JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 106, NO. Cll, PAGES 26,917-26,928, NOVEMBER
15, 2001
Pathways of cross-frontal exchange
in the North
Atlantic
Current
StephanieDutkiewicz,• Lewis Rothstein, and Tom Rossby
Graduate Schoolof Oceanography,Universityof Rhode Island, Narragansett,Rhode Island, USA
Abstract. The North Atlantic Current (NAC) formspart of the boundarybetweenthe
subtropicaland subpolargyresin the North Atlantic Ocean. The current has
topographicallycontrolledstationarymeandersthat appear to grow and decay.A region
east of the current
in the Newfoundland
Basin contains
water
of mixed
subpolar/subtropical
properties,suggestingthat there is exchangeacrossthe NAC. This
studyconsidersdata from isopycnalRAFOS floats launchedin the NAC region from 1993
to 1995. We use the RAFOS data to define the "frontal zone" as a pressurerange where
the jet is most likely to be found. This definition requiresa latitudinal dependenceas the
NAC shoalsto the north. Floats shallowerand deeper than this range are defined to be
on the subpolarand subtropicalside, respectively.These definitionsare used to estimate
mixingthat occursbetweenthe current and its surroundingsand to estimatethe relative
quantity of exchangeof water parcelsbetween the two gyres.Only small quantitiesof
massexchangefrom one gyre to the other are found, but there is a distinctasymmetry
leadingto a mean flux from the subpolarto subtropicalsides.We also find that floats
spendsignificanttime in the frontal region and are frequentlyexchangedbetweenfast and
slow movingwaters, particularlyat the meander extrema. Diffusion, while in the jet, leads
to eddy cross-frontalexchangewhich is important for the exchangeof properties across
the NAC.
1.
refer to thiswater as "subtropical."Colder, fresherwater from
Introduction
the Labrador
Basin enters the Newfoundland
Basin from
the
Recentstudieshaveexploredthe mechanisms
and frequency
of exchange between strong oceanic jets and surrounding
slowermovingwaters and the exchangeof water parcelsand
water propertiesbetween two different physicalor chemical
regions.One of the most intenselystudied dynamicfronts is
that associatedwith the Gulf Stream.This oceancurrent separates the warm, salty SargassoSea from the fresher, colder
slopewater. Thesewatersalsohavedifferentconcentrations
of
tracers,suchasoxygenand biologicalspecies.Clearlythen, the
Gulf Stream actsas a boundarybetweenthe two water masses,
and yet observationsof oxygenconcentrations[Boweret al.,
1985] and Lagrangiansubsurfacedrifters [Songet al., 1995]
have shownthat exchangedoes occurwithin the Gulf Stream
system.Thesestudieshavespurredseveralanalyticaland modeling investigationsof the nature of mixing and stirringwithin
a meandering,propagatingjet [Bower,1991; Satnelson,1992;
north. This water is of subpolarorigin, and we will refer to it as
"subpolar."The Newfoundland Basin is therefore a meeting
point of the subtropicaland subpolargyres,and the two very
different water massesflow together,creatinga strongthermal
front. There appears to be a slow eastwardmoving body of
water of inhomogeneouscharacter to the east of the NAC
[HarveyandArhan, 1988;Kearns,1996], suggesting
a mixingof
subtropicaland subpolarproperties.Exchangeacrossthe jet is
fundamentallyan interactionbetweenthe subtropicalgyreand
the subpolargyre systemsand therefore has important consequencesfor the gyre-scaledynamicsof, for instance,the potential vorticity budget [Lozier and Riser, 1990; Dutkiewicz,
1997]and the transportof heat from low to high latitudesand
in the distributionof chemicaltracersand biologicalspecies.
A strongmeanderpathwayseenin previousstudiesof the
NAC region [Kearns,1996;Lozier et al., 1996] was also identified in a RAFOS float study conducted during 1993-1995
Dutkiewicz et al., 1993; Lozier and Bercovici, 1992; Lozier et al.,
[Anderson-Fontana
et al., 1996; Rossby,1996]. The data col1997].
lected by these floats are discussedin section2. We differenIn this study we considerLagrangian observationsof extiate betweentwo processes:
the lossof water parcelsfrom the
change in the continuation of the Gulf Stream, the North
fast movingjet to slowermoving surroundingwaters and the
Atlantic Current (NAC). The NAC bringswarm water of subexchangeof mass and propertiesbetween the two gyresto
tropicalorigininto the NewfoundlandBasin(topographicfeaeither sideof the NAC. In section3 we definespecificregimes,
turesof the basinare givenin Figure 1). This currentcan be suchas the "jet" and the "frontal zone," to aid in the investiviewed as an extensionof the subtropicalgyre, and we will gation of these processes.In section4 we determine statistics
from the float data that provide a measureof the mixing. In
•Nowat Department
ofEarth,Atmospheric
andPlanetary
Sciences, section 5 we discuss and summarize these results.
MassachusettsInstitute of Technology, Cambridge, Massachusetts,
USA.
Copyright2001 by the American GeophysicalUnion.
2.
Paper number 1999JC000089.
From June 1993 to June 1995, approximately100 RAFOS
floatswere deployedin the NewfoundlandBasin. The details
0148-0227/01 / 1999JC000089509.00
26,917
RAFOS Float Experiment
26,918
DUTKIEWICZ
ET AL.: PATHWAYS
OF CROSS-FRONTAL
NEWFOUNDLAND
EXCHANGE
BASIN
54
52
5O
48
• 46
4O
38
-ss
-so
-4s
_4o
-as
-ao
longitude
Figure 1. Topographicfeatures of the NewfoundlandBasin. The 200-, 1000-, and 4000-m isobathsare
shown.Circlesindicateconductivity-temperature-depth
probe sitesfor three sectionstaken duringthe summer 1993by the RV Oceanus.
of the experiment and the data recoveredare discussedby
Anderson-Fontana
et al. [1996].For the purposesof this paper
a few key pointswill be noted.The RAFOS floatsare designed
to track water parcelsalong constantdensitysurfacesand to
record temperatureand pressureeveryhalf day. The temperature measurements are accurate to within 0.1øC, and the
this study is interestedin the velocity measurements,floats
farther than 0.1 at from their target densitywere not used.A
set of 39 floats on the 27.2o-t surfaceand a set of 25 floats on
the 27.5tr t surfaceprovidedata for this study.The trajectories
of thesefloats are shownin Figure 2.
