.88,ando•.5= 34.66,
respectively. However, conditionsinfluencingCLSW formation,
Beforeproceeding
withthedetailsof theuptakeandspreading which include preconditioningwater in the central LabradorSea
of transienttracersin NADW an overviewis presentedof where during the previousyear and the strengthof the Noah Atlantic
thesetracersare currentlyfound within the NADW components. Oscillation[Curry et al., 1998], vary from year to year. Thus the
This can be seenusing two zonal sectionsof CFC-11, one in the temperature,
salinity,anddensitycan vary from year to year, and in
subpolar
NorthAtlanticextendingfromtheIrmingerBasineastward someyears,surfacewater does not becomedenseenoughto form
andonein the subtropicalNorth Atlanticalong24øN(Plate 1). The CLSW [Lazier, 1995; Dicksonet al., 1996].
highestCFC-11 concentrations
are found in the near-surfacewater
ULSW has only recentlybeen recognizedas a distinctwater
alongbothsections,whichis expectedsinceCFCs enterthe ocean mass and a componentof NADW. Its discoverystems from
at the surface; however, here we are interested in the concentrations
observations
of an extensive
subsurface CFC
maximum
at 1200-
in thedeepwatercomponents.
1500 m in the subtropicalNorth Atlanticduringthe 1980swith a
Along the subpolar section, high CFC-11 concentrations potentialtemperature
rangeof 4ø-5øC[Weisset al., 1985;Fine and
extendquitedeep(-2000 m) westof the Mid-AtlanticRidge. The Molinari, 1988; Smethie, 1993] (Plate lb). This feature is also
water between -500
and -2000
m is Classical Labrador Sea Water
(CLSW) that most likely advectedinto the IrmingerSea from its
formationregionin the LabradorSea. CLSW is also observedeast
of the Mid-AtlanticRidgeas a layerof nearlyhomogeneous
CFC11 concentrationbetween -700 and 1500 m, although the
concentration
is lessthanwestof the ridge. In the IrmingerBasin
thereis a maximumat the bottom along the Greenlandcontinental
rise extendingto the middle of the basin. This is Denmark Strait
Overflow Water (DSOW) that flows over the Greenland-Iceland
Ridge. In the easternbasin there is a similar maximum at the
bottomalong the easternflank of the ReykjanesRidge. This is
Iceland-ScotlandOverflow Water (ISOW) that has flowed across
the Iceland-Scotland
Ridge. It has a lower CFC concentrationthan
the DSOW in the westernbasin. ISOW flowsthroughthe Charlie
Gibbs FractureZone into the westernbasin just south of this
section.Tritiummeasurements
nmdealonga similarsectionduring
TTO in 1981 [Ostlundand Root& 1990] showbasicallythe same
structure.
There are two prominentsubsurfaceCFC-11 maxima in the
westernbasin along the subtropicalsection(Plate lb). Both are
intensified in the west and extend well into the interior of the
westernbasin. Initially the uppermaximumappearedto be CLSW
andthe deepmaximumDSOW or ISOW. As will be elaboratedon
below,the deepmaximumis a mixtureof DSOW and ISOW, which
will be referred to as Overflow Water (OW), but the upper
maximumlies at a lighter densityhorizonthan CLSW. This latter
water mass,which has only recentlybeen recognized,is Upper
Labrador Sea Water (ULSW), which appearsto form in the
southwestern
LabradorSea [Pickart, 1992a;Pickart et al., 1996].
4. Formation
of NADW
observedas a maximumin verticalprofilesof tritiummeasurednear
the westernmargin of the subtropicalAtlantic in the 1970s and
1980s [Jenkins and Rhines, 1980; Ostlund, 1984; Olson et al.,
1986; Ostlund and Grall, 1987; Pickart et al., 1996], but their
significancewas not recognizeduntil the moreextensiveCFC data
setswerecollected. Pickart [ 1992a]was the first to recognizethat
this was not a variety of CLSW but a distinct water mass that
formed at a densityless than CLSW and hencereferredto it as
ULSW. (This water mass has also been referred to as Shallow
Upper NADW by Rhein et al., [1995]). RecentlyPickart et al.,
[ 1996] observednewly formedULSW nearits sourceregionin the
southernLabradorSea. They observeda smallweaklyrotatingeddy
that had an anomalouslylow temperatureand salinity(2.9øC and
34.78) andhigh CFC and tritium content,embeddedin warmerand
saltierwaterof thesamedensityflowingequatorward
in theDWBC.
Althoughits potentialtemperatureis similar to that of CLSW, its
salinityis significantlyfresher,and its densityis in the rangeof the
4ø-5øCwater with high CFC and tritium concentrations
observedin
the subtropicalAtlantic. This eddy was being erodedrapidly by
lateralmixing, and its lifetime was estimatedto be severalmonths.
Eddiessuchas this one apparentlyform by deepconvectionduring
winter near the southwest•narginof the LabradorSea, and Pickart
et al. [1997] have shown that this could occur in the Labrador
Current. Therethey quicklybecomeentrainedinto the equatorward
flow of the DWBC, are cappedby lessdensewater, and become
completelyabsorbedin the DWBC by the time it flows aroundthe
Grand Banks. The end result is a water mass that is warmer, more
salty,and lessdensethan CLSW.
The densestcomponents
of NADW form behindand overflow
the Greenland-Iceland-Scotland
Ridge. They are ISOW that enters
the easternNoah Atlantic acrossthe Iceland-ScotlandRidge and
DSOW that enters the western Noah Atlantic across the Greenland-
NADW is a complex of severalwater masses,as was first
demonstrated
by Wiist [1935] who, on the basisof salinity and
oxygenconcentrations,
classifiedNADW into three types,upper
NADW, middle NADW, and lower NADW. Today, it is known
that thereare four primaryNorth Atlanticwatermassesthat make
up NADW, two that form by deepconvectionin the openoceanand
two that are derived from the overflow of dense water across the
Greenland-Iceland-Scotland
sill. Wiist [1935] alsorecognizedthat
therearedeepwatersof non-NorthAtlanticoriginthatinfluenceits
characteristics. Water from the southern ocean, which is often
referred to as Antarctic Bottom Water, and Mediterranean Overflow
Waterarethe two mostimportant.
The least densecomponentsof NADW form by deep open
ocean convection during winter. The best known of these
componentsis CLSW, which forms by convectionextendingto
IcelandRidge.On the basisof tritium measurements
from the early
1970s, Swirl et al. [1980] showed that DSOW is much better
ventilatedthan ISOW, which will be elaboratedon below.
ISOW originatesfrom a densityhorizonof ~o0 = 28.06, which
lies near900 m in the NorwegianSeaandapproaches
the surfacein
the GreenlandSea [Swirl, 1984]. The primaryinput of this water
(-0.55øC and 34.91 in 1981 [$methie et al., 1986]) to the
northeastern
Atlanticis thoughtto be the FaeroeBankChannelwith
a sill depth of 850 m, but some flow also occurs acrossthe
shallower Iceland-FaeroeRise. This water has relatively low
anthropogenic
tracerconcentrations
[Bullister,1984; $methieet al.,
1986; $methieand Swirl, 1989], and a residencetime of ~45 years
behindthe Iceland-Scotland
Ridge has been estimatedfrom these
low concentrations[$methie and Swirl, 1989; $methie, 1993].
ISOW entrainssurroundingwater after enteringthe easternNorth
14,300
SMETHIE ET AL.: TRACING THE FLOW OF NORTH ATLANTIC DEEP WATER
Atlantic. Swirl [1984] deducesthat a mixtureof roughly60% pure
ISOW/40% northeast Atlantic water forms, which then flows into
the westernNorth Atlantic throughthe CharlieGibbsFractureZone.
CLSW is also entrainedinto this mixturebut primarily after the
water enters the western basin. Harvey and Theodorou [1986]
conclude
thatCLSWis entrained
priorto entering
thewestern
basin
and that this water is a mixture of 45% pure ISOW, 25% CLSW,
21% northeast Atlantic water, and 9% North Icelandic and Arctic
IntermediateWater. Deep water massescan also contribute,and
McCartney [1992] has shown that the southwardflowing water
along the eastern fl•nk of the Reykjanes Ridge contains a
componentof deepwaterthat hasbeenadvectedfrom the Southern
Hemisphere. The entrained northeast Atlantic water is well
ventilateddownto the depthsof inflow of pureISOW (-850 m) by
deep convectionduring winter [Harvey, 1982; Robinsonet al.,
1980]. Swirl[1984], Srnethieand Swirl [1989], Srnethie[1993] and
Doneyand Jenkins[ 1994] haveshownthat thisentrainmentis also
the major sourceof transienttracersin water flowing into the
westernbasinthroughthe Charlie GibbsFractureZone. This water
is lessdensethanDSOW enteringthe westernbasinandoverridesit
asit flowscyclonicallyaroundthe IrmingerBasin.
DSOW is the densest,coldest,and freshestcomponentof
NADW. It formsbehindtheGreenland-Iceland
Ridgeandentersthe
westernNorth Atlantic throughDenmarkStrait,which has a depth
of-600 m. Swirlet al. [ 1980] haveprovidedevidencethat DSOW
is formed primarily from upper Arctic IntermediateWater (AIW)
(-0.5øC and 34.75-34.85). Upper AIW is found in the upper few
hundred meters of the western Greenland Sea and the Iceland Sea
whereit outcropsat the surfaceduringdeepconvection
in winter.
A denser water mass also appearedto contribute-10% to the
formationof DSOW. Newly formedDSOW wouldbe expectedto
be well ventilatedand have a high anthropogenic
tracer content
becauseof the exposureof upperAIW to the atmosphere.This was
confirmedby the observationof high tritium concentrations
during
the GEOSECS expedition[Swirlet al., 1980] andcanbe seenin the
high CFC-11 concentrationat the baseof the Greenlandcontinental
slopeat 60øN (Plate lb). Livingstonet al. [1985] and Srnethieand
Swift [1989] showedthat the DSOW layer 450 km downstream
of
Denmark Strait was no older than 2 years,demonstrating
its rapid
inputto the North Atlanticfrom its formationregion.
The formation
of both ISOW
and DSOW
discussed above
involves the conversionof surfacewater to deeper water in the
Greenland/Iceland/Norwegian
region. This water flows acrossthe
Greenland-Iceland-ScotlandRidge with ISOW residing much
longer behind the ridge than DSOW before overflow. Recently,
Mauritzen [1996] has proposedthat the overflow watersoriginate
primarily in the Arctic Ocean. The sourcewater is warm salty
surface water that enters the Arctic Ocean from the North Atlantic in
the NorwegianAtlanticCurrent. This waterlosesits buoyancyand
enters the Arctic Ocean in two branches,the Fram Strait Branch and
the BarentsSea Branch [Schaueret al., 1997]. Both branchesare
modified
in the Arctic
Ocean and flow back toward the North
Atlantic as denserwater massesthroughFram Strait. The Barents
Sea branch flows into the Norwegian Sea and feeds ISOW. The
Fram Strait Branch flows toward the Denmark Strait along with
Atlantic
water that has recirculated
within
Fram Strait to feed
DSOW. Mauritzen [1996] refers to this water as Arctic Atlantic
Water. This mechanismprovidesa steady(nonseasonal.]
input of
water of the appropriatedensityto the overflowregions. Dickson
and Brown [ 1994] showedthat the flow of DSOW was steadywith
respect to seasonfrom 1986 to 1991. However, using recent
hydrographicand acousticDoppler current profiler data, Bacon
[1997] suggeststhat this transportdecreasedsignificantlyafter the
early 1990s, and Dicksonet al., [1999] presentmore recentdata
indicatingthat the characteristics
of DSOW havechangedsincethe
It could alsobe formedby a combinationof both processes.For
example,Strasset al., [ 1993]havepresented
evidence
thata portion
of DSOW forms by mixing, within the East GreenlandCurrent,
betweenupperAIW andAtlanticwaterthathasrecirculated
within
Fram Strait.
All of the components
of NADW eitherformin the subpolar
westernNorth Atlantic or are advectedto this region. DSOW
entrainsoverlyingGibbsFractureZoneWater(GFZW) asit enters
the North Atlantic and further mixes with it as both water masses
flow alongthe westGreenlandcontinental
slopeandthenaround
the LabradorSeain a deepwesternboundarycurrent(alsoreferred
to as the DeepNorthernBoundaryCurrentby McCartney[ 1992]).
