The bomb 14C transient
in the Pacific
Ocean
Keith B. Rodgers•
Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York
Daniel P. Schrag
Department of Earth and Planetary Sciences,Harvard University, Cambridge, Massachusetts
Mark
A. Cane and Naomi
H. Naik
Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York
Abstract. A modeling
studyof the bomb14Ctransientis presented
forthe Pacific
Ocean. A primitive equation ocean circulation model has been configuredfor a
high-resolutiondomain that accountsfor the IndonesianThroughflow(ITF). Four
separateruns were performed: (1) seasonalforcingwith 20 Sv of ITF transport,
(2) seasonalforcingwith 10 Sv of ITF transport, (3) seasonalforcingwith no ITF
transport, and (4) interannualforcingwith 15 Sv of ITF transport. This study
has two main objectives. First, it is intended to describe the time evolution of
the bomb14Ctransient.This servesas a tool with whichonecanidentifythe
physical
processes
controlling
the evolvingbomb14Cdistributionin the Pacific
thermocline
and thusprovides
an interpretiveframework
for the databaseof A14C
measurementsin the Pacific. Second, transient tracers are applied to the physical
oceanographicproblem of intergyre exchange. This is of importance in furthering
our understanding of the potential role of the upper Pacific Ocean in climate
variability.We usebomb14Cas a dyetracerof intergyreexchange
betweenthe
subtropical
gyresandthe equatorial
upwelling
re,ionsof the equatorial
Pacific.
Observations
showthat whilethe atmospheric
A14Csignalpeakedin the early
to mid-1960s,
the A14Clevelsin the surfacewaterwatersof the subtropical
gyrespeakednear1970,andthe A14Cof surfacewatersin the equatorialPacific
continued to rise through the 1980s. It is shown that the model exhibits skill in
representing
the large-scale
observed
featuresobserved
for the bomb14Ctransient
in the Pacific Ocean. The model successfullycaptures the basin-scaleinventoriesof
bomb14Cin the tropicsaswellasin the extratropics
of the NorthPacific.Forthe
equatorial Pacific this is attributed to the model's high meridional resolution. The
discrepancies
in the three-dimensional
distributionof bomb14Cbetweenthe model
anddata are discussed
withinthe contextof the dynamicalcontrolson the A14C
distribution of bomb 14C in the Pacific.
1. Introduction
layersof the Pacificthermocline.The primary pathway
A high-resolutionprimitive equationoceanmodelhas
been used to simulate the bomb •4C transient in the
Pacific Ocean between 1955 and 1980. Over the period since the peak in atmospheric testing of nuclear
by whichbombt4C reaches
the subsurface
layersof the
subtropicaland tropical Pacific is through subduction
in the subtropical
gyres.Our investigation
focuseft
primarilyon bombtaC as a tracerof intergyreexchange
weapons
in the 1950sand 1960s,i4C hasbehaved
es- betweenthe subductingregionsof the extratropicsand
sentially as a dye tracer within the directly ventilated
the equatorial pycnocline of the Pacific•Ocean.
Recently, it has been proposed that intergyre ex-
changeof thermal anomaliesbetweenthe subducting
regions of the
1Now
at Max-Planck-Institut
fiir Meteorologie,
Hamburg,
extratropical Pacific and the equatorial
pycnoclineprovidesan advectivemeansof ch.anging
the
temperature of the upwelling water in the easternequa-
Germany.
Copyright1999by the AmericanGeophysical
Union.
torial Pacific[Deseret al., 1996; Gu and Philander.
1997: Zhanget al., 1998]. The effectiveness
of this
mechanismwill dependon the amplitudeof diapycnal
Paper number 1999JC900228.
0148-0227/99/1998JC900228509.00
8489
8490
RODGERS
ET AL.: BOMB •4C TRANSIENT
fluxes associatedwith intertyre exchangebetween the
subtropics and the equatorial thermocline. Prebomb
IN THE PACIFIC
OCEAN
andsurface
oceanwereof order30%0. The bombx4C
transientintroducedlateral and vertical gradientsin the
A•4C levelsrecordedby coralsin the equatorialup- seasurfaceand thermoclinedistributionof A•4C, which
welling regionsuggestthat significantamountsof water
from below the directly ventilated thermocline are en-
were an order of magnitude larger than the prebomb
gradients.
Severaldifferent ocean modelshave previouslybeen
trainedinto the equatorialupwelling[Toggweiler
et al.,
1991]. AlthoughprebombA•4C can provideus with usedwith a globaldomainto studythe bombx4Ctranet al., 1989b;Duffy et al., 1995]. In
important qualitative clues as to the extent of dilution sient[Toggwei!er
associatedwith intertyre exchange, other tracers are
needed to provide a quantitative assessment.
This study extends our earlier processstudy of the
dynamical mechanismscontrolling seasonalvariability
their discussion of the bomb A x4C distribution for the
upper Pacific Ocean these studies have focusedon the
model's Ax4C distribution in the northern and south-
ern subtropics.In particular, they comparedtheir sim-
of seasurfaceA•4C in the equatorialPacific[Rodgersulated distributionof Ax4C in the subtropicalgyres
OceanSectionsSurvey(GEOSECS)
et al., 1997]. There we identifiedseasonalvariability with Geochemical
in the surface currents of the Pacific Ocean as the dy- data. However, these coarse-resolutionmodels cannot
namical mechanism responsiblefor the high seasonal give a satisfactoryrepresentationof equatorialthermoAx4C variabilityduringthe 1970sand 1980srecorded cline ventilation as they do not represent the critical
by corals. Given that the boundariesof the Pacific do- scalesof circulation in the equatorial Pacific.
In addition, the models tend to be overly diffusive.
main used in that study were 30øN and 30øS, we were
unable to represent the subtropical subduction process They are typically tuned in sucha way that the ampliat the polar end of the shallow meridional cell within tude of the global-scalethermohaline circulation produces a A•4C distribution that matches GEOSECS
the thermocline.
of the prebombAx4C distributionin
A•4C has provenusefulas a tracer of oceancircu- measurements
lation on timescales ranging from the seasonal cycle the deep Pacific. Such models achieve the apparent
[Moore.et al., 1997;Rodgerset al., 1997;and Guilder- 1500 year ventilation timescalefor the deepPacific,and
son et al., 1998;and Guildersonand $chrag,1998]to
the ventilationage of the deepPacific[Stuiveret al.,
1983]. Our purposehere is to demonstratethe utility
of A•4C asa tracerof upperoceanprocesses
onE1NifioSouthernOscillation(ENSO) to decadaltimescales
in
thustheobserved
deep-Pacific
A x4Cvaluesof roughly250%0,at the expenseof an overlydiffusethermocline.
The thermocline
distributionof bombx4Cgenerated
by
such modelsis far from satisfactory.
In configuringour model domainfor the currentstudy
the Pacific Ocean. As the timescale of radioactive de- we specificallychosehigh vertical resolutionin the upcayfor x4Cisotopes
is morethan 2 ordersof magnitude per severalhundred meters, high meridional resolution
larger than either the timescale of Pacific thermocline within the equatorial thermocline, and high zonal rescirculation or the bomb transient, we can ignore ra- olution for the western boundary currents in order to
dioactivedecayas a sink of x4C in our treatment of achieve model skill for the thermocline
bomb•4C as a perturbationto the prebombA x4Cin ciated with intertyre exchange.
circulation
asso-
our analysis. However, it is important to describe first
the observed
prebombdistributionof Ax4Csinceit has 2. Zkl4C As a Tracer of Ocean
much to teach us about circulation in the upper Pacific Circulation
Ocean.
Beforeanthropogenic
perturbations,A•4C levelsin
Ax4Cis definedas the per mil deviationof the ratio
the oceanand atmospherewere approximately in steady of the isotopicspeciesx4C and x2C in CO2 from the
state. By definitionthe A•4C of the prebombatmo- ratio in the preanthropogenicatmospherewith a masssphere was zero per mil. It is generally assumedthat dependentfractionationusingthe stableisotope
the natural sourceswere roughly balanced by natural The equilibrationtimescalefor carbonisotopesbetween
sinks,with the principalsink of •4C beingradioactive the mixed layer and the overlying atmosphereis of ordecay within the ocean, and the principal sourcebeing der 10 years, and this is due to the bufferingeffect of
spallation in the upper atmosphere. Deep waters in the dissolvedinorganiccarbon in the ocean.
Pacific Ocean have a radiocarbon content that is •20%
The atmospherictesting of hydrogenbombsduring
(i.e., 200%ø)lessthan that of the preanthropogenic
at- the 1950sand early 1960sproducedsufficientx4C to
mosphere,
which,giventhe 5730yearhalf life of •4C, increaseatmospheric
A x4Clevelsin the atmosphere
by
more than 50%. Figure I showsa time seriesof atmo-.
yields an apparent ventilation age of 1500 years.
