GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 9, PAGES 1329-1332, MAY 1, 1999

Pacific thermocline bridge revisited
NiklasSchneider• , Stephan
Venzke2, ArthurJ.Miller1,DavidW. Pierce
1,Tim P. Barnett
1,
ClaraDeser3, andMojibLatif2
Abstract. The coupling on decadaltime scales of the midlatitude and tropical Pacific via an oceanic 'bridge' in thethermocline is investigated using ocean general circulation
model hindcasts and a coupled ocean atmosphere model.
Results indicate that in the tropics decadal anomalies of
isopycnaldepthare forced by Ekman pumping and are largely
independent of the arrival of subductedanomalies in thethermocline that originate in the mid-latitudes of either
hemisphere.In the coupledmodel, temperatureanomalieson
isopycnals show little coupling from the tropics to thenorthern hemisphere,but are lag correlatedbetween southernhemisphere mid- and low-latitudes. However, anomaly
magnitudeson the equatorare small.Theseresultssuggestthat
the oceanic'bridge' to the northernhemisphereexplains onlya small part of the observeddecadalvariancein the equatorial
Pacific. Couplingto the southernmid-latitudesvia temperature
anomalieson isopycnalsremains an intriguing possibility.

Introduction

It has been hypothesizedthat decadalclimate variability
can be causedby coupling of the central North Pacific to theequatorialPacific via thermal anomaliesthat propagatein theoceanic thermocline (Gu and Philander 1997).

Time mean

pathways of subductedwaters deducedfrom trace gases(McPhaden and Fine 1988), oceanic density (Johnson and
McPhaden1999) and oceangeneralcirculation models(Liu etal. 1994, McCreary and Lu 1994, Rothstein et al. 1998) areconsistent with this oceanic 'bridge'. In the northernhemispheremost paths connectingthe mid-latitudeswith theequatorial regions pass through the low-latitude westernboundary currents. In contrast, large parts of the southernhemispheremid-latitudesare connectedto the equatorialregion
througha direct,mid-oceanroute(e.g. Johnsonand McPhaden1999).

indicate that decadalvariability within the tropics and theequatorialregion is dominatedby tropical wind forcing and islargely independentof the arrival of thermal anomaliesfrom
thenorthern

mid-latitudes.

This

is

inconsistentwithsuggestionsthat Pacific decadalclimate variability resultsfrom coupling of the northern mid-latitude and equatorial
regionvia an oceanic'bridge' in the thermocline.An influenceof southern-hemisphereanomalies on the equatorial region
remains a possiblity.
The propagationof thermalanomaliesin the upper oceanis
governedby the three dimensionaloceaniccirculationand isthereforebest viewed in a referenceframe of constant density
surfaces(isopycnals).Thermal anomaliescan either be caused
by undulationsof isopycnals, governed by planetary wave
dynamics(Huang and Pedlosky 1999, Liu 1999a, b), or bychangesof temperatureon the isopycnal in the presenceof
compensating salinity anomalies, in which case thermal
anomalies behave like a passive tracer. Since historicalobservations are currently inadequateto estimate subsurface
salinity anomalies (T. Suga and K. Hanawa, personalcommunication)with the exception of a few sections (Kessler
1999) these two effects cannot be distinguished from dataalone. Rather, a combination of data, ocean model hindcasts

and coupledocean-atmospheremodel simulation needsto beconsideredand is presentedin the following.
Oceanic

Observations

Oceanic density and the depth of isopycnal surfacesare
estimated from observations (White