DSOW also entrains some Antarctic Bottom Water that has entered
theregioneitherthroughtheCharlieGibbsFracture
Zoneor froma
deep flow along the westernflank of the Mid-AtlanticRidge
[McCartney,1992]. DSOW mayalsoentrainsomeCLSWandless
denselocal watermassesas it flows into the Atlanticjust southof
Denmark Strait. GFZW is sandwiched between DSOW below and
CLSW above and thus mixes with both of these water masses.
Finally,ULSW entersthesystemneartheoutflowfromthesubpolar
Atlanticto the subtropicalAtlantic.
5. Construction of Sectionsand Maps
Theseresultsare basedon a synthesis
of mostof the CFC data
collectedin the North Atlantic through1992. Sinceour analysisis
focused on the basin-wide distribution of CFCs, we have relied
mainlyon datacollectedin the late 1980sandearly 1990sto obtain
a quasi-synoptic
picture of the three-dimensional
distributionof
CFCs in the North Atlantic Ocean. A listingof all of the dataused
in this analysisis given in Table 1.
To
illustrate
how
CFCs
track
the
flow
of
the
various
components
of NADW in the DWBC, 12 CFC-11 sectionsnormal
to the flow of the DWBC (Figure3) takenbetween1988 and 1992
are presented(Plate 2). Thesesectionsstartnearthe origin of the
DWBC in the subpolarNorth Atlanticand extendto 10øS. Each
sectionis accompanied
by a plot of O/Sand0/CFC-11 (Figure4) to
illustratehow the variouscomponents
of NADW fit into the largescale hydrographic structure and are modified during the
equatorwardtransit. In theseplots the coresof the variouswater
massesare highlightedas describedin the Figure4 caption,and a
summaryof the corepropertiesis givenin Table2. TheseCFC-11
data have not been normalized to a common date, which must be
taken into accountwhen comparingCFC concentrations
between
sections.CFC-12 data for thesesectionsshowessentiallythe same
featuresandarenot presented.
AlthoughCFCs have been measuredin muchof the North
Atlantic,therehas not been a synopticsurvey,and the basin-wide
distributioncan only be mappedusing data from a number of
different cruises taken during different years. This presentsa
problemwhenusingCFCsandothertransienttracerssincetheinput
varies with time and the distributionis not in steadystate. To
investigatethe basin-scalecirculationpatternof recentlyformed
NADW, we preparedmapsof CFC-11 for the coreof ULSW and
the core of Overflow Water (OW) (Figure 5) usingdata collected
between 1988 and 1992 and one cruise in 1986 (Table 1). The
mapswere constructedby finding the maximumconcentrationat
each stationbetween34.5 and 34.7 Ol.5 with 0 >3.5øC for ULSW
and between 45.8 and 45.9 o4 below 2500 m for OW. The
maximumconcentrations
werethenmultipliedby a correctionfactor
(givenin Table 1) to normalizethe concentrations
to 1990. All the
data usedwere adjustedto 1990 becausemuch of the data were
takenin that year,and it is alsothe midpointof the 1988-1992time
period. The normalizationfactor was determinedas follows. The
late 1980s.
profilesof CFC-11 and CFC-12 in each of the upperand lower
The exact mechanismby which DSOW formsis not known, maximawere vertically integrated,and the CFC-1l:CFC-12 ratio
but thereis evidencesupportingboth mechanisms
discussed
above. was calculatedfor the integratedvalues. This seawaterratio was
SMETHIEET AL.: TRACINGTHE FLOW OF NORTH ATLANTIC DEEPWATER
14,301
Table 1. Data Used in This Study
Correction
Cruise
Reference
a Stations
STACS 3
1
1-61
Dates
U/Lb
2/89-3/89
1.22/1.20
65-79
STACS 4
Trident
2
3
1-67
1-39
Principal
Use
•
Investigator
CU,CL,AU,AL,VS
R. Fine
CU,CL,AU,AL
CU,CL, AU,AL, VS
R. Fine
R. Fine
1.17/1.17
6/90-7/90
8/92-9/92
40-61
62-71
1.00/1.00
0.74/0.74
0.71/0.70
0.74/0.74
OCE 134
4
1-76
6/83-7/83
NA
AU,AL
W. Smethie
WBEX
4
1-51
4/86-5/86
__a
CU,CL
W. Smethie
18-51
EN 214
EN 223
5,6
7,8
1-44
1-10
HE 06
9
1-74
AU,AL
6/90
3/91-4/91
11-44
7/92-8/92
75-112
1.0/1.0
0.95/0.86
CU,CL,AU,AL,VS
CU,VS
W. Smethie
W. Smethie
0.95/0.95
CL,AL
0.71/0.70
CU,CL,AU,AL, VS
W. Smethie
0.74/0.74
METEOR 14
METEOR 16
METEOR 18
10
10
11
627-682
286-343
558-622
10/90
5/91-6/91
9/91
1.0/1.0
NA
0.95
CU,CL,AU,AL
AU,AL.
CL,VS
M. Rhein
M. Rhein
W. Roether
METEOR 22
10
474-537
10/92-11/92
0.70/0.70
CU,CL, AU,AL,VS
M. Rhein
3'TONAS_leg
7
TTOTAS_leg1
TFOTAS_leg
2
12,13
13
13
220-249
4-32
55-94
9/81-10/81
12/82
12/82-1/83
NA
NA
NA
AU,AL.
AU,AL.
AU,AL.
R. Gammon
R. Weiss
R. Weiss
SAVE 1
14
11-170
11/87-3/88
1.39/1.39
CU,CL, AU,AL
A. Putzka
(Legs1,2,3)
SAVE2
14,15
309-379
4/89
1.20/1.20
CU,CL,AU,AL
(Leg6)
HUDSON 92014
OCE 202
R. Weiss
W. Smethie
R. Weiss
W. Smethie
16
17
1-52
1-17
6/92
7/88-8/88
0.91
NA
CL,VS
VS
E.P. Jones
J. Bullister
al, Molinariet al., [1992];2, Johnset al., [1997];3, R.A.Fine(personal
communication,
1999);4, Smethie
[1993];5,
PickartandSrnethie
[1993];6, Pickartet al., [1992];7,Pickartet al., [1996];8, McKeeet al., [1995];9, Brydenet al.,
[1996];10,Rheinet al., [1995];11, W. Roether
andA. Putzka(personal
communication,
1999);12, Physical
and
Chemical
Oceanographic
DataFacility[1986];13,Weiss
et al., [1991];14, Weisset al., [1993];15,Srnethie
et al.,
[1992];16,E.P.Jones
(personal
communication,
1999);and17,DoneyandBullister[1992].
•dultiplication
factors
used
toadjust
theCFCconcentrations
to 1990,fortheupper
CFCmaximum
(U) andlowerCFC
maximum(L). Seethetextfor detailedexplanation.
cCdenotes
thatthecruisewasusedin a mapof CFC-11concentration;
A denotes
usagein a mapof CFC-11/CFC-12
derivedage;U denotes
usagein a mapof theupperCFCmaximum;
L denotes
usagein a mapof thelowerCFC
maximum;andVS denotesusagein a verticalsectionof CFC-11.
dMultiplication
factors
used
toadjust
theWBEXCFCconcentrations
to 1990fortheupper
CFCmaximum:
stations
1-9,
correction
1.54;stations
10-18,correction
1.74;stations
19-22,correction
2.08;stations
23-25,correction
1.98;stations
26-36,correction
2.05;station
37, correction
1.83;stations
38-49,correction
1.92;andstations
50-51,correction
1.82;
and for the lower CFC maximum:stations5-12, correction1.80; stations13-17, correction1.90; stations18-23,
correction
2.00; station24, correction
2.10; station25, correction
2.30; station26, correction
2.10; stations27-28,
correction
2.00;station29, correction
1.90;stations
30-31,correction
2.00;stations
32-33,correction
2.10; andstations
34-50, correction 1.95.
thenconvertedto an atmospheric
ratio by dividingby the CFC1l:CFC-12 solubilityratio [Warnerand Weiss,1985], andthe year
of formationwas determinedfrom Figure 1. The annualpercent
changefor theyearof formationwasestimated
fromFigure2, and
this rate of changewasmultipliedby the time differencebetween
Age maps(Figure6) wereconstructed
usingdataacquired
between 1981 and 1992 at the maximum CFC- 11 concentrationsin
ULSW andOW for the intervalsgivenabove. The seawater
CFC11:CFC-12 ratio was calculatedfor each stationand convertedto an
atmospheric
ratio of the partialpressure
of CFC-11/CFC-12as
i990 and the cruise date to determine the normalization factor. For
described
in theprevious
paragraph.The resulting
partialpressure
waterformedsince1978 it wasnot possibleto calculatethe yearof ratios were then comparedwith the Northern Hemisphere
timehistoryof theratios(Figure1) to givetheyearof
formationusingthe CFC-1l:CFC-12 ratio becausethe ratio was atmospheric
fromthecruisedateto givean
nearlyconstantfrom 1978 to 1990. The averageannualrate of formation.Theyearwassubtracted
whentheconcentration
of eitherCFC
changefor CFC-11 from 1978to 1990was-4.5%, andthisrateof age.Ageswerenotestimated
of thelargeerrorin theratioat these
changewas usedto determinethe normalization
factorfor water was<0.02pmolkg'• because
formed after 1978.
low concentrations.
Also,ageswerenotcalculated
for thesubpolar
14,302
SMETHIE
ET AL.' TRACING
THE FLOW OF NORTH ATLANTIC
-...--...--.:.z..•'?
:::..-..-•:
DEEP WATER
5O
':-':•.
•.• .......
....
......
.........
. -.'" '
7:;:;".
-•
': '•
..
....:*:6•":':'.:.
"
.
"7.:.
•::
../½
•.
';'-:?'"':
-;: . ....
..
'•.
.
;..-;.*:' ::-.
-%
**•
::
:-.,:.
,:-:'?--
2O
**7*******
'• '•**':"}'::":
.
......... •i:i;•!'
..-•-.x-.-..--9•
• ":::::.:'•:
'
' .:::•
:: •
..,:½'.*'
::-:;::(;•
•.-%.,
,..:,.,...
:--......
:...:.. •..•.•..,.
::':,-
*%X'"'"::::•.:
;:.*'
•:*•.
10
--
...,z;,-,,':.•s,•-,:::•:?
...,'•**:,
,:,.
.......
..,::.,:..
......:
2O
.'.....x:...-........
.
•.
80
70
60
50
.
,.
:.,.
•.:..:,1,:•;,½,:.::---:-•
.:::
40
30
20
10
0
10
Figure 3. Stationlocationmap for 12 sectionsthatcrossthe flow of North AtlanticDeep Water (NADW) in
the Deep WesternBoundary(DWBC). The sectionshave been labeledA-L. Data sourcesare as follows:
sectionA, Oceanus-202,July 1988 [Doneyand Bullister, 1992]; sectionsB and C, Meteor 18, September
1991 (A. Putzkaand W. Roether,unpublished,1999); sectionD, Hudson92014, June 1992 [Pickart et al.,
1996]; sectionsE and F, Endeavor223, April 1991 [Pickartet al., 1996]; sectionsG and H, Endeavor214,
July 1990 [Pickart and Srnethie,1993]; sectionI, Trident,August1992 (R.A. Fine, unpublished,1999),
sectionJ, STACS 3, February1989 [Molinari et al., 1992]; andsectionsK andL, Meteor 22, November1992
[Rheinet al., 1995]. Also plottedare stationlocations(stars)for the subpolar(Meteor 18) and subtropical
(Hesperides
06) trans-Atlanticsectionspresentedin Plate 1.
region becauseULSW is not found in this region and the CFCData from the 20øW sectionoccupiedin 1988 [Doney and
1l:CFC-12 ratios in OW for this region indicatedthat this water Bullister, 1992] are not used for the mapsbecauseULSW and the
formedafter 1978 when the ratio did not increasemonotonically mixtureof GFZW and DSOW that form OW is confinedmainly to
with time.
the western basin. At the far northern end of the 20øW section there
Maps of dilutionfactors(Figure7) wereconstructed
usingthe
age data presentedin Figure 6. CFC ratio-derivedages were
converted to concentrationsusing the Northern Hemisphere
atmospherictime history (Figure 1) and the Warner and Weiss
[1985] solubilityfunctionwith the estimatedpotentialtemperature
and salinity at the time of formation. For ULSW a formation
temperatureand salinityof 2.9øC and 34.78 were usedon the basis
is water with a CFC maximum near 36.56 o•.s, and it has OIS
properties similar to ULSW. However, the water derives its
propertiesfrom incompletemixing between ISOW and northeast
Atlantic water [Doney and Bullister, 1992]. Also, Pickart et al.