The NAC pathway and variability derived from the float
pressuremeasurementsare accurateto within 10 dbar. Every data are discussed in the remainder of this section. The float
half day the floats listen for signalsfrom four soundsources data have been compiledinto 0.5ø x 0.5ø meansand standard
moored in the region. The travel time from two or more of deviations.The bins were required to contain data from at
thesesourcesprovidesthe float'slocation.The floatswere also leasttwo floats,andno singlefloat waspermittedto contribute
designed
to period!cally
profileup anddown0.1at unitfrom more than five measurementsin any bin: Where there were
their surface,measuringtemperatureand pressureat the top, more measurementsin a bin, the chosendata were evenly
middle, and bottom. The floatswere deployedon three cruises spacedthroughoutthe record, and no bin with <10 measureand had averagemissionlengthsof 10 months,providingcov- mentswasconsidered.The aboverequirementsprovidea view
erage in the NewfoundlandBasin for 2 years.
of the NewfoundlandBasinmeanvelocity,eddykineticenergy,
The floatshave the samecompressibilityas seawater,so by meanpressure,and meantemperaturestructurealongthe two
definition they will follow a specificvolume anomalysurface. densitysurfaces(Figures3 and 4). The fieldswere smoothed
They were ballastedfor one or the other of two specificvolume usinga simpleweightedaveragemethod where each bin was
anomalieswhich,for convenience,are referencedin thispaper weightedone and the eight adjacentbins were weightedone
by the isopycnalsurfacesthey approximate,27.2o-t and 27.5 o-t eighth.In Figures3c, 3d, 4c, and 4d the contoursindicatethe
[Carr et al., 1997]. The lower surface(27.5trt) doesnot out- meanvalues,and the shadingindicatesthe standarddeviation.
crop in this regionandwas,in general,not affectedby seasonal In all of the figuresthe thin solidlinesindicatethe 200-, 1000-,
forcing.The uppersurface(27.2trt) doesoutcropin the north- and 4000-m isobaths.
As the waters of the Gulf Stream reach the Southeast Newern part of the NewfoundlandBasin and will certainlybe
affectedby seasonalforcing.To preservethe floatsagainstthe foundlandRise (locationsof topographicfeaturesare shownin
damage encounteredwhen on the surface,the floats were Figure 1), severalbranchesseparatefrom the main current
programmedto operate in isobaricmode once they reached system[Mann, 1967;Arhan, 1990].Hogg[1992]suggested
that
200 dbar or less. By using conductivity-temperature-depth a branch of water movessouth and west, forming part of the
(CTD) castsconductedat the time of deployment,Carr et al. Gulf Streamrecirculation.A secondbranchappearsto sepa[1997]estimatedthe actualdensitythat the floatstracked.The rate southof the Grand Banks,forming the eastwardmoving
temperatureand pressuremeasurementswere then corrected Azores Current [Kleinand Siedler,1989]. However, a greater
backto the target densitysurfaceusingthe profile information. part of the Gulf Streamwater continuesnorthwardalongthe
The corrections were on the order of 0.3øC and 60 dbar. Since
eastflank of the GrandBanks.The meanvelocities(Figures3a
DUTKIEWICZ
Floats
ET AL.: PATHWAYS
OF CROSS-FRONTAL
EXCHANGE
Floats
on 27.2 surface
26,919
on 27.5 surface
5ll
..
•[..
.',•,,,. .' ...............-......
ß.-...
..
.
....
.- .....
..
-:'"*'
48--.'.'.'.'
.........
:-"'
i"....."..
'
•
'..
.'.' .
...•.............
.'-'•' '•. .....
'
'--... ..-...' ....
=•
......
.'".:'
.•46
.•,- ...:::::-'"'.:...
......
i' ..•'.-,-'g!½•:.•3.
•:.'<' - . .•.'"
::::i'
: '......
;.•,.)'3.
' -:'
'-' '
'..
.•
:- .
/• .
':: i.
•
.-
..?..
"'2'"':""
......•'•' •.
-
'....
[5 •
-44
(a)
-42
-40
-38
-36
-34
-32
" '
•
38
'
-SO
-30
-48
•"PJ'
-46
-44
'
-42
" ':::
':'"
-40
(b)
longitude
t:'"'
-
40
-46
.:---'
...
!. .-'":
"'"
• •t• ...... :'
4:>
40'" ....
•
,•.
-':-
'•I ""- ' '"
l.:::"
-48
" ....
" "' -.7'.:--
......
'.•i"::'",
':.'.'.'.::..'.':'
38
-50
--... ,. ..'.......
"
50i'
.....
50
-38
-36
-34
-32
-30
longitude
Figure 2. RAFOS float trajectorieson the (a) 27.2(r, surfaceand (b) 27.5(r, surface.Launchpositionsare
shown as thick crosses.Dotted lines indicate the 200-, 1000-, and 4000-m isobaths.
and 4a) show a strong current there, with a semistationary Maximumvaluesareover1000cm2 s-2 for theupperlayerand
meander pattern and associatedrecirculations.Background over600cm2 s-2 for thelowerlayer.Maximumvaluesaround
eddykineticenergy
valuesare-200 cm2s-2 and-100 cm2s-2
1250cm2 s-2 werefoundfor the 50- to 100-mlayerfromthe
for the upper and lower layers,respectively(Figures 3b and
4b). Higher eddy kinetic energyis associatedwith the NAC:
International
Ice
Patrol
and
the
Institut
fdr
Meereskunde
drifter data setsfor this region [Rossby,1996], and a similar
EddyKineticEnergy
(cm2/s
2)
Upper Surface: Mean Velocity (cm/s)
1000
50
• "•'•"
•'-:.•
ß':.".-:"
7.:.;Y•
:•:•(:•
ß•j
45
'.•
ß .....
•;•..•"'•'•:
.....
•:'"...."•"•
40
(b)
MeanT (øC)andstandarddeviation
-45
ß•'••
-40
4OO
200
-35
Mean P (dbar) and standard deviation
2OO
.....
*':•::'*
'"'"•i•
'':%
'
:•,'-'•':•:.:.i:•:;•:,,....i
i"':*:"%•
::.:.½•;•:...
:..4 1.5
*t•-.?'*"•:":•
"* '•'•• o'
• 45
i'":.
