JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. C2, PAGES 3369-3378, FEBRUARY
15, 2000
Modeling the evolution of the Arctic mixed layer during the fall
1997 Surface Heat Budget of the Arctic Ocean (SHEBA)
Projectusingmeasurements
of 7Be
David
Kadko
Marine and AtmosphericChemistry,RosenstielSchoolof Marine and AtmosphericSciences,Universityof
Miami, Miami, Florida
Abstract. During the SurfaceHeat Budgetof the Arctic Ocean (SHEBA) projectin
October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used
to reconstructthe evolutionof the mixed layer over the previousseasonand confirmed
that the reservoirof heat beneaththe fall mixed layer was emplacedin the summer,rather
than input cumulativelyover severalseasonsor advectedin from distantsources.As
suggested
by McPheeet al. [1998], severaltimes as much heat was emplacedat SHEBA
than at Arctic Ice DynamicsJoint Experiment 20 yearsearlier. A likely mechanismfor this
would be substantiallygreaterlead openingcoupledwith the positivefeedbackloop
betweenincreasedopen water, increasedheat absorption,and further ice melting.In
addition,
a rateof netprimaryproduction
>_13gCm-2 for thepreceding
spring-summer
period was derived usingthis mixed layer historywith an oxygenprofile from the fall.
SHEBA mixed layer was the result of cumulativeinput over
severalsummersor was advectedin from elsewhere.Basically,
at SHEBA duringthe fall of 1997offeredonly
One of the important considerationsin the summersurface the observations
were made
energybalance of the Arctic Ocean is the fate of the energy a "snapshot"of the systembecauseno observations
input (mostlyshortwaveradiation) in the heterogeneousice- duringthe precedingsummer.It is thereforedifficultto reconoceansystemduring the melt season.Downwellingsolar radi- structthe mixedlayer evolutionand heatinghistoryof this site
ation throughleadsis the primarysourcefor the directheating prior to the initial SHEBA occupation.
To addressthis question,the heat exchangebetween the
of the ocean,but the fate of the solarenergyabsorbedthrough
leads
and underlyingmixed layer of the SHEBA site is evaluthe leadsis not well understood[Rind et al., 1995]. It is not
ated
here
usinga naturallyoccurring
radioactive
tracer,7Be.
knownfor example,the extentto whichthisheat laterallymelts
the ice in contactwith the leads,is conveyedto depth and melts Beryllium7 is a cosmicray-producedspeciesthat is delivered
the ice from below, or is transportedaway from the ice. The to the Earth's surface and, as such, is a form of proxy for
fate of the heat has direct consequencetherefore upon the incomingsolar radiation. Becauseof its 53.3 day half life, it is
1.
Introduction
persistenceof the ice cover and the associatedsummertime
albedo.
The SurfaceHeat Budget of the Arctic Ocean (SHEBA)
programwas undertakenin October 1997 to studythe feedbacksassociatedwith the ice-wateralbedosystem,in whichthe
dispositionof the summerheat input is of primary importance
[e.g.,Ingramet al., 1989].The initial observations
made in the
Fall of 1997 indicatedthat the upper oceanwas lesssalineand
warmer than expected[McPheeet al., 1998].The temperature
abovefreezing(tST) of the mixedlayerduringSHEBA (1997)
had increasedby a factor of 2.5 over that which occurred
during an earlier program,Arctic Ice DynamicsJoint Experiment (AIDJEX) (1975). Additionally,the mixedlayer salinity
in 1997was ---27.6ppt, comparedto 29.7 ppt in 1975 [Maykut
and McPhee;1995;McPheeet al., 1998].From this data it was
concludedthat the oceanicheat flux was considerablygreater
duringthe 1997 summer.Becausethe albedoof openwater is
considerablylessthan that of seaice, it wassuggested
that the
percentageof open water between1975 and 1997 had tripled
to allow a greater amount of heatingto occur [McPheeet al.,
1998]. It is possible,however,that the heat stored in the
an ideal tracer of water that has been in contact with the sea
surfaceover the pastseason[Kadkoand Olson,1996].Profiles
of 7Bemeasured
in thefallprovideanintegration
of themixed
layer behaviorover the previoussummerand can offer insight
into the mixed layer evolutionand heatinghistory.
2.