Within the Pacific thermocline, measurements col-
sphericAx4Cbetween1955and 1990[Broecker
et al.,
intothreedifferent
lectedin theopenoceanduringthe 1950s[L/nick,1978], 1995].The Ax4Csignalis separated
in additionto A•4C valuesrecorded
by corals[Dru•½l, latitude bands. During the early and mid-1960sthe atA•4C signalis largestin the NorthernHemi1987],showthat the rangeof A•4C wasbetween-70 mospheric
and -40•/• for the surfaceoceanand upper thermo- sphere, which is due to the fact that the majority of
cline. Lateral and vertical gradientsin the thermocline the abovegroundhydrogen bomb tests were conducted
8491
RODGERSET AL.: BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
900
,
,
'
'
'
'
'
8O0
700
I
600
I'
500
400
3OO
2OO
lOO
o
1955
1960
1965
1970
1975
1980
1985
199o
995
YEAR
Figure1. Timeseries
ofAz4C
values
fortheatmosphere,
separated
intothree
latitude
bands
[Broecker
etal..1995].
Thesolid
lineisforlatitudes
north
of20øN,
thedashed
lineforlatitudes
between
20øN•nd 20øS,andthedashed-dotted
lineforlatitudes
southof 20øS.
there.At the timeofpeakatmospheric
A•4Cvaluesin the westernboundaryregionsas well as in the vicinity
1964and 1965the interhemispheric
gradientwithin the of the Peru upwelling.This is the samegrid spacing
atmosphere
wasmorethan200ø/øø.
Uponentering
the we usedfor our study of the effect of the Indonesian
(ITF) on equatorial
thermocline
ventilaoceanthis bomb •4C producedlarge lateral and ver- Throughflow
et al., 1999]. Therewe foundthat the
ticalgradients
in A•4C withinthe surface
oceanand tion [Rodgers
the thermocline.
As a resultof this,the bomb•4C sig- mixing ratio of northernto southerncomponentwanal overwhelmed
the gradients
presentin the prebombter in the equatorialthermoclineis in best agreement
ocean,andthusbomb•4C in the upperoceanserves
as with the value inferred from equatorial salinity meaa dyetracerthat taggedsubducting
thermocline
waters surementswhen the imposedITF transport is between
10 and 20 Sv. The same two domains used for that
with high-A•4Cvalues.
studyareagainusedhere.They differin that the first
3. Model Experiment Description
domain doesnot allow for flow between the Pacific and
3.1. The Circulation
ingNewGuineaandthe Asiancontinent,
whereas
the
Model
Indian Oceansthroughthe IndonesianStraitsseparat-
The model usedfor this study is the primitive equa- seconddomain doesallow for flow through the Indone-
tion Lamont Ocean-Atmospheric
Mixed Layer Model
sian Straits.
usingthe parameteriza(LOAM),whichisa descendent
oftheGentandCane Verticalmixingis achieved
tion
of
Pacanowski
and
Philander
[1981]in additionto
[1989]
model.Modelcalculations
areperformed
onan
convective
adjustment.
Lateral
mixing
of temperature
A grid,anda levelvertical
grid'is used,withrealistic
and
salinity
(as
well
as
passive
tracers)
is
achieved
using
bathymetry
anda barotropic
solver
[Naiket al.,1995].
The model domain extendsmeridionallyfrom 68øSto a modifiedversionof the GriJfies[1998]implementa-
of Gent
62øNandzonallyfrom100.5ø to 300øE.The total num- tionof theoceaneddymixingparameterization
and
Mc
Williams
[1990].
The
eddy
transfer
coefficient
ber of verticallayersis 30• with 15 of theselayersin
the upper300m. The meridional
resolution
alongthe usedwith the Gent and McWilliamsparameterization
because
of grid
equatoris "•0.33
ø, taperingoff to almost2.5ø in the varieslinearlywith modelresolution
stretching.
The
minimum
value
for
the
eddy
transfer
extratropics.The zonalresolution
is almost2ø in the
extratropics
but is stretched
to slightlylessthan 1ø in coefficientis "•200m• s-•, and the maximumvalueis
8492
RODGERS ET AL.: BOMB 14C TRANSIENT
IN THE PACIFIC OCEAN
"•1600m2 s-1. A fourth-order
Shapirofilter [Shapiro,Hemispheric thermocline water in the equatorial under1970]is usedto removetwo gridpointwavesin the mo- current increaseto a value greater than unity.
For each of the runs the model is initialized
mentum equations. In addition, a lateral diffusivity of
with Lev-
1500m2 s-1 hasbeenusedwith the momentumequa- itus and Boyer [1994]temperatureand salinityfields,
tions in order to tune the structure of the equatorial and then spunup for 10 yearsin robust diagnosticmode
undercurrent(EUC), whichis too strongunderthe in- [Sarrnientoand Bryan, 1982]. A restoringtimescale
fluence of the Shapiro filter alone.
for temperature and salinity used within the thermoSurface heat fluxes are calculated using the atmo- cline is 180 days during this period. The model is then
sphericmixedlayermodelof Seagetet al. [1995],with restarred in 1955, at which point the transient tracer
monthly climatologiesused for surface solar radiation fields are initialized, without any restoring of subsur[Li and Leighton,1993],fractionalcloudiness
[Rossowface temperature and salinity fields.
and Schiffer,1991],and surfacewindswithin the AML
[Gibsonet al., 1977].A stormtrackparameterization
of 4. Tracer
is now included in the AML model, and a description of
this as well as its effect on the model's
simulation
Formulation
The treatment
of ex-
of A14C as a tracer follows that of
Toggweiler
et al. [1989a]. The gasexchangeformulatratropicalwintertimeseasurfacetemperatures(SSTs)
tion of Broeckeret al. [1985]is usedin conjunction
is describedby Hazela#eet al. [1999]. The boundary
with the atmosphericA14C time seriesof Broeckeret
condition used for sea surface salinity is relaxation to
al. [1995](shownin Figure1) to represent
fluxesat the
the Levitus monthly climatology in the surface layer
with
a timescale
3.2. Description
of I month.
sea surface. The wind speed climatology used is from
the EuropeanCentre for Medium-RangeWeather Fore-
casts(ECMWF) analysis[Gibsonet al., 1977].A•4C is
of Experiments
initialized usingthe empirically derivedrelationshipbe-
A total of four model runs were performed for this
tweenthermoclineSiO2and prebombA14Cof Broecker
study. They differin the choiceof wind stresswind forcet al. [1995],
ing (seasonal
versusinterannualwind stressdata set of
daSilvaet al. [1994]),as wellas in their representation
of the IndonesianThroughflow. These differencesare
summarized in Table 1. A simple naming convention
is usedto distinguishbetweenthe runs: S is usedto
denote runs forced with seasonallyvarying winds and
is followedby a number representingthe net imposed
ITF transport in Sverdrups;likewise,I15 indicatesinterannualwind forcingwith 15 Sv of imposedITF. The
set of runs consistsof SO,S10, S20, and I15; S10 is the
control
A14C: --70- •qi02,
(1)
and the griddedLevitus SiO2 distribution of Levitus and
Boyer[1994].
5. Model
for the
Results'
Control
Bomb
14C Transient
Case
First, the results for the seasonallyforced control run
(caseS10), for which10 Sv of ITF wereimposed,are
run.
Run SO does not allow for any lndonesian Through- presented. Our purpose here is to describethe threestructureofthebomb•4Ctransientmoving
flow, and is forcedat the seasurfacewith the seasonally dimensional
varyingmonthlywind stressclimatologyof daSilvaet through the upper ocean between 1955 and 1985. This
al. [1994]. The otherthreeexperiments
usea domain section begins with a descriptionof the the evolving
that allowsfor flow throughthe IndonesianStraits con- seasurfaceA•4C distributionbeforemovingon to the
nectingthe Pacificand Indian Oceans.Run S10,which evolution of the thermocline distribution of A 14C.
we define to be the control run, imposesa time-invariant
barotropicflowof 10 Sv throughthe IndonesianStraits,
and run S20imposes20 Sv of barotropicflow. Both S10
5.1. Sea Surface
A•4C
and S20 are forced at the sea surface with seasonal wind
The annualmean seasurfaceA14C at 5 year intervals from 1960 to 1985 are shownin Figures 2a-2f. By
stressclimatologyof daSilvaet al. [1994].
1960a relativemaximumof high A•4C has developed
Run I15 imposes15 Sv of ITF transport and is forced at the sea surface in the subtropical and subpolar re-
at the seasurfacewith the daSilvaet al. [1994]inter- gions,with low Ax4Cvaluesin the equatorialregion.
annuallyvaryingmonthlywind stressclimatology.The The meridionalgradientin sea surfaceA14C between
wind stressanomaliescontainedin the data set will gen- the tropics and the extratropics increasessignificantly
erate interannual variability in ocean circulation. Al- by 1965(Figure2b).