..............
1
..
.
..,.**¾.,;-•:-'
•*•:•
,:/%
0.5
...
40
.
....:
:.
..
..
(c)
..
-•2•
•
-4,5
longitude
'
"
0
50
-•***.."'
...•
45
'"150
'•'•'"•
lOO
40
. ...=...
-'m..•.-*:..
:* •50
(d)
-45
-40
longitude
-35
Figure 3. Climatologyderivedfrom 2 yearsof RAFOS float data alongthe 27.2rr, surface:(a) meanvelocity
(arrows)
andspeed(shading)
(cms-•), (b) eddykineticenergy(cm2s-2),(C)meantemperature
(contour
with
0.5øCinterval)and standarddeviation(shading),and (d) mean pressure(contourwith 100-dbarinterval) and
standarddeviation(shading).Lines indicatethe 200-, 1000-,and 4000-misobaths.
26,920
DUTKIEWICZ
ET AL.: PATHWAYS OF CROSS-FRONTAL EXCHANGE
EddyKinetic
Energy
(cm2/s
2)
Lower Surface: Mean Velocity (cm/s)
8O
......
'•:•i•'•:,•
•:-•"::':• .•::•:;•5•
•
;'::½:'•:•
60
600
50
500
4O
400
45
/•.....•½-:.-*-:•:•,•%•:::t•:'m?
""•'
2O
3OO
4O
200
(a)
-45
-40
(b)
-35
MeanT (øC)andstandarddeviation
-45
-40
-35
Mean P (dbar) and standard deviation
I
1.5
5O
I
200
150
5O
100
45
5O
0.5
4O
4O
0
(c)
-4s
40 -3s
longitude
-
(d)
-4s
0
40 -3s
longitude
Figure 4. Climatology
derivedfrom2 yearsof RAFOS floatdataalongthe 27.50't surface:(a) meanvelocity
(arrows)
andspeed
(shading)
(cms-h),(b)eddykineticenergy
(cm2s-2),(C)meantemperature
(contour
with
0.5øCinterval)andstandarddeviation(shading),and(d) meanpressure(contourwith 100-dbarinterval)and
standarddeviation(shading).Linesindicatethe 200-, 1000-,and 4000-misobaths.
maximumwas found from ERS-1 altimeter data [Heywoodet tendencyto grow and decay.A secondtroughthat formsjust
al., 1994].The meanpressure(Figures3c and4c) andthe mean eastof FlemishCap has even highereddykinetic energythan
temperature(Figures3d and4d) showwest-eastgradientsas- the 44øNtrough.This feature alsocoincideswith the location
sociatedwith the NAC. There is also a north-southgradient of one of the eastward"branches"that hasbeen suggestedby
variousauthors[e.g.,Arhan, 1990].The currenteither continassociatedwith the shoalingof the isopycnals.
The strong anticycloneat 42øN appears as a permanent ues north and retroflectsin the NorthwestCorner or will join
feature that has also been encounteredby various hydro- the SubpolarFront to the northeast,leavingan isolatededdyin
graphicstudiesandwasfirst documentedas an eddyby Mann the NorthwestCorner [Carr and Rossby,1995;Lazier, 1994].
[1967]. It has therefore often been referred to as the Mann Lazier [1994]found that an eddyof --•100km would frequently
Eddy.The currentpathfollowsthe northernedgeof the Mann form and persistin the NorthwestCorner for 1 to 2 months.
Eddy and forms a steep trough, at 44øN,whose amplitude After the Northwest Corner the current turns eastward and
appearsto growand decayovertime. However,the troughis exhibitslarge meanderingand temporallydifferent pathways
found to be a more persistentfeature duringthe 2-yearfloat and crossesthe Mid-Atlantic Ridge in the regionof the Charexperimentthan it is in longerclimatologies
[Carret al., 1997]. lie-Gibbs Fracture Zone. This eastward moving current apThe area between42øNand 44øNalsoshowsa localizedhigher pears more diffusethan the current along the Grand Banks
eddykineticenergy(Figures3b and4b), whichis an indication [Rossby,1996;Kearns,1996;Rowley,1996] and is termed the
of the NAC transportand
of highvariabilityin the locationof strongfronts[Rossby,
1996; SubpolarFront. Further discussions
Johnset al., 1995].There is alsohigher pressureand temper- the Lagrangianstatisticsare providedby Carr and Rossby
ature variabilityassociated
with the trough.Float trajectories [2001] and Zhang et al. [2001].
This studyfocuseson the current betweenthe Southeast
additionallyindicatea recirculationwithin the trough (Figure
2). Between43øNand44øN,thereis colderwaterflowingfrom NewfoundlandRise and the NorthwestCorner. In this region
the southwest,
whichis probablythe LabradorCurrentjoining the currentappearsas a coherentfeature, and the float data
the path of the NAC. After the 44øNtroughthe currentforms providegood coverage.A modelingstudy[Dutkiewicz,1997]
a crestagainstthe Grand Bank, southof Flemish Cap. The demonstratesthat muchof the cross-frontalexchangeoccursin
eddykineticenergyassociated
with the crestis lower than the the region where the front and associatedinstabilitiesare
trough,whichsuggests
that the crestdoesnot haveas stronga strong,and lessexchangeoccurswhenthe front becomesmore
DUTKIEWICZ
ET AL.: PATHWAYS
OF CROSS-FRONTAL
EXCHANGE
(a) 48 N transect:o (kg/m=)
26,921
cm/s
0
...:. 100
500
:•
.2•
100
:
I
0
0
i
20
0
40
i
0
IO
60
"
80
(b)Flemish
Cap- MilneSeamounts
transect
:o(kglm
3)
':•"
.......
ß.......
ß
..................
•.......... '•..-•i:g•$
"•'"•'"•:"•:•
= '='•'-
500
O•
•r'2
100
0
0•
0
50
•
100
0
100
cm/s
'i'i
:'"'"•
.... = ..........
'•"-
-27.2
•
27'8
• - 27.5
•
0
•
0
•
150
200
(c)43-41Ntransect
:o(kg/m
3)
cm/s
,•, 0 .•=.• :;•.%.
..•;:•
ir- --•'::='
...........
:2.•-'
--:•..'
•::•:•...•..
........
•,1•.=,.•
..
100
•=..:•
.....