Background
Beryllium 7 is a cosmicray-producedradioactivenuclide
with a radioactive
meanlife of 76.9days(the meanlife is defined
as the 1/e decaylevel or half life/0.693) and, as such,is well
suited for studyingseasonalphenomenon.Beryllium 7 is deposited upon the Earth's surface by precipitation and is
homogenizedwithin the surfacemixedlayer of the oceanrapidly with respectto its decayrate [e.g.,Silker,1972;Youngand
Silker,1980;KadkoandOlson,1996].In snowthe 7Beactivity
(equivalentto concentration,here expressed
as disintegrations
perminute(dpm)perm3) is ---2ordersof magnitude
greater
thanthat in the oceanmixedlayerbecause
a given7Befluxis
Paper number 1999JC900311.
diluted in mixed layerstypicallytens of meters deep but depositedin snowlayers(over a 77 day period) that are at least
an order of magnitude shallower,thereby concentratingthe
nucliderelativeto the surfaceocean[e.g.,Cooperet al., 1991].
In the oceanthe mixedlayer depth is a criticalparameterthat
0148-0227/00/1999JC900311509.00
largelydetermines
the7Besurfaceactivitybecause
for a given
Copyright2000 by the American GeophysicalUnion.
3369
3370
KADKO: MODELING
THE ARCTIC
MIXED
LAYER USING 7BE
Al
7o
Barrow
Dead
Horse
Tuktoyaktuk
65
18,0
12O
•
1so
Figure 1. Locationmap of the SurfaceHeat Budgetof the Arctic Ocean(SHEBA) projectsite.Point A is
the initial deploymentlocationin October1997.Point B is the locationin July 1998.
7Beinput,the activityis concentrated
in shallow
mixedlayers placedin a marenellibeaker,whichin turn wasplacedover a
and dilutedin deepermixedlayers.As will be describedbelow, low background
germanium
gammadetector.The 7Behasa
waterin contactwiththe oceansurfaceis "tagged"
with7Be, readilyidentifiablepeak at 478 keV. The detectoris calibrated
and profilesof this isotopeprovide a record of the seasonal for this geometryby addinga commerciallypreparedmixed
changesin the mixed layer depth.
solution of known gamma activities to an ashed fiber and
countingit in the marenelligeometry.The countingefficiency
for the 478 keV gamma in this configurationwas 0.03. The
3.
Methods
limit of detection used in this work defined at the 99.7% con-
fidence level, is SL = --> 3frO, where O'b is the standard
Verticalprofilesof 7Bewerecollected
in the BeaufortSea
deviationof the blank measurements[Rubinson,1987]. The
from October8 to 12, 1997(Figure 1). To collectthe samples,
detection
limitof a sample
increases
withtimebecause
of 7Be
a hydroholewas melted through -2 m of ice throughwhich
samples
for7Bewerecollected
bypumping.
In thisprocess,
700
L of seawaterwere passedthrough iron-impregnatedacrylic
fiberspackedin cylindricalcartridges[e.g.,Krishnaswami
et al.,
1972;Lal et al., 1988;Lee et al., 1991;Kadkoand Olson,1996].
The water wastaken at variousdepthsthrougha 1.5"hoseat
the endof whichwasa conductivity-temperature-depth
(CTD)
system.The pumpingwas achievedwith a centrifugalpump,
poweredby a gasoline-fueled
generatoron the ice,at a rate of
-14 L min.-• Thuseachsampleddepthrequired-50 min.In
mostcases,doubleor triple sampleswere collectedat any one
depth and later combined.On one cast (October8), where
only singlecartridgeswere usedat eachdepth,three samples
from different depthsin the mixed layer were combinedas
representativeof the mixed layer activity.The fiber efficiency,
testedwith stableBe in the lab or by countingtwo cartridges
placedin serieswas 0.90 _+.02. This is an improvementover
earlier fibers [Kadkoand Olson, 1996] and reflectsa higher
retention of iron in the fibersproducedin the laboratoryfor
this study.
On land the fibers are dried
and then ashed. The ash was
decayand,for oursystem,
is(12.7/V) x[exp(XD)] dpmm-3,
whereD is the numberof daysafter samplecollection,V is the
volume(m3),andX is thedecayconstant
(0.013d-•).
To estimate
theatmospheric
deposition
of 7Be,samples
of
snow(m2 in area)weretaken,andalso,plasticbuckets
were
deployedto collectfallout over knowntime periods.The snow
was melted, and 0.5 mL of concentrated HC1 and stable Be
yieldtracerwereadded[e.g.,Dibb,1990].The 7Bewasremoved by coprecipitationwith iron hydroxide, dried, and
countedby gammaspectrometry.