A14Chasalreadybegunto
though this global data set containsentries for each By 1965the atmospheric
month sincethe beginningof 1945,the numberof wind drop, as can be seen in Figure 1, although it is still
observationsin the western equatorial Pacific is small in excessof 500%0 at all latitudes. The maximumsea
before the 1970s. For I15 the imposed transport for surface
A•4Cat thistimeisslightlylargerthan300%0,
15 Sv wasmotivatedby the Rod#erset al. [1999]find- and thusthe gradientin A•4C acrossthe seasurfaceis
ing that only whenthe mean ITF transportis between still quite large. The year 1965 is also characterized
10 and 20 Sv does the ratio of Southern
to Northern
by a hemisphericasymmetry in the distribution of sub-
RODGERSET AL.. BOMB•4CTRANSIENTIN THE PACIFICOCEAN
I
-
I
8493
' I • ' I,,,
I
z
-ii!iii•
1) 100
120'E
150'E
180'
150'W
120øW
90 øW
60 'W
120øE
'
150øE
180
ø
x
180'
150•/V
120•/V 90•W
x
60•/V 120'E
150øE 180'
(b)
150'E
180
ø
120øW
90øW
60øW
150øW
x
150•'V
120•/V 90øw
x
60•/V
(e)
...........
•:iii::11ili::iii::i::iiii::::!•i•::•:•:•
......
50
120'E
x
(d)
(a)
120øE 150'E
150øW
•::%::i•::•::%::•::
120øW
.• ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
-
90øW
60øW
120øE
100
150•E
180
ø
150•'V
x
120•'V
900W
.
60•'V
(c)
Figure2. Annual
mean
seasurface
/•14C
in(a)1960,
(b)1965,
(c)1970,
(d)1975,
(e)1980,
and(f) 1985for thecontrolcaseS10.
of the Northern
tropical
seasurface
Ax4Cabouttheequator.In 1965 zonalgradientsacrossthe subtropics
the maximumvaluesin the NorthernHemispheric
sub- Hemisphereare significantlylarger than thoseacross
tropics
arein excess
of 100%olargerthanthevaluesthe subtropicsof the SouthernHemisphere.
By 1970 the surfaceA•4C valuesin the subtropin the Southern
Hemispheric
subtropics,
reflecting
the
ics
have increased,with the amount of increasebeing
large
meridional
gradient
inatmospheric
A•4Cthatexlarger
for theSouthern
Hemispheric
subtropics
thanfor
istedin theearly1960s(Figure1). Notetoothat the
8494
RODGERS
ET AL.. BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
the Northern Hemisphericsubtropics. Although atmo-
sphericAz4Cvaluespeakedin the mid-1960s,1970represents the time of maximum sea surface A z4C in the
subtropics. The lag between the peaks is due to the
5-10 year air-seaequilibrationtimescalefor A •4C isotopes. In the eastern equatorial Pacific, regions with
A14C valuesof <0ø/oopersistin the upwellingregion
between 160ø and 100øW, as well as in the region of
upwelling off the coast of Peru.
SubtropicalseasurfaceA •4C valueshavedroppedbetween1970and 1975(Figure 2e), whereasvaluesin the
equatorial Pacific have risen. The region of negative
seasurfaceA•4C valuesin the tropicsis nowconfined
to the Peru upwellingregion.
5.2. Thermocline
Distribution
of/k14C
In Figure3 we consider
the distributionof A•4C asit
evolvesacross32øN in 1960, 1970, and 1980. The distributions shownin Figure 3 representsnapshotson January i of the respective years. This section lies within
the subtropics and was the approximate latitude across
which the GEOSECS program sampledeight stations.
The model results will be compared with data in Section 7; here we present the transient as representedby
the model.
In 1960 (Figure 3a) we see that the surfacevalues
/
x
_-.
are already highest in the eastern part of the basin.
By 1970 (Figure3b), when surfacevaluesin the east-
i
150øE
180 ø
ern subtropics have reached their maximum values, the
x
verticalgradientat 130øWis of order 200%0 overthe
(b)
I
150•N
120•N
150'v,/
120•N
upper 200 m. Although a significantzonal gradient ex-
ists in seasurfaceA z4C at this time, zonalgradients
on constant depth horizons are small at depths >200
m. This remains true below 200 m depth through 1980,
althoughthe seasurfaceA•4C gradientdecreases
significantly between 1970 and 1980.
The corresponding
A •4C distributionalong180øW
is shown in Figure 4. During the 1960s the sea su-
face /k14Cincreaseis largestbetween20ø and 45øN,
which correspondsto the subtropics. By 1970, tongues
representing
subsurface
maximain A•4C can be seen
to extend from the subtropics of both hemispherestoward the equatorial thennocline. In the mid 1970s the
subsurfacemaximimum in the northern tongue exceeds
200%0, and the subsurface
maximumin the southern
tongueexceeds100%o. However,as we shallseein the
150'1=
180'
x
(c)
along32øN
coral comparison, maxi•na of this order are never seen Figure 3. The evolvingA14Cdistribution
in
(a)
1960,
(b)
1970,
and
(c)
1980.
at the sea surfacein the eastern equatorial Pacific in the
1980s. The model resultsalong this sectionwill alsobe
comparedwith GEOSECS measurementsin Section 7.
The plots shownin Figures 5a-5c show snapshotsof ever,A•4C is presentthroughmuchof the subtropical
the evolvingdistributionof A•4C on the ors=25.0sur- region, with maximum values in the Northern Hemiface. The shadedregionsof the open oceancorrespond spherealong the easternpart of the outcrop. The maxto regionswhere the isopycnalsurfacehas outcropped imum valuesin the SouthernHemispherestretch along
at the seasurface.Theseplotsrepresentthe Az4C dis- the outcropbetween180ø and80øW.By 1975the A•4C
tribution on January i of 1965, 1975, and 1985. The valueson this surfacehave increasedsignificantly,with
initial prebombA14Cvaluesin 1955wereuniformlylow maximum values in the eastern subtropics and minialongthe ors=25.0surface.By 1965 (Figure 5a), how- mum values in the upwelling regions. The zonal gra-
8495
RODGERSET AL.' BOMB •4C TRANSIENTIN THE PACIFIC OCEAN
asonemightexpectfromanidealized
zonallyaveraged
meridionalcell with downwellingin the subtropics,up-
wellingin the tropics,anda ventilation
timefor the
equatorialpycnocline
of order10 years.
It shouldbe pointedout for the isopycnal
surfaces
shown
in Figure5 that someof thewaterthat subducts
20'S
10'S
0'
10"N 20'N 30*N 40'N 50'N 60'N
Y
(a)
o • ' I ' I ' I ' I ' I ' I•
I ' ! ....
!...............
1
_-,..
.......
. ,,_.._.;...:,:...........,,,,,,,,,,
.,,'
,,-..,'
',,, ' .....•o-" ,,'
120'
E
i
150'
E
180'
150'W
x
120'W
90øW
(a)
120'
E
150'
E
180'
x
150'W
120'W
90'W
(b)
20'S
10'S
0'
10'N
20'N
Y
30'N
40'N
50'N
60'N
150
(c)
Figure4. Theevolving
A•4Cdistribution
along
180øW
in (a) 1960,(b) 1970,and(c) 1980.
120
dientin Ax4Cin subtropical
seasurface
Ax4Creflects
the zonalgradientin seasurface
conditions
alongthe
outcrop
ratherthanzonalgradients
in the ventilation
rate within the subtropicalthermocline.
150
120'E
150'E
180'
x
150'W
120'W
90'W
By1985thesubtropical
values
havediminished,
which
(c)
isexpected
given
thatatmospheric
values
haddecreased
monotonically
sincethe 1960s(Figure1). However, Figure 5. The Ax4Cdistribution
onthe•re=25.0surfor the controlrun (S10).
thereisnogreatincrease
in equatorial
Ax4Cvalues
be- facein 10 yearincrements
shown
are(a) 1965,(b) 1975,and(c) 1985.
tween 1975 and 1985 within 5ø latitude of the equator, Theyears
8496
RODGERSET AL.' BOMB •4C TRANSIENTIN THE PACIFIC OCEAN
WøOa
WøOe
WøOR
t
wøoat
'o8t
:looat
:1øog
t
x
z
120øE
150øE
180ø
150øW
120øW
90øW
60øW
x
(b)
Figure6. (a)Difference
in seasurface
A14Cbetween
theS10andSOcases
fortheannual
mean
during
1974.Negative
contours
indicate
lower
Ax4Cvalues
whentheITF transport
is 10as
opposed
to0 Sv.Themodel
inventories
ofbomb
x4Cinunits
of100atoms
cm-2 forJanuary
1
1975areshown
for(b)theSOcase,
(c)theS10case.
and(d)theS20case.
inthesubtropicical
North
Pacific
recirculates
within
theperience
intergyre
exchange.
Thetongue
ofhigh-AX4C
subtropical
gyre,whilesome
advects
toward
theequa-waterseen
along
theequator
ontheere-25.0
surface
torial
thermocline.
Thepartitioning
between
these
twoin 1985
istracing
oneoftheimportant
pathways
of
ventilation
pathways
islargely
dependent
upon
thelon-equatorial
thermocline
ventilation
forS10.
namely,
subgitude
atwhich
a water
parcel
subducts.
Water
sub-duction
intheeastern
subtropical
North
Pacific,
advecducting
inthewestern
partofthebasin
ismore
likelytiontothewestern
boundary
viatheNorth
Equatorial
,
toundergo
recirculation
within
thesubtropics,
whereas
Current,
equatorward
advection
withintheMindanoa
water
thatsubducts
totheeast
ismore
likely
toex- Current,
andtheneastward
advection
withtheEUC.