,6..•':•
......
:.;•.:;,•::•..
- _-':•
• 500
--
ß
• 1000•
COCO C •C
0
100
ß
C C
50
•
CC C
200
'•
300
0
27.5
•
400
0
•
500
0
distance along transect (km)
Figure 5. CTD sections(locationsshownin Figure1) from (a) 1000-misobathat 48øNtendingeastfor ---100
km, (b) FlemishCap to Milne Seamounts(---250km), and (c) 1000-misobathat 43øNtendingeast-southeast
to 41øN(---500km). Circlesindicatepositionof eachCTD}station.
Contours
indicatedensity(at, kg m-3);
shading
shows
horizontal
velocity(cms-•) fromacoustic
Dopplercurrentprofilerdownto ---400m. Thick
horizontal
linesindicate
third-order
polynomial
fit P•(0) for the27.2rrt (upper)and27.5rrt (lower)surfaces.
diffuse.Therefore the NAC is a potentiallyimportant region
for the exchangeof subpolarand subtropicalproperties,possiblymore important than the SubpolarFront.
part of the NAC at each latitude, 0, using the RAFOS float
data. We will refer to this "center"of the jet as the "dynamical
front."
From the climatologiesshownin Figures3 and 4 it is possible to define the most likely corridor of the jet's path. The
3. Definitions: Jet and Dynamical Front
corridor is divided into 1.5ø latitude bins (Figure 6), which
Studiesof exchangein the Gulf Streamhave been aided by givesa goodcoverageof the current,with reasonablenumbers
the constantdownstreamprofile of that current [Halkin and of float observationswithin each bin. We plot the horizontal
Rossby,1985].For instance,the definitionof the centerof the
speed,pressure,and temperatureof the float data in eachbin
Gulf Stream was taken as the halfway position between the
(Figures7 and 8). We haveallowedeachfloat to provideonly
12øCisothermcrossing400 m and 600 m [Halkin and Rossby,
10 data pointsper bin, thusminimizingthe biasthat can occur
1985].However,sucha definitionwith no downstreamdepenbecauseof a singleslowor fast movingfloat. In Figures7 and
dence is not possiblefor the NAC where, for example, the
8 the fastest25% of the data are denotedwith darker symbols,
pressurealong the isopycnalsurfaces(Figures 3d and 4d)
clearly indicatesnot only an east-westgradient of pressure revealinga subsetof fast movingfloat data that are then used
associatedwith the jet but also a shoalingtoward the north. to estimatethe pressureof the fast movingcore of the NAC.
This shoalingis alsoevidentin CTD sections(Figure 5). Sim- The mean pressurevaluesfor this subsetin each bin (diailarly, the temperatureis colder both to the west and to the monds)and for one standarddeviationon either side (bars)
north (Figures3c and 4c). The isopycnals
associated
with the are shownin Figures7b and 8b for the two isopycnalsurfaces.
polynomial
to the mean,Pf(O), (solid
jet shoalsharplybetween42.5øNand 46.5øN,remain at a more We fit a third-order
deviation
on eitherside,P•(0) _+
constantdepth near 48øN,and shoalagainfarther north. For lines)andto onestandard
lines)for eachof theisopycnal
surfaces
as
the purpose of examiningexchangeacrossthe current, we s•(0), (thickdashed
therefore require a definition of the current that takes this a function
of latitude.Thesepolynomials
Pf( 0) showa similar
latitudinal dependenceinto account.We accomplishthis by trend to that seenalongCTD transectsand in the float-derived
estimatingthe rangeof pressuresthat encompasses
the central climatologies.
We plotthevaluesof Pf(O) for the27.5rrt and
26,922
DUTKIEWICZ
ET AL.' PATHWAYS
OF CROSS-FRONTAL
Mean Velocity (cm/s): upper surface
EXCHANGE
Mean Velocity (cm/s): lower surface
54
54
52
52
50
48
48
.-_46
.-_46
44
44
42
42
4O
38
-50
-45
-40
(a)
-35
38
-50
-30
-45
-40
(b)
longitude
-35
-3O
longitude
Figure 6. Latitudebinsof themostlikelypathof the NorthAtlanticCurrentsuperimposed
onmeanvelocity
fieldsfoundfrom RAFOS data for the (a) 27.2% surfaceand (b) 27.5% surface.Linesindicatethe 200-,
1000-, and 4000-m isobaths.
27.2o-t surfacesfor the appropriatelatitudeswith thick hori- P:(0) andhavinga rangeof ___s:(0).
Thusa floatis on the
zontallinesin Figure 5. Note that the centerof the jet appears "subpolarside" of the current if
to be farther onshore for the lower surface and farther offshore
Pftoat
%Pf(O) -- sf(O)
for the upper surface.
The frontal zone is therefore defined as being centeredat
Horizontal Speed
and on the "subtropicalside"if
Pressure
52
(b)
Temperature
52
(c)
ß- '7;'J'-•,:7
'•"•'-'
50
5O
':E'•'
'7..7 7 -
48
48
46
46
44
44
44
42
42
42
48
= 46
•"
40
0
ß
'
50
cm/s
'
1 O0
40
200
40
600
dbar
1000
4
'
6
'
8
'
10
12
oc
Figure7. Floatdata(dots)frombinsin Figure6 for the27.2% surface:
(a) horizontal
speed(cms-•), (b)
pressure(dbar), and (c) temperature(øC).Darker dotsindicatefastest25% of datain eachbin. Thin dashed
linesmark latitude of eachbox and diamondsshowmeanpressureor temperaturein eachbin, with one
standard
deviation
(bar) shownon eitherside.Third-order
polynomial
fit P:(0) is appliedto bin mean
pressure
(solidline)andto onestandard
deviation
s:(0) on eitherside(thickdashed
lines).
DUTKIEWICZ
ET AL.' PATHWAYS
Horizontal Speed
OF CROSS-FRONTAL
EXCHANGE
Pressure
26,923
Temperature
52
(c)
50
48
46
44
42
•i•i
.:.:.