1995) of anomalous

temperatureand meansalinityin the upper400 m of the Pacificnorth of 20øS from 1969 to 1996. This was done using, atevery horizontal position, both the mean vertical salinity
profile and the mean temperature-salinity relationship.
Consistentwith the compensatingeffect of salinity on density

at the depthof the 25.5 15osurface,isopycnaldisplacements

While observedupperoceantemperatureanomaliesappear
to propagatefrom the northernmid-latitudesto the equatorial
region (Zhang et al. 1998), consideration of wind stressanomaliesusing simple steady-statemodels of the oceanic
circulationindicatesthat tropical variability is mainly driven
by low-latitudewinds(Schneideret al. 1'999).To resolvethis
controversy we investigate simulations with a full-physics
oceanmodel driven by observedatmosphericforcing,and witha sophisticated coupled ocean-atmospheremodel. Results

are reducedby use of the latter techniquecomparedto theformer, most prominently polewardof 15øN. However, allresultspresentedhere are robust. In the following, isopycnal
displacementsfrom the first technique are shown, sincetemperatureanomaliespolewardof 15øNarecausedby diabatic
processes(Schneideret al. 1999) and it is thereforeunclearif
the temperature-salinity
relationshipremainsconstant,Allestimateswere low-passfilteredto focuson decadalvariability
with time scaleslonger than 6.5 years.

Scripps
Institutionof Oceanography,
La Jolla,California
Max-Planck-Institut
ftirMeteorologie,
Hamburg,
Germany
National
CenterforAtmosphericResearch,
Boulder,
Colorado

latitudeand equatorialPacific(Johnson and McPhaden1999,
Lysne et al. 1997). Its depth anomalies have enhanced
variancein the Kuroshioregion at 30øN and in a broad swath
that extendssouthwestward
from the subductionregion in thecentralNorth Pacific to the westernsubtropicaland equatorial
Pacific (Miller and Schneider1998). In this region coherent
anomalies propagate from the central North Pacific to theThe25.5kgm'3150
isopycnal
connects
thenorthern
mid-

Copyright1999by the AmericanGeophysicalUnion.
Papernumber1999GL900222.
0094-8276/99/1999GL900222505.00

1329

1330

SCHNEIDER ET AL.: PACIFIC THERMOCLINE

BRIDGE REVISITED

enhanced variance between the western coast of the Pacificand160øE.The result (Figure 2 a) showsa positivedepthanomaly,
corresponding to anomalous deep isopycnal and warm
conditions, that originated in the central North Pacific from1972 to 1977 and propagatedto the southto reach 18øN about
eight yearslater. At the sametime, a positive depth anomaly
occupiedthe region south of 18øN, whose maximum predated
the arrival of the signal from the mid-latitudes. After the
climate shift of 1976/77 (Trenberth and Hurrell 1994, Graham

120"E

150"E

180"

1994, Miller et al. 1994), a negative depth anomaly,
correspondingto shallow isopycnals and cold temperature
anomalies, propagated with similar speedsfrom the central
North Pacific to the southand appearedto spreadall the way tothe equatorial western Pacific. Analysis of simple, steady150"WFigure1:Depthanomalies
ofthe25.5kgm-3 oOisopycnal.state, wind-forced models (Schneider et al. 1999), rather thanShown is the contour correspondingto a shallowing of thesurfaceby 10 m. Heavy lines denotedata from years 1986,
1990 and 1994 (as indicated). Anomalies of the in-between

years 1988 and 1992 (not shown) are consistent with acoherent propagation from the central North Pacific to thesubtropical western Pacific of the depth anomaly over theeight year period(Miller and Schneider1998). Dashedlines are

2.4 and4.5-10'lø m'2 s'l isoplethsof potentialvorticity
estimated from the vertical distance of the mean 25.3

of general circulation models consideredhere, indicated that
this apparentspreadresultsfrom tropical wind forcing and is
not a result of the arrival of subductedsignals from the midlatitudesas concludedelsewhere(Zhanget al. 1998).
Ocean

Hindcasts

and 25.7

kg m'3 oo isopycnals.
Shading
denotes
areaof zonal
averagingusedin Figure 2. Shadedareasin the inset show
ventilationpathsof the coupledmodeland are usedin Figure3.

