kuroshio extension variability and forcing of the paciﬁc

VOLUME 33 DECEMBER 2003J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

q 2003 American Meteorological Society 2465

Kuroshio Extension Variability and Forcing of the Pacific Decadal Oscillations:Responses and Potential Feedback

BO QIU

Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii

(Manuscript received 27 January 2003, in final form 22 May 2003)

ABSTRACT

A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region ofthe western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonalmean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trendreversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticitydynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by windstress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). Tobe specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansionsof negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomaliesnorth of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the KuroshioExtension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting theKuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescaleof ;12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it isfound that this dominant timescale is consistent with the preferred timescale under a stochastic white-noiseatmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and thelatitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the NorthPacific midlatitude coupled system on the decadal timescale.

1. Introduction

The Kuroshio Extension is the region of the westernNorth Pacific occupied by the Kuroshio after it separatesfrom the Japan coast near 358N, 1408E (e.g., Mizunoand White 1983). Figure 1, adapted from Teague et al.(1990), shows the climatological surface dynamic heightfield, in which the axis of the eastward-flowing KuroshioExtension is well represented by the 170 dyn cm con-tour. Free from the constraint of coastal boundaries, theKuroshio Extension has long been observed to be azonal jet accompanied by large-amplitude meanders andvigorous pinched-off eddies. While many past studieshave focused on the mesoscale variability of this en-ergetic zonal jet, several recent coupled modeling stud-ies have highlighted the active role the Kuroshio Ex-tension can play in generating or enhancing the decadal-to-interdecadal variability of the coupled ocean–atmo-sphere system in the midlatitude North Pacific.

Based on coupled model simulations, Latif and Bar-

Corresponding author address: Dr. Bo Qiu, Department of Ocean-ography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu,HI 96822.E-mail: [email protected]

nett (1994, 1996) show that increased advection by theKuroshio generates warm temperature anomalies in theKuroshio Extension and reduces the meridional sea sur-face temperature (SST) gradient in the central NorthPacific. This reduced SST gradient works to weaken themidlatitude westerlies, resulting in a reduction in thebasin-scale wind stress curl field. As the subtropical gyrespins down due to this changing wind stress field, thecorresponding reduction of heat transport by the Ku-roshio reverses the above effect, causing oscillations ofthe coupled ocean–atmosphere system on decadal time-scales. Using a hierarchy of coupled models with dif-ferent complexity of ocean dynamics, Barnett et al.(1999) indicate that while the modeled midlatitude de-cadal signals are driven stochastically by internal at-mospheric variability (e.g., Frankignoul et al. 1997),they are enhanced by a coupled mode that has its cou-pling center over the Kuroshio Extension region. Inclu-sion of full ocean dynamics in the coupled models isshown to increase significantly the decadal variance inthe midlatitude oceanic and atmospheric variables.

Adopting an improved version of the coupled modelused by Latif and Barnett (1994, 1996), Schneider etal. (2002) have furthered the investigation into themechanisms underlying the decadal signals in the mid-

2466 VOLUME 33J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y

FIG. 1. Surface dynamic height field (dyn cm) relative to 1000 dbar based on the climatology of Teague etal. (1990). Dashed lines denote the 4000-m isobaths, and areas shallower than 1000 m are shaded.

latitude North Pacific. In the Kuroshio–Oyashio Exten-sion (KOE) region east of Japan, they find that it is thechange in meridional position of the subtropical–sub-polar gyre boundary, in response to the varying basin-scale wind stress curl field, that is responsible for thewintertime SST anomalies in the region. Their exami-nation of the surface ocean heat budget reveals that theSST anomalies in the KOE region are controlled byvertical entrainment processes. This result of Schneideret al. is in contrast to that of Latif and Barnett (1994,1996), who found the SST anomalies in the KOE regionto be generated by anomalous Kuroshio advection ofthe warmer tropical-origin water. The importance of me-ridional displacement of the subtropical–subpolar gyreboundary in creating SST anomalies in the KOE regionis also emphasized by Seager et al. (2001) in their hind-cast modeling study of the decadal North Pacific SSTvariations. Instead of vertical entrainment, Seager et al.find that the KOE SST anomalies are largely determinedby the mean SST advection by anomalous horizontalcurrents.