The stableBe in the precipitate wasthen measuredby atomicabsorptionto calculatethe
7Berecovery
duringprecipitation.
Thebuckets
weredeployed
for periodsrangingbetween 1 and 3 weeksin October 1997
and July-August1998.After collectionthe bucketswere rinsed
with diluteHC1.Subsequently,
the HC1rinsewastreatedasthe
snowsamplesdescribedaboveand counted[e.g.,Baskaranet
al., 1993].The countingsystemis calibratedfor all samplesby
preparinga commercialstandardin geometriesidenticalto the
samples.
The error for each measurementis the statisticalcounting
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3371
error•r andthe uncertainty
in the blank,X/o-2 + •r2t,,multi- Table 1. Beryllium 7 From SHEBA, October 1997
plied by
[(XCT)/[1 - exp (-XCT)]][exp (XD)/(CE. FE ßPE)],
where CT is the countingtime, CE is the countingefficiencyof
the 478 keV gamma, FE is the fiber (or precipitation)effi-
Date
Depth,m
7Be,dpmm-3
Oct. 8
10, 20, 25
100.8 _+16.6
Oct. 9
Oct. 11
ciency,
andPE isthephotonemission
fraction(0.104)for 7Be.
4.
Results
and Discussion
Oct. 12
24
103.5 _+ 15.0
35
25
33
39.6
45.5
25
34
45
24.8 _+ 17.7
133.45 + 14.0
30.2 _+ 9.2
19.1 _+ 6.4
5.95 + 6.00
104.7 _+ 5.9
17.8 _+ 3.75
12.5 +_ 5.2
Comments
three samplescombined
D.L. a=
30.6
D.L. a=
12.4
The typicaltemperatureand salinityprofile from SHEBA in
October 1997 is shownin Figure 2. The mixed layer extended
aD. L. is the detection limit.
to •30 m depth, below which was found a layer •0.5øC
warmer than the mixed layer. McPheeet al. [1998] note that
this layer is appreciablywarmer than a similarlayer measured
during AIDJEX in 1975. The salinity of the mixed layer in allow phytoplanktonactivity.Second,while the initial oxygen
SHEBA is alsoconsiderablyfreshet than that of AIDJEX. The contentis not known and an absolute02 utilization rate cannot
?Bemeasurements
in October1997are presented
in Table 1 be determined,the persistenceof the oxygenpeak well into
that the bacterialrespirationrate is low. As
and displayedwith the •T (temperatureabovefreezing)profile October suggests
in Figure 3a. In two casesthe valuesare closeto or below the discussedbelow, the data and model presented here allow
detection limit defined earlier but are retained because of
someestimateof primary productionfor the summerof 1997
consistency
withtheothervalues(Table1). The remnant?Be
to be determined.
belowthe mixedlayer marksthe warm water ashavingbeen in
contactwith the sea surfacewithin the previous77 days.Thus
the heated layer beneath the October mixed layer at SHEBA
was remnant of an earlier, deeper mixed layer and was the
resultof very activeheat input duringthe previouslate spring-
The flux determinationsare presentedin Table 2. For 1997,
if it is assumedthat by October13 snowhad beenaccumulating
for 3 weeksat the SHEBA site,then the flux determinedby the
snowinventoryagreeswith two out of the three bucketdeployments at that time. The average for these measurementsis
summer.The ?Beshowsthat the higherheatlayercouldnot
0.0020+_0.0009dpmcm-2 d-•. In the summerof 1998the
have been advectedfrom afar or emplacedcumulativelyover
average flux from nine bucket deploymentswas 0.0122 _+
several
0.0070dpmcm-2 d-•.
seasons.
Corresponding
to theheatand7Bebeneaththemixedlayer
is a peak in oxygen(Figure 3b; profile collectedby E. Sherr).
4.1.
The 7Be givessomeinsight,at leastqualitatively,
into the
Thedistribution
of ?Becanprovidequantitative
information
on mixedlayerevolutionbecause
featuresof the 7Bedistribu-
history of this as well. First, it suggeststhat there had to be
substantialprimary productionearly in the melt seasonwhen
the mixedlayerwasdeep(of the order of 50 m). The presence
of substantialopenwater, a suggested
by McPheeet al. [1998],
would have contributed to sufficientlight being available to
Modeling the Mixed Layer Evolution
tion demandcertain aspectsof mixed layer history.For exam-
ple,thepresence
of 7Beat a depthof 50 m requiresthemixed
layer to havebeen that deep early in the melt season.Furthermore, the mixedlayer had to persistlong enoughat that depth
to accumulate sufficient 7Be to be detected months later in
October.In the followingthe 7Be is usedto derivea mixed
layer historythat then can be appliedto other tracers,notably
Potential Temp ('C)
-1
-2
0
0
1
•
•
2
heat.