To test the hypothesis that low-latitude isopycnal depth
anomaliesare due to wind forcingratherthan dueto the arrivalof subducted anomalies from the mid-latitudes, an ocean

general circulation model (Wolff et al. 1997) was forced byobserved fluxes (Da Silva et al. 1994) of momentum and heat

subtropics(Figure 1)along contoursof potential vorticity.
This indicatesthat anomaliesare advectedby the mean flow assuggested previously by an analysis of isothermal
displacements(Schneideret al. 1999).
In orderto observethe propagationalong this path for theentire data set, the zonal averageof depth anomaliesis formed
along this path as delineatedin the main part of Figure 1.
Poleward of 15øN the region is marked by contours ofpotential vorticity andequatorward
of 15øNby the region of

for the time period from January 1949 to December 1993
(Venzke 1999). In addition, surfacetemperatureswererestored
to observed, time-dependentvalues (Parker et al. 1995). This
restoration partially overrides the forcing by anomalous
surface heat fluxes but ensures that anomaliesof sea surface

temperature conform to observations. The simulation of
isopycnal displacement averaged over the same area as
describedabove compareswell with observations(Figure 2 b).
To determineif the anomalies equatorwardof 18øN can be

30'N

20

10

20'N
0

10øN

-10

-20

o
70

80

90

70

80

90

70

80

90

70

80

90

70

80

90

Figure2:(a)Decadal
anomalies
ofthedepth
ofthe25.5kgm'3oe isopycnal
zonally
averagedin theshaded
regionof themain
partof Figure1. (b) Sameas(a) butfromresultsof anoceanic
circulation
model(Venzke1999)forcedwithobserved
anomaliesof
surfacewind stress(Da Silva et al. 1994),heatflux (Da Silva et al. 1994) andsurfacetemperature(Parkeret al. 1995). (c) Results

froman experimentin whichtheanomalous
atmosphericforcingwasrestricted
to the areaequatorward
of 18ø latitude.To avoid
artificiallygenerated
cudof the windstressa lineartransitionto zero anomalous
forcingextendedfrom 18ø to 23ø latitude(indicatedbydashedlines).
(d)Differenceof 25.5kgm'3•o isopycnal
depth
anomalies
of simulations
withfullforcing(b)andtropicalforcing(c). The additionaldottedcontoursmarkdifferenceswith magnitudesof 3 m. (e) Depthanomaliesobtainedby

forcingtheoceangeneralcirculation
modelwithwindstressanomalies
assembledby theFloridaStateUniversity(Stricherz
et al.
1992, 1997). Anomalouswind stresswasestimatedfrom the pseudo-stressby a constantdragcoefficientof 10-•. As in (c) windstressanomalieswereappliedwithin 18ø latitudeof the equator,andrampedto zero at 23ø latitude. In all plots the contour
intervalis 5 m, light shadingdenotesnegativedepthanomalies,corresponding
to a shallowingof the isopycnal,and dark
shadingrepresents
positivedepthanomalies(seethe gray-scaleon the fight).The verticaldottedlines in (a) to (d) markthe endof the modelexperiment.All resultshavebeensmoothedusinga low-passfilter with a half powerat 6.7 years.

SCHNEIDER ET AL.: PACIFIC THERMOCLINE BRIDGE REVISITED

explained by the arrival of mid-latitude anomalies or bytropicalwind forcing, an additionalexperimentwas carriedout
in which the anomalousatmosphericforcing and relaxation of
surface temperatureanomalies were restricted to the area
equatorwardsof 18ø latitude in both hemispheres. The
resulting isopycnal depth anomalies poleward of 18øN are
indeedreducedto nil, while the simulation equatorwardof 18ø

H

betweenthesimulations

indicates

that

20øN

uncertainties

of oceanic

wind

1.0

-

10øN

stress as shown

0.8
0.6

O

mid-

latitude anomalies from the north (Figure 2 d)propagate into
the equatorial region (Lysne et al. 1997). However, theirmagnitude,of the same order as reported by Lysne et al.
(1997), is less then 10% of the observedequatorialvalues andexplains little of the variance there. Remaining differences
betweenthe simulation and the observations, especially thelarge shallowing in the early 1990s, are within theobservational

T

30øN

latitudeis largelyunchanged(Figure2 c) in bothhemisphei'es.
The difference

13 31

0.4

.........
....
.':.."'
.....
o.o

O

::•i::.':' •.•:

10øS

-0.2

20"S

30'S
..

,....,

....

,

....

,...

