GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 9, PAGES 1329-1332, MAY 1, 1999
Pacific thermocline bridge revisited
NiklasSchneider
• , Stephan
Venzke
2, ArthurJ.Miller1,DavidW. Pierce
1,Tim P. Barnett
1,
ClaraDeser
3, andMojibLatif2
Abstract. The coupling on decadaltime scales of the midlatitude and tropical Pacific via an oceanic 'bridge' in the
thermocline 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 the
thermocline that originate in the mid-latitudes of either
hemisphere.In the coupledmodel, temperatureanomalieson
isopycnals show little coupling from the tropics to the
northern hemisphere,but are lag correlatedbetween southern
hemisphere mid- and low-latitudes. However, anomaly
magnitudeson the equatorare small.Theseresultssuggestthat
the oceanic'bridge' to the northernhemisphereexplains only
a 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 the
equatorialPacific via thermal anomaliesthat propagatein the
oceanic 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 et
al. 1994, McCreary and Lu 1994, Rothstein et al. 1998) are
consistent with this oceanic 'bridge'. In the northern
hemispheremost paths connectingthe mid-latitudeswith the
equatorial regions pass through the low-latitude western
boundary currents. In contrast, large parts of the southern
hemispheremid-latitudesare connectedto the equatorialregion
througha direct,mid-oceanroute(e.g. Johnsonand McPhaden
1999).
While observedupperoceantemperatureanomaliesappear
to propagatefrom the northernmid-latitudesto the equatorial
region (Zhang et al. 1998), consideration of wind stress
anomaliesusing 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 with
a sophisticated coupled ocean-atmospheremodel. Results
indicate that decadalvariability within the tropics and the
equatorialregion is dominatedby tropical wind forcing and is
largely independentof the arrival of thermal anomaliesfrom
the
northern
mid-latitudes.
This
is
inconsistent
with
suggestionsthat Pacific decadalclimate variability results
from coupling of the northern mid-latitude and equatorial
regionvia an oceanic'bridge' in the thermocline.An influence
of southern-hemisphereanomalies on the equatorial region
remains a possiblity.
The propagationof thermalanomaliesin the upper oceanis
governedby the three dimensionaloceaniccirculationand is
thereforebest 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 by
changesof temperatureon the isopycnal in the presenceof
compensating salinity anomalies, in which case thermal
anomalies behave like a passive tracer. Since historical
observations are currently inadequateto estimate subsurface
salinity anomalies (T. Suga and K. Hanawa, personal
communication)with the exception of a few sections (Kessler
1999) these two effects cannot be distinguished from data
alone. Rather, a combination of data, ocean model hindcasts
and coupledocean-atmosphere
model simulation needsto be
consideredand 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 Pacific
north of 20øS from 1969 to 1996. This was done using, at
every 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
are reducedby use of the latter techniquecomparedto the
former, most prominently polewardof 15øN. However, all
resultspresentedhere are robust. In the following, isopycnal
displacementsfrom the first technique are shown, since
temperatureanomaliespolewardof 15øNarecausedby diabatic
processes(Schneideret al. 1999) and it is thereforeunclearif
the temperature-salinity
relationshipremainsconstant,All
estimateswere low-passfilteredto focuson decadalvariability
with time scaleslonger than 6.5 years.
The25.5kgm'3150
isopycnal
connects
thenorthern
midScripps
Institution
of Oceanography,
La Jolla,California
Max-Planck-Institut
ftirMeteorologie,
Hamburg,
Germany
National
CenterforAtmospheric
Research,
Boulder,
Colorado
Copyright1999by the AmericanGeophysicalUnion.
Papernumber1999GL900222.
0094-8276/99/1999GL900222505.00
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 the
centralNorth Pacific to the westernsubtropicaland equatorial
Pacific (Miller and Schneider1998). In this region coherent
anomalies propagate from the central North Pacific to the
1329
1330
SCHNEIDER ET AL.: PACIFIC THERMOCLINE
BRIDGE REVISITED
enhanced variance between the western coast of the Pacific
and
160øE.The result (Figure 2 a) showsa positivedepthanomaly,
corresponding to anomalous deep isopycnal and warm
conditions, that originated in the central North Pacific from
1972 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 to
the equatorial western Pacific. Analysis of simple, steady
150"W
Figure
1:Depthanomalies
ofthe25.5kgm-3 oOisopycnal.state, wind-forced models (Schneider et al.
Shown is the contour correspondingto a shallowing of the
surfaceby 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 a
coherent propagation from the central North Pacific to the
subtropical western Pacific of the depth anomaly over the
eight 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
1999), rather than
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 arrival
of subducted anomalies from the mid-latitudes, an ocean
general circulation model (Wolff et al. 1997) was forced by
observed fluxes (Da Silva et al. 1994) of momentum and heat
subtropics(Figure 1)along contoursof potential vorticity.
This indicatesthat anomaliesare advectedby the mean flow as
suggested previously by an analysis of isothermal
displacements
(Schneideret al. 1999).
In orderto observethe propagationalong this path for the
entire 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 of
potential 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 anomalies
of 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
Figure
2:(a)Decadal
anomalies
ofthedepth
ofthe25.5kgm'3oe isopycnal
zonally
averaged
in theshaded
region
of 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 experiment
in whichtheanomalous
atmospheric
forcingwasrestricted
to the areaequatorward
of 18ø latitude.To avoid
artificiallygenerated
cudof the windstressa lineartransitionto zero anomalous
forcingextendedfrom 18ø to 23ø latitude
(indicated
bydashed
lines).