RODGERS ET AL.: BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
8497
z
10
12
/
i
120øE
150øE
180'
150øW
120øW
90'W
120øW
90øW
60'W
(c)
z
4••
i
120øE
150øE
180'
150øW
i
60øW
(d)
Figure 6. (continued)
Along the equator,in the westernPacificthe approx- ventilation and intergyre exchange. Next we consider
imate depth of this surfaceis 200 m. This advective the wayin whichthe treatmentof the IndonesianThroughpathwaydoesnot existin courseresolutionmodels,as flowinfluencesthe bombz4Cresponseof the equatorial
Pacific.
they do not capturethe criticalscalesfor the EUC.
Figure 6a showsthe differencein annual mean sea
6. Comparison of Cases With Different
ITF
Transports
surface A•4C between the S10 and SO runs for 1974.
Clearly,thereis a significantoffsetin excessof 30%0
for most of the surface ocean between 20øN and 20øS,
The AZ4C distribution calculated for the control case with a meanoffsetfor this regionof 55%0. There is a
S10 has illustrated the pathwaysof model thermocline maximum differencelocated at •160øW along the equa-
8498
RODGERS
ET AL.: BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
60.1
20
5ON'
•"
100
4.ON
-5O
-2:O
: 55
3ON
20N
-10
15
1ON
-55
-20
-35
EQ
-75
-35
-20
--5O
lOS
15
2os
60
ß
•2'OE
•¾OE
•'OE
•0
lSbW
14bW
12bW
10bW
80W
Figure ?. Geochemical
OceanSections
Study(GEOSECS)seasurfaceA14C;modelminusdata
for S10.
tor. CaseSO,whichhas no IndonesianThroughflow,is 7. Discussion
characterizedby sea surfacebomb A x4C levels,which
are muchhigher than for S10. There is almostno differ- 7.1. Data Comparison: Sea Surface A14C
ence in the sea surface A•4C distribution between the
S 10 and S20 runs for 1974.
Figure 7 showsthe differencebetween the modeled
A14Cdistribution
for the I15, caseandthe seasurface
Figures 6b, 6c, and 6d show the vertically integrated valuesmeasuredduring GEOSECS betweenAugustof
inventoriesof bomb•4C for the SO,S10, and S20 cases 1973 and June of 1974. To account for the fact that the
on January1 1975. The units for theseplots are 10ø GEOSECS data were collectedat differenttimes, the
atomscm-2. A comparisonof the S10 casewith data model values at the location of each GEOSECS station
will be addressedin Section 7; for now we wish to point
out
the similarities
and
differences
between
the three
cases.
The
differences
between
the SO and S10 cases are
were chosento correspondto the particular month during which the station was sampled. This method of
comparisonallowsus to avoid aliasingof seasonaland
interannualvariabilityin seasurfaceA14C.
quite pronounced. Along the equator, inventoriesfor
Between 20 ø and 35øN the difference in sea surface
the SOcaseare more than 20% larger than for the S10 A•C increases
from20%o at ~160øE,to 95%0 at
case. This is consistent with what we saw for the sea
160øW.That the seasurfaceA•C in the subtropics
surfaceA•4C difference
plot in Figure6a. The inven- of the North Pacificis too large in the model is at least
tories for the SOcaseare also larger in the subtropics in part due to the fact that convectiveadjustmentis
not penetratingdeepenoughin this regionduring winof 30øN, the signof the differencechanges,with inven- tertime. The modeloverestimation
of 95%o at 160øW
toriesfor the S10casebeinglarger than for the SOcase. is approximately consistentwith the overestimationobThe sensitivityof the bomb •4C inventoriesin the ex- tainedin the simulationof Toggweiler
et al. [1989b].
tratropical North Pacific to whether the ITF is open For the two GEOSECS stationsin the subtropicalNorth
was certainly not an expected result of this study.
Pacific closestto Japan between140ø and 155øEthe sea
In comparison, the differencesbetween the S10 and surface A•C in the model is less than the GEOSECS
$20 casesare relatively insignificant. This is not only measurements.One possibleexplanationfor this is that
true of the depth-integrated inventories. The three- the model's horizontal resolution is still insufficient to
dimensional distribution of bomb •4C in the Pacific therresolvethe westernboundarycurrentsin the subtropics.
across 20øN than they are for the S10 case. Poleward
toocline is nearly identical when examined on either
depth levels or isopycnal horizons for the S10 and S20
The model's seasurfaceA•C between 10øN and 10øS
in the equatorial Pacific is lessthan the GEOSECS mea-
cases.Althoughthe thermoclineA•4C distributionis surements.The point on the equator(125øW,0øN) in
quite sensitiveto whether the ITF is open or not, it is the eastern Pacific where the model underestimates the
nearly insensitiveto whether the imposed tranport is GEOSECS measurement
by 75ø/øøis perplexing. As
10 or 20 sverdrups.
was pointedout in an earlierprocessstudy [Rodgers
RODGERS ET AL.
BOMB •4C TRANSIENT
IN THE PACIFIC OCEAN
8499
55N
50N
45N
40N
ß
84.•• 11.8
55N
50N
1
25N
20N
•-12---•-
.•12
15N
1ON
5N
55
12oE
140E
160E
180
160W
140W
120W
100W
80W
Figure 8. Model •4C inventoryfor January1, 1975 (caseS10), with valuesinferredfrom
GEOSECSmeasurements
superposed.
The unit:•are 100atomscm-2.
et al., 1997)],the seasurface
GEOSECSA14Cvalue the modelcontours,and the unitsare 109atomscm-2.
measured at the surface on the equator in the eastern
The GEOSECS survey intersected the equator at
Pacific is as high as the value measuredon the equator both 180 ø and 125øW in the Pacific Ocean. The data
in the western Pacific, whereas corals clearly indicate reveal a zonal gradient in the inventories across the
that a meanzonalgradientin seasurfaceA•4C ex- equatorialPacificof order 2 x 109 atomscm-2, and
isted acrossthe equatorial Pacific in the 1970s. What- this is captured by the model. Of course, one must
ever processin the real oceanacted to producesuch bear in mind that the vertical samplingdensity for the
an anomalously
largeA x4C valueat this samepoint GEOSECS stations in the equatorial thermocline typiin May 1974 has not been capturedby the model. As cally includesonly four or five measurmentsin the upwe shall seein Figure 8, the model adequatelycaptures per 200 m, but nevertheless,the basin-scalefeatures are
the depth-integratedinventoryat 125øW, 0øN. As the captured by the model.
discrepancybetweeenthe model and the observations The spatial structure of the model inventory within
existsprimarilyfor the surfaceAx4Cvalue,we reiter- the subtropical North Pacific is also in good agreement
ate what we statedin our earlierstudy [Rodgerset al., with the model. However, the comparison is compli1997],namely,
thatthesampling
strategy
ofGEOSECS cated by the fact that the GEOSECS measurements
complicates
model-data
comparisons
for particulargrid tend to coincidewith regionsof large meridional gradients in the model. The tongue of maximum inventories
points at particular times.
In the westernequatorialPacific,the modelunderes- across the central subtropical North Pacific appears to
timates the seasurfaceGEOSECS A•4C measurements be shifted too far south in the model, although by how
by 35-50%0.This is consistent
with the offsetthat we
much is difficult
to assess from the GEOSECS
data.
will see between the Nauru coral time series at 165ø
and the model in a later section. One possible reason
for this offsetis that the gasexchangeformulationunderestimatesthe flux of x4Cfrom the atmosphereto the
Table 1. Summaryof Bomb•4C SimulationRuns
ocean in the warm pool region.
ITF Transport
7.2. Data Comparison: Bomb 14CInventories
0 Sv
10 Sv
15 Sv
20 Sv
and
GEOSECS
Data
A comparison
of themodel's
bomb14Cinventory
for
January1, 1975, with the inventorycalculatedusing
SeasonalForcing
Interannual Forcing
SO
S10
115
S20
For the casewith seasonalwinds, the seasonalclimatology
Pacific GEOSECS measurementsis shown in Figure 8.
The inventories calculated from GEOSECS measure-
of da$ilva[1994]is used.For the casewith interannualwinds
the interannual climatologyof daSilva is used to force the
ments[Broecker
et al., 1995]havebeensuperposed
on
ocean model.
8500
RODGERS
ET AL-
BOMB •4C TRANSIENT
Table
2. Pacific Inventories for •4C Atoms
IN THE PACIFIC
OCEAN
Table 4. Geochemical
OceanSectionsSurvey(GEOSECS)
Indian Ocean Inventories, Estimated Local Input From
Latitude Range
Broeckeret al. [1985]
Lamont Model
40ø-65øN
15ø-40øN
10øS to 15øN
7.3
35.9
23.7
7.2
32.9
23.4
Total
66.9
63.5
Unitsarein 102614Catomscm-2.