Themajorfeaturesof oceanic
7Beprofilescanbegenerated
by accountingfor the seasonaldeepeningand shoalingof the
mixedlayer on the basisof empiricalobservations
of the mixed
10
layerhistory.
For a giveninputof 7Bethemixedlayerdepthis
a criticalparameterthat largelydeterminesthe 7Be surface
activity;
shallow
mixedlayersconcentrate
7Bewhilein deeper
20
mixedlayersthe concentrationis diluted.The model usedhere
is not dynamicallydriven, in that there are no surfaceforcing
terms,butsimplyillustrates
how7Be(andothersurfacetracers) will respondto a prescribedmixedlayer history,an input
function, and relevant tracer decay terms. With this under50
-
60
-
70
26
standing
an observed
7Beprofilealongwithknowledge
of 7Be
input can yield insightinto the mixed layer history.
Details of the model are presentedelsewhere[Kadko and
Olson, 1996]. It is basedon the finite differenceform of the
•
I
•
27
28
29
30
SALINITY
Figure 2. Temperatureand salinityprofilesfrom October 11,
1997.This profile was typicalof thosetaken betweenOctober
8 and 12.
one-dimensional
analyticalexpression
governing
the 7Bedistribution:0c/0t= gzO2C/OZ
2 - •c, wherec is the concentrationof 7BeandKz is the verticaleddydiffusivity.
The 7Beis
assumedto be mixed rapidly with respect to its radioactive
decay in the mixed layer and is mixed with a finite K z in the
pycnocline.As an illustration,the model representationof a
3372
KADKO: MODELING THE ARCTIC MIXED LAYER USING 7BE
Be-7 (dprn/rn"5)
o
20
o
40
60
80
ml/I
100
120
140
•
0 0
0.2
o
(^)
oxygen above safurafion
0.4
0.6
!
(B)
10
lO
2O
20
3O
5o
[]
ß
v
ß
40
10/8/97
10/9/97
10/11/97
10/12/97
40
50
50
60
I
0.0
0.5
•
I
60
1.0
I
[
I
1.5
DelfoT (øC)
Figure
3. (a)The7Be
measurements
inOctober
1997
aredisplayed
withthe•T profile.
Theprofile
issimilar
to thatexpected
fromthemodel
(Figure
4c).(b)Thedifference
(02 concentration
minus
02 saturation)
plotted
versus
depth
forOctober
24,1997(datacourtesy
ofE. Sherr,
Oregon
StateUniversity).
generic
seasonal
cycleof mixedlayerdepthisshownin Figure shoalas a resultof heatandfreshmeltwaterinputuntil
4a,whereat sometime(Tmax)
themixedlayerisat itsdeepest coolingrecommences
anddeepening
occurs.
(Lmax).
In thisexample,
Tmax
is50 m, andKz for thepycno- As the mixedlayeronceagaindeepens
(Figure4c), the
clineis assigneda value of 10-6 cm2 s-•.
mixedlayer?Beconcentration
decreases
because
of dilution,
Monthlyprofiles
of ?Beactivity
aregenerated
asthemixed andtheupperthermoclineactivityistruncated.Thisisbecause
layershoals,leavingwaterbeneaththe mixedlayerthat is the?Bedeposited
muchearlierin theyearhasnowdecayed
isolatedfrom the atmospheric
inputI, whichis chosento be away.Thisiswhatwasexpected
at SHEBAin October1997,at
constant
inthisexample.
The?Beactivity
intheshoaling
mixed whichtime the leadsand melt pondswerefrozenandmixed
layerincreases,
whilethe?Beisolated
at depthdecays
radio- layerdeepeninghad begun.
actively,
whichproduces
theobserved
profiles
(Figure4b).In
In thisworkthebasicmodelof KadkoandOlson[1996]is
the Arctic,in late SpringandSummer,
the mixedlayerwill modifiedfor the SHEBA site.The parameters
usedfor the
Table
2.
Flux Measurements
Collection
Sample
Bucket
1
Bucket 2
Bucket 3
Snow 1
Snow 2
Collection
Oct.
Oct.
Oct.
Oct.
Oct.