[ 1996] find that the eastwardextentof ULSW is largelyblockedby
the North Atlantic Current. Along 20øW between45ø and 25øN,
water at the density of oks = 36.56 is warmer and saltier than
ULSW and is apparentlymostly MediterraneanWater. Although
of the observationsof Pickart et al. [1996]. For OW the O/S
characteristics
of pureDSOW (-0.5øCand 34.80) wereusedbecause there is a CFC maximum at the bottom in the northeasternbasin, the
mostof the tracercontributionis from the DSOW component(see
discussionthat follows on OW and Smethieand Swirl [1989] and
Smethie[1993]). Thesecalculatedconcentrations
were divided by
the measuredconcentrations
to get a dilution factor.Only data with
CFC-11andCFC-12concentration
>0.02pmolkg'• wereused.The
OW in the westernbasin derives most of its CFC signal from
DSOW, sotheseconcentrations
are not includedin the map.
6. Discussion
dilutionfactorswere then multipliedby 0.7 for both ULSW and
The equatorwardpathways followed by the NADW
OW to take into consideration
the best estimateof the percent
equilibration
with the atmosphere
at the time of formation[e.g., componentsare quite complex. There is a decreasein the
Pickartet al., 1996](seealsothefollowingdiscussion
on DSOW).
CFC concentrations
in the equatorwarddirectionthat reflects
SMETHIE ET AL.' TRACING THE FLOW OF NORTH ATLANTIC DEEP WATER
(A)
14,303
M18
CFC-11 (pmol/kg)
, ...........
ß .il
'
:
.' :
ß
., ,•-
'.• .
2.50
'i
i.. 0%
2.00
ß
ß
. .. 211.•. .
ß
:
12.00
1.50
.-
: •.00
0.50
0.00
0
1000
DISTANCE FROM STN. 558 (km)
He-06
(B)
CFC-11 (pmol/kg)
o
'"1- i ',, i, .n==a,
'ql.•5' '
:.. .
.
ß• 0 '
ß
' "ß
0.10
i,
ß
ß
.
.
,t.
,.,. ,. ' ' .
5
. ß ..
ß
-2
0.2
0:.
.
.
''
i
-4
,,
l,
ß
ß
o
-5
ß
5
. . .
ß
'0.05'
....
'
'
.o
•1o
ß
-6
o
1000
2000
3000
4000
5000
DISTANCE FROM STATION 101 (km)
Plate 1, Verticalsections
of CFC-11 in (a) the subpolarNorthAtlanticOceanand (b) the sub-tropical
North
AtlanticOcean.The subpolar
sectionis fromtheMeteor18cruisein 1991,andthesubtropical
sectionis from
theHesperides
06 cruisein 1992. SeeFigure3 for stationlocationsß
.
14,304
SMETHIEET AL.:TRACINGTHE FLOWOF NORTHATLANTICDEEPWATER
124
0
-I
:
-2
O0
.
-3
-5
I00
2oo
300
4o0
50o
600
I
700
1oo
DISTANCE (kin)
200
3.00
400
500
DISTANCE 0tin)
(D)
3.5
.35
-1
3.0
,
3.5O
3.L66
.
2.5
ß
3.00
o
2.O
-.3
1.5
ß
-
1.0
O.5
oo
-5
100
200
300
400
DISTANCE (km)
500
600
0
100
200
300
400
DISTANCE (km)
Plate2. Vertical
sections
ofCFC-lI (pmol
kg-•)across
theDWBC.SeeFigure
3 forsection
locations.
Five
density
lines
arealso
plotted.
They
are,from
shallowest
todeepest:
34.56
and34.66c•.5,45.82,
45.86,
and
45.90 c•4.
SMETHIE
ETAL.'TRACING
THEFLOWOFNORTHATLANTIC
DEEPWATER
14,305
-1
-3
.
t .60
- 0.80
-5
-5
200
IOO
300
400
0
500
100
(G)
300
o)
(H)
10
38
41
200
DISTANCE (kin)
DISTANCE (kin)
7
4
1
o
2
•.:•
ß
1
s
8
11
o
-oo- • -
,.
.
.
.
' O0C60
-1
-I
• '•
!:•0 '
0.60
•.'
o
No
0.40
-2 1•'' ., •
.
ß
•
'"'
--"'•
-4
.-4
i U
0.0
i
-3
4 m
0.30
5.90
-5
100
200
DISTANCE (kin)
300
0
100
DISTANC•
200
(kin)
ß
0.
ß.
300
o
IOO
200
DISTANCE (km)
Plate 2. (continued)
14,306
SMETHIE
ET AL.: TRACING
THE FLOW OF NORTH ATLANTIC
DEEP WATER
(J)
(L)
48
0 j
n
46
"
"
,
$37 $36 $34
,
No
Data
ß
-1
44
0 [ ' ' '..........
[
.
08
$32
53o
.
No
ß Dntn
o.o
0.10 • •: '
I
ß
104
.,4
t,
-2
0.04
0.04
() 04
0.08
01
i
!
-5
-5
0
100
200
300
400
500
600
100
700
DISTANCE (km)
200
DISTANCE (km)
(K)
508
503
5
501 499
495
492
0 S
No
Data
.
-1
No
Data
-5
100
200
300
400
500
600
DISTANCE (kin)
Plate 2. (continued)
700
800
100o
300
SMETHIEET AL.'TRACINGTHEFLOWOFNORTHATLANTICDEEPWATER
a
SectionA (NortheastAtlantic)
SectionA (NortheastAtlantic)
SectionB (NortheastAtlantic)
SectionB (NortheastAtlantic)
14,307
9
8
7
6
.-. 5
o
.•.•
f4
3
2
b 8
'•4.7
34.8
34.9
35.0
3•.10
Salinity(psu)
!
'
CFC-11 (pmol/kg)
•
Figure4. Potential
temperature/salinity
andpotential
temperature/CFC-11
plotsforthesections
presented
in
Plate2. Linesof constant
density
areplotted
onthepotential
temperature/salinity
plots,oi.5is usedforthe
upper
portions
ando4isused
forthelower
portions.
Potential
temperature/salinity
ranges
areplotted
for(a)
pureIceland-Scotland
Overflow
Water
(ISOW)andNortheast
Atlantic
Water
(NEAW),
and(c)Upper
Arctic
Intermediate
Water(UAIW)andArcticAtlanticWater(AAW).Notethatthesalinityscalefor Figure4a is
shifted
toa higher
range
thaninFigures
4b-41
toinclude
NEAW.Points
forthecores
ofthefollowing
water
masses
aredesignated
bydifferent
symbols
asfollows:
Classical
Labrador
SeaWater(CLSW),square;
Upper
Labrador
SeaWater(ULSW),circle;section
E western
boundary
slopewater,triangle;
ISOW,inverted
triangle;
Gibbs
Fracture
ZoneWater(GFZW),
star;incompletely
formed
ISOW,diamond;
Denmark
Strait
Overflow
Water(DSOW),
hexagon;
andAntarctic
Bottom
Water(AABW),plussign.Seetextfordiscussion.
andmaps(Plate2 andFigures4-7). Note, however,
the temporallyincreasingatmospheric
sourcefunction, sections
were normalizedto the year
advection,mixing,entrainment,
and recirculation
in gyres. that the map concentrations
Thesections
presented
arefromIcelandto 10øS(Plate2) and 1990, but the section concentrationsare presented as
haveseveralfeaturesin common.The highestconcentrations observed.
are observedin the inshore region coincidentwith the
DWBC. Intermediate concentrations are observed in the 6.1UpperandClassical
LabradorSeaWater
adjacentregionnearthe interior,wherethe spreading
of
tracers
ispredominantly
in deepgyrerecirculations.
Seaward
of therecirculations,
concentrations
are nearblanklevels,and
6.1.1.Subpolarto 38øN.CLSWhashighCFCconcentrations
andis observedas a maximumalongall of the sectionsto 38øN
CFCconcentrations
andlowest0 and
the tracer distributionis probablydominatedby lateral (Plate2, C-F). Thehighest
values
in CLSWarefoundonsection
D extending
fromthe
mixing.Thediscussion
belowincludes
references
to boththe salinity
14,308
SMETHIEETAL.:TRACINGTHEFLOWOFNORTHATLANTICDEEPWATER
SectionC (IrmingerSea)
(2 8
SectionC (IrmingerSea)
5
34.8
35.0
34.9
35.10
1
2
Salinity
(psu)
3
4
5
4
5
CFC-11(pmol/kg)
SectionD (Labrador
Sea)
SectionD (LabradorSea)
6
5
7•-• -34'5
•
I
* ' GFZW
i
4.7
34.8
34.9
35.0
Salinity
(psu)
e
8
35.10
I
Section
E (Newfoundland
Basin)
2
3
CFC-11
(pmol/kg)
Section
E(Newfoundland
Basin)
' '
i
. '
I
'
i
'
T
,
ß. ...:... i ..
4
•'
'-"
' 7:
,
'
' •'."":'
' :.'-*--O'--"-:.
:;:,%,• • .
'.""•':",
i.
o:;
..'"
] •
'
'"•
Salinity
(psu)
I
Figure 4. (continued)
':i •'"•
CFC-11
(pmol/kg)
5
SMETHIE ET AL.: TRACING THE FLOW OF NORTH ATLANTIC DEEPWATER
SectionF (55'W)
SectionF (55'W)
t
I-
•'-•
I
14,309
'
J.-• • --•,
2
Salinity(psu)
SectionG (Northof Gulf StreamCrossover)
SectionG (Northof GulfStreamCrossover)
g 8
i
t4'.7
]4.8
]4.•
½•.o
0.5
•.8.o
1.0
1.5
2.0
Salinity(psu)
CFC-11 (pmol/kg)
SectionH (Southof GulfStreamCrossover)
SectionH (Southof GulfStreamCrossover)
h 8
.......Fi 34.1•i,
_.-•...... ,.
• © ß DSOW•
,. e ßUL-SWI
-•4'.7
34.8
34.9
35.0
35.0.0
•.•o
CFC-11 (pmol/kg)
Salinity(psu)
Figure 4. (continued)
2.0
14,310
SMETHIE ET AL.: TRACING THE FLOW OF NORTH ATLANTIC DEEP WATER
SectionI (24'N)
SectionI (24'N)
I.
I
,
34.7
34.8
34.9
35.0
I
35.0.0
2.0
Salinity(psu)
CFC-11 (pmol/kg)
SectionJ (5-10øN)
SectionJ (5-10'N)
j 8
i
,
, © 'IDSO,W
•ULSW
I
-h'.7
35fl.00
•
I
0.•$
0.04
0.12
0.16
0.20
CFC-11 (pmol/kg)
k 8
SectionK (35' W)
•'
't 3.9
__--.-34.1
SectionK (35'W)
!'--......
,
_L........ •"
!"'''!':' _%- "-7:"•"•:
_;34.5
-4
ß '•:.:
--;':.•;".""..t"
::' '
_ 4-34.7
] '-'•I,;--_•
,,•• J.......
-•--.•..•,..•46.0
•-'
,
......:•__.
_-i --,
........---r------
I
.
; .-46.2
',
34.8
,
......
34.9
35.0
35 fl.00
i
0.04
Salinity(psu)
!
6
O. 8
i
i
0.12
CFC-11 (pmol/kg)
Figure 4. (continued)
0.16
0.20
SMETHIE ET AL.: TRACING THE FLOW OF NORTH ATLANTIC DEEP WATER
14,311
SectionL (10'S)
SectionL (10'S)
I 8
'
ß
'
;
,,
34.8
34.9
Salinity(psu)
35.0
352.00
I
I
/ e ' ULSW
I
I
i c'DSOW
''4-+
-•4'.?