By assimilating observed SST and subsurface tem-perature data into an upper-ocean mixed layer model,Kelly (2004) emphasized recently the importance of ad-vective heat transport convergence associated with theKuroshio Extension variability in determining the up-per-ocean heat storage rate in the western North PacificOcean. On the interannual-to-decadal timescales, shefound that the upper-ocean heat storage rate is betterrelated with the advective heat transport convergencethan the local surface heat fluxes.

In addition to the modeling studies, decadal modu-lations in the strength of the Kuroshio Extension arealso examined through analyses of in situ observationaldata (e.g., Yasuda and Hanawa 1997; Miller et al. 1998;

Deser et al. 1999). In particular, it is found that thestrengthening of the Kuroshio Extension in the 1980slagged 4–5 yr behind the intensification of the basin-scale surface wind forcing. This 4–5-yr lag is attributedby Miller et al. (1998) and Deser et al. (1999) to bethe oceanic adjustment time required by the baroclinicRossby waves to propagate from their excited regionin the eastern North Pacific to the western boundaryof the Pacific basin. From their data analysis, Deser etal. further show that the strengthening of the KuroshioExtension is confined to the relatively narrow band of308–388N (see their Fig. 8). Their explanation of thisconfined response is that the intensified basin-scalewind forcing increases the Sverdrup flow in the sub-tropical and subpolar gyre interior, which in turn in-creases the transport of the western boundary currentsand, subsequently, their downstream extensions. Be-cause of the data constraints, Deser et al. (1999) com-posite the observational data in the decades before andafter the North Pacific climate shift. This necessarilylimits the analysis to an averaged depiction of the Ku-roshio Extension’s response to the basin-scale windforcing.

Measurements of the global sea surface height (SSH)field by the Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeter satellite over the past decadeprovide us now with a unique opportunity to investigatethe detailed evolution of the Kuroshio Extension on theinterannual-to-decadal timescales. Fortuitously, the lastdecade is also marked by clear phase changes in thePacific decadal oscillation (PDO: Mantua et al. 1997;Zhang et al. 1997). For example, Fig. 2a shows the firstEOF mode of the surface wind stress curl field over theNorth Pacific Ocean in the last two decades. As notedby many previous investigators (e.g., Trenberth and

DECEMBER 2003 2467Q I U

FIG. 2. (a) First EOF mode of the NCEP wind stress curl anomalies (1028 N m21) during 1982–2001 over the North PacificOcean: the spatial pattern and its weighting function. This mode explains 17.8% of the total variance. (b) Time series of thePDO index anomalies during 1982–2001. Based on Mantua et al. (1997).

Hurrel 1994; Miller et al. 1994; Deser and Blackmon1995; Nakamura et al. 1997; Tourre et al. 1999), thecenter of action of this mode is located near 408N overthe eastern North Pacific and its temporal variability isdominated by decadal modulations of the Aleutian lowpressure system. The weighting function of this EOF

mode reveals a shift from predominantly negative windstress curl anomalies to positive anomalies in late 1992and a sharp transition from positive to negative anom-alies in early 1998. Both of these transitions correspondwell to the reversals in the PDO index (see Fig. 2b).

The objective of this study is to understand how the


FIG. 3. Time series of zonally averaged SSH difference, ^dh&, across the Kuroshio Extension jet. For eachT/P cycle, SSH differences are first evaluated along the ground tracks and are then averaged between 1428Eand 1808. Solid marks indicate the ^dh& values from individual cycles and its low-pass filtered time seriesis given by the solid line.

Kuroshio Extension jet responds to the basin-scale sur-face wind forcing both through the T/P measurementsand within a simple dynamic framework. We will showthat the observed decadal modulation in the zonal meanKuroshio Extension jet is attributable to the emergenceof oppositely signed SSH anomalies across the KuroshioExtension jet that are forced in the eastern North Pacificby the PDO-dominated surface wind fluctuations. Thedifference in the baroclinic Rossby wave speed betweenthe southern and northern limits of the Kuroshio Ex-tension jet is found to play a key role in generating theoppositely signed SSH anomalies. Because of the po-sition of the Kuroshio Extension jet relative to the max-imum atmospheric forcing, we will further show thatthe modulated Kuroshio Extension jet contributes to setthe timescale of the North Pacific midlatitude coupledsystem.