Dates
2-10, 1997
3-15, 1997
4-15, 1997
13, 1997
13, 1997
Time,
Flux,
days
dpmcm-2 d-i
7.9
11.8
10.9
21"
21"
Average Oct. 1997
Bucket
4
Bucket
6
Bucket
Bucket
Bucket
Bucket
Bucket
Bucket
Bucket
7
8
9
10
11
12
13
July 5-20, 1998
July 5-20, 1998
July 6-20, 1998
July 12-29, 1998
July 20 to Aug. 8, 1998
July 20 to Aug. 8, 1998
July 20 to Aug. 8, 1998
July 21 to Aug. 10, 1998
July 30 to Aug. 10, 1998
Average JulyAugust 1998
aAssumed snow accumulation time.
15.2
15
14
17
17
17
17
20.1
10.75
0.0020
0.0005
0.0021
0.0031
0.0022
0.0020 _+ 0.0009
0.0092
0.0028
0.0058
0.0233
0.0192
0.0177
0.0127
0.0143
0.0052
0.0122 _+ 0.0070
KADKO:MODELINGTHE ARCTICMIXED LAYERUSING?BE
I
F
10
_1
40
x
50
I
i
I
i
120
180
240
3373
I
_
Tmo•
i
6o
0
60
i
.300
360
DAYS
o
(Bq
IIII I I+1120
I I II III
I•
-
I I I•
20
..••=-"•-•+
180
t
......__J
-80
days
.//J/ +60
_. _ t
-50days
40
1/i'-ox
lO
ZO
days
5o
max
I
6o
o
-2 -1
-2d-1
Xo.006
dpm
cm
d I II--0•,006
dpm
cm
I
100 200
I
300
I
400
I
500
I
600
L
0
I
100
I
200
I
300
i
400
i
500
i
600
Be-7
Be-7 (d pm/m"5)
Figure
4. (a)Themodel
representation
ofanidealized
seasonal
cycle
ofmixed
layer
depth
where
atsome
time(Tmax)
themixed
layer
isatitsdeepest
(Lr.ax)'
Inthis
example
theinput
of7Be,
I, isconstant
over
the
entire
seasonal
cycle,
and
Kzinthepycnocline
ischosen
tobe10-6m2s-•. (b)The7Be
activity
forashoaling
mixed
layer.
Profiles
of7Bearegenerated
forvarious
times
(+ days
after
Tmax).
Asshoaling
progresses,
the
7Beactivity
ofthemixed
layer
increases,
while
the?Be
isolated
atdepth
decays
radioactively,
which
produces
theobserved
tailing.
(c)Asthemixed
layer
once
again
deepens,
themixed
layer
7Beconcentration
decreases
because
ofdilution
andtheupper
thermocline
activity
istruncated.
Thisisbecause
the7Bedeposited
much
earlier
intheyear
hasnow
decayed
away.
Profiles
thatmight
beexpected
atSHEBA
inthefallduring
mixed
layerdeepening
wouldbesimilar
to thesemodelcurves.
1997summer-fall
in SHEBAaredisplayed
in Figure5, andthe
19toSeptember
20,thereisonlyatmospheric
input.In stage
3,
of icecover.Themodelisrun
model
output
isshown
inFigure
6.Monthly
profiles
of7Beare thereiszero7Beinputbecause
derived
asa function
of mixedlayerevolution
and7Beinput. until October 11, at which time the SHEBA site was comi.e., isolated
fromatmoInitially,
it isassumed
thatthereisno7Beintheocean
follow- pletelyfrozenandsnowcovered,
spheric
or
melt
input.
Likely,
it
had
been
in
this
stateof the
ingthewinterwhentheice-covered
upperoceanhadbeen
isolatedfrom atmosphericinput.
orderof 3 weeks.Therefore,from September20 to October
Theinputfunction
hereissomewhat
different
thanthatof 11,thereiszero*Beinput(i.e.,thereisonly*Bedecay).
fluxis determined
asfollows:
onOctober
KadkoandOlson[1996]in thatinputof 7Beisnotconstant. Theatmospheric
11
the
depth-integrated
standing
stock
of
7Be
was
0.357dpm
Hereit iscomprised
notonlyof atmospheric
inputbutaddithisto September
20,thestanding
stockof
tionally,
duringlatespring,
of rapidinputfrommelting
snow cm-2. Correcting
wouldbe0.463dpmcm-2, corresponding
thathasaccumulated
?Beoverthewintermonths.