I
I
0.04
0.08
•
0.12
I
0.16
0.20
CFC-11 (pmol/kg)
Figure 4. (continued)
Table2. CorePotential
Temperature,
Salinity,DensityandCFC-11Concentration
fortheVerticalSections
Crossing
the
DeepWesternBoundaryCurrent
Potential
Water
Section
A
B
C
D
E
F
G
H
I
J
K
L
Salinity,
CFC-11,
øC
psu
pmolkg'•
Formed ISOW
CLSW
ISOW
CLSW
ISOW
CLSW
GFZW
DSOW
CLSW
GFZW
DSOW
4.3-4.6
3.5-3.8
2.7
3.3-3.6
2.6-3.0
2.9-3.2
3.0-3.3
1.1-2.0
2.9
2.8-3.0
1.8
35.00-35.01
34.90-34.91
34.98
34.88-34.89
34.97-34.98
34.85-34.87
34.99-35.00
35.87-35.88
34.88
34.92
34.89
2.5-2.8
1.4-1.8
2.4
1.7-2.3
1.6-1.9
3.3-3.6
1.3-1.6
2.9- 3.1
3.6-3.8
1.2-1.4
2.1-2.3
34.51-34.59
34.60-34.65
SlopeWater
Mass
Temperature,
Density
(71.5
(74
Incompletely
45.81
34.62-34.65
45.75-45.81
34.65-34.66
45.68-45.71
45.88-46.02
34.68
45.72-45.75
45.91
3.0-3.7
34.85-34.87
3.2-3.7
34.50-34.65
ULSW
CLSW
GFZW
DSOW
ULSW
CLSW
OW
3.2-4.0
3.0-3.2
3.2-3.4
1.8-1.9
4.4-4.6
3.4-3.9
2.0
34.80-34.82
34.86-34.88
34.94-34.95
34.895
34.89-34.93
34.88-34.92
34.905
3.1-3.5
2.6-3.0
0.5-0.8
1.5-2.0
2.2-2.4
2.4-2.5
0.8-1.0
34.49-34.60
34.65-34.66
ULSW
CLSW
OW
ULSW
OW
ULSW
OW
ULSW
OW
ULSW
OW
ULSW
OW
AABW
4.1-5.0
3.8-3.9
2.0-2.4
4.1-4.8
1.9-2.5
4.2-4.9
2.0-2.2
4.0-4.4
1.9-2.1
3.6-4.3
1.9-2.2
3.4-4.0
1.9-2.3
0.2-0.4
34.96-34.98
34.95
34.91-34.92
34.99-35.04
34.895-34.93
35.005-35.04
34.90-34.91
34.97-35.005
34.900-34.905
34.98-34.99
34.89-34.91
34.975
34.89-34.91
34.70-34.71
1.4-1.6
0.9-1.3
0.6-0.75
0.8-1.0
0.4-0.5
0.6-0.7
0.3-0.4
0.08-0.17
0.06-0.09
0.08-0.11
0.06-0.08
0.04-0.06
0.02-0.03
0.03-0.04
34.49-34.60
34.62-34.65
45.65-45.69
45.91-45.92
34.49-34.52
34.59-34.64
45.89
45.83-45.88
34.55-34.62
45.82-45.90
34.55-34.62
45.85-45.88
34.58-34.65
45.86-45.89
34.59-34.69
45.86-45.90
34.62-34.71
45.84-45.88
46.03-46.04
14,312
SMETHIE
ET AL.' TRACING
THE FLOW OF NORTH ATLANTIC
DEEP WATER
5O
......
...
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,....-:•.•..:.•:•.:
3O
:..:•:"•:',..:.-.
..%..:...:-..•:•:
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::
4O
.•.,.•.:•
..........
•
2O
..• :•:-::
..•[..:.•:•:
.:.•:'
,.. •..:::....
10
,
i
....
.....:?...
.•.•:j'"'"':•:::.:•..
•:.•
:
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.
•
. '•%:.
.':'::.•::L•'...
*
• ......
:•:::.:•'
10
,..
•
(pmol/kS)
.
/:......
......
:•::::.•.
-•--::•
:::::::::::::::::::::::::::
....
•?'? ':
•::•..:.:•::•:•:•:
:: .•j::•i-•:::.
2O
,%•:•:•::•
......
80
70
...•:.
60
50
40
. o oo
30
.........
20
..:..::•:•.-,•.
10
0
10
Figure 5a. Lateralmapof the maximumCFC-11 concentration
in UpperLabradorSeaWater(ULSW). See
Table 1 for datasources.Thesedatahavebeenadjustedto a commondateof 1990asdescribed
in thetext.
80
70
60
50
40
30
20
10
0
10
Figure 5b. SameasFigure5a but for overflowwaters(OW).
14,313
5O
4O
3O
;::.:"-"•:'..-.-.'*-•;-
20
ß
:;:.•.•;•'...
....
"Z'. ......
-• •
•'"-";.:•:
:. -•....
--,.*,,-...:
•.,,
....
'4 .'.:%•&ß
2O
--• -.- --
•,,-•:-:•;•;:;..::
............
::,:;:-..•.-.•
....... "-- ,, -
.,-..'-"'
*
..:':.•.
.-....:::.:.
:% •
'•;'•:½•-%•:•:•
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..,•..•
. .................
.:
,•..if:, ½-
........-..
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v,-.................
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: ' •
½½•½'
'.......
.
.
..::::.
.,.;•:.-":•;:.... ::'•;::•:::•2:•'•,:-:
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:.
x:,.;::T;:.
=.:;
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•:=:::::•...%.
,.'• . :..--•.•.•---:•.,:
•:•.. . ......
;.::
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:•.,:;.;...•:
.:':'::•:•;&•,...:
.............
.• •:.:.
........
.
CFC
age
:-:.
?7' ,-•--
(years)
ß
•:•;
__
.•-•?
:•
:...;.lO
....
;?•"
:.
.,
........
:::
80
70
60
50
40
30
20
.
::
10
0
10
Figure 6a. Lateralmapof theCFC-11:CFC-12ratioagefor ULSW. SeeTable I for datasourcesandthe text
fortheprocedure
used
tocalculate
ages.Thedashed
linerepresents
the0.02pmolkg-I isoline.Ageswerenot
calculated
forCFCconcentrations
<0.02pmolkg'l because
of thelargeuncertainty
in theCFC-II:CFC-12
ratio at low concentrations.
"o
,<,•....5•'•*,.•ii•i:t'::-'-'-':..:'"•.',;-'•'•i,
;.:."..•.,
.. .:•::•':'"•---'
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50
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*;. ':..7•'••.-:.•4;•.:=====================
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'ß
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-:?;;i;.'•:;;•:::;•:•;.;:.;
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.-i5: .......
•:::•::•::•½•:E.'"':"--..5:-'
.:-::'-•
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..........
:'?":,"';
..... .:•-½g"•-';•..
25 •
":
::""•.:.;•E
:•
.:•::::,..•..•.%•..,......:•::•::
•.•:. .
• .... ..
• "--•:-•5•%::;4:(
•-•L--ß ß •
• ;,:..Z 30
'• ':.-,.
•:•::•:•,
•.,::•:•.•:•:•:;;•.•;•:::;•E•.•:?
-'.••..':•;•i•:."':
?•g;•;•:
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7? ß.,•;•.........
.,.:.:•:
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......
::%:.,.
-% .•
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,•..
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•g•;.:•.. •:..,:::::.:..-:,
..................
.%.,•:.?::..
....
:•.?½•,•
:•..•.
ß-'
CFCage
.......
:.... •
(years)
70
•
......................
......
?
.:
.•.:.•.
..d?>.
%
80
.
-:.:•..,/•.•,:•:•::•;;;•?:':.
• ..:::
'?:"•:::•-.:
.....
.
.
.
"
...:.,
60
50
40
-:•;::.
•:.•:.
30
20
10
Figure 6b. SameasFigure6a but for OW.
0
10
14,314
•
.......
ß....
• ?•-,:.•': ....•a•
.:2.::•i%:'?
, ß "=., :.•:-•'=.;•;.:'.•:.
'=•
<•}F"•"•'•;=•m:.•/•:•:'•
.............
-=:•:.==.::
'-.....%;..;-•?'"'•='•'•=
'"'77.":C:::::::::::::::::::::::::::::::::::::::::::::•.'
........
"•}•.:•..
,•
,,...:=.•:•::==<:i
..........
-'•:•........
= =•
'=•:"• ß• ' '•.•k.':.
•
'- • '" • ß s, •'57•?'
:":•<=•-• . •-*.-=a•*•()•:g•'•?•D;.
• •g?.•;•:'=
•½:=•gS•=•BE•:•B
'•
'=:::::•'.==37•-•
ß-.
' t,.
•,
............
. q•e=::=•'T'.:??•:'•:[75
....
.........................................
•
.
• •...-
ß
Zq:•:•='?•:==...............
•..........
'5•:•:•:•'•:;:
.:,?:=::'
*•:•=
.x.-:.::•.-•?-,':,
....
ß
y'=•-...:::....=..'
.......
----•=.•=
?=:,•
.• .%•
......
:•=•.
....x
...
•-.--.-%..
;:
..
Dilution
80
:.•..;--.
:.;
.... ,-
•:•,:-.
....
?:---: •[
ß
-•.
•:.,: ULSW
.',-:%
,/
.•:•-•.•
...................
:.....
-.........
:-• .....
70
60
50
40
30
:.,:•.:":•:•':'•':':'•'•;•i•'•:•:-•5•
•':'•:::"
....
,, 20
20
10
0
10
Figure 7a. Lateral map for dilution factorsfor ULSW. See Table I for data sourcesand the text for the
procedure
usedtocalculate
dilution
factors.
Thedashed
linerepresents
the0.02pmolkg-I isoline.Dilutions
werenotcalculated
forCFCconcentrations
<0.02pmolkg'l because
of thelargeuncertainty
in theCFC11:CFC-12 ratio and henceage at low concentrations.
5O
4O
:"
..::..,•
1'•/...
:..'...' '-.•i::"'::.
'.
-':•iC•'"o•, ø o
' o ....: ø t
........
• '•:. /' "t,. .,:
..
_- -
3O
_
:..
"--"'if
i
.. 4k,,,•...
i.:!:: I
ß
2O
I
•:• ..•..
'•......
..
....
ß
:..."'•...
•%• .::. •--
"-•'."7":
:::.'•'
ß....
'"'•X
10
-. •:... '•
...
........
,.•
':4j•..i:•.
i
10
Dilution
Factor
ow
80
70
60
2O
50
20
10
0
Figure 7b. SameasFigure7a but for OW.
10
SMETHIE ET AL.' TRACING THE FLOW OF NORTH ATLANTIC
30
DEEP WATER
14,315
Along section E in the Newfoundland Basin, high CFC
concentrations
extend
down
to -2000
m and both CLSW
and
ULSW canbe identifiedasCFC maximain the upper1800 m (Plate
25
2e). CLSW is warmer and saltier than is observedin the Labrador
Sea,andthe CFC-11 concentration
is lower (Figure4e andTable 2).
Thesepropertiesare all consistentwith transportof CLSW in the
DWBC from its source region and mixing laterally with older,
warmer,and saltierCLSW. ULSW is observedat a singlestation,
15, in an eddy suchas the one discussedin the sectionon NADW
e/
<
15
o
formation, but farther downstream from the formation site. Its CFC-
lO
11 concentration
and potentialtemperatureare slightlyhigher,and
its salinityand densityare lower thanthe underlyingCLSW (Table
ß CFC-11.CFC-12 Age
ß CFC-113:CFC-11 Age
0
'
i
i
i
i
0
i
5000
i
i
I
i
i
10000
i
i
i
2).
i
15000
Distance from southern Labrador Sea (km)
The southerntip of the Grand Banksrepresentsthe boundary
betweenthe subpolarand subtropicalAtlantic. Along sectionF at
55øWjust southof the Grand Banksthe CFC distributionis similar
to that in the Newfoundland Basin (Plate 2f).
High CFC
concentrations extend down to -1800 m with two maxima, one
corresponding
to ULSW and one to CLSW. Potentialtemperature
and salinity for CLSW have increasedmoderately(Figure 4f and
Table 2) as a resultof lateral mixing with warmer,saltierwater in
the interior. However, thereis a sharpchangein O/Spropertiesfor
ULSW comparedto the eddy observedalong section E in the
30
25
o
.-.
•
ß
20
ß
Newfoundland
15
10
ß
o Tritium/He-3
Age
ß CFC-11:CFC-12 Age
ß CFC-113:CFC-11 Age
i
0
0
!
i
5000
!