Gas Exchange,and difference
Latitudes
Inventory
Gas Exchange Input
Deficit
25øN to 15øS
15ø S to 50 øS
13.2
40.5
24.1
20.3
-10.9
20.2
Units are 1026 atoms. This table is taken from Broecker
et al. [1985].
A more thorough assessmentof the model's simulated
A14C distributionin the subtropicswill requirea de-
Straits
tailed comparisonof the vertical structure of the simu-
run. The fluxes were calculated using the perturbation
is tabulated
as a function
of time
for the S10
lated and observed A14C distribution.
x4Ctracer. The first columnof modeloutputgivesthe
The integratedbomb14Cinventories
for the Pacific flux for a particular year, and the secondcolumn gives
are represented in Table 2, where they are compared
with the calculated budgets obtain from GEOSECS
year. Thus the modelsendsa flux of 9 x 1026atomsto
data [Broeckeret al., 1985]. The agreementbetween
the Indian
the model and the data is very good, especially considering that Broecker et al. estimate the error bars in
in 1974,whichmeansthat 1/8 of the bombx4Catoms
their calculations to be of order 5-20%.
Next we consider the flux of bomb 14C atoms from
the Pacific to the Indian Ocean through the Indonesian Straits. In Table 3 the flux of bomb 14C atoms
from the Pacific to the Indian
Ocean via the Indonesian
the cumulative flux integrated from 1955 to the given
Ocean between
1955 and Pacific GEOSECS
deliveredto the Pacific Ocean via gas exchangebefore
the GEOSECS program were exported to the Indian
Ocean via the ITF.
By the time of Indian OceanGEOSECS(early1978),
this accumulated flux of 14C had increased to •13 x 1026
atoms. The Indian OceanGEOSECS data [Broecker
et al., 1985]are shownin Table 4, and comparingthe
model flux with the data tells us that •25% of the total
Table 3. Flux for Each Year and Cumulative Export
IndianOceaninventory(53.7x 102•atoms)hascome
of x4C Atoms From the Pacific to the Indian Ocean
from the Pacific Ocean. In fact, our calculated flux
CalculatedUsingthe Perturbationx4CTracer
from the Pacific
Year
Flux of 14C Atoms
Cumulative 14C Flux
Broeckeret al. [1985]between15øSand the northern
1955
0.1
0.01
1956
1957
1958
1959
1960
O.3
O.5
O.7
0.9
1.4
O.O4
1961
2.0
0.6
1962
2.6
0.8
1963
3.2
1.2
1964
3.9
1.6
1965
4.7
2.0
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
5.6
6.5
7.5
8.4
9.1
9.4
9.7
10.0
10.3
10.5
10.4
10.3
10.2
2.6
3.2
4.0
4.8
5.7
6.7
7.6
8.6
9.7
10.7
11.8
12.8
13.8
1979
10.1
14.8
0.1
O.2
0.3
0.4
to the Indian
Ocean between
1955 and
1978 is nearly equal to the total inventory calculated by
boundary of the Indian Ocean. Our model domain is
able to tell us nothing about the eventual ventilation
pathwaysof bombx4C-ladenPacificthermoclinewater
upon entering the Indian Ocean. Sufficeit to say that
our results imply that the role of the ITF should not
be ignored when consideringIndian Ocean inventories
of transient
tracers.
7.3. Data Comparison:
G EOSECS
Sections
The model distribution of A14C across 32øN on Jan-
uary 1, 1975, is shown in Figure 9a, with the observed
GEOSECSvalues[Ostlundand$tuiver,1980]alongthis
transect superimposed. For the sake of comparisonwe
shallconsiderAx4C to be a dye tracer,with the depth
of the 0ø/ooisolinerepresenting
the penetrationdepth
of the dye. Importantly, across the western and central regionsof the subtropical North Pacific, the depth
of the 0ø/ooisolineas recordedby the GEOSECSmeasurementshas penetratedat least 50% deeperthan the
0ø/ooline in the model. Of course,one shouldkeep
in mind that for this region the GEOSECS measurements occured •1 year earlier than the model output
The unitsfor the fluxesare 1025atomsyr-x, and for the field shown in Figure 9. However, this only helps to
cumulative fluxes are 1026 atoms
emphasize our point that the model is underestimat-
RODGERS
224
ET AL.- BOMB x4C TRANSIENT
223
225
226
214
IN THE PACIFIC
213
OCEAN
204
8501
202
201
50
140
lOO
15o
200
250
300
............
11• .................... 50.....
,
- 5o"
...... 50.............20--"
.
--75
,,
10
350
,,
ß
ß
..
-50
.-- .......
-100
o
•
400
.• ....... -.• ........
ß
,,
.
•
•
450
,
lOO........
-.
,,
--15
............
--12O
.
'-..... "--25
-lOO
5OO
lOO....
--20
2O
55O
1•'OE
14'OE
269
150E
26,3
16'OE
257
17'OE
251
24-6
17bw
16'ow
24-1
229
90
134)
I
125 95
50
1õ0
150w
2,31
14'OW
13'ow
214-
12bw
217
218219
1
100
• l'l••y"
150
200
175 "/ /"'/;""';'"'
--''"r,i
:-"
,,'
250
,,,,
X. /
"... 7o ---o-
:300
'"-
""
,,
m'"'
,
350
\
,,
'ß
.-.
/
',,
,;
\
400
....-"_ -,,o ....
.....- -7o
.-.....-•
',,
,-
--70
,/
-•5
.
..
",,
,,'
....
.,'
,.'
/
'
,'
ß
--10•
/
...',m ,,,.."'
',
-•oo-•o
,'
"
5OO
,
;
ß
ß
ß
-,
-'-135--'•'•0.
"'
,-"•o6 -•o
,'
,,'/
,-'"-2o
"-100
,,
/
'
,,/
,
,.'
,
,,
/
26s
.-
,,'
......
,,'"
,
ß
,,'ß
lO
55O
.. ..... ,'
......- •oo-•"•.•oo
../
/
ß
"-•f
45O
-70
,,,,...
'"-,•---o•
/
--120 --19o
16s
E'Q
,/
--180
.'"
16N
26N
36N
46N
55N
Figure 9. A14C(a) as a functionof longitudeand depthacross32øNfor the modelin 1975,
alongwith GEOSECSmeasurements,
and (b) as a functionof latitude and depthalong180øW.
GEOSECS data are givenin bold print, and GEOSECS station numbersare givenacrossthe top
of the distributions.Alsoshownare (c) the simulatedJanuarytemperatures
across32øNand (d)
the Janauryvaluesfor the samesectionfromthe LevitusandBoyerI1994]monthlyclimatology.
8502
RODGERS ET AL.- BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
ing the rate of ventilationof the westernand central olution,is knownto suppressconvection[Largeet al.,
subtropicalPacific thermocline.
19971
.
Given our interest in the penetration depth of transient tracers in the western
Pacific and the observation
in Figure 9a that the subtropicalGEOSECS sampling
of inventoriescoincideswith regionsof large gradients
in the model, the question arises as to the validity of
our claim with Figure 9a that the dye tracer is not
penetrating to sufficient depth. This is addressedin
Another factor contributing to the discrepancybetween the model
be that
and observations
our model
in midlatitudes
has •2.5 ø horizontal
could
resolution
in
the extratropics. Followsand Marshall [1996]comparedthe bomb x4Cinventoriesin the North Atlantic
for two different versions of the Community Modeling
Effort (CME) model, one with 1ø and one with 1/3ø
Figure9b, whichshowsthe A•4C distributionaccord- resolution.They foundthat eddytransfersin the 1/3 ø
ing to the model on January 1, 1975, along 180øW. As
versionof the model significantly enhancedthe ventilacan be seen with the GEOSECS data near 32øN, once tion of the western subtropicalgyre. In fact, the eddyagain,the modelA•4C is not penetratingto the same permitting experiment of Follows and Marshall is the
depth as that recorded by GEOSECS measurements
anywhere within the subtropicsalong the dateline. In
fact, this problem appears to be characteristicof both
hemispheresalong 180øW in our model simulation. As
with the Northern Hemisphere, the penetration depth of
only simulationof the bomb x4C transientin the subtropics that has provided a satisfactory representation
of tracer inventories in the western part of the subtrop-
ical thermocline.The Gent and McWilliams[1990]parameterization used in our study is intended to account
the A•4C equals0ø/oo
isolinein the westernpart of the for this effect, but it's clearly inadequate.
South Pacific subtropicalgyre This is consistentwith
Despite the fact that the model is not adequately sim-
the findingsof Craig et al. [1998],who simulatedthe ula.ting the penetrationof bomb•4C into the lowertherdistribution of CFC's in a global 1ø versionof the Geo- toocline of the western subtropical North Pacific, it is
physicalFluid DynamicsLaboratory(GFDL) Modular doinga reasonablejob in maintainingthe observedtherOceanModel (MOM). In their study,the penetration mal stratification in this region. Figures 9c and 9d show
depthof the 1 pmolkg-• isolinefor Fll is only half of the distrubtion of January temperature across 32øN,
what is seen in CFC data meausured between 1988 and
whichis the samesectionshownin Figure9a for A•4C
1990 along 170øW at both 35øN and 35øS. The same
problem exists in their modeled Fll field at 165øE. Nevertheless,the Fll distribution in their equatorial pcynocline is in much better agreement with observations.