(d)Difference
of 25.5kgm'3•o isopycnal
depth
anomalies
of simulations
withfullforcing
(b)and
tropicalforcing(c). The additionaldottedcontoursmarkdifferences
with magnitudes
of 3 m. (e) Depthanomaliesobtainedby
forcingtheoceangeneral
circulation
modelwithwindstress
anomalies
assembled
by theFloridaStateUniversity
(Stricherz
et al.
1992, 1997). Anomalouswind stresswasestimatedfrom the pseudo-stress
by 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-scale
on the fight).The verticaldottedlines in (a) to (d) markthe end
of the modelexperiment.All resultshavebeensmoothed
usinga low-passfilter with a half powerat 6.7 years.
SCHNEIDER ET AL.: PACIFIC THERMOCLINE BRIDGE REVISITED
H
explained by the arrival of mid-latitude anomalies or by
tropicalwind 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
between
the
simulations
indicates
that
20øN
uncertainties
of oceanic
wind
1.0
-
10øN
stress as shown
0.6
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
anomalous tropical wind stress derived from a different
analysis(Stricherz et al. 1992, 1997).
In summary,the hindcastexperimentssuggestthat Ekman
pumping in the tropics rather than the arrival of subducted
anomalies from the mid-latitudes of either hemisphere
explainsthe observeddecadalvariabilityof isopycnaldepthin
the equatorialregion.
Coupled Model Results
Sincethe observedsubsurfacetemperaturerecordcomprises
only one warm and one cold event in the mid-latitudes,it is too
short to unequivocallydeterminethe importance of coupling
of the centralNorth Pacific-tothe tropicsvia anomaliesin the
thermocline.In addition, an assessmentof the role of salinity
compensatedtemperatureanomaliesand of coupling to the
southernhemispheremid-latitudesare not available from data.
To obtainmoredegreesof freedomandto investigatethe roles
of salinity compensatedtemperatureanomalies and of the
southernhemisphere, a 130 year integration of a coupled
oceanatmospheremodel(Frey et al. 1997) is considered.The
simulation producesboth realistic E1 Nifio and decadal
variability in the North Pacific (Pierceet al. 1999).
As in observations,North Pacific decadalvariability in the
0.8
O
mid-
latitude anomalies from the north (Figure 2 d)propagate into
the equatorial region (Lysne et al. 1997). However, their
magnitude,of the same order as reported by Lysne et al.
(1997), is less then 10% of the observedequatorialvalues and
explains little of the variance there. Remaining differences
betweenthe simulation and the observations, especially the
large shallowing in the early 1990s, are within the
observational
T
30øN
latitude
is largelyunchanged
(Figure2 c) in bothhemisphei'es.
The difference
13 31
-5
....
0
,
....
5
,...
10
-5
0
5
10
Log/yr
Figure 3: Lagged correlation with anomalies at 32 ø latitude of
(leftcolumn)
isopycnal
depth
anomalies
ofthe25.5kgm'3oe
isopycnal and (right column) of salinity compensated
temperature
anomalies
between
the25.0and26.0kg m'3 oe
isopycnals.Anomalieswere zonally averagedon paths (shown
in the inset of Figure 1) that are designedto capture the
varianceof temperatureon isopycnalsand that follow contours
of potential vorticity. Correlations were formed with
anomaliesequatorwards
of 32ø latitude for the northern (top)
and southern (bottom) hemisphere. Contour interval and
shading are explained by the gray scale on the right. Results
are from an extended integration of a coupled oceanatmospheremodel (Frey et al. 1997, Pierceet al. 1999).
overwhelmedby the effect of anomalousadvectionacrossthe
mean temperaturegradienton isopycnals. On the southern
hemisphere,the signal can be traced to the edge of the
equatorialundercurrent,even though the magnitudeof the
temperatureanomaliesthere are again small (1/10 to 1/20øC).
Summary
The couplingof extratropicaland tropicalPacific Oceanvia
coupledmodelis associatedwith the generationof subducted an oceanic 'bridge' involves two distinct processes, the
anomaliesthat 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 temperature
anomalies on isopycnals that are accompanied by
inset 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. This
anomaliesequatorward
of 10-15ø latitudeare independentfrom implies that tropical Ekman pumping overwhelmsisopycnal
propagatinganomaliesoriginatingin the mid-latitudes(Figure depthanomaliesthat originatefrom the mid-latitudes.Lack of
3) 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 extendedcoupled
propagateto 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
the 25.5 oe surface
alongthe subduction
path(shownin the
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 the
previously(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. These
andD. 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 the
arrivalof thermalan0maiieson isopycnalsto the stateof the
equatorial
Pacific
_n•6dsto be
modeling studies.
Oberhuber, Interdecadal variability of the Pacific Ocean: Model
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determined
fromtargetedParker,
D. E., C. K. Folland, A. C. Bevan, M. N. Ward, M. Jacksonand
K. Maskell, Marine surfacedata for analysisof climatefluctuations
Acknowledgments. This work was supportedby NSF (OCE9711265), by DOE (DE-FG03-98ER62605), by the NOAA via the
ExperimentalClimatePredictionCenter (NA77RJ0453ECPC) and via
the Consortiumfor the Ocean'sRolein Climate(NA47GP0188),and by
the EuropeanUnion's SINTEX programme.
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(ReceivedJanuary08, 1999; revisedMarch 11, 1999;
acceptedMarch 15, 1999.)