(Figure 2 e)by a repeat of the ocean hindcast using only
-5
0
5
10 -5
0
5
10
anomalous tropical wind stress derived from a different
Log/yranalysis(Stricherz et al. 1992, 1997).
In summary,the hindcastexperimentssuggestthat Ekman Figure 3: Lagged correlation with anomalies at 32 ø latitude of
isopycnal
depth
anomalies
ofthe25.5kgm'3oepumping in the tropics rather than the arrival of subducted (leftcolumn)
isopycnal and (right column) of salinity compensatedanomalies from the mid-latitudes of either hemispheretemperature
anomalies
betweenthe25.0and26.0kg m'3 oe
explainsthe observeddecadalvariabilityof isopycnaldepthin
isopycnals.Anomalieswere zonally averagedon paths (shown
the equatorialregion.
in the inset of Figure 1) that are designedto capture thevarianceof temperatureon isopycnalsand that follow contoursof potential vorticity. Correlations were formed with
Coupled Model Results
anomaliesequatorwardsof 32ø latitude for the northern (top)and southern (bottom) hemisphere. Contour interval andSincethe observedsubsurfacetemperaturerecordcomprises
shading are explained by the gray scale on the right. Results
only one warm and one cold event in the mid-latitudes,it is too are from an extended integration of a coupled oceanshort to unequivocallydeterminethe importance of coupling atmospheremodel (Frey et al. 1997, Pierceet al. 1999).
of the centralNorth Pacific-tothe tropicsvia anomaliesin thethermocline.In addition, an assessmentof the role of salinity
compensatedtemperatureanomaliesand of coupling to the overwhelmedby the effect of anomalousadvectionacrossthesouthernhemispheremid-latitudesare not available from data. mean temperaturegradienton isopycnals. On the southern
To obtainmoredegreesof freedomandto investigatethe roles hemisphere,the signal can be traced to the edge of theof salinity compensatedtemperatureanomalies and of the equatorialundercurrent,even though the magnitudeof thesouthernhemisphere, a 130 year integration of a coupled temperatureanomaliesthere are again small (1/10 to 1/20øC).
oceanatmospheremodel(Frey et al. 1997) is considered.The
simulation producesboth realistic E1 Nifio and decadal Summary
variability in the North Pacific (Pierceet al. 1999).
As in observations,North Pacific decadalvariability in the
The couplingof extratropicaland tropicalPacific Oceanvia
coupledmodelis associatedwith the generationof subducted an oceanic 'bridge' involves two distinct processes, theanomaliesthat travel equatorwards
in the oceanicthermocline propagation of isopycnal depth anomalies, governed by(Pierceet al. 1999). The zonal averageof depth anomaliesof
planetary wave dynamics, and the advection of temperaturethe 25.5 oe surface
alongthe subduction
path(shownin the anomalies on isopycnals that are accompanied byinset of Figure 1)indicates that the meridionalspeedsof the compensatingsalinity anomalies.Ocean hindcastexperiments
anomalies on both hemispheresare close to observations suggest that tropical wind-stress anomalies force decadal(Figure 3). As in observations, simulatedisopycnal depth variability of isopycnal depth in the equatorialPacific. Thisanomaliesequatorwardof 10-15ø latitudeare independentfrom implies that tropical Ekman pumping overwhelmsisopycnal
propagatinganomaliesoriginatingin the mid-latitudes(Figure depthanomaliesthat originatefrom the mid-latitudes.Lack of3) and are forced by the tropical wind stress(Barnett et al. basin-wideobservationsof oceanicsalinity and surfacefresh
1999, Pierce et al. 1999).
water flux precludesan analysis from observationsor ocean
The lagged correlation of temperature anomalies on hindcasts of salinity compensated temperature anomalies
isopycnals along the subductionpath shows that they along isopycnals. However,resultsfrom an extendedcoupledpropagateto the equatorialPacific and indicatesthat the model model integration suggest that tropical wind forcing also
correctly simulatesthis pathway (McPhadenand Fine 1988, causes anomalous advection across the mean temperature
Johnson and McPhaden 1999, Liu et al. 1994) on both
gradienton isopycnalsthat dominatesover anomaliesformed
hemispheres (Figure 3). However south of 10øN on the in the northern mid-latitudes. Thus, tropical wind forcing
northernhemisphere,the magnitudeof the correlationdropsto rather than an oceanic 'bridge' to the northern mid-latitudes
0.3 (Figure 3) and the amplitudeof the subductedtemperature dominatesthe generationof thermalanomalieson decadaltime
anomalies is only 1/20øC. The mid-latitude signal is scalesin the equatorialPacific ocean.In contrast,anomaliesof