The ?Be 7Bebeforeisolation
to
flux
of
0.463X
or
0.006dpmcm-2 d-•, whichis theinput
inputfunction
consisted
ofthreestages
(Figure
5).In stage
1,
used
between
July
19
andSeptember
20 (stage2). It is then
fromJune21 (summer
solstice)
untilJuly19,?Bewasinput
that the atmospheric
fluxaccumulating
in the snow
through
atmospheric
fluxandsnow
melt.In stage
2,fromJuly assumed
3374
KADKO: MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
0.020
I
-
Stage
1
I
-
I
_
-•
ß
_
50
_
60
x
:•
I
,
_
I
,
'---------C--0.012
',
I
I
Stage 2
_
•Stage5
i
I
i
i
i
i
180
Jun
0.008
-
I
- •
i
0.010
_
I
i
i
,
200
,
i
i
i
i
•
220
21
i
!
!
!
240
i
i
z
.-•
c
o.006
I
I
i
_
I
I
70
•
0.014
-
40
I
0,016
I
ß
m
0,018
!
260
i
!
i
.
i
0.004
3
0.002
•
o.ooo
3
28(J
DAYS
Oct.• •
Figure5. Parameters
usedin theSHEBA?Bemodelasa functionof time.Thethreestages
of ?Beinputare
shown.
during the winter monthswas the same as during the stage2
Inputtotal= InpUtmelt+ Inputatto
flux,andthusthe standing
stockof 7Bein the snowwouldbe
0.463dpmcm-2. Assuming
snowmeltoccurred
predominately
overa month'stime,this7Beis inputinto thewatergradually
= {(0.463/28)[1 - exp(-(X28)]/(X28)}
+ 0.006dpmcm-2 d-•.
overthe 28 dayperiodof stage1. The formulationfor stage1 is
20 /2s
• •0
,m.,
tm 40
Max ML = 50 m
50
60
0
20
40
60
80
100
120
140
160
i
180
200
i
116
-•
'1 •z,
9/15
_
•o/•/•
10/9/97
10/11/97
10/12/97
60
0
20
40
60
80
100
120
•
140
•
160
180
200
Be-7 (d pm/m"3)
Figure6. The modeloutputshowing
the evolutionof the 7Beprofileat SHEBAwithtime.Data fromthe
period October8-12 are superimposed
upon the model profiles.The modelprofile for October 11 is shown
as a dashed line.
KADKO:
MODELING
THE ARCTIC
MIXED
LAYER
USING 7BE
3375
10
20
30
---0 m2/s
10-6 rn2/s
- -- 10-5m2/s
40
!
50
Max
ML-
50
m
2
Input - .006 dpm/cm
60
0
i
20
i
40
60
80
100
'
120
/d
140
Be-7 (d pm/m"S)
Figure7. The modeloutputfor the 7Beprofileat October11 for threedifferentvaluesof Kz in the
pycnocline:
0, 10-6, and10-s m2 s-].
Fromthemaximum
depthof the7Beit is surmised
thatat the 4.2. Modeling the Evolution of Mixed Layer Temperature
onset of melting the maximumdepth of the mixed layer was
50 m. This assumes
that mechanisms
suchasdownwellingwere
not a factor, which is reasonable as the SHEBA site was not in
the centerof an anticyclonicgyre. StartingJune 21 (summer
solstice)this depth is maintainedfor 12 daysfollowingwhich
the mixed layer decreasesto a minimum of 10 m on July 19.
With the 7Beinputfluxuseda late springmixedlayermaintained at this depthfor this time period is requiredto achieve
the 7Beprofileobserved
in October.The maximummixed
layer depth modeled here is similar to that reported for the
CanadianBasinat ice islandT-3 from 1970 to 1973 [Morison
and Smith,1981]and for the AIDJEX projectin 1975[Maykut
and McPhee,1995].
The effectof differentpycnoclineKz on the modelresultsis
shown in Figure 7. These results suggestthat in the highly
stratifiedwater belowthe mixedlayer (see salinitygradientin
The evolution of the 7Be distribution in the model described
aboveis similarto the evolutionof bT describedbyMaykutand
McPhee[1995].In late winter,there is a deep,well-mixedlayer.