40
0
'
50
'
1 O0
42
40
200
42
40
600
cm/s
1000
4
'
6
dbar
'
8
'
10
'
12
oC
Figure8. Floatdata(dots)frombinsin Figure6 for the27.5o-t surface:
(a) horizontal
speed(cms-•), (b)
pressure(dbar), and (c) temperature(øC).Darker dotsindicatefastest25% of data in eachbin. Thin dashed
lines mark latitude of each box and diamondsshow mean pressureor temperature in each bin, with one
standarddeviation(bar) shownon eitherside.Third-orderpolynomial
fit Pf(0) is appliedto bin mean
pressure
(solidline) andto onestandard
deviation
sf(0) on eitherside(thickdashed
lines).
> P/O) + s/O).
Front, for both of which the definitions above are not accurate.
The studyregiontherefore coversfrom 50øWto 36øW,except
We find that much of the data discussed in section 4 indicate
for regionsto the southeastand northeast.The boxesdenote
that floatsare not alwaystravelingquicklywhen in this region; the studyarea in Figures9, 10, 11, 13, and 14.
a float can be in the right pressurerangefor the front and yet
As an initial example,we considerfloat 269 on the 27.5o't
be movingrelativelyslowly.This suggestsa breakdownof the surface.Figure 9a showsthe trajectoryof the float within the
tight frontal structures,a phenomenonthat was observedin NewfoundlandBasin superimposedon the mean velocityfield
the Gulf Stream[Halkin and Rossby,1985].We thereforealso found from all the floats on the 27.5o-t surface. Float data
use a thresholdvelocityto define the jet: A float is "in the jet" outside of the study region in both Figures 9a and 9b are
if IVf•oat]
> Vthreshol
d.Threethreshold
velocities
areconsidered; indicatedby thin lines. Float 269 is launchednear 42øN and,
thesespeeds
(v• = 30 cms-•, v2 = 45 cms-•, andv3 = 60 after a loop southaroundthe Mann Eddy, movesnorthwardto
cm s-•) are takenasfractions
of the fastestspeeds
foundin the SubpolarFront. Figure 9b showsthe correspondingLaFigures 7a and 8a.
grangiantime seriesof pressure(dbar), temperature(øC), and
A tight frontal structureasfound in the Gulf Streamstudies horizontal
speed(cm s-•) for thisfloat.The thin line in the
[e.g., Halkin and Rossby,1985; Bower et al., 1985] would be pressure
recorddenotes
P•(0), wherethe latitude0 is taken
necessaryto calculatecross-frontaleddypropertyexchange.In from the float track,and the dashedlinesindicatePf(O) _
this studywe insteadestimatethe positionof the floatsrelative st(0). Thesefrontalzonelinesareonlyplottedwhilethefloat
to the NAC and investigatehow frequently these positions
change.We have two ways of defining a float's position:Its
pressureindicateswhere it is in relation to the front, and its
horizontalspeedindicatesif it is in the fast movingcurrent or Table 1. Number of Data for RAFOS Floats Referenced
to Pressure Data
in the slowermovingsurroundingwaters.
Number of Data
4.
Float
Statistics
The region of study is the NAC between 40.5øN and the
Northwest Corner, 52.5øN; however, we wish to exclude the
recirculating portion of the Mann Eddy and the Subpolar
27.2% Surface
27.5o't Surface
Study domain
6940
7864
Frontal zone
Subtropicalside
Subpolarside
2658(38%)
3531(51%)
751 (11%)
1863(24%)
5378(68%)
623 (8%)
26,924
DUTKIEWICZ
27.5 surface:
FLOAT
ET AL.: PATHWAYS OF CROSS-FRONTAL EXCHANGE
269
27.5 surface
54
200
..
-
'
•'
ß FLOAT
'L'_-" :...::-r-L:-:
....
269
' ...... :::...:':
• .... }--•.4
...............................
400
52
•
7?: ....
ß
p,,:,
.
. • !
5O
600
8oo
.
1000
"
1200
0
50
100
150
200
250
300
0
50
100
150
200
250
300
50
100
150
200
250
300
48
.-,_46
.
.
44
I
'4
42
80
4O
•'4o
.
38
i
i
i
-50
-45
-40
(a)
,,
•2o
.
,
o
-30
-•$
(b) o
Ion•ltud•
days
Figure 9. Float 269 (a) trajectorysuperimposed
on meanvelocityfield, and (b) valuesof pressure(dbar),
temperature
(øC),andhorizontal
speed(cms-•). Circles
areevery10days.Forthepressure
record,
P( 0) and
P(O) _+sf(O) (thinsolidandthindashed
lines,respectively)
areshown
whilethefloatisin thestudyregion.
For thespeedrecordthethreshold
speed(v• - 30 cms-•) isindicated
(dashed
line).Floatdataaredenoted
by a thick line (within the studyregion)or a thin line (out of the studyregion).Dotted linesin Figure 9a
indicate the 200-, 1000-, and 4000-m isobaths.
iswithin the studyregion.The dashedline on the speedrecord 90 after crossingthe dynamical front, and its temperature
denotes
thethreshold
speed(v• = 30 cms-•) for the27.5o-t increasesby 0.8øC.However, it had alreadycrosseda stronger
surface;any speedgreaterthan thisdenotesthat the float is in temperaturefront as it moved off the Grand Banks before
encounteringthe dynamicfront. A strongtemperaturefront
the jet. The float is launchedon the subtropicalside,Pitoat>
Pf(O) + sf(O). Fromdays60 to 80 the float'spressure
de- inshoreof the dynamicfront in this regionwas found in CTD
creases,and there is a corresponding
temperaturechange.We sectionsof this region [e.g., Rossby,1996] and suggeststhe
could,mistakenly,assumethat the float crossedthe dynamical presenceof LabradorCurrent water which,after retroflecting
front. However,the pressureof the front,Pf(O), alsode- and turningnorthward,flowsoff the Grand Banksand joins
creasedas the float moved northward, and the float was found the NAC pathway.However,Kraussand Kiise [1984]suggest
that there are two Gulf Stream fronts at 50øW, separated
to stayon the subtropicalsidefor its entire mission.
A secondexampleis float 295, which is alsoon the 27.5o-t meridionallyby a couplehundredkilometers,and they suggest
surface(Figure 10). The float is launchedin coldwater on the that the southernfront is composedof Gulf Stream water,
subpolarsideof the front, movesinto the NAC, and is caught while water in the northern front contains a mixture of Gulf
in the 44øNrecirculation.It is expelledto the eastaroundday Streamand slopewater from the continentalshelf.