!
i
!
i
,
,
10000
i
i
15000
Distance from Denmark Strait (km)
Figure 8, CFC-11:CFC-12 ratioagesandtritium/He-3ages[Doney
and Jenkins, 1994] for the core of (a) ULSW and (b) OW in the
DWBC versusdistancefrom the sourceregion.
Labradorcoastinto the centralLabradorSea(Plate2d, Figure4d,
and Table 2), which is not unexpectedsince these stationsare
closestto the formationregionin the centralLabradorSea. High
CFC concentrations
extenddownto 2000 m depthindicativeof the
deep convectionthat forms CLSW. The most extremevaluesare
locatedjust northeast
of the westernboundary,indicatingthat the
recentlyformedCLSW has becomeentrainedin the equatorward
flowing DWBC. However, high CFC concentrations
and low
potential temperatureand salinity values are also found at the
stationin the centralLabradorSea. Potentialtemperature,
salinity,
and density of these waters are colder, fresher, and greater,
respectively,than the typicalvaluesbecauseof the intensewinters
that occurredin the region startingin the late 1980s with the
increasein the North AtlanticOscillation(NAO) index [Curry et
al., 1998]. The CFC-11 concentrationis at -62% saturation,similar
to that reportedby Wallaceand Lazier [1988] for the summerof
1986. In the IrmingerSea (Plate 2c and Figure 4c) the CFC-11
concentration
is similarto, and potentialtemperature
and salinity
slightly greater than, the values in the Labrador Sea. In the
northeasternbasin (sectionsA and B), CLSW is warmer and saltier
and has a much lower CFC-11 concentration(Plates 2a and 2b,
Figures4a and 4b, and Table 2). Most of the differencein CFC-11
concentration,potential temperature,and salinity between the
eastern and western basins is due to the increased convective
activity in the late 1980s. While the newly convectedwater is
observedin the westernbasinin 1992, it had not yet reachedthe
easternbasinin the 1988observations
[Syet al., 1997].
Basin.
The water south of the Grand Banks is much
warmer and saltier but retains its density, indicating extensive
isopycnalmixing with warmerand saltierwaterflowing northward
in the North Atlantic Current [Pickart et al., 1996].
It is importantto notethatduringthe 1980stherewasonly one
CFC maximum at 55øW, and it was in ULSW [Smethie, 1993].
There was no CFC signal in CLSW, although Talley and
McCartney [1982] had mapped its low-salinity and potential
vorticitycharacteristics
into the subtropicsalongthe westernmargin
on the basisof hydrographic
datacollectedbetween1954 and 1964.
The flow of CLSW with relatively high CFCs aroundthe Grand
Banks appearsto have begun about 1990 [Pickart and Smethie,
1998] after an increasein its productionduring the late 1980s
[Lazier, 1995].
The earlierdata,whichhad a betterspatialresolution[Smethie,
1993, Figure 17] also suggested
a split in the coreof the ULSW at
the Grand Banks with one branchflowing to the southwestalong
the western boundary and the other flowing more directly
southward,similar to the circulationpatternshownby Schmitzand
McCartney[ 1993]. The differencesin pathwayscouldbe relatedto
temporalvariabilityor to the spatialresolutionof the datacollected
in 1990not beingsufficientto showthe splitpathway(Figure5a).
FarthersouthalongsectionG at 38øN, 72øW in 1990, thereis
only a singleCFC maximumin the upper2000 m (Plate2g), which
is ULSW. This water is warmerand saltier(Figure4g and Table 2)
than at 55øW, but its density range is similar, indicating further
isopycnalmixing. CLSW is presentin the O/Splot asa minimumin
salinity, but there is not a CFC maximum associatedwith it.
However,the CLSW salinityminimumis not presentfarthersouth
along sectionH (Figure 4h) suggestingthat the leading edge of
newly formed CLSW extendedno farther than -38øN in 1990.
Recentlycollecteddata show the signalhad arrived at 26.5øN in
1996 [Molinari et al., 1999].
6.1.2. 38øN to the tropics. At -36øN the northeastward
flowing Gulf Stream crossesover the southwestwardflowing
DWBC, and them is a distinct boundaryin the ULSW CFC
distribution.ContrastingsectionG (Plate2g) and O/Sand 0/CFC-11
plots (Figure 4g) north of the crossoverwith sectionH (Plate 2h)
and O/S and 0 /CFC-11 plots (Figure 4h) south of the crossover
revealsa 40% drop in CFC-11 concentration(thesesectionswere
takenwithin 2 weeksof eachother),an increasein densityfrom o•.5
= 34.57 to o•.5 = 34.63, and an increasein the depth of the
maximumfrom -800 to 1300 m. This abruptchangein propertiesis
due to a combination
of the interaction between the Gulf Stream and
the DWBC andthe increased
contributions
of SouthernHemisphere
14,316
SMETHIE
and Mediterranean
Water
ET AL.: TRACING
south of the crossover.
THE FLOW
Pickart
and
Smethie [1993] showed that when the Gulf Stream and DWBC
collide,the ULSW is split, with water lessdensethan o•.5 = 34.57
being entrainedby the deepGulf Streamand recirculatedinto the
interior and water with greaterdensitymostlyflowing beneaththe
Gulf Stream. Theseresultsconfirm the modelingstudiesof Spall
[1996a,b] that suggestthe propertiesof both the Gulf Streamand
DWBC
should be altered at the crossover.
ULSW is transported
into the interiorin the subtropics
in two
ways, cold core Gulf Streamrings [Smethie,1993] and the deep
Gulf Stream recirculationgyre [Schmitz and McCartney, 1993].
The latter appearsto be the major pathway.Water recirculatingin
the gyre is found southof the BermudaRise (Figure5a) with a high
CFC-11 concentration[M. Baringer, personal communication,
1999]. Johnset al., [1997] showthat severalsverdrupsof ULSW
OF NORTH
ATLANTIC
DEEP WATER
equatorandnorthwardbulgebetween40ø and50øS)plottedon the
34.64 Ol.5 density
surface,
whichis closeto thedensity
of the
ULSW CFC maximumin the equatorialregion.
There is a dramaticchangein the O/S structureof ULSW
betweenthe subtropics
andthetropics. In the subtropics
theULSW
is observedat eithera slightsalinityminimumor in a regionwhere
salinityincreases
as temperature
increases
(Figures4fi 4g, 4h, and
4i), but in the tropicsit is observedat a salinitymaximum(Figure
4j, 4k, 41). Also, the salinityof the ULSW, which increases
monotonicallyin the southwarddirection in the subtropics,
decreases
in the southwarddirectionin the tropics(Figure4j, 4k, 41;
Table 2). The maximumsalinityof ULSW in the data presented
hereoccursat 24øN, whichis the latitudeof the high-salinitytongue
extendingwestwardfromtheMediterranean
Seaat thisdepth[Reid,
1994,Figure13c]. Lateralmixingwith thishigher-salinity
waterto
from the Gulf Stream recirculation are carried westward across the
the east can accountfor the increasein salinity with decreasing
axis of the Blake BahamaOuter Ridge. From thereit is channeled latitudeto thispoint. The abruptchangebetweenthe subtropics
and
back into the equatorwardflowing DWBC at ~29øN. The same tropicsis the resultof intermediate
watersof NorthAtlanticorigin
flow patternis seenin Reid's[1994] adjustedstericheightmap on beingdisplacedby northwardspreadingintermediatewatersfrom
the 34.64 o•.5 surface[Reid, 1994, Figure 13b], which is slightly the SouthAtlantic. Fine and Molinari, [1988] suggested
the water
deeperthan the CFC maximumat this latitude. Along sectionI at from the South Atlantic was Antarctic Intermediate Water, but it
24øN (Plate 2i and Figure 4i) the O/Spropertiesare essentiallythe appearsto be Upper CircumpolarWater which lies between
same as just southof the crossoverand this marks the highest Antarctic IntermediateWater and ULSW. The decreasein salinity
salinity(-35.03) thattheULSW obtainsalongthewesternboundary in the southwarddirectionis causedby the upper part of ULSW
(seediscussionbelow).
being entrainedinto the northwardflowing Upper Circumpolar
The decreasein the CFC concentration
of ULSW equatorward 'Water,thuserodingthe top portionof the O/Sstructure(Figures4j,
andeastwardis the resultof two processes.
First, waterfarthestfrom 4k, 41), not by additionof fresherwater into ULSW by diapycnal
the sourceregionhasa lower concentration
becauseit formedat an mixing with the Upper CircumpolarWater. This also causesthe
earliertime when the atmosphericconcentration
was less.Second, densityof the ULSW coreto increase,whichis the oppositeof what
the water farther downstream has had more time to mix with low
would result from diapycnalmixing with the less denseUpper
traceror tracer-freewater alongthe flow path. The effectof mixing CircumpolarWater. The densityof the ULSW core in the tropics
and dilution can be examinedby calculatingdilution factors as
hasincreasedto Ol.5= 34.66, whichis coincidentwith thedensityof
describedin section6.1.2. The map of dilution factorsfor ULSW CLSW.
(Figure 7a) has a distributionsimilarto the CFC-I 1 concentration
In the tropicsthe coreof ULSW is associated
with salinityand
map (Figure5a) but the relativegradientsare lesssincethe effectof
oxygenmaximumsin the watercolurn. This waterhasbeenwidely
the temporallychangingatmosphericsourcehas been removed. referredto as upperNADW, and its origin has beenthoughtto be
North of ~30øN, the dilutionfactorsare 2 or less,indicatingthat the
CLSW [Wiist, 1935]. This is not surprisingsinceit has the same
waterhasmixedwith an equalvolumeof waterformedprior to the
densityas CLSW, althoughit is warmerand saltier.UsingCFCs to
input of CFCs. The high dilutions south of the Gulf Stream
Crossover
are consistent
with
the conclusions
of Pickart
and
Smethie[1993] regardingits effect on the propertiesof the upper
partof the DWBC andthe recirculation
of the upperpartof ULSW
trace its O/S evolution, we have shown that at the time of our
observationsthe upper NADW CFC signal is not derived from
CLSW
but instead from the less dense ULSW.
The dilution factor increasesfrom 2 to 10 along the western
into the interior with the Gulf Stream.
boundaryto the equatorwith high valuesin the equatorialplume
6.1.3. Tropics. In the tropicalAtlanticthe ULSW CFC signal
(Figure7a). This is consistentwith the reversingcurrentsalongthe
remainsa prominentfeature(Plates2j, 2k, and 21). There is a
equator observedwith SOFAR floats [Richardsonand Schmitz,
strong
decrease
in CFC-11concentration
from-0.6 pmolkg'• at 1993], which would resultin enhancedmixing with older water in
24øNto -0.15 pmolkg'• at 6øNfor thecoreof ULSW,butthis theequatorialband.
differenceis exaggerated
by temporaldifferences
becausethe 24øN
measurements were made in 1992 and the 6øN measurements were
made in 1989. In the map (Figure 5a) where concentrations
have
beennormalizedto 1990 the concentration
changeis moregradual,
decreasing
by abouta factorof 2. The mostprominentfeaturein the
map and the sectionalong 35øW (Plate 2k) is the splittingof the
ULSW into two brancheswith one extendingeastwardalong the
equatorandthe othersouthwardalongthewesternboundary,as first
observedby Weisset al., [ 1985] in CFC datacollectedin 1983. In
agreementwith C. Andtie et al., [1998], the CFC maximumis
locatedjust southof the equatorin the westernbasin coincident
with the salinity maximum[Tsuchiyaet al., 1992]. However,the
CFC maximumappearsto shift northwardto the equatorin the
easternbasin. A moresubtlefeatureis the northwardbulgingof the
6.2
Overflow
Water
6.2.1. Subpolar. As discussedpreviously, there are two
overflow water massesthat feed NADW: ISOW, which enters the
northeastern basin, and DSOW, which enters the northwestern
basin.
6.2.1.1. ISOW: Only ISOW is observedin the northeastern
basin.