This paper is organized as follows. Following a briefdescription of T/P SSH data processing in the next sec-tion, we describe in section 3 the decadal modulationdetected in the zonal mean Kuroshio Extension jet andits kinetic energy field. The dynamics underlying thisobserved modulation is examined in section 4 by con-sidering wind-forced baroclinic adjustment processes.In section 5, we explore the potential roles of the mod-ulated Kuroshio Extension jet in setting the preferredtime scale of the North Pacific midlatitude coupled sys-tem. Results of the study are summarized in section 6.

2. T/P altimeter data

TOPEX/Poseidon altimeter data from the period ofOctober 1992–July 2002 (repeat cycles 2–362) are usedin this study. For all of the ground tracks crossing theNorth Pacific Ocean, the raw altimeter data are firstadjusted for various environmental corrections, includ-ing the dry troposphere, the tides, and the inverse ba-rometer effect. After these corrections, the height dataare interpolated to a common latitude grid with an along-track resolution of 5.6 km. The along-track anomalousSSH profiles are then calculated by subtracting fromeach height profile the temporally averaged profile(namely, the geoid plus the 9.8-yr mean SSH).

With the cold and dry air from the Eurasian continentoverriding the warm waters carried by the westernboundary current, the Kuroshio Extension region iswhere the largest air–sea surface heat exchanges takeplace in the Pacific Ocean (Talley 1984; Qiu and Kelly1993). As a result, a significant part of the altimetricallymeasured SSH anomaly signals in this region is due tothe seasonally varying surface heat flux forcing thatcauses expansion or contraction of the water column(e.g., Gill and Niiler 1973; Stammer 1997; Vivier et al.1999). Because these steric height changes are not ofinterest to this study, they are removed from the along-track T/P SSH anomaly signals by using the daily netsurface heat flux data from the National Centers forEnvironmental Prediction (NCEP) reanalysis (Kalnay etal. 1996). For the detailed procedure of this removal,readers are referred to Qiu (2002, his section 2). Oncethe SSH anomaly field is obtained for individual cycles(each cycle is separated by 10 days), the absolute SSHfield is taken to be the sum of the mean climatologicalSSH field (cf. Fig. 1) and the anomaly field.

3. Low-frequency variability of the KuroshioExtension

To describe the large-scale changes in the KuroshioExtension system, we first evaluate the strength of theeastward-flowing jet of the Kuroshio Extension. Figure3 shows the time series of the SSH difference, ^dh&,across the Kuroshio Extension jet averaged from 1428Eto 1808. Here, dh is calculated along individual T/Ptracks by first identifying the center of the KuroshioExtension jet (where the eastward surface current ismaximum), averaging the SSH values over the 1.58 binscentered 18 north and south of the jet’s center (hN andhS), and then taking the difference, hS 2 hN. By takingthe zonal average along the jet path (denoted here byangle brackets), the time series in Fig. 3 gives a measurefor the overall intensity of the surface Kuroshio Exten-sion jet. From Fig. 3, it is clear that the jet strength ofthe Kuroshio Extension underwent a low-frequencymodulation over the past decade. The ^dh& value was;0.65 m in late 1992 and had a decreasing trend over


the period from 1993 to 1996. The mean ^dh& value in1996 was only 0.45 m, a 30% drop from the 1992 level.This decreasing trend in ^dh& reversed after 1997 andthe overall strength of the Kuroshio Extension in recentyears has returned to the level of 1992/93.

The long-term modulation in the strength of the Ku-roshio Extension can also be seen explicitly by com-paring the SSH maps in different phases of the KuroshioExtension jet strength. For example, Fig. 4a shows theSSH field in March 1993 when the Kuroshio Extensionhad a relatively large ^dh& value. The coherent KuroshioExtension jet in this case can be recognized to extendto the date line along 348N. In contrast, the jet structureof the Kuroshio Extension was weaker and more dif-fused in March 1996, when ^dh& was relatively small(Fig. 4b). After the restrengthening of recent years, onecan again identify the coherent jet structure of the Ku-roshio Extension extending to the date line in Fig. 4c(March 2000).