After 2 months,spanningthe summersolstice,there is warming throughoutthe mixed layer and fresheningdue to melt
water introduced at the surface.By late summer the mixed
layer has shoaled,leavingdeeperwater behindthat had been
heatedearlier in the season.The maximumin bT is interpreted
to be the temperature to which the mixed layer was heated
aroundthe time of the solsticeand,like the deep7Beis a
remnantfeature of the deepermixedlayer of late spring/early
summer. In a sensethis is a form of subduction in which water,
initially in contactwith the surface,becomesisolatedfrom the
atmosphere.The heat is trappedin this layer until subsequent
wintertime
convection.
In the modelingshownbelow the temperaturemaximumis
Figure2), verticalmixingis not muchgreaterthan10-6 cm2
similarly
formed, i.e., as a consequenceof meltwater capping
s-• andthat the 7Beprofileis mainlya response
to the fall
and
represents
a residual mixed layer. However, it is shown
deepeningof the mixedlayer.Higher valuesof Kz would draw
that the bT maximumlikely corresponds
to the temperatureto
more7Befromthe mixedlayerintothepycnocline
water.
The fluxusedin themodel(0.006dpmcm-2 d-•) liesbe- whichthe mixedlayerwasheatedby mid-July(as opposedto
tween the flux determinationsof October 1997 (0.0020 dpm the solstice)andmarksthe depthof the mixedlayerat that time.
The parametersused in the temperatureevolutionmodel
cm-2 d-•) andJuly-August
1998(0.0122dpmcm-2 d-•). As
such, the model value is not unreasonable, but there is no are shownin Figure 8. The model is the sameas that usedfor
historical 7Be flux data from this area of the world with which
to compareit. Taken at facevalue, the ratio of the model flux
(basedon the summer1997water inventory)to the measured
summer1998flux (--•0.5)couldsuggestthat in 1997the summertimelead coveragewasof the order of 50%. This appears
7Beexceptthatthereis no radioactive
decaytermandit uses
the mixedlayerevolutionderivedfrom the 7Bedistribution
(compareFigures5 and 8). The heat input functionwas derived from the pattern of summerheat content of the water
columnduringAIDJEX in 1975 (Figure 9 [from Maykutand
large and is certainlybiasedby 7Be input from meltwater McPhee,1995,Figure9]). At AIDJEX (Snowbirdsite),starting
runoff. However, the large heat inventorymeasuredin Octo- approximatelyJune 21, the heat contentincreasedat a rate of
ber 1997 suggeststhat indeed, substantialopen water must 9.64W m-2 for a periodof 28 days(Figure8, linea). Forthe
next39 daysthenetheatincrease
wasat a rateof 0.48W m-2
havebeen present.This will be discussed
in section4.2.
3376
KADKO: MODELING
•,
THE ARCTIC MIXED
LAYER USING 7BE
0
40
-
I
_
I
-
I
-
I
•
I
I
_
I
C
ß
40
/
o
•
60
•.
20
z
10.•
-->. 5o -_,
ß'o
.•o
......Inpu
ear
I
_
-10
-
-20
i
-
i
...........
X
180
200
220
240
260
)
2801
• 70,i,•....•....I....I....I....,,
Jur,
2•
DAYS
oot. l•
Figure 8. The parametersusedin the temperatureevolutionmodelplottedasa functionof time. The model
usesthe mixedlayerevolutionderivedfromthe 7Bedistribution.
(Figure 8, line b). Beyondthat date, there was a net heat loss 4.3. Modeling the Evolution of Mixed Layer Oxygen
of -4.4 W m-2 (Figure8, linec).To achieve
thetemperature and Implications for Primary Production
profile observedon October 11, 1997, with this model, the
same relative heat flux for each period was used, but the
absolute
valueshadto bequadrupled
(39,2, and-18 W m-2,
respectively).
This is consistent
with McPheeet al. [1998],who
suggestthat the lead openingsin 1997 relative to AIDJEX
(1975) tripled, and with observations
made at SHEBA in October that indicatedextensivesummertimemelting. These includedthinnerice than anticipated(--•1.2 m asopposedto 3-4
meters),extensivemelt pond (by then frozen) coverage,and
melt pondsthat had melted through to the underlyingocean
(D. Perovich,personalcommunication,
1998).This is alsocon-
sistentwith the high7Beinventory(relativeto flux) of the
By analogywith the temperatureprofile the peak in oxygen
measuredin October 1997 is remnant of spring-summerproduction,while the water aboveit has subsequentlydegassed
duringmixedlayer deepening.Neglectingfor the momentloss
effectsthrough respiration,the upper 50 or so meters must
havebeenat least0.5 mL L-• or 35 g m-2 abovesaturation.