Sections A and B extend across the ISOW
that flows
southward
alongtheReykjanes
Ridgein theeasternAtlantic(Figure
3 and Plate 2). Three varietiesof ISOW are observedalongthese
sections.(1) There is a lobe of high CFC waterat the baseof the
Icelandcontinentallope (sectionA, station4) that hasa potential
CFC-11 isoplethsbetween40ø and 50øW. In agreementwith temperature
and salinity~4.3øCand35.00, whichis incompletely
Andtieet al., [ 1998]thissuggests
thatsomeULSW is recirculating formed ISOW [Doney and Bullister, 1992] (discussedearlier).
salinity,and densitycharacterfrom the westernboundaryto the interiorbeforereachingthe Althoughits potentialtemperature,
equator,supporting
theexistence
of a deeprecirculation
gyre[Johns isticsare similarto thoseof ULSW, it is not thoughtto be a source
et al., 1993; McCartney, 1993; Schmitzand McCartney, 1993; of this water mass. (2) At sectionA, station6, ~75 km farther
FriedrichsandHall, 1994]in theGuianaBasin.Reid[ 1994,Figure south,themis a slightCFC-11 maximumjust abovethebottomthat
13d] showsa very similar patternfor oxygen(splittingat the hasO/Svaluesof 2.66øC/34.976anda densityof o2 = 37.058 (04 =
SMETHIE
ET AL.: TRACING
THE FLOW OF NORTH ATLANTIC
45.825). Althoughthereis only onedatapointin thisfeature,there
is a similarfeatureat stations580-584alongsectionB (Plate2b and
Figure4b), which has nine data points. This water is slightly
warmerand saltier(Table 2) and hasa densityof c•2= 37.03-37.05
(c•4= 45.77-45.81). Swirl[1984] reportsthatthe maximumdensity
that extendsfrom the easternbasin through the Charlie Gibbs
FractureZone is c•2= 37.04, so this water appearsto be the water
that flows throughthe fracturezone. Althoughthe O/Schanges
DEEP WATER
14,317
Assumingthe mixturesestimatedin the aboveparagraphform
rapidlywith respectto the changinginput of CFC-11 (-1 year),the
concentrationof CFC-11 in pure DSOW can be estimatedfrom the
CFC-11
concentrations
measured in DSOW
C (Plate 2c and Table 2).
and GFZW
on section
For the 45:55 mixture of UAIW and
GFZWtheestimated
concentration
in pureDSOWis5.4pmolkg'•,
which correspondsto a percentsaturationof 74%. For the 60:40
mixtureof AAW andGFZW the pureDSOW CFC-11 concentration
is estimated
to be 4.3 pmolkg'•, whichcorresponds
to 60%
concentration
is fairlylarge,2.4pmolkg'• forsection
A comparedsaturation. The fractionof the CFC-11 signalthat is derivedfrom
to 1.8 pmolkg-• for section
B. Thisdifference
wouldbe even the pure DSOW componentcan also be estimatedand is 78% for
between sections A and B are small, the difference in CFC-11
greater if the concentrationswere normalized to the same date
(sectionA was taken in 1988, and sectionB was taken in 1991).
the UAIW:GFZW
mixture and 85% for the AAW:GFZW mixture,
which agreeswith earlierfindingsthat DSOW is the main sourceof
This decreasein CFC concentration
indicatesmixingwith adjacent CFCs to OW that flows into the subtropicalAtlantic [Srnethie,
olderwaterwith similarO/Spropertiesasthe waterflows southward 1993].
Along section D in the Labrador Sea the GFZW CFC-11
alongthe ReykjanesRidge. (3) At stationsfarthersoathandeaston
is similarto that in the IrmingerSea, but the core is
both sectionsthere is a CFC maximumalong the bottom. This concentration
bottommaximumhasO/Svaluesof 2.2ø-2.3øC/34.975
anda density cooler and fresher(Figure 4d). This is probablycausedby the
of c•2>37.08 (c•4greaterthan 45.85), which is too denseto flow upperportion of the GFZW layer mixing with CLSW during the
throughthe CharlieGibbsFractureZone. Thusit appearsthatthe wintertime deep convection. The maximum CFC concentration
densestvarietyof ISOW formeddoesnot flow directlyinto the occursin the DSOW layerat thebaseof the continentalslope(Plate
2d). It has decreasedby -30% from values in the lrminger Sea
western basin but fills the bottom of the northeastern basin.
Water that flows throughthe CharlieGibbs FractureZone is (Figure 4d), and the DSOW core is warmer and less dense. It is
fresherthan the mixing line betweennortheastAtlantic water and possiblethat the densestwaterobservedin the IrmingerSeahasnot
pure ISOW (Figure 4a), althoughboth of thesewater massesmust been transportedto the Labrador Sea, which would occur if the
be major components.Thus it appearsthat pure ISOW entrains densestDSOW follows deeperisobathseastof the LabradorBasin.
CLSW in additionto northeast
Atlanticwateras shownby Harvey Diapycnalmixing with overlyingGFZW would also decreasethe
maybe at work.
and Theodorou[1986]. FromFigure4a and4b, a composition
of density,andboth mechanisms
Along section E in the Newfoundland Basin the CFC
45% pure ISOW, 20% northeastAtlantic water, and 35% CLSW
in both the GFZW and DSOW layers(Plate 2e and
can be estimatedfor waterexpectedto flow throughthe Charlie concentrations
Gibbs FractureZone,whichis in goodagreement
with Harveyand Figure 4e) have decreased relative to the Labrador Sea
(Plate 2d) as expectedin the downstreamdirection.
Theodorou's[1986] estimate,exceptthat they concludethat North concentrations
Icelandic and Arctic Intermediate water contribute 9% and CLSW
The O/Spropertiesfor DSOW areessentiallyunchanged(Figure4e).
However,potentialtemperatureand salinityare warmerand saltier
contributesonly 25%.
In the lrminger Basin, three of the major components
of for GFZW and are closeto the valuesfor the IrmingerSea. This
that someGFZW bypasses
the LabradorBasinenrouteto
NADW are clearly evident (Plate 2c). The ISOW horizon is suggests
relativelywell mixed horizontally,is saltier,and has a lower CFC the Newfoundland Basin.
concentration than the CLSW and DSOW that it is sandwiched
There is an abrupt decreasein the density and CFC-11
concentration
betweenthe subpolarand subtropicalbasins,whichis
between. The O/S characteristics for ISOW between the eastern and
westernbasinsis different(Figures4b and 4c) with the western a result of the higher-density,higher CFC bottom water in the
basin variety being fresher and slightly warmer; this can be subpolarbasinsnot flowing southwardaroundthe Grand Banksin
explainedby mixingwith DSOW and CLSW after enteringthe the DWBC (see section6.2.2). This indicatesthat the denserwater
in the subpolarbasinswith water siphonedoff the top
westernbasinthroughthe Charlie Gibbs FractureZone. There is recirculates
ß
not a verticalCFC-11 maximumin the ISOW horizonalongthe
westernflankof the Reykjanes
Ridge,whichwouldbe expected
if
the water flowing throughCharlie Gibbs FractureZone flowed
northwardalongtheridgeundiluted.Thissuggests
thatolderwater
from the ISOW horizonis rapidlyentrainedinto the newerwater
enteringthe basinfrom the fracturezone. This modifiedISOW will
be referredto asGFZW [BroeckerandPeng,1982].
6.2.1.2. DSOW: The DSOW underlyingthe GFZW in the
IrmingerBasinhasa densitygreaterthanc•4= 46.00 andis tagged
with a highlevelof CFCs(Plate2c) reflectingits recentinputfrom
Denmark
Strait.
It is observed as a CFC-11
maximum and
into the DWBC and replenishedat the bottomby inflow from
Denmark Strait.
Some bottom water also flows around the Grand
Banksat deeperdepthsthantheDWBC asdiscussed
previously.A
deep recirculatinggyre has been proposedby a number of
investigators.Worthington[1976] proposeda deep anticyclonic
gyre in the NewfoundlandBasin, but McCartney [1992] and
Schmitzand McCartney[ 1993] suggestthat this recirculationgyre
doesnot exist in the very bottomwater. Reid's [1994] plots of
adjustedstericheightfor this regionindicatethat a cyclonicgyre
circulatesbetweenthe LabradorandNewfoundlandBasinsfor deep
andbottomwater. Althoughthe directionof the recirculationgyre
cannotbe determinedfrom the CFC data, the data do strongly
temperature
andsalinityminimumat thebottomextending
eastward
from the Greenlandcontinentalrise to beyondthe centerof the suggestthata deeprecirculationgyredoesexistin thisregion.
basin. Althoughthe DSOW is the coldestwaterobserved,it is not
as cold or as fresh as upper Arctic IntermediateWater (UAIW)
Between35ø and42øN and52ø and68øW, OW recirculatingin
the NorthernRecirculationGyre causesthe easternbulgeof the 0.4
(Figure4c), whichhasbeenproposed
to be its precursor
[Swirland pmolkg-• isopleth
(Figure
5). Thisrecirculation
feature
shows
up
Aagaard, 1981;Swirl, 1984]. Its O/Spropertiesalongthis section much better in the more extensive OCE-134 data set collected in
could be explainedby roughly a 45:55 mixture of UAIW and this region in 1983 [Hogg et al., 1986; Srnethie,1993]. The 0.2
GFZW. If Arctic Atlantic Water (AAW) is the precursoras pmolkg-• isopleth
extends
much
farther
south
thanthe0.4pmolkg'•
suggested
by Mauritzen [ 1996], it would alsohaveto entrainGFZW
isoplethin the interiorof the westernbasin. There is not sufficient
to obtain the observedO/S characteristics
but in a smalleramount, data in the northeastern
part of the subtropicalbasinto determine
60% AAW:40% GFZW.
For either formation mechanism the
the pathwayfor this CFC signal,but it couldhavebeentransported
majorityof the CFC signalcomesfromthe DSOW precursor
since to this locationby a southwardinterior flow that splits from the
its CFC concentrationis much higher than the GFZW westernboundaryflow at the point where OW roundsthe Grand
concentration.
Banks as proposedby Schmitzand McCartney [1993]. Another
14,318
SMETHIE
ET AL.: TRACING
THE FLOW OF NORTH
possibilityis the deep cyclonicsubtropicalgyre shownby Reid's
[ 1994] adjustedstericheightmaps,whichextendsnorthwardto the
subpolargyrein theNewfoundland
andLabradorBasins.If thereis
an exchangeof waterbetweenthesetwo gyres,thenthe subtropical
gyre would transpoarelativelyhigh CFC water southwardin its
ATLANTIC
DEEP WATER
In the area between the Bermuda Rise and the mid-Atlantic
Ridge, thereis evidencefor modificationof OW by mixing with
AABW (M.S. McCartney,personalcommunication,1999). Speer
and McCartney [ 1991] suggestthat the salinityminimumbetween
15ø and25øN over the mid-AtlanticRidgecouldbe directevidence
eastern limb.
of veaical mixing betweenOW and AABW. A poaion of this
The mapof dilutionfactorsfor OW (Figure7b) revealsthatthe modifiedwaterjoins the DWBC at the Blake BahamaOuter Ridge
waterin the DWBC just southof the GrandBankshasbeendiluted (M. Baringer,personalcommunication,
1999) to flow equatorward
by a factor of 2-3. The water circulatedto the interior by the [Amoset al., 1971]. The effect of AABW on modifyingthe OW
Noahern RecirculationGyre has been diluted by a factor of 3-4. (estimatedto contain20% AABW by Wiist [ 1935] and Broeckeret
This dilutionis greaterthanfor ULSW at the samelocation(Figure al., [1991]) is the reasonfor the large zonal gradientsin CFC
7a), demonstrating
the stronginfluencethe Gulf Streamhas on concentrations(Figure 5b) in the subtropics.The effect can be
recirculatingULSW into the interior. Along the westernboundary observedin the O/Sdiagramthroughoutthe subtropicalandtropical
the dilution factorincreasesmonotonicallyfrom southof the Grand western Atlantic Ocean. There is an inflection at -2øC, often
Banksto valuesgreaterthan 10 in the tropicalAtlantic.
referredto as the 2ø discontinuity[Broeckeret al., 1976] with water
6.2.2. Subtropics. In the subtropicalNoah Atlantic the deep colderthanthe inflectionpointfalling on a straightline betweenthe
CFC maximumoccursadjacentto the continentalslopebetween inflection point and AABW. The deep CFC maximumin the
-3000 and 3800 m, and the concentrationcontinues to decreasein
DWBC occurseitherat thisinflectionpointor at a slightlywarmer
theequatorward
direction(Plates2f-2g). The CFC-11 isolinesof the temperature
throughoutthesubtropical
andtropicalwesternAtlantic
OW maximum(Figure5b) are nearlyparallelto the westernmargin (Figures4f-41).