One consequence of this long-term modulation in theoverall strength of the Kuroshio Extension jet is that theregional kinetic energy level also underwent a clear de-cadal modulation. Figure 5 shows the yearly averagedkinetic energy distributions estimated from the along-track SSH data by assuming geostrophy and the isotro-pic condition. As the SSH gradients are dominated bysignals of the SSH anomalies, the kinetic energy dis-tributions shown in Fig. 5 reflect largely the distribu-tions of the mesoscale eddy field. The interannual trendsfound in the kinetic energy field are similar to thoseshown in Figs. 3 and 4. Specifically, the zonal penetra-tion of the high kinetic energy band decreased persis-tently from 1993 to 1996 (Figs. 5a–d) and, over theentire Kuroshio Extension region, the kinetic energylevel was at its minimum in 1996. Following 1997, thetrend in kinetic energy level reversed and the high ki-netic energy band expanded progressively eastward(Figs. 5e–j). Like the time-varying strength of the Ku-roshio Extension shown in Fig. 3, the kinetic energylevel in years 2000–02 had similar magnitude and spa-tial patterns to those observed in 1993.

With the Kuroshio Extension jet subject to persistentbaroclinic instability (e.g., Hurlburt et al. 1996), the cor-respondence between the interannual trends in the ^dh&time series and that of the kinetic energy level is nosurprise. A larger ^dh& in Fig. 3 signifies a more intensezonal jet, whose larger surface transport and greater zonalpenetration are likely to give rise to a higher level ofeddy kinetic energy because of enhanced baroclinic in-stability. Since the large-scale changes in the mean andeddy field of the Kuroshio Extension region can exert agreat influence on the regional SST field, which in turnmay impact the overlying atmospheric circulation, a solidunderstanding of the causes for the observed changes in^dh& is clearly called for.

4. Dynamic response of the Kuroshio ExtensionTo clarify the causes for the low-frequency signals

shown in Fig. 3, it is helpful to look into the along-

stream development of the time-varying Kuroshio Ex-tension jet. Figure 6 shows the anomalies of the SSHdifference across the Kuroshio Extension jet, dh9, as afunction of time and longitude. Here, primes denotedeviations from the time average. Although noisy dueto the presence of the mesoscale eddy variability, thedh9 signals, especially those associated with the inter-annual trends in ^dh&, have a consistent pattern: dh9anomalies tend to appear first in the downstream Ku-roshio Extension region and then propagate westward.This is the case for the large, negative dh9 anomaliesthat resulted in the ^dh& minimum of 1996, and it is alsotrue for the positive dh9 anomalies that caused the in-creasing trend in ^dh& following 1997. From the dh9slopes in Fig. 6, the westward propagating speed of thedh9 signals is found to be ;0.04 m s21 (denoted in thefigure by a straight diagonal line). This estimated speedfalls in the range of the phase speed of first-mode baro-clinic Rossby waves in the latitude band of the KuroshioExtension (e.g., Mizuno and White 1983; Qiu et al.1991), suggesting a potential role played by baroclinicadjustment processes in carrying externally forced SSHsignals from the east. Given the significant PDO-relatedchanges in the surface wind stress field over the easternNorth Pacific Ocean (recall Fig. 2), it is natural to askto what extent the observed ^dh& signal could be ex-plained by the variability in the basinwide wind stressfield.