Thisis of the orderof 13 gC m-2 net production
but likely
representsa minimum because(1) if this productioncommenced in June, then the water column started out below
saturationasa resultof respirationduringthe dark season,(2)
02 is lost throughgasexchange,and (3) respirationoccurred
subsequentto the summer.The 1998 spring-summerobserva-
water columnmeasuredin October 1997,whichrequiressub- tions at SHEBA are consistentwith this estimate. Although
stantialleadopening
for the7Beto entertheupperocean. direct comparisonmay not be possiblebecausethe SHEBA
The model output is shownin Figure 10 comparingthe case sitehad drifted far westfrom the October1997location(Figof AIDJEX
and SHEBA.
It is seen that the &T maximum on
ure 2), the June-Julynet productionof oxygensuggested
a net
October11 corresponds
to the temperatureto whichthe mixed community
primaryproduction
of --•17gC m-2 (B. Sherr,
layerwasheatedby mid-Julyandmarksthe depthof the mixed personalcommunication,1998).
layer at that time. These resultsare consistentwith the conclusiondrawnby McPheeet al. [1998] that the mixedlayerwas
5.
Conclusions
of the order of 50 m deepearlyin the summer,wasextensively
heatedbefore shoaling,and by Octoberhad givenup its availThispaperreportsthefirstmeasurements
of 7Beevermade
ableheat.Byusing7Be,similarconclusions
areindependentlywithin the Arctic Ocean. The profile from October 1997was
drawn.
usedto reconstructthe evolutionof the mixed layer over the
i
,
'
'- .....
,
!
40 .....
!
i
......
-1 ......
'b
i
r
.......
'
i
E 20
o
140
1,,Fe!5 180
200
220
240
260
280
300
Day of 1975
Figure 9. The pattern of summer heat content of the water column during Arctic Ice DynamicsJoint
Experiment(AIDJEX) in 1975 [from Maykutand McPhee,1995].The heat input for SHEBA (1997) was
derivedby quadruplingthe slopeof the three linespicturedhere.
KADKO: MODELING
(a)
THE ARCTIC MIXED LAYER USING 7BE
delta T (oC)
0 0
0 1
0
I
10
delta T (oC)
0.2
ß
3377
0 0
0.1
0.2
I
I
I
75
973
8 6
2o
•
3O
1
5
DJEX 1975
40
50
•"/
0
I
I
I
•
I
I
I
10 75
•
6/ 8
/
•
I
I
19
I
I
9/13
8
/SHEBA
50
0.2
I
7
40
0.0
I
I
/•
0.4
0.6
0.8
1.0
delta T (oC)
0.0
0.2
•
•
0.4
0.6
0.8
1.0
delta T (oC)
Figure 10. The modeltemperatureoutputfor (a) AIDJEX and (b) SHEBA. Note the changein temperaturescale.It is seenthat the &T maximumon October11 corresponds
to the temperatureto whichthe mixed
layerwasheatedbymid-July(asopposed
to the solstice)
andmarksthe depthof themixedlayerat thattime.
previousseasonand confirmedthat the reservoirof heat beneaththe fall mixedlayerwasemplacedin the summer,rather
than input cumulatively
over severalseasons
or advectedin
from distantsources.In addition,a reasonablerate of primary
Acknowledgments.This work was supportedby NSF Polar Program (grant OPP-9701067)and by the NSF ChemicalOceanography
Program(grant OCE-9809168).The authorwould like to thank J.
Morison, M. McPhee, and B. Welch for many valuable discussions
in
regardsto thispaperand two anonymous
reviewersfor their helpful
productionfor the spring-summer
periodwas derivedusing comments.Thanks also to E. Sherr and B. Sherr for permissionto
presenttheir oxygendataandto M. Stephens
for field andlaboratory
thismixedlayerhistorywith an oxygenprofilefrom the fall.
assistance.
Finally, the author acknowledges
the crew of the CCGC
As suggested
byMcPheeet al. [1998],severaltimesasmuch DES Groseilliersand the SHEBA logisticsgroup for their support
heatwasemplacedat SHEBA than at AIDJEX 20 yearsear- duringthe field operationof SHEBA.
lier. A likely mechanismfor this is substantially
greaterlead
openingcoupledwith the positivefeedbackloopbetweenincreasedopenwater,increasedheat absorption,andfurtherice References
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KADKO: MODELING
THE ARCTIC MIXED
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(ReceivedJanuary23, 1999;revisedSeptember10, 1999;
acceptedOctober6, 1999.)