27.5 surface:
FLOAT
27.5 surface
295
54
... 200
ß FLOAT
295
....
• 400
e 600
52
800
•1000
5O
=1200
0
50
100
150
200
250
300
0
50
100
150
200
250
300
250
300
48
/"
,••
42
4O
"x
'•
-
80
• -- •
,
,
30 c•s
.
38
(a)
i
-50
i
-
-40
longitude
-35
-30
(b)
0
50
100
150
200
Figure 10. Float 295 (a) trajectorysuperimposed
on meanvelocityfield,and (b) valuesof pressure(dbar),
temperature
(øC),andhorizontal
speed(cms-•). Circles
areevery10days.Forthepressure
record,
P( 0) and
30 0
cm
s_s(l
Float
data
are denoted
by
a thick
lineFor
(within
the study
region)
or a thinspeed
line (out
P(
)_+
0is)indicated.
are
shown
while
the
float
isinthe
study
region.
the
speed
record
the
threshold
(v•of=
the studyregion).Dotted linesin Figure10a indicatethe 200-, 1000-,and 4000-misobaths.
DUTKIEWICZ
Floats
54 ÷
i
ET AL.: PATHWAYS
Table
on 27.2 surface
i
i
OF CROSS-FRONTAL
2.
EXCHANGE
Statistics for RAFOS
26,925
Floats Defined
27.2crt
27.5crt
Surface
Surface
Number of portionsof trajectoriesthat enter and
leave the frontal
48
38
39
9
5
15
9
6
11
12
6
1
20
5
16
zone
Time in frontal zone, days
Mean
Median
'= 46
Number of floatsthat enter from subpolarside
Leave on subtropical
Leave on subpolar
Number of floats that enter from subtropicalside
Leave on subpolar
Leave on subtropical
44
42
-5'0
-52
(a)
-34
longitude
Floats
8% for the lowersurface,are clearlyon the subpolarsideof the
front; the remainingdata fall in the frontal zone.
For the purposeof consideringthe likely fate of floatsthat
enter this frontal zone,we consideronlythoseportionsof float
trajectoriesthat entered and exited the frontal zone while in
the domainof study.The trajectories(38 for 27.2rrt and 39 for
27.5rrt), and some statisticsfor these cases,are shown in
Figure 11 and Table 2. The histogramin Figure 12 showsthe
length of time (>2 days) that floats remain with pressures
on 27.5 surface
54
52
5O
48
betweenPf(O) - sf(O) andPf(O) + sf(O). Floatson the
• 46
(b)
as in Frontal
Zone
i
-52 -5'0 -418 -416 -414 -412 -4'0 -318 -3'6
-34
longitude
Figure 11. Trajectoriesof floatswith pressuresin the frontal
zoneasdefinedin section3, for the (a) 27.205 surfaceand (b)
27.505 surface.Light shadedtrajectoriesindicate floats that
enter the frontal zonefrom the subpolarside;solidtrajectories
indicate floats that enter from the subtropicalside. Dashed
lines indicate
that the float left the frontal
lower surfaceappear to spendlonger in the dynamicalfront
(mean of 15 daysand median of 9 days) than floats on the
upper surface(mean of 9 daysand median of 5 days).However,thismaybe biased,sincefloatson the uppersurfacemove
faster and may leave the domain more frequentlybefore exiting the frontal zone.
On the 27.205 surface, 17 of these trajectory portions
startedon the subpolarsideof the front and 21 startedon the
subtropicalside (light and dark lines, respectively,in Figure
11a). Of thoseenteringfrom the subpolarside,-35% crossthe
dynamicalfront and leave the frontal zone on the subtropical
zone to the side of
(a) days in frontal zone: 27.2 surface
the front that it entered from; dark shaded lines indicate that
the float
crossed from
one side to the other.
Dotted
lines
median
indicate the 200-, 1000-, and 4000-m isobaths.
We now examinethe float data with regard to our two sets
of definitionsbasedon (1) float pressurerelated to the analyzed range of pressureswhich describethe dynamicalfront
associated
with the NAC and (2) float horizontalspeedreferenced to various (arbitrary) thresholdspeeds.The observationsare composedof floatsthat remain for 2 daysin a specific
speedor pressureregion.This requiresthat at least five data
points showsimilar values,thereby reducingthe problem of
4
6
mean
8
10
12
14
16
18
20
22
24
26
>28
(b) days in frontal zone: 27.5 surface
median
mean
noise and/or small-scale turbulence.
4.1.
Referenced
to Pressure
For the floats in the studydomainwe have 6940 pressure
4
6
8
10
12
t4
16
18
20
22
24
26
>28
days
data points for the 27.2rrt surfaceand 7864 points for the
27.5% surface(Table1). The largestportionof these,51% for Figure 12. Histogram of length of time floats stay in the
the uppersurfaceand 68% for the lower surface,are pressure frontalzone,asdefinedin section3, for the (a) 27.205 surface
valueson the subtropical
side,Pf•oat> Pf(O) q- sf(O). Only and (b) 27.505 surface.Mean and mediannumberof daysin
a smallamountof the float data, 11% for the uppersurfaceand the frontal zone are indicated.
26,926
DUTKIEWICZ
27.5 surface:
54
FLOAT
i
ET AL.: PATHWAYS
OF CROSS-FRONTAL
Table
307
3.
EXCHANGE
Statistics for RAFOS
..
52 '
::
50
'•N.\"
•- x;.V•.
48 :.
ß '
ß..
.*,_46
..
...
ß .
.. ß
ß.
ß
ß
'
".
ß /!
..-'/•
ß
'.
,/
&,l
/•.. • ,...
\ ,,. .....
' '•!x-'•_ •'•..x \
V > V•
V > V2
V > V3
.
ß.,
.'..ß
... '
ß
42'.
I
•
' ' I --•N
ß.' ,,..55
"-L .....
.''
-'"-'/
"3ocnvs
ß
ß
..
..
I
38 _5•0
(a)
,- '
•,-x///
-45
-
Reference
to
10
I
27.2o't
27.5o't
Surface
Surface
7101
7944
Number of Float Data GreaterThan lIthreshol
d
'-'"
. • ,.,,.•..
ß" :xfi•..x \ x•
Number of data in studydomain
...
..