1332

SCHNEIDER L• AL.' PACIFIC THERMOCLINE BRIDGE REVISITED

equatorial ocean circulations- The subtropicalcell, J. Phys.
temperatureon isopycnalsreachthe equatorialsystemfrom the
Oceanogr.,24, 455-497, 1994.
southern hemisphere via a direct path as hypothesized
McPhaden,M. J. and R. A. Fine, A dynamicalinterpretationof thepreviously(Johnsonand McPhaden1999) but are small upon
Tritium maximum in the Central Equatorial Pacific, J. Phys.
arrival. The testing of this model result requireslong term
Oceanogr.,18, 1454-1985, 1988.
Miller, A. J. and N. Schneider,Interpretingthe observedpatternsof
observationsof temperatureand salinityin the upperoceanof
PacificOceandecadalvariations,in BioticImpactsof Extratropical
the southern hemisphere and their analysis in terms of
ClimateVariabilityin the Pacific,editedby G. Holloway,P. Mtiller
isopycnal depth and temperature on isopycnals. TheseandD. Henderson,pp. 19-27, Universityof Hawaii, 1998.
observationsare currently not available except for a few, Miller, A. J., D. R. Cayan, T. P. Barnett, N. E. Graham and J. M.

limited sections(Kessler1999). Finally, the importanceof thearrivalof thermalan0maiieson isopycnalsto the stateof the

Oberhuber, Interdecadal variability of the Pacific Ocean: Model
responseto observed heat flux and wind stressanomalies, Clim.
Dyn., 9, 287-302, 1994.
equatorial
Pacific_n•6dsto be determined
fromtargetedParker,
D. E., C. K. Folland, A. C. Bevan, M. N. Ward, M. Jacksonand
modeling studies.
K. Maskell, Marine surfacedata for analysisof climatefluctuations
Acknowledgments. This work was supportedby NSF (OCE9711265), by DOE (DE-FG03-98ER62605), by the NOAA via theExperimentalClimatePredictionCenter (NA77RJ0453ECPC) and via
the Consortiumfor the Ocean'sRolein Climate(NA47GP0188),and by
the EuropeanUnion's SINTEX programme.

References
Barnett,T. P., D. W. Pierce,M. Latif, D. DommengetandR. Saravanan,Interdecadalinteractions
betweenthetropicsand midlatitudes
in thePacificBasin,Geophys.
Res.Lett.,26, 615-618, 1999.
Da Silva, A.M., C. C. Young and S. Levitus,Atlas of surface marine
data 1994, Volume 1: Algorithmsand procedures,NOAA Atlas 6,
U.S. Departmentof Commerce,NOAA, NESDIS, 83pp,1994.
Frey, H, M. Latif andT. Stockdale,The coupledmodelECHO-2. Part I:
The tropicalPacific,Mon. Wea.Rev., 125, 703-720, 1997.
Graham, N. E., Decadal scale variabilityin the 1970's and 1980's:
Observations
andmodelresults,Clim.Dyn., 10, 135-162,1994.
Gu, D. F. and S. G. H. Philander, Interdecadal climate fluctuationsthat

dependon exchangesbetweenthe tropicsand extratropics,Science,
275, 805-807, 1997.

Huang, R. X. and J. Pedlosky,Climate variability inferred from alayeredmodelof the ventilatedthermocline,
J. Phys.Oceanogr.,29,779-790, 1999.