Our resultsat 32øN are alsoconsistentwith the findings
for the model
and
data.
For
these distributions
the
simulated depths of the 12ø and 14ø isotherms are only
10% shallower than the values derived from the Levi-
tusandBoyer[1994]monthlyclimatologyfor the same
time. Thus the 50% disagreement
betweenthe model
and data in the penetrationdepthof bomb•4C for this
of the OceanModel Intercomparison
Project (OCMIP)
program(J. Orr, personalcommunication,
1999),where regionis not a reflection of problemsin maintaining the
the vertical structure of A•4C inventories in the North
Pacific
is underestimated
stratification of the extratropical thermocline.
for all of the ocean models
compared in the project.
The extentto whichthe errorin extratropicalA•4C
resultsfrom shortcomingsin the model'srepresentation
of dynamicalprocesses
and the extent to whichit representsproblemsin the parameterizationusedfor gasexchangewill be addressed
in a futurestudy.The modeled
vahles in the surface waters of the eastern subtropics are
largerthan observations
by •100ø/oo.aswe sawin Fig-
7.4. A•4C on Isopycnal Surfaces
The A•4C distribution on the as=25.0 surface for
the S10 run is shown in Figure 10a. Vertically interpolated values from GEOSECS stations are superimposedon the model fields. Given the density of vertical
samplingduring GEOSECS, one must exercisecaution
in making detailed comparisonswith the data. Thus,
for the latitude belt between 20øSand 10øN the agreement between the model and the data is good since
errorsare less than 25ø/øø. However.in the Northern Hemispheric subtropics the differencebetween the
ure 7 wherewe comparedmodelresultswith GEOSECS
data. This is largely due to the fact that the model is
not adequatelycapturing the seasonalcycle in mixed
layer depth for much of this region. Wintertime con- modelA x4C distributionandthe verticallyinterpolated
vectionis seldompenetratingto as nmchas 50 m depth GEOSECSdatais > 50ø/øø.
with largerdifferences
in the
in the eastern subtropics, so that the upper ocean re- eastern part of the gyre.
mains stratified through the year. The model'sshallow
Figure 10b showsthe model and GEOSECS data on
wintertime
convection at these latitudes is inconsistent
the •s=26.0 isopycnalsurface. On this isopycnalsur-
with the data [Huangand (•i'u, 1994].whichshowsa face the model is underestimatingthe A•4C levelsin
mixed layer depth in this regionwhich is twice that of the SouthernHenrisphere
and overestimating
the A•4C
the model. This could be an artifact of the eddy mix-
levelsin the easternpart of the Northern Henrispheric
ing parameterizationof Gent and McWilliams [1990], subtropicalgyre. In the westernpart of the subtropical
which when used in conjunctionwith high vertical res- gyrewe seethat the modelis underestimating
Ax4C,
RODGERS ET AL.' BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
8503
o
o
'-
18
o
o
N
o
4
o
8,,•
o
o
o
o
150øE
180
øX
150øW
120øW
(c)
o
o
o
o
8
o
o
o
o
150øE
180'
150øW
x
Figure
9. (continued)
120øW
8504
RODGERS
ET AL.: BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
210
<D
25N•'"'-/"•'90•'
•,•,,•150._...
95 •-"180
15N
•
I
5N
•.
105
160 5•2;
'• _ an •120
-
•150•
ou•
-90•
60-
•5
•zu
,
60
50-
-
120E
•
120 •••-------•••
150
140E
1•OE
180
160W
140W
120W
100W
86W
140E
160E
180
160W
140W
120W
100W
80W
(b) ""
50N
40N
30N
20N
120E
Figure 10. A14Con January1, 1975,for the S10run on (a) the ere=25.0isopycnal
surfaceand
(b) the ere=26.0isopycnalsurface.GEOSECSdata are givenin bold print.
which is the same problem we saw at depth in the west- are the same;namely,the model significantlyunderesern subtropicsin Figure 9a.
timates the depth of the isosurfacein the wetsern sub-
7.5. Penetration Depth of A14C Isolines
tropicsof the Northern Hemispherebut comparesreasonablywith data in the easternsubtropicsand within
The depth of the A14C=0 isosurface
on Januray1, the equatorial thermocline.
1975,is shownin Figure 11, with GEOSECS data super-
posed. Consistentwith what we have seen before, the 7.6. Data Comparison:
Experiment
model is significantly underestimating the penetration
depthof the bomb14Csignalin the westernsubtropical
North Pacific
The Pacificcomponentof the globalGEOSECSsur-
North Pacific. The simulated penetration depth in the vey represented
the first synopticsurveyof A14Cin the
eastern subtropical North Pacific, on the other hand, is PacificOcean.However,the verticalsamplingdensity
in muchbetteragreement
with the GEOSECSmeasure- within the equatorialthermoclineduring GEOSECS
ments.Thepenetration
depthoftheA14C--0isosurfacetypicallyincluded
onlyfoursamples
in theupper200m.
in the tropicsis alsoin reasonably
goodagreement
with It wasnot until the North PacificExperiment(NORthe data.
PAX) survey[Quayet al.. 1983]along150øand160øW
To demonstratethat our conclusionwas not biased during 1979and 1980 that the equatorialthermocline
by the arbitrarychoice
of the A14C--0isosurface,
this wassampled
with sufficient
densityto resolve
thevercomparison
wasrepeated
againforboththeA14C---25ticalstructure
of A14Cin theequatorial
pycnocline.
and-50%0isosurfaces.
The resultson thesesurfaces Themeridional
section
plotsof z•14Cin theequato-
RODGERS ET AL.: BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
8505
55N
50N
45N
40N
35N
3ON
391
3•0
25N
15N
1ON
'35C
•250
5N
-230
2,35
•
225
1•
5S
255
1
EO
120E
140E
160E
180
160W
140W
0
50•
120W
100W
80W
Figure 11. ThedepthoftheAx4C-0isosurface
onJanuary
1, 1975,fortheS10run. GEOSECS
data are givenin bold print.
rial thermocline
from the studyof Quayet al. [1983] 7.7. Data Comparison: Coral Time Series
are shownin Figure 12. Figure 12a showsthe distri-
bution
asa function
oflatitude
anddepth,
andFig- Nextwecompare
themodel
results
withexisting
coral
ure12bshows
thedistribution
asa function
oflatitudeAz4CdatafromthePacific
Ocean.
Timeseries
forfour
anddensity.
TheAxaCdistribution
measured
alongsites
areshown
in Figure
13:French
Frigate
(23øN,
thistransect
isrelatively
symmetric
about
theequator.
166øW)'
Galapagos
(1øS,
90øW),
Fiji(18øS,
l?9øE),
It is clearfor the latitude versusdensityplot that the andNauru(1øS,167øE).FrenchFrigateliesin the sub-
maximum
subsurface
AxaC
signal
onthedensity
rangetropics
oftheNorthern
Hemisphere,
FijiinthesubtropHemisphere,
Galapagos
in the eastsurroundingere=25.0doesnot extend to the equator, icsof the Southern
butrather
forms
a subsurface
tongue
which
extends
in erntropics,
andNauru
inthewestern
tropics.
Foreach
fromthesubtropics.
Nevertheless,
theAx4C
values
on siteweshow
themodel
output
fortheI15run(black)
the equator on the ae=25.0 density horizon are well andthe coraldata (shaded).
above
theirprebomb
levels
of~-70ø/oo.
Firstweconsider
French
Frigate
(Figure
13a).The
Some
interhemispheric
differences
intheAxaC
distri-coraldata[Dvuffel,
1987]
represents
annual
mean
valbution
along
150øW
warrant
mention.
Thedistribution
ues.Thecoraldatapeakat 180ø/oo
in theearly1970s.
ischaracterized
bya subsurface
maximum
inAz4Cfor Themodel
simulation
peaks
a fewyears
earlier,
and
theNorthern
Hemisphere
thatiscentered
between
the themaximum
offset
between
themodel
andthedatais
ae-24.5
and25.0isopycnal
layers.