To address this question quantitatively, we adopt inthis study the 1½-layer reduced-gravity model that gov-erns the large-scale, baroclinic ocean response to surfacewind forcing. Under the long-wave approximation, thelinear vorticity equation of the model is

]h ]h g9k · = 3 t2 c 5 2 , (1)R]t ]x r g fo

where h is the SSH of interest, cR is the speed of thelong baroclinic Rossby waves, g9 is the reduced gravity,ro is the reference density, f is the Coriolis parameter,k is the unit vector in the vertical direction, and k · =3 t is the curl of the wind stress vector. Integrating Eq.(1) from the eastern boundary (xe) along the baroclinicRossby wave characteristic, we have

h(x, y, t)

x 2 xe5 h x , y, t 1e1 2cR

xg9 x 2 x91 k · = 3 t x9, y, t 1 dx9.E 1 2r g fc co R Rxe

(2)

To hindcast the h(x, y, t) field from Eq. (2), we use themonthly wind stress data from the NCEP reanalysis(Kalnay et al. 1996). The horizontal resolution of theNCEP data is 1.9048 lat 3 1.8758 lon. Along the easternboundary, h(xe, y, t) 5 0 is assumed. Justification for


FIG. 4. Sea surface height fields in (a) Mar 1993, (b) Mar 1996, and (c) Mar 2000.


FIG. 5. Yearly averaged kinetic energy distributions estimated from the along-track absolute SSH data. Contour intervals are 0.1 m 2 s22.Light-shaded areas indicate energy levels greater than 0.2 m2 s22 and dark-shaded areas indicate energy levels greater than 0.5 m2 s22.

this assumption follows Fu and Qiu (2002) who, byusing the same reduced-gravity model and analyzing theT/P data, found that the forcing along the eastern bound-ary is of little importance in determining the large-scaleSSH signals in the interior midlatitude North PacificOcean. For the latitude-dependent baroclinic Rossbywave speed cR, we use the values shown in Fig. 7. Thesevalues are estimated by calculating autospectra of theT/P SSH anomaly data in the frequency–zonal wave-

number space (e.g., Tai and White 1990) along eachlatitude of the wind stress curl grid. Similar cR valuescan be found in Fig. 14 of Fu and Chelton (2001). Noticethat the Rossby wave speed at 32.48N on the southernside of the Kuroshio Extension jet (cR 5 0.041 m s21)is nearly 2 times as fast as that on the northern side ofthe jet (cR 5 0.023 m s21 at 38.18N). This large changein cR is due not only to the difference in latitude (cR }f 22 and / 5 1.33), but also the difference in equiv-2 2f fN S


FIG. 6. SSH difference anomalies, dh9, across the Kuroshio Extension jet as a function of timeand longitude. Contour intervals are 0.2 m and shaded areas indicate positive dh9 anomalies. Thestraight diagonal line denotes the westward propagating speed of 0.04 m s21.

alent depth He on the two sides of the Kuroshio Exten-sion jet. (He is larger on the subtropical side of the jetthan on the subpolar side.) As we will find below, thisdifference in cR plays a central role in determining thetimescale of the Kuroshio Extension variability. Last,g9 5 0.027 m s22 is used in Eq. (2) as the reducedgravity value.

Figure 8 compares the yearly averaged SSH anomalyfields from the T/P data (left column) with those cal-culated from Eq. (2) (right column). As the hindcastmodel only predicts the SSH changes that are driven bythe time-varying surface wind stress signals, the com-parison is likely to be unfavorable in regions where otherdynamic processes, such as instability and eddy-meanflow interaction, contribute to the SSH signals. Asidefrom the detailed SSH structures in the vicinity of the

Kuroshio Extension path, Fig. 8 reveals that the hindcastmodel is able to reproduce many aspects of the observedSSH patterns favorably. For example, both the T/P dataand the model show that the SSH field surrounding theKuroshio Extension in 1993 is dominated by the pres-ence of positive height anomalies (Fig. 8a). From 1994to 1996, both show a progressive, westward expansionof negative SSH anomalies south of the Kuroshio Ex-tension jet (Figs. 8b–d). The westward expansion ofthese negative SSH anomalies gradually reduces theSSH difference across the Kuroshio Extension path andis responsible for the decreasing trend of ^dh& shown inFig. 3. In 1997, both the observed and modeled SSHfields are dominated by the presence of negative heightanomalies (Fig. 8e) and the westward expansion of pos-itive SSH anomalies south of the Kuroshio Extension


FIG. 7. Westward phase speed of first-mode baroclinic Rossbywaves, cR, as a function of latitude. The cR values are estimated fromautospectra in the frequency–zonal wavenumber space of the T/P SSHdata.

jet in the following years (Figs. 8f–i) is responsible forthe increasing trend of ^dh& after 1997.