;x,"• , ': •':"'"
•
Floats With
Three ThresholdSpeeds(l/threshold)
,
Average time > vt
Average time > v2
Averagetime > v3
Median time > vx
Median time > v2
Median time > v3
3448(49%)
1674(23%)
685 (10%)
Numberof Days in Jet
7
6
4
6
5
3
2000(25%)
593 (7%)
123(2%)
7
4
4
6
4
3
-, 10
-35
longitude
27.2 surface
c' 2øø/
'
i1000
•'1200
0
, .- _ _ ,...
'
•
'
ß FLOAT 307
,__.. ,.._ _ _ _•..._
__.•./.-, • a•...•
'
•
f
'
'
20
40
60
80
100
120
140
160
180
20
40
60
8'0
100
I 20
140
150
180
•8
0
80
.....
•60
o
4.2. Referenced to Speed
.....................
¾4o
0
i
(b) 0
Fi•,r•
l•.
i
40
i
i
,
8odays ,00
i
,40 ,60 ,80
•1oat 30? (a) tmj•cto• •up•fi•po•d
o•
(øC),a•d horizontal
•p•d (c• •-•). Ckcl• a• •w• •0 da•.
•o• tb• p••
•co•d, •(• ) a•d •(• ) • •r(O) (da•b•da•d
dotted I•
•tud• •o•.
exchange
tendto spendlongerin the frontalzoneandlose/gain
more heat alongtheir trajectorythan thosethat crossinto the
frontal zone and exit again on the same side. Floats on the
lower surfacegain or lose 0.5øCas they crossbetweengyres.
Farthernorth,and especiallyon the 27.2o't surface,floatsthat
enter the frontal zone tend to exit againto their originalgyre.
However,thesefloatsstill lose/gainheat duringtheir exposure
in the frontal zone: -0.3øC for subpolarfloats and about
-0.05øC for subtropicalfloats.Suchbehavior,discussed
further in section4.2,will leadto propertyexchange
alongthejet.
•p•cfiv•l•) a• •bow• wbJl• tb• float
•o• tb• •p•d •co•d tb• tb•bold •p•d (•:
We found that many floats spendsignificanttime with a
pressureindicativeof the frontalzone.For instance,float 307
(Figure 13), launchedin s•ubpolar
water at 43øN,passesinto
the frontal zoneat day20 and remainswithin it for mostof the
time it is in the regionof study.While the float is within the
frontalzone,severalexcursions
in pressureare associated
with
temperaturechanges.For instance,the float has a pressure
decreaseof -50 dbar at day 100with a corresponding
temperature increase. At the same time, the float showsseveral ex-
cursionsbetweenfasterand slowermovingwater (Figure 13b,
bottom). Suchparticlesin the frontal zone will tend to mix
30 c• •-•) • J•dJcatcd.
•1oatdataa• d•ot•d b• a thick
(w•tbJ• tb• •tud• •o•)
a•d a tbJ• I•½ (out o5 tfi• •tud• propertiesas they travelbackand forth acrossthe front, with
m•Jo•). Dotted I•
• •J•u• ]3b J•dJcat•tb• 200-• ]000-• subsequent
lossesfrom the jet on eithersidetransportingthese
a•d 4000-•
J•obatb•.
propertiesinto differentregions.In a numericalmodelstudy
[Dutkiewicz,1997]thismechanismwasalsofoundto be important in the mixingof tracerbetweentwo gyres,providedthat a
propertyfront was present.We note that this
side(solidlinesin Figure 11), denotingthe exchangeof mass corresponding
from one gyreto the other.The remainingfloatsenteringfrom typeof exchange
is the sameprocessasthe "dissipative
meanbyLozierandRiser[1990]in the contextof a
the subpolarsidereturn to the subpolarsideafter sometime dering"discussed
midlatitudejet.
spentin the dynamicalfront (dashedlinesin Figure 11). Of quasi-geostrophic
To estimatethe amountof suchan eddy exchangeof propthosethat start on the subtropicalside,only 1 (5%) crosses
to
the subpolarside; the remainder are expelledagain to the ertiesthat occursdue to meanderingwithin the frontal zone,
subtropicalregion. There is therefore an asymmetryin the we now examinethe positionof the floatswith respectto fast
exchange
of massbetweenthe twogyres.Thisis alsotrue of the and slowmovingregions.We considerthree arbitrarythresh27.5rrt surface(Figure 11b), where 66% of the floatscross old speedsandshowthe resultsin Table 3. We find that 49%,
from the subpolarto the subtropicalgyre after enteringthe 23%, and 10% of the float data on the 27.2rr t surface are
thethreethreshold
speeds
(30,45,and60cms-•). The
dynamicalfront andonly23% crossfrom the subtropicalto the above
lowerlayerhasslowermovingwaterwith 25%, 7%, and 2% of
subpolarside.
On both surfaces, exchangeoccurring within the 44øN the data beingabovethesethree speeds.Floatsremainin the
troughappearsto be predominantlymassexchange(solidlines fastmovingregionsfor only a few dayson average(seeTable
in Figure 11), whichsuggests
a meanmassflux from the sub- 3).
In Figure 14 the trajectoriesof these excursionsinto fast
polar to subtropicalgyre. Floats that are involvedin mass
DUTKIEWICZ
Floats
ET AL.' PATHWAYS
on 27.2 surface
54
52
5O
48
= 46
44
4o
,
i
'6
(a)
'
-4'2
'0
longitude
Floats
on 27.5 surface
52
48
• 46
44
40
-52 -5'0 -4'8 -4'6 -4'4 -4•2 -iO -3'8 -3•6 -34
longitude
Figure 14. Trajectories of floats with speed in the jet, as
defined
in section
3, forthe(a) 27.2rr,surface
(45cms-•) and
(b) 27.5rr,surface(30 cms-•). Asterisks
indicatewherethe
floats leave the fast moving water. Dotted lines indicate the
200-, 1000-, and 4000-m isobaths.
water are plotted.Asterisksindicatewhere the floatsleavethe
fastermovingwater. For the two surfaceswe chosethe thresh--1
old speedfor approximatelyone fourth of the data (30 cm s
for the 27.5rrt surfaceand45 cm s-• for the 27.2rrt surface;
seeTable 3). Expulsionof floatsfrom the jet generallyoccurs
to the eastwhen approachingtroughsand to the west when
approachingcrests;however,lossesappear mostlyto the offshoresideof the current;that is, into the subtropicalgyre.The
lossesto the eastat the 44øNand47øNtroughscorrespondwith
the branchingof the NAC suggested
by Arhan [1990]. Those
floatslost onshoreof the current are frequentlycaughtin the
recirculations
associated
with the troughs(e.g.,44øNand47øN)
and later becomereentrainedin thejet. The NorthwestCorner
is also a region of considerableexpulsionsfrom and entrainments into the jet.