and extend beneath the Gulf Stream DWBC crossover,in contrast to
6.2.3. Tropics. Overflow water is transpoaedin the DWBC
the ULSW maximum whose isoplethsare normal to the western from the subtropicalAtlantic to the tropicalAtlantic. Reid [1994]
boundaryin the regionof the crossover.This is causedby ULSW and Schmitzand McCartney [1993] both presentevidencefor an
being recirculatedinto the interior at the crossoverregion as elongatedrecirculationgyrein the GuianaBasinwith flow from the
discussed
previously,while OW flowsmoreeasilybeneaththeGulf westernboundaryinto the interiornear the equatorand flow from
Stream[Pickart and Srnethie,1993].
the interior to the westernboundarynear 25øN. This recirculation
Althoughthe effectsof the DWBC/Gulf Streamcrossover
and has been relatedto an increasein transpoaof the DWBC in the
recirculationgyre appear to be less for OW than ULSW, the Guiana Basin [Johnset al., 1993]. The CFC distributionreflects
oppositeis the casefor the Abacogyre.A cyclonicgyrecenteredat the GuianaBasingyre. The CFC concentration
gradientalongthe
26.5øN, offshoreof Abaco Island, the Bahamas[e.g., Lee et al., westernboundaryis low (Figure5b) becauseof the rapidtranspoa
1990],isoutlined
bythe0.2pmolkg-• contour
(Figure
5b). Johns in the DWBC, andthe isoplethsturneastwardnearthe equator.The
et al.'s [1997] detailedstudyof the deep water flow pathsin the returnflow of the gyre extendsto the noaheastalongthe flank of
region showsthat they follow the isobathsand recirculatein the the Mid-Atlantic Ridge and transpoasa mixtureof AABW (which
AbacogyremorefaithfullythandoesULSW.
contains essentially blank level CFC concentrations)and the
AlthoughOW with a densitygreaterthan o4 = 45.9 is not recirculatedOW. The low CFC influenceof AABW appearsto be
foundin the subtropicalAtlanticDWBC, it is foundat the bottom mostpronounced
between20ø and 30øN with a westwardpointing
fartheroffshore. $rnethie[1993] observedlobesof high CFC/low minimumcenteredat 25øN reflectingthe region where the gyre
silicawateralternatingwith lobesof low CFC/highsilicawaterat circulationtums westwardtoward the westernboundary. Similar
the bottomalong55øWbetween37ø and42øN. This wasattributed patternsare observedin tritium [Doney and Jenkins,1994] and
to OW that had a greatercomponentof DSOW and hence was salinity,nutrients,and oxygen[Fukumoriet al., 1991; Reid, 1994;
denser than OW found in the DWBC, interleaving with low Speer,1993].
CFC/high silica water of southernorigin flowing into the region
In the equatorialregion the CFC distribution(Figure 5b)
from the mid-oceanridge to the east. The 0/$ propertiesof the revealsspreading
of OW eastwardalongthe equatorandsouthward
offshore water were 1.75ø-1.88øC/34.884-34.896,and its density alongthe westernboundary.This is similarto the distributionfor
the ULSW, but the isoplethsdo not extendas far to the eastbecause
rangedfromo4= 45.908 to c•4 = 45.915.
Within the DWBC the deep CFC maximum is observedat at this deptheastwardflow is affectedby the RomancheFracture
theequator
differentdensities.A comparisonof the 0/$ plot for sectionF at Zone [Andrieet al., 1998]. AlongsectionK thatcrosses
55øW (Figure 4f) with the O/S plot for section E in the at 35øWthedeepCFC signalis confinedto thechannel,whichruns
alongtheequatorandis theconduitfor bottomwaterflow
NewfoundlandBasin(Figure4e) revealsthatthe CFC maximumhas east-west
between
the
western North and South Atlantic Oceans (Plate 2k).
shiftedto a shallowerdensity. In the NewfoundlandBasinas well
by
as the other subpolarsectionsthe DSOW maximum occursat a The CFC-11 concentrationin the OW CFC maximumdecreases
a factorof 3 betweensectionK at the equatorand sectionL at 10øS
densitygreaterthanc•4 = 45.90 (Plates2c, 2d, and2e andFigure4c,
(Table 2), and these two sectionswere both taken in 1992. This
4d, and 4e). Water denser than o4 = 45.90 does not enter the
sharpdecreasemay be the resultof the deepflow splittingat the
subtropicalNorth Atlantic from the NewfoundlandBasin in the
equator,as is the casefor the ULSW, with a significantpoaion of
DWBC. Along sectionF the CFC maximumoccursat a densityof
the flow enteringtheRomanchFractureZone [Andrieet al., 1998].
-c•4 • 45.88. However, farther south in the subtropicalwestern
As discussedin the previousparagraph,the O/Spropeaiesof
Atlanticthe CFC maximumis foundat a lowerdensityof-45.86 o4 the OW CFC maximumare the sameas in the subtropicalNorth
(Plates2g, 2h, and2i). In the tropicsthe CFC maximumis again Atlantic. However,this wateris now underlainby waterenriched
found at 45.88 c•4 (Plates2j and 2k). During the mid-1980sthe with Antarctic Bottom Water (AABW) as can be seenfrom the O/S
CFC maximumin the DWBC occurredat 45.88 c•4 throughout
the trendbelow the 2ø discontinuity
(Figure4k and 41). The CFC-11
subtropics[Fine and Molinari, 1988; $rnethie,1993]. This vari- concentrationalso increaseswith decreasingtemperaturebeneath
ability in densitywith time in the subtropicsis probablydue to the2ø discontinuity,
andthisis particularlyevidentat 10øS(Figure
densityvariationsin differentvintagesof OW beingtranspoaed
in 4k-41). This CFC signalis from AABW, whichoriginatedin the
theDWBC [Pickart,1992b].The variabilitybetweenthe subtropics SouthernOcean, and in 1992 it approachedentry to the Noah
and tropicsmay alsobe due to temporalvariabilityin the AABW Atlantic. See Smythe-Wright
and Boswell[1998] for additional
[Hall et al., 1997].
information
on CFCs in AABW.
SMETHIE
ET AL.: TRACING
THE FLOW OF NORTH
ATLANTIC
DEEP WATER
14,319
7. CFC Agesand SpreadingRates
flowing AABW. Although a correctionis made for this, this
correctionbecomeslargeanddifficult to makeaccuratelyin regions
The previousdiscussion
hasdemonstrated
how newly formed wherethe mantlecomponentis high and the tritiogeniccomponent
componentsof NADW are transportedand mixed into the North is low, which is the case in the interior and far downstreamfrom the
AtlanticOceanfrom their sourceregionsand how this affectsthe sourceregion.
Apparentcurrentspeedscan be calculatedby dividing the
hydrographicstructure. Since this discussionwas built aroundthe
spreading
of the CFC signalin the NorthAtlantic,onecanreadily distancefrom the sourceby the tracer age. For ULSW and OW
see that the timescale involved is several decades,i.e., the time of
these
speeds
range
from1 to2 cms't [Weiss
etal., 1985;Srnethie,
the measurable
CFC transient.However,as discussed
previously, 1993;Doneyand Jenkins,1994]. This canbe easilyseenin a plot
the CFCscan be usedto estimatewatermassageson a finer scale of tracerage in the DWBC core versesdistancefrom the source
than several decades.
region(Figure8). For ULSW the slopein the subtropics
indicates
current
speed
of .-.1.1cms't. In thetropics
theslope
Maps of the CFC-11:CFC-12 ratio age(hereafterreferredto as anapparent
to decrease,
whichwouldindicatea higherapparentcurrent
the CFC age) for ULSW and OW are presentedin Figure6. These appears
agesare calculatedassumingthat the CFC-1l:CFC-12 ratio (not speed,but the ages in the tropicshave large errors becauseof
concentration)
in the sourcewaterwasin solubilityequilibriumwith uncertainties in the CFC ratio at low CFC concentrations. For OW
the atmosphereduring the year of formation. However, if the there is very good agreementbetweentritium/He-3 agesbasedon
made between 1981 and 1983 [Doney and Jenkins,
surfacewater does not reach equilibrium during the formation measurements
madebetween1983
process,as occurs during deep convection,the ratio in newly 1994] and CFC agesbasedon measurements
and
1992.
The
overall
slope
based
only
on
CFC
ratio agesyieldsan
formed water may not be the same as is predicted from the
current
speed
of 1.9cms'l compared
to 1.7cms'• onthe
atmospheric
ratio and the solubilities,and the newly formedwater apparent
will havean apparentage. For ULSW an apparentageat formation basisof the tritium/He-3age [Doneyand Jenkins,1994]. The slope
has been estimatedto be .-.3-5 years from the CFC-113:CFC-11 of the combinedCFC ratio agesand tritium/He-3 ages(Figure 8b)
anapparent
current
speed
of 1.8cms'l, extending
fromthe
ratio (in a mannersimilarto the one usedto calculateagesfrom the yields
CFC-11:CFC-12 ratio [Schlosserand Smethie,1995]) and from the subpolarregionto the tropics.
Measuredcurrentspeedsin the DWBC are considerably
higher
tritium:He-3ratio [Pickart et al., 1996]. Newly formedOW may
currentmetermeasurements
in the
alsohavean apparentage. On the En223 cruise(Table 1), CFC-113 than1-2 cm s'1. Long-term
was measured as well as CFC-11 and CFC-12 at four stations
DWBC along the North Americancontinentalslopeshow speeds
from5 to 10cms'1 [Watts,1991],andspeeds
ashighas
crossingthe flow of DSOW aroundthe southerntip of Greenland. generally
[e.g.,MillsandRhines,
1979;Johns,
The age of this water based on the CFC-113:CFC-I 1 and CFC- 40 cms-1havebeenobserved
113:CFC-12ratioswas 3.6 years,whichwouldrepresent
an upper 1993]. The primary reasonfor the differencebetweenthe traceris that the tracers
limit for the apparentage at its entry into the North Atlantic. Thus derivedspeedand absolutevelocitymeasurements
the agespresented
in the mapsincludea "relicage"acquiredat the give an effective spreadingrate [Doney and Jenkins,1994; Fine,
timeof formationthat couldbe as high as 3-4 years. Also, mixing 1995], which includesthe effectsof mixing and recirculation. In
can resultin the CFC age of the DWBC beingoverestimated
[e.g., the mean the water does not flow directly equatorwardin the
Pickartet al., 1989; Rhein, 1994]. It shouldbe kept in mind that
theseagesare not the averageage for the entire water massbut are
for the mostrecentlyformedcomponent
thathasbeentaggedwith
CFCsduringthepastfew decades.
The CFC agefor ULSW is ~10 yearsin the DWBC just south
of the Grand Banks (Figure 6a). It increasesmore or less
monotonically
to between20 and25 yearsat 10øNandto greater
than 25 years in the equatorialplume and along the western
boundarysouthof the equator. Agesin the interiornorthof 30øN
range from <15 years to 20 years, reflectingthe circulationof
DWBC but makes excursions into the interior, mixes with older
water,and thencomesbackto the boundary;the DWBC transports
both newly formed water and older recirculatingwater. The
effective spreading rates derived from tracer data clearly
demonstrate
the importanceof exchangewith the interiorin aging
the water. The tracer-derivedage representsthe rate at which a
climateanomalyor perturbationin the formationof NADW would
enter the ocean.
8. Summary and Conclusions
ULSW into the interior at the Gulf Stream/DWBC crossover. The
The compilationof availableCFC data for the westernNorth
15 year isoplethextendsto 30øN near the Mid-AtlanticRidge,
suggesting
flow directlyto the interiorfrom the subpolarregion. Atlantic provides an opportunity to contribute details to our
However,this flow patternis not supportedby the 1990 CFC-11 knowledge of the large-scale circulation pathways that the
componentsof NADW follow and the related timescales. The
formationof the major co•nponents
of NADW are reviewedwith a
bias towardunderstanding
the sourcesof the most recentlyformed
tropicssuggests
a 10 yeartransittimeforULSW to flowthroughthe and hencehigh tracercomponents.A summaryof this review and
subtropical
Atlanticalongthe westernmargin. Recently,Molinari synthesisfollows.