To examine the geographical dependence of the mod-el’s hindcast skill, we plot in Fig. 9 the linear correlationcoefficient r between the yearly averaged T/P and mod-eled SSH fields. In general, the correlation betweenthese two fields is high, r . 0.4, in the downstreamKuroshio Extension region east of ;1608E. High cor-relation is also found south of the upstream KuroshioExtension jet. The wind-driven model, however, fails tocapture the SSH signals along the path of the upstreamKuroshio Extension and in the area to its north. Theupstream Kuroshio Extension is where the highest eddykinetic energy is observed (see Fig. 5), and the SSHsignals here are likely controlled by processes of dy-namic instability and the nonlinear interaction betweeneddies and the mean flow. Being adjacent to the Japancoast, the upstream Kuroshio Extension path and itsSSH signals are also likely affected by changes in theinflow Kuroshio condition. The area north of the up-stream Kuroshio Extension jet is where the subpolargyre western boundary current, the Oyashio, convergesfrom the north (Fig. 1). As noted by many previousinvestigators (e.g., Kawai 1972; Sekine 1988), this so-called ‘‘mixed water’’ region is frequented by anoma-lous intrusions of the subpolar waters; the governingdynamics here are clearly more complicated than thelinear vorticity relation (1).

Since the variability in the North Pacific surface windfield has its center of action near 1508W (see Fig. 2a),it is important to clarify how the SSH changes foundin the Kuroshio Extension region are related to the sur-face wind forcing centered in the eastern North Pacific.To do so, we show in Fig. 10 the time–longitude plotof the modeled SSH anomalies along 32.48N versusthose along 38.18N. These two latitudes straddle theKuroshio Extension path west of the date line. Whilechanging in amplitude along their journeys, most of theinterannual SSH signals in the west at these two latitudescan be traced to their origins in the eastern North Pacific

where the surface wind forcing is at a maximum. Noticethat in the eastern basin, the wind-forced SSH anomaliesat the two latitudes tend to have the same sign. Forexample, near 1508W, the SSH anomalies at both lati-tudes change from predominantly positive to negativein 1992 and back to positive in 1997–98. This SSH shiftin the eastern North Pacific near the center of the max-imum wind forcing is a direct response to the local windstress curl field (see Fig. 10c), with positive curl anom-alies lowering the SSH and vice versa. Due to the dif-ferent propagation speeds of the baroclinic Rossbywaves at 32.48 and 38.18N, however, Fig. 10 shows thatthe SSH signals arriving in the west no longer have thesame sign. In fact, it is the fast-propagating, positiveSSH anomalies along 32.48N (generated by the negativek · = 3 t anomalies after the 1998 PDO transition)catching up to the slow-moving, negative SSH anom-alies along 38.18N (generated by the positive k · = 3t anomalies prior to the PDO transition) that cause thelarge ^dh& across the Kuroshio Extension jet in recentyears. Similarly, arrivals of the fast-propagating nega-tive SSH anomalies along 32.48N and the slow-movingpositive SSH anomalies along 38.18N are the cause forthe small ^dh& across the Kuroshio Extension jet around1996. In this latter case, the negative SSH anomalies tothe south are generated by the positive k · = 3 t anom-alies in 1992–95, whereas the positive SSH anomaliesto the north are generated by the negative k · = 3 tanomalies prior to 1992 (see Fig. 10c).

Given the success of the wind-driven reduced-gravitymodel in simulating the large-scale SSH anomaly sig-nals over the T/P period, it is of interest to extend thehindcast to a longer time period. In Fig. 11a, we plotthe time series of ^dh9& based on Eq. (2) with the useof the NCEP monthly wind-stress curl data availablefrom 1948 to 2001. Here, ^dh9& is defined as the modeledSSH anomaly difference between 32.48 and 38.18N av-eraged from 1428E to 1808. Thus defined, ^dh9& providesa measure for the relative intensity of the zonal-meanKuroshio Extension jet. From Fig. 11a, it is clear thatthe low-frequency modulation of the Kuroshio Exten-sion jet detected by the T/P SSH measurements is byno means unique to the recent decade. Similar modu-lation can be seen to have occurred over the past 45years. A spectral analysis of the ^dh9& time series revealsthat the modulation of the Kuroshio Extension jet hasa dominant timescale of ;12 yr (see Fig. 11b).