Discussion and Summary
Parcelsof water that crossthe entire frontal zone not only
exchangepropertiesbut movemassfrom one sideof the front
26,927
show that
floats often
enter
the
frontalregionand after sometime returnto their originalgyre.
Suchbehavioris more likelyto occurnorth of the 44øNtrough
and especiallyin the Northwest Corner. Since these floats
diffusetheir propertiesin the frontal zone and are frequently
lost to slowermoving surroundingwater, propertieswill be
exchangedbetweenthe two sidesof the jet. Floats within the
jet, and thus possiblyinvolved in this "property exchange"
within the frontal zone, are lost from the fast movingjet into
the recirculationregionsassociatedwith the meander extrema
to the east from the 44øN and 47øN troughsand to the west
in the Northwest
Corner.
This mecha-
nism of exchangingpropertiesis mechanisticallythe same as
the dissipativemeanderingdiscussed
byLozierandRiser[1990]
as the main mechanismfor the exchangeof potentialvorticity
betweengyresin a quasi-geostrophic
model.Dutkiewicz[1997]
demonstratedthat when the property and dynamicfronts are
in closeproximity,this type of exchangehas a significantimpact on the flux of tracer. In the NAC this diffusivemechanism
is important for the flux of subtropicalproperties into the
subpolarregions,especiallyin the northern part of the study
domainwhere the temperatureand dynamicalfronts appear
closertogether(see Figures3 and 4). "Massexchange"in the
44øN trough may, however, dominate the flux of subpolar
propertiesinto the subtropicalwater.
Subpolarpropertieslost to the subtropicalzone through
both massand property exchangeare diffusedinto the waters
of the Newfoundland Basin by the slow eastwardmoving region of water found to the east of the NAC. This accountsfor
the mixedpropertiesof the water as found by variousauthors
[e.g.,Arhan,1990;Kearns,1996].In contrast,propertieslost to
the subpolarsidewill not mix over aswide an area. We found
that many of the floats lost to the jet on the subpolarside
would be reentrainedinto the jet. An exceptionis the water
lost from the Northwest Corner that moves away from the
region to the north and west within eddies.Therefore along
much of the length of the NAC the subtropicalside of the jet
has more diffuse properties,and the subpolarside maintains
high property gradients.
This study has consideredthe cross-frontalexchangethat
occursalong the length of the NAC. Although the amount of
"mass"exchangeis small comparedto the transport of the
NAC, there is a net mean flux from the subpolargyre to the
subtropicalgyre.This appearsto be causedby the more temporarily changingnature of the meanderson the east side of
the jet (as can be noted in the eddy kinetic energy,Figures3c
and 4c). Hopefully,future studiesof the currentsystemsin the
far North
5.
observations
from the retroflection
5O
EXCHANGE
to the other. The float observationssuggestthat in the NewfoundlandBasin,there is movementof subpolarwater mass
into the subtropicalregion but very little mass flux in the
oppositedirection.The preferred site for floats to crossthe
dynamicfront is the 44øN trough and, to a lesserextent, the
47øNtrough and the NorthwestCorner. Floats on the 27.5o-t
surfaceloseor gain •--0.5øCasthey crossbetweengyres.Floats
on the upper surfacealso appearto lose/gainheat during the
crossing,althoughwith significantlysmalleramplitudes.The
lack of a tight structureto the NAC makes it difficult (i.e.,
beyond the scopeof this paper) to calculatemore definite
cross-frontalproperty exchangevalues.
Additional
42
(b)
OF CROSS-FRONTAL
Atlantic
and Labrador
Sea will be able to address the
open questionof where and when this asymmetricflux is compensatedby a net flux in the opposite direction. The most
commoncauseof intergyre exchangeof properties,however,
26,928
DUTKIEWICZ
ET AL.: PATHWAYS
appearsto be eddycross-frontalexchangein the form of water
parcelsmovinginto and out of the NAC.
The NAC is a uniquecurrentsystemin that it bringswaters
of such different properties into contactwith one another.
Exchangeof these properties acrossthe front will have an
effecton the propertiesof the surrounding
waters,especiallyto
the eastof the current,and thereforewill potentiallyhave an
impact on the gyre-scaledynamics[Lozier and Riser, 1990;
Dutkiewicz,1997].With suchstrongdifferencesin temperature
and salinitythis mixingwill potentiallybe responsiblefor inducing density changesthrough cabbeling processes.The
cross-frontalexchangeof propertieswill also causethe property gradientsalongthe NAC to becomeweaker,especiallyon
the subtropicalside, when the current turns eastwardas the
SubpolarFront and crossesthe Mid-Atlantic Ridge.
Acknowledgments.The authorswould like to expresstheir appreciationto Jim Fontaine,who built the floats,and SandyFontana,who
analyzedthe data. Many people helped in building,ballasting,and
deployingthe floats;othershelpedin processing
the data and discussing the results;our thanksto them, especiallyMary-Elena Carr, Clark
Rowley, Mark Prater, and Ed Kearns. We owe a particulardebt of
gratitudeto Mary-Elena Carr for her initial ideason this studyand to
Mark Prater for his suggestions
on improvingthe paper. Comments
from two anonymousreviewers are gratefully acknowledged.This
projectwasfundedjointly by the National ScienceFoundation(NSF)
and the Office of Naval Research(ONR) and was administered
throughONR.
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S. Dutkiewicz,Department of Earth, Atmosphericand Planetary
Sciences,MassachusettsInstitute of Technology,77 Massachusetts
Avenue, 54-1533 MIT,
Cambridge, MA 021028, USA.
([email protected])
T. H. Rossbyand L. Rothstein,GraduateSchoolof Oceanography,
Universityof Rhode Island, South Ferry Road, Narragansett,RI
02882,USA. ([email protected];
lrøthstein@gsø'uri'edu)
(ReceivedOctober8, 1999;revisedMay 15, 2000;
acceptedJanuary2, 2001.)