According
tothe 50ø/øø
in !968.BythetimeofPacific
GEOSECS
the
March
seasurface
temperatures
andsalinities
[Levitus
offset
between
themodel
andthedataisquite
small.
andBoyev,
1994],
thiscorresponds
toa March
outcrop Nextweconsider
themodel's
simulation
ofthebomb
between
25ø and30øN.FortheSouthern
Hemisphere
transient
at Galapagos
(Figure
13b).ThecoralA[4C
themaximum
A•4Cis centered
between
theere=25.0datasetshown
isthatof Guilderson
and$chrag
[1998].
and25.5density
layers.Thiscorresponds
to a SouthernIn contrast
to French
Frigate,
theA•C values
at GalaHemispheric
winteroutcropbetween
30øSand35øS. pagos
exhibita gradualincrease
through
the1970s.The
The modelresultsshownin Figure12crepresent
a agreement
between
thesimulation
andtheobservations
snapshot
of modelconditions
for January
1, 1980,for isquitegood,although
themodel
slightly
overestimates
theS10case.Themodel
reproduces
themajorfeaturesseasurface
z•[•Cin thelate1960s
andslightly
underofthedata,namely
thesubsurface
tongues
ofhighAx4C estimates
thevalues
forthelate1970s
andearly1980s.
extending
fromthesurface
watersof the subtropics
in Guilderson
and$chrag
[1998]
havepointed
outthatseaeitherhemisphere
intotheequatorial
thermocline,
and sonallyminumumcoralAx•C is characterized
by an
it captures
thesetongues
at thecorrect
depthlevel.The abruptjump in 1976. This is not reproduced
by the
asymmetry
abouttheequator
in themaximum
subsur-model,whichshows
a gradualincrease
in its seasonally
faceAz4Cvalues
is alsocaptured
by the model,with minimum
A•4Cthrough
the 1970s.
largervaluesto the northandsmallervaluesto the Nextweconsider
theFiji site(Figure13c).Herethe
south.
coraldata [Toggweiler,
1983]areseento be in good
8506
RODGERSET AL.. BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
a
15"5
I0
I
•,
I
•oo I•_
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ß
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0
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,,,.,.•
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20"N
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F,////
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22,
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._.• . _--__--_'•,_,.,v•
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ß __ .I, --
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+,,,o
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-----"--- • • / I
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/._--.--•.,,,
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/
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/
/
20øS
I1
\
/
10øS
0'
10øN
20øN
30'N
40øN
50øN
60øN
Y
Figure 12. (a) TheAx4Calong150øWmeasured
in 1979duringtheHawaiito TahitiShuttleas
a funtionof latitudeanddepth[Quayet al., 1983].(b) The samequantityplottedasa function
of latitudeand potentialdensity.(c) The model'sA[4C at 150øWas a functionof latitudeand
depth on January 1, 1980.
RODGERS ET AL.' BOMB •4C TRANSIENT
IN THE PACIFIC OCEAN
8507
(a)
a,
25o
I
I
I
I
i
I
200 -
150 -
.•-
100-
v
o
-9
50-
0-
-50
-100
1955
1960
1965
1970
1975
1980
1985
1990
YEAR
(b)
b
250
200
150
100-
50
-50
-100
1955
1960
1965
1970
1975
1980
1985
1990
YEAR
Figure i3. Model(black)and coral(shaded)Z•14Cfor (a) FrenchFrigate,(b) Calapagos,
(c)
Fiji, and (d) Nauru.
agreement with the model simulation. As with the resolutiondata of Guildersonet al. [1998].The offset
French Frigate coral, this subtropicalsite peaks in the betweenthe modeland the data is •40ø/oothroughthe
1970s. This offset is consistent with the offset meaearly 1970sbefore declining through the 1980s.
Finally, we considerthe A14C signalfor Nauru Is- suredat 180øW during GEOSECS [Ostlundand $tuland (Figure 13d). The coral curve is from the high- iver, 1980].In particular,the modelcapturesthe spike
8508
RODGERS ET AL' BOMB •4C TRANSIENT IN THE PACIFIC OCEAN
(c)
c
25O
I
I
I
I
I
I
200
150
.•'
E
lOO
v
-•
50
-50
-100
1955
•
•
•
•
•
•
1960
1965
1970
1975
1980
1985
1990
YEAR
d
25O
I
I
I
I
I
I
200
150
•'
ß
100
-•
50
E
0
-50
-100
1955
1960
1965
1970
1975
1980
1985
1990
YEAR
Figure 13. (continued)
associated
with the 1972-1973E1 Nifio, althoughthe
peak in the model occurs some 5 months earlier than
There are severalpossible
reasons
why the model's
Az4Calongtheequator
during
theearly1970s
could
be
the peak in the data. In the modelthe spikeis caused lowerthanthe valuesmeasured
duringGEOSECSand
by an eastwardsurgeof surfacewaterswith highAz4C the valuesrecorded
by equatorial
corals.Onepossibilfromthe MindanaoCurrentto the eastalongthe equa- ity is that the gas exchangeformulationusedfor this
tor.
studyis inadequate.This is quitelikelyat Nauru since
RODGERS
2.5
ET AL.: BOMB •4C TRANSIENT
I
I
I
IN THE PACIFIC
I
OCEAN
I
8509
I
2
•
1.5
•
1
:• 0.5
0
z
rr
0
LU -0.5
-1
-1.5
-2
-2.5
1955
•
•
•
•
I
•
1960
1965
1970
1975
1980
1985
1990
YEAR
Figure 14. Comparison
of modeled(black)and observed
(shaded)[Kaplanet al., 1998]NINO3
SST anomalies
between
1955 and 1990.
deeperwatersand dilutingthe A•4C signal.It is most
the wind speedsover the warm pool in the ECMWF
climatologyare not very large. However, local tropical likely that excessivemixing within the equatorial unis dilutingthe seasurfaceA•4C signalin the
wind speedsshould not be so important at Galapagos dercurrent
sincethe supplyof bomb •4C via intergyreexchangeequatorial Pacific in the early 1970s.
A comparison of the SST anomalies in the eastern
should be more important.
A secondpossibleexplanation is that by imposing a equatorial Pacific for the I15 case and the dataset of
fixed ITF transport for theseruns we have compromised Kaplan et al. [1998]is shownin Figure 14. For both
the meanA14C of the surfacewatersfor the equatorial cases, temperature anomalies are calculated over the
Pacific. We have seen that for the SO case the mean sea NINO3 region, i.e., over a box ranging in latitude from
surfaceAz4C for the equatorialPacificis significantly 5øSto 5øN and from 90ø to 150øW in longitude.
It should be kept in mind that wind stress data for
larger than for the other cases.In fact, the sea surface
the
equatorial Pacific is quite sparse before the 1970s,
A•4C distributionfor the SOcaseis in better agreement
so
that
the model's interannual variability in sea surface
with the GEOSECS data than the others. However, we
temperature
during this period should not be expected
rule this possibilityout by the argument that when the
to
match
observations.
In addition, for the I15 case we
ITF is closed, the mixing ratio of northern to southforced
the
model
with
interannually
varying winds, but
ern component
watersin •he equatorialthermoclineis
heat
fluxes
were
calculated
using
seasonally
varying clidistorted[Rodgers
et al., 1999].
matologies.
Given
these
limitations,
the
match
between
A third explanation for the model's equatorial sea
the
model
and
the
data
for
this
region
is
quite
satisfacsurface
A14Cbeingsomewhat
lowerthantheGEOSECS
tory. It is interesting to note, however, that although
measurements
is that the model is too diffusive.
We
have seenin our comparisonof the A•4C distribution the model representsa shift in SST in 1976, it fails to
measuredin the equatorial thermocline during NOR-
do sowith A14C,as wasseenin Figure13b.
PAX (Figure 12a) and the Az4C for the modelfor
7.8. The Subtropical Cell and Intergyre
January 1, 1980, at the samelocation(Figure 12c)
that if anything,the modeloverestimates
the A•4C at
10øN and 10øS.This points to the low-latitude western
Exchange
It
was mentioned
earlier
that
the
evolution
of the
boundarycurrents(LLWBCs) and the EUC as poten- bomb A •4C transient in the Pacific thermocline has a
tial locationsof spuriouslylarge diapycnal diffusivity in bearing on climate questions. For the zonally averaged
the model, which would have the effect of entraining circulation in the Pacific a meridional subtropical cell
8510
RODGERS
ET AL.: BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
maximain A14Cdo not
(STC)exists
in thesurface
andthermocline
waters.The system,sincethe subsurface
surfacewater that subducts in the eastern part of the
manage to ever reach the EUC. A quantificationof this
subtropicalgyresmovesequatorward
within the ther- entrainment will be important for understandingthermocline and, upon upwelling along the equator some mal budgets.
A secondpathwayby whichlow-AX4Cwater can be10-20yearslater, returnsto the midlatitudesvia the
surfaceEkman flow. Wyrtki [1981]usedmass,heat, come entrained into the STC is via Ekman flow at the
and salinitybudgetsto arguethat the strengthof the ocean surface. A sizeable fraction of the water that
equatorialupwellingis •50 Sv. Assuming
that only 5 subductsin the subtropical convergenceof both hemiSv of this representslocal recyclingof equatorialwater
impliesthat the strengthof the STC is 45 Sv.
The northern boundaryof the STC coincideswith the
zero wind stresscurl line separating the subpolar and
subtropicalgyresof the North Pacific,or approximately
the outcropof the crs-26.2 isopycnal,and the southern
boundarycorrespondsto the outcropof the sameisopycnal. The crs-26.2 will define the lower boundary of
spheresof the Pacific Ocean is suppliedby the equatorward Ekman flow from the subpolar gyre of the North
Pacific and from the Antarctic Circumpolar Current
(ACC) regionof the SouthernHemisphere.That this
watermaintainsits low-A•4C signatureuponsubduct-
ing in the subtropicsis due to the decadalair-seaequilibration timescalesfor carbon isotopes,whereasits temperatures and salinity properties are significantly modour STC as it delimits the lower bound of the directly ified through heat and freshwater fluxes. A quantificaventilatedthermocline
[HuangandQiu,1994],with the tion of the effectof this process,which involvesintergyre
exchangebetween the subtropical and subpolarregimes,
upper boundarybeing the seasurface.