Since the wind stress curl forcing over the midlatitudeoceans has largely a white frequency spectrum (e.g.,Frankignoul et al. 1997), a question arising naturally iswhat determines this decadal timescale dominating theKuroshio Extension jet modulation. In other words, doesthe ocean simply integrate passively the atmosphericwind forcing? Or, does the ocean dynamics play a moreactive role in selecting the decadal timescale shown inFig. 11b?


FIG. 8. (Continued on next page)


FIG. 8. Yearly averaged SSH anomalies from the T/P data (left column) vs those hindcast from the linear vorticity model forced by theNCEP monthly wind stress data (right column). Contour intervals are 0.04 m and shaded areas indicate positive SSH anomalies.

5. Kuroshio Extension modulation versus timescaleselection

To answer this question, it is relevant to first pointout that while the meridional scale of the wind stresscurl anomalies in the eastern North Pacific is as largeas 208 in latitude, the forced oceanic signals in the west

can have a much smaller meridional scale. This reduc-tion in scale is due to the latitudinal dependence of thebaroclinic Rossby wave speed and can be quantified asfollows. Assume the wind forcing in the east is periodicwith a period T and let W be the meridional lengthbetween latitudes A and B. As shown schematically in


FIG. 9. Linear correlation coefficient r between the yearly averaged SSH anomaly fields from the T/P dataand those from the wind-driven hindcast model (see Fig. 8). Shaded areas indicate where the coefficient ishigher than 0.4.

FIG. 10. SSH anomalies along (a) 32.48 and (b) 38.18N from the wind-driven hindcast model as a function of time and longitude. Contourintervals are 0.05 m. (c) Time series of the weighting function for the first EOF mode of the wind stress curl anomalies. The time series isthe same as that presented in Fig. 2a.

Fig. 12, W is located at a distance X from the initialphase line (assumed to be meridional), and it separatesthe two bordering phase lines with a phase differenceof p. If the baroclinic Rossby wave speeds at A and Bare cRA and cRB, respectively, it is easy to show

T 1 15 X 2 . (3)1 22 c cRB RA

Expanding cRB in a Taylor series around the latitude A,we have


FIG. 11. (a) Time series of the zonally averaged SSH anomaly difference, ^dh9&, between 32.48 and 38.18Nacross the Kuroshio Extension jet. The hindcast of Eq. (2) is based on the NCEP monthly wind stress curldata from 1948 to 2001. (b) Frequency spectrum of the time series shown in (a).

FIG. 12. Schematic showing the phase propagation of a mono-chromatic baroclinic Rossby wave initiated in the east. Each phaseline is separated by p, and the initial phase line (u 5 0) is assumedto be meridional. W denotes the meridional scale between the twophase lines separated by p at the distance X west of the initial phaseline.

2TcRAW 5 2 . (4)(2X 1 Tc )]c /]y |RA R A

Using the parameter values appropriate for the KuroshioExtension region: A 5 358N, cRA 5 0.03 m s21, X 54500 km (from 1508W to 1608E), and ]cR/]y | A 5 22.493 1028 s21 (from Fig. 7), and assuming decadal windforcing: T 5 10 yr, we find W 5 5.68lat. This estimatedW value agrees well with the meridional scales of theSSH anomalies found both in the model and the T/Pdata (see Fig. 8). With this shrinking of the meridionalscale in the west,1 the decadal variations in the broad-scale wind forcing of the eastern North Pacific becomehighly effective in modulating the strength of the zon-ally aligned Kuroshio Extension jet.

That the PDO-related surface wind stress forcingshould produce an effect on the Kuroshio Extension

1 Validity of the long-wave approximation requires W k pLR,where LR is the baroclinic Rossby radius of deformation. Because LR

. 35 km in the region of our interest (see Fig. 6 in Chelton et al.1998), this ‘‘smaller’’ meridional scale W 5 616 km is still muchlarger than pLR.