The shallowestcomponentof NADW is ULSW, which forms
et al., [1999] have useda time seriesof hydrographic
and CFC
concentrations
at 26.5øNin the DWBC to showthat the transport by deep convectionin the southernLabradorSea, possiblyin the
time of the recentlyformedCLSW (discussed
previously)is 8-10 Labrador Current [Pickart et al., 1997]. Its existence was
yearsfrom the LabradorSea to 26.5øN, which is aboutthe sameas discoveredonly recently [Pickart, 1992a] from observationsof a
CFC maximum [Fine and Molinari, 1988; Smethie, 1993] and a
ULSW transittimederivedfromthemaps.
The CFC ageof OW (Figure6b) is 12-15yearsin the DWBC tritium maximum [Jenkins and Rhines, 1980; Olson et al., 1986;
southof theGrandBanksandincreases
alongthewesternmarginto Pickart et al., 1996] obtainedduringthe 1980sin the North Atlantic
-25-30 yearsat 8øN. The OW agesin theinteriornorthof 30øNare subtropicalDWBC at a temperaturetoo warmto be CLSW. It hasa
higher than for ULSW and range from 20 to 25 years. A core densityof •1.5 = 34.56. Newly formed ULSW, which has a
comparison
of Doneyand Jenkins'[1994] tritium/He-3ageswith CFC saturationof-70%, advectssouthwardfrom the sourceregion
Figure6 revealssimilaragesalongthewesternboundary,
but in the in small eddies that become rapidly entrainedinto the DWBC
interiorthetritium/He-3agesaregreaterthantheCFC agesby 5 to [Pickart et al., 1996]. This maximum has also been observedin the
10 years. This differencemay be causedby mantle-derived
He-3, DWBC in the tropics[Rheinet al., 1995] and in a plumeextending
which is transportedinto the North Atlantic by the northward eastwardalongthe equator[Weisset al., 1985]. The upperportion
distribution(Figure 5a) perhapsbecausethere are no data east of
63øW for this time. The age data eastof 63ø W are from earlier
cruises. The age differencebetween the Grand Banks and the
14,320
of
the
ULSW
SMETHIE ET AL.: TRACING THE FLOW OF NORTH ATLANTIC DEEP WATER
recirculates into
the interior
at
the
Gulf
Stream/DWBCcrossover[Pickartand Smethie,1993], but someof
this flow appearsto rejoin the DWBC at the Blake-BahamaOuter
entersthroughDenmark Strait [Swirl, 1984]. DSOW has a much
higherCFC signalthanISOW andis the primarysourceof CFCs in
the bottomwaterof the subpolarwesternbasin[Smethieand Swirl,
Ridge [Johnset al., 1997; M. Baringer,personalcommunication, 1989; Smethie, 1993]. Both ISOW and DSOW flow around the
1999]. Fromthe combinedCFC datasetpresented
herewe show Irmingerand LabradorBasinsin a deep boundarycurrent[Swirl,
the following.
1. ULSW is a major water massin the westernNorth Atlantic
Ocean.
It is a continuous feature in the DWBC
from the Grand
1984; McCartney, 1992], and a mixture of thesetwo water masses
entersthe subtropicsin the DWBC [Smethie,1993] producinga
deep CFC maximumthat is observedin the DWBC throughthe
subtropics
[Fine and Molinari, 1988; Smethie,1993] andtropicsto
Bankstojust southof theequatorandextendsfar into theinteriorof
the westernsubtropicalAtlantic.
10øS [Molinari et al., 1992; Rhein et al., 1995]. The OW
2. The O/S propertiesof the ULSW CFC maximum in the recirculatesinto the interior of the subtropicalAtlantic with the
DWBC showthatthe salinitygraduallyincreases
in the southward northernrecirculationgyre [Hogg et al., 1986; $methie,1993], but
directionto ~24øNbecause
of mixingwith higher-salinity
water
from the east. Southof this latitude,themis an abruptchange
causedby the upperportionof the ULSW beingentrainedinto
northward
spreading
upperCircumpolar
Water,andthesalinityand
temperature
of the CFC maximumdecreasewhile the density
increases.In the equatorialregionthe densityof the ULSW CFC
maximum
is essentially
thesameasCLSWin thesubtropics,
butthe
temperature
andsalinityaregreaterthanthatof CLSW. Although
CLSW mayhavebeenimportantin the pastfor formationof upper
NADW, duringthe 1980s, ULSW was the major sourceof upper
NADW.
3. The equatorialplume is muchbetterdefinedthan in earlier
work, extending eastward to at least 3øE with the maximum
generallyoccumngat 1ø-2øS.
4. The high CFC concentrationsin the interior of the
subtropicsindicateextensiverecirculationfrom the boundaryto the
interior, particularlywith respectto the Gulf StreamRecirculation.
The data also supportrecirculationat the Blake Bahama Outer
Ridge and in the GuianaBasin.
5. Dilution factorsof ULSW range from -2 along the western
boundaryof the subtropicsto 10 at the equator,with values>10 in
the equatorialplume. The largeregionin the subtropicswherethe
dilutionrangesfrom 2 to 4 reflectsthe extensiverecirculationin this
region.
CLSW formsby deepconvectionextendingto depthsof >2000
m, andits historical
coredensityis o•.5 = 34.66 [Talleyand
south of this recirculation, it crossesbeneath the Gulf Stream with
little impedance[Pickart and Smethie, 1993]. Farther south, at
~26øN,thereis anotherrecirculationgyre off AbacoIsland[Johns
et al., 1997]. Fromthe combinedCFC datasetpresented
herewe
showthe followingfor lowerNADW.
1. The maximumdensityof recentlyformedISOW that flows
throughthe Charlie Gibbs FractureZone to the westernbasin to
formGFZW is 44.77-45.81o4 (37.03-37.05o2), whichis in good
agreementwith Swifi's [1984] determinationof 37.04 o2 on the
basisof datacollected
in the 1970s. It is a three-component
mixture
of 45% pure ISOW, 20% northeastAtlantic water, and 35% CLSW.
The densestISOW, which was observedto be 45.85 o4 in the
combineddata set, doesnot enterthe westernbasinbut spreads
alongthebottomof the easternNoah Atlanticwhereit produces
a
bottom CFC maximum.
2. The percentCFC saturationof newly formedDSOW was
estimated
to be 74% takingAIW to be the sourceof pureDSOW
[Swirland Aagaard, 1981]. However,recently,Mauritzen[ 1996]
proposedthat AAW is the sourcefor DSOW, and 60% saturation
was estimatedfor this case. For eithersource,DSOW provides
-80% for the CFC signal to the DSOW/GFZW mixture that is
transportedsouthwardin the DWBC.
3. The dense,high CFC bottom water in the Newfoundland
Basin (45.92 04] was not observed south of the Grand Banks and
apparentlyrecirculatesin the NewfoundlandBasin,with someof the
waterflowingsouthwardalongthe bottomoffshoreof the DWBC as
observedpreviously[Smethie,1993].
McCartney, 1982].
The very deep convection results in
undersaturation
of CFCs in newly formed water, and Wallace and
4. Thedensityof thedeepCFC maximumin thesubtropics
and
Lazier [1988] reported60% saturationin recentlyformedCLSW in
tropicsrangedfrom45.86 to 45•8804 bothspafiallyandtemporally
the central Labrador Sea in 1986. However, in the subtropics wheretherewerereoccupations
of sectionsacrossthe DWBC. This
during
the1980sthe34.66Ol.5density
surface
wascloseto a deep variabilitycouldbe causedby temporalvariabilityin the source
CFC minimum, indicating that recently formed CLSW had not wateras suggested
by Pickart [1992b] or by vaciabilityin mixing
entered the subtropicsin significant quantities [Smethie, 1993]. with AABW encountered as the NADW flows southward.
Yet, in the Newfoundland Basin, CLSW was observed near the
westernboundary,and asreportedby Pickart and Smethie[1998], it
was observedjust southof the Grand BanksunderlyingULSW in
1991. This demonstratedthat CLSW, which formed during the
5. In additionto therecirculation
thatoccursin thesubtropics,
the CFC distributionin the tropicssupportsrecirculationin the
GuianaBasingyre.
6. The dilutionfactorfor OW is estimated
to be 2-3 alongthe
intense winters associated with the rise in the NAO index in the late
westernmargin of the subtropicalNorth Atlantic and 3-4 in the
1980s and early 1990s, was just beginning to flow into the
subtropicsin the early 1990s. From the combinedCFC data set
presentedhere we showthe following for CLSW.
1. High CFC concentrationsextendeddown to 2000 rn in both
the Labrador Sea, which had a CFC saturationof 62%, and the
Irminger Sea. Lower concentrations
were observedin CLSW in the
subtropical
interior.It increases
to values>10 in thetropics.
The CFC agesrevealthat ULSW and OW bothspreadfrom
their subpolarsourceregionsto the tropicsin 25-30 yearswith
youngeragesgenerallyfound along the westernboundaryrather
than in the interior. Effectivespreadingratesin the DWBC were
eastern basin.
estimated
fromplottingageagainst
distance
to be -1.1 cms'l for
ULSWand1.8cms-• forOW,in goodagreement
withprevious
2. The leadingedgeof the CLSW formedduringthe intense estimates
of 1-2 cm s'• fromtracerstudies[Weisset al., 1985;
wintersof the late 1980sappearedto extendto 38øNin the DWBC Smethie,1993;Doneyand Jenkins,1994]. Theseeffectivespreadin 1990, but there was no CFC maximum in CLSW. The CFC
concentration in CLSW at 38øN was well above the minimum
observed in the 1980s but was less than that of ULSW.
ing rates are low comparedto direct long-termcurrent meter
measurements
in theDWBC,whichrevealspeeds
of 5-10cm s'l
[Watts,1991]. The tracer-derived
effectivespreading
ratesintegrate
The densestcomponents
of NADW are the overflowwaters theeffectof mixingandrecirculation
processes
on theequatorward
that enter the North Atlantic from behind the Greenland-Iceland- flow. This differencedemonstrates
the importance
of recirculation
ScotlandRidge. ISOW is the leastdenseof the overflowwaters, gyresin transportingnewly formedNADW into the oceaninterior,
andit entersthewesternbasinthroughtheCharlieGibbs Fracture whereit mixeswith older waterbeforereenteringthe DWBC to
Zone where it ovemdes and is entrainedinto the denserDSOW that
continueits southward
journey. Thisprocess
retardsthe spreading
SMETHIE ET AL.: TRACING
THE FLOW OF NORTH ATLANTIC
rateof NADW in the relativelyfastflowingDWBC andrenewsthe
DEEP WATER
14,321
Current and Abyssal Western North Atlantic: Estimatesfrom tritium
and3Hedistributions,
J. Phys.Oceanogr.,
24,638-659,1994.
interior water of the North Atlantic Ocean.
Elkins, J., T. Thompson,T. Swanson,J. Butler, B. Hall, S. Cummings,D.
Fisher, and A. Raffo, Decreasein the growth rates of atmospheric
Acknowledgments,We wouldlike to thankDebbieWilley,
chlorofluorocarbons - 11 and - 12, Nature, 364, 780-783, 1993.
Hoyle Lee, andFrankZhengfor combiningall of the datasetsin a Fine, R. A., Tracers, time scales, and the thermohaline circulation: The
commonformat for us to work with and for preparationof the
lower limb in the North AtlanticOcean,U.S. Nat. Rep. lnt. Union of
figurespresented
in thismanuscript.This workwassupported
by
Geod.Geophys.1991-1994, Rev.Geophys.,33, 1353-1365, 1995.
the NOAA Atlantic Climate ChangeProgram, NOAA grants Fine,R.A., and R.L. Molinari, A continuousdeepwesternboundarycurrent
NA26GP0231
to W. Smethie and NA67RJ 1049
betweenAbaco(26.5øN) and Barbados(13øN), Deep Sea Res.,Part A,
to R. Fine, and NSF grantsOCE 89-17801 and OCE 90-19690 to
35, 1441-1450, 1988.
Fine, R.A., M.J. Warner, and R.F. Weiss, Water mass modification at the
WMS
and NA46GP0168
and OCE
94-13222
and OCE
98-11535
to RAF.
This is
Lamont-DohertyEarth Observatorycontributionnumber6028.
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