That this cell is not closedcan be seen by consider-
on A•4C inventories in the STC will be addressed in a
inga simplebudgetcalculationinvolvingthe Indonesian future study.
Enhanced equatorial resolution in ocean models is
Throughflow.The ITF representsa meanflux of thermocline water out of the Pacific basin, and given that
the estimatesof ITF transportare in the range5-16 Sv
also important in simulations of the oceaniccarbon cy-
cle. Aumontet al. [1999]haveshownthat whenmerid-
[Godfrey,1996],this constitutesa flux that is equiva- ional resolution of 0.5 ø is used in the vicinity of the
lent to 10-35% of the strengthof the STC. The mass equator. the extent of nutrient trapping is greatly reflux associated with the ITF
outflow must be balanced
duced.
in steadystate by an entrainmentof colderwater into
the STC.
Toggweiler
et al. [1991]haveusedmeasurements
of
prebombA•4C to arguethat thereissignificant
entrain-
8. Conclusions
The bomb •4C transient for the Pacific Ocean has
ment of deeperwater into the STC. One important dif- been simulated using a high-resolutionprimitive equaferencebetween our analysisand that of Toggweileret tion ocean circulation model. The control run, which
al. lies in the sourceof the low prebombA14C water was forced with seasonallyvarying winds and buoyancy
recordedby Galapagoscorals. Toggweileret al. ar- fluxes, reproducesthe observedlarge-scalefeatures of
guedthat the 13ø thermostadwater that liesbelowthe the bomb A•4C transient in the Pacific thermocline.
equatorial
undercurrent
[Tsuchiya,
1981]upwellsoffthe SeasurfaceAx4C peakedwithin the subtropicsof the
coast of Peru and then advects over Galapagoswithin Pacific Ocean in the late 1960s and early 1970s but
the South Equatorial Current. However,for the high continued to rise in the equatorial Pacific through the
resolution model used in our study the surface water 1970sand into the 1980s. The lag in the rise of trop-
surroundingGalapagosis suppliedalternativelyby the ical seasurfaceA•4C with respectto the subtropicsis
EUC and the southwardadvectionof a Ax4C front ly- controlledby the advective timescalefor intergyre exingjust to the north of the equatorthroughthe seasonal changeof thermoclinewater betweenthe subtropicsand
cycle.The water that upwellsoff Peruis advectedpole- the tropics of the Pacific Ocean.
ward within the surface Ekman flow in the Southern
The model's basin-wide bomb•4C inventories for the
Hemisphere. Thus, whereasToggweileret al. looked Pacific Ocean are in very good agreementwith the into the Peru Upwelling as the sourceof low prebomb ventories calculated from GEOSECS measurements. In
A14C recordedby Galapagoscorals,our model impli- particular, the model inventories in the equatorial Pacates thermocline water within the EUC.
cificare consistentwith the data in absolutemagnitude,
There are two important pathways by which low- and the model capturesthe zonal gradient in the invenA14C waters could be entrained into the STC. The first
tory between 125ø and 180øW. We attribute this success
of theseis via diapycnalfluxeswithin the thermocline. to the high meridionalresolutionfor the equatorialPaDiapycnalfluxescausecolderwater from deep thermo- cific used in our study, which allows us to resolve the
cline to intermediate depths to mix with water in the critical scalesof equatorial circulation.
pycnocline.
The plotshownfor the A•4C asa function This study was motivated by our interest in intergyre
of latitude and potential densityanomalyat 150øW in exchangeas an oceanographicprocesswith important
1979 (Figure 12b) suggests
that significantdilution is implications for climate variability. The same mixing
occuringin the latitude belt of the equatorialcurrent and advective processesthat control the intergyre ex-
RODGERS
ET AL.: BOMB •4C TRANSIENT
IN THE PACIFIC
OCEAN
8511
changeof A•4C alsoimpactsthe propagation
of thermal
The distribution of bomb radiocarbon in the ocean, J.
Geophys. Res., 90, 6953-6970, 1985.
controlrun aregenerallyconsistent
with the A•4C mea- Broecker, W. S., S. Sutherland, W. Smethie, T.-H. Peng,
and G. Ostlund, Oceanicradiocarbon: Separation of the
surementscollectedduring the GEOSECS and NORnatural and bomb components,Global Biogeochern.CyPAX surveys. The most important difference between
cles, 9, 263-288, 1995.
the model and the GEOSECS
is reflected in the fact
Craig, A. P., J. L. Bullister, D. E. Harrison, R. M. Chervin,
and A. J. Semtner Jr., A comparison of temperature,
that the simulatedA•4C is too small by an amount
salinity, and chlorofiuorocarbonobservationswith results
that cannot be accounted for by aliasing in the samanomalies
within
the ocean.
The
model
results for the
pling. This discrepancymost likely indicates that the
from a 1øresolutionthree-dimensional
globaloceanmodel,
J. GeophysRes., 103, 1099-1119, 1998.
daSilva, A., C. Young, and S. Levitus, Atlas of surface maAs such, the modeling study serves two purposes.
rine data 1994, vol. 1, Algorithms and procedures,NOAA
Atlas NESDIS 6, U.S. Dep. of Cornruer., Washington,
First, the controlrun providesa diagnostictest of model
D.C., 1994.
performance.Model skill in reproducingcoral A•4C
Deser, C., M. A. Alexander, and M. S. Timlin, Upper-ocean
time series depends not only on the the local dynamithermal variations in the North Pacific during 1970-1991,
cal responseof the equatorial ocean to wind forcing but
J. Clirn., 9, 1840-1855, 1996.
model's
EUC
is somewhat
too diffusive.
also on many nonlocal processes,ranging from subduc- Druffel, E., Bomb radiocarbon in the Pacific: Annual and
seasonaltimescale variations, J. Mar. Res., 19, 35-46,
tion in the subtropicsto equatorward geostrophicflow
1987.
within the thermocline to upwelling in the equatorial
Pacific.
Second,the model results illustrate the way in which
specific dynamical processesimprint themselveson the
Duffy, P., D. Eliason, A. Bourgeois,and C. Covey, Simulation of bomb radiocarbonin two global ocean general
circulation models, J. Geophys.Res., 100, 22,545-22,563,
1995.
seasurfaceA•4C signalrecordedby equatorialcorals. Follows,M. J., and J. C. Marshall,On modelsof bomb•4C
in the North Atlantic, J. Geophys. Res., 101, 22,577The strong sensitivity to the openingof the Indonesian
22,582, 1996.
Throughflowindicatesthat the treatment of this boundGent, P. R., and M. A. Cane, A reducedgravity, primitive
ary condition has far reaching consequences
for tracer
equation model of the upper equatorial ocean, J. Cornput.
budgets in the equatorial thermocline. Also, the sensiPhys., 81, 444-480, 1989.
tivity of seasurfaceA•4C to interannualvariabilityin
Gent, P. R., and J. C. McWilliams, Isopycnal mixing in
wind stressforcing provides us with a tool to identify
past variability in ocean circulation due to changesin
ocean circulation models, J. Phys. Oceanogr., 20, 150155, 1990.
Gibson, J. K., P. Kallber, S. Uppula, A. Hernandez, A. Nowind forcing. The atmosphericA•4C forcingfunction mura, and E. Serrano,ECMWF re-analysisproject report
(Figure 1) is smoothlyvaryingin time, so the interanseries:ERA description,Tech. Rep., 1, 72 pp., Eur. Cent.
nual A•4C variabilityrecordedby coralscanbe usedto
for Medium RangeWeather Forecasts,Reading,England,
U.K., 1977.
identify past changesin ocean circulation.
Godfrey,J. S., The effect of the Indonesianthroughflowon
In a future study we will use a global domain with
oceancirculation and heat exchangewith the atmosphere:
similarly high meridional resolution in the tropics to
A review, J. Geophys.Res., 101, 12,217-12,237, 1996.
addressin more detail intergyre exchangebetween the Gu, D., and S. G. H. Philander, Interdecadal climate fluctuations that depend on exchangesbetween the tropics and
South Pacific and the equatorial Pacific for the 40 year
the extratropics, Science,275, 805-807, 1997.
timescale between the beginning of the atmospheric
Guilderson, T., and D. P. Schrag, Abrupt shift in subsurtesting of weaponsand the World Ocean Circulation
face temperatures in the eastern tropical Pacific associ-
Experiment(WOCE) Pacificprogram. The expanded
ated with recent changesin E1 Nifio. Science, 281, 241-
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changein the South Pacific, but it will also allow us to Guilderson,T., D. P. Schrag,M. Kashgarian,and J. Southon,
Radiocarbon variability in the western equatorial Pacific
study interbasin exchangeof thermoclinewater between
the Pacific
Acknowledgments.
and Tom
inferred from a high-resolution coral record from Nauru
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