FIG. 13. Schematics showing the SSH phase maps under a mono-chromatic wind forcing in the east. The forcing period is short in (a),intermediate in (b), and long in (c). In the schematic, L denotes thelength of the boundary jet, C is the distance from the forcing to thecenter longitude of the jet, and A and B denote the southern andnorthern latitudes bounding the jet, respectively. Each phase contouris separated by p and the initial phase line (u 5 0) is assumed to bemeridional.

region on the scale of the jet width suggests more thana coincidental relation between the two. In fact, themagnitude of the wind-induced SSH difference acrossthe Kuroshio Extension jet, ^dh9&, is very sensitive tothe period of the wind stress curl forcing. As shownschematically in Fig. 13a, forcing with a short waveperiod (hence, a short wavelength) would be ineffectiveat producing a large ^dh9& because averaging dh9 alongthe zonal jet cancels out alongstream SSH anomalies ofopposite signs. Likewise, forcing with a too-long waveperiod (Fig. 13c) would encompass the entire zonal jet

with SSH anomalies of similar phase, resulting again insmall ^dh9&.

To quantify the dependence of ^dh9& on the forcingperiod and other physical parameters, it is useful toconsider Eq. (1) as a stochastic forcing problem (Has-selmann 1976; Frankignoul and Hasselmann 1977):

]h9 ]h92 c 5 F(x)W(t), (5)R]t ]x

where the time-varying function W(t) has a white fre-quency spectrum. For simplicity, let F(x) 5 d(x 2 xo)(namely, the forcing is localized at x 5 xo) and, withoutloss of generality, we set xo 5 0. A more realistic formof F(x) will be considered later on. West of x 5 0, theforced SSH anomaly field is simply

01 x 2 x9h9(x, t) 5 F(x9)W t 1 dx9E 1 2c cR Rx

1 x5 W t 1 . (6)1 2c cR R

Averaging h9 over the jet length L along latitude A (seeFig. 13),

2C1L /21 1 x^h9 &(t) 5 W t 1 dx,A E 1 2L c cRA RA2C2L /2

and taking the Fourier transform, we obtain

2 vL ivCˆ^h9 &(v) 5 W(v) sin exp , (7)A 1 2 1 2vL 2c cRA RA

where C is the distance from the forcing to the longi-tudinal center of the zonal jet and W(v) is the Fouriertransform of W(t). A similar expression can be writtenfor ^ &(v) by replacing cRA with cRB in Eq. (7).h9B

Taking the SSH difference across the zonal jet andassuming white noise forcing (i.e., | W(v) | 2 5 1), wefind the power spectrum for ^dh9& as follows:

2T pL pL2 2 2| ^ dh9 &(v) | 5 sin 1 sin

2 2 1 2 1 2[p L Tc TcRA RB

pL pL2 2 sin sin1 2 1 2Tc TcRA RB

2pC 2pC3 cos 2 . (8)1 2]Tc TcRB RA

In Eq. (8), 2pC/TcRB 2 2pC/TcRA denotes the phasedifference between the two sides of the jet at the jet’scenter longitude, x 5 2C. When 2pC/TcRB 2 2pC/TcRA

5 np with n being an odd integer, the SSH anomaliesacross the jet at x 5 2C have opposite signs. Althoughthis may hint at a maximum in | ^ & | 2, the actualdh9power spectrum of Eq. (8) is more complicated becauseof the averaging over the jet length. Notice that, in the


FIG. 14. Power spectra of | ^ & | 2, defined in Eq. (8), as a functiondh9of the forcing period T. Solid lines show the result with parametervalues most appropriate for the North Pacific Ocean: L 5 408, C 5508, A 5 32.48N, and B 5 38.18N. Dashed lines show the resultswhen (a) the jet length L is adjusted, (b) the distance between theforcing and the longitudinal center of the jet C is adjusted, and (c)the latitudes bounding the jet, A and B, are adjusted.

high-frequency forcing limit, the power spectrum hasan