Johnson,
G. C. and M. J. McPhaden,Interior pycnoclineflow from thesubtropicalto the equatorialPacific Ocean, J. Phys.Oceanogr.,inpress,1999.
Kessler,W. S., Interannualvariabilityof the subsurfacehigh-salinity
tonguesouthof the equatorat 165øE,J. Phys. Oceanogr.,29, inpress, 1999.

Liu, Z., Planetarywavesin the thermocline:
Non-Dopplershift mode,
advectivemodeand Green mode,Quart.J. RoyalMeteor.Soc.,inpress, 1999a.

Liu, Z., Forcedplanetary
waveresponse
in a thermocline
gyre,J. Phys.
Oceanogr.,in press,1999b.

Liu, Z. G., S. G. H. PhilanderandR. C. Pacanowski,
A GCM studyof
tropical-subtropicalupper-ocean water exchange, J. Phys.
Oceanogr.,24, 2606-2623, 1994.
Lysne,J. A., P. ChangandB. Giese,Impact of the extratropicalPacificon equatorialvariability,Geophys.
Res.Lett.,24, 2589-2592,1997.
McCreary, J.P. and P. Lu, Interactionbetweenthe subtropicalandon interannual-to-century
time scales,in NaturalClimateVariability
on Decadal to CenturyTimeScales,NationalAcademyPress,241250, 1995.

Pierce, D. W., T. P. Barnett and M. Latif, Connectionsbetween the
Pacific Ocean Tropicsand Midlatitudeson decadaltime scales.J.
Climate, submitted,1999.
Rothstein,L. M., R.-H. Zhang, A. J. Busalacchi and D. Chen, A

numericalsimulationof the meanwater pathwaysin the subtropicalandtropicalPacificOcean,J. Phys.Oceanogr.,28, 322-342, 1998.
Schneider,N., A. J. Miller, M. A. Alexander and C. Deser, Subructionof decadalNorth Pacific temperatureanomalies:Observationsand
dynamics,J. Phys.Oceanogr.,29, in press,1999.

Stricherz,J. N., J. J. O'Brien and D. M. Legler, Atlas of FloridaState
University tropical winds for TOGA 1986-1985, Florida State
University,Tallahassee,
FL, 250 pp., 1992.
Stricherz,J. N., D. M. Legler and J. J. O'Brien. TOGA pseudo-stressatlas,1985-1994.VolumeII: PacificOcean,FloridaStateUniversity,
Tallahassee,
FL, 158 pp., 1997.
Trenberth, K. E. and J. W. Hurrell, Decadal atmosphere-ocean
variationsin the Pacific,Clim. Dyn., 9, 303-319, 1994.
Venzke, S., Ocean-atmosphereinteractionson decadal time scales,
Ph.D.dissertation,
99 pp., Universit/itHamburg,Germany,January
1999.

White,W. B., Designof a globalobserving
systemfor gyre-scaleupper
oceantemperature
variability,Prog. Oceanogr.,36, 169-217,1995.
Wolff, J.-O.,E. Maier-Reimerand S. Legutke,HOPE, the Hamburg
OceanPrimitiveEquationModel,TechnicalReport,DKRZ, 1997.
Zhang,R.-H., L. M. Rothsteinand A. J. Busalacchi,Origin of upperocean warming and E1 Nifio changes on decadal scales in thetropical Pacific Ocean,Nature, 391,879-883, 1998.

T. P. Barnett,A. J. Miller, D. W. Pierce, and N. Schneider,Scripps
Institutionof Oceanography,9500 Gilman Drive, La Jolla, CA
92093-0224. (e-mail: tbamett@ucsd.edu, ajmiller@ucsd.edu,
dpierce@ucsd.edu,nschneider@ucsd.edu)

C. Deser,NationalCenterfor AtmosphericResearch,P.O. Box 3000,
Boulder,Colorado80307-3000. (e-mail: cdeser@ucar.edu)
M. Latif and S. Venzke, Max-Planck-Institutftir Meteorologie,
Bundesstr.
55, D-20146 Hamburg,Germany.(e-mail: latif@dkrz.de,venzke@dkrz.de)(ReceivedJanuary08, 1999; revisedMarch 11, 1999;
acceptedMarch 15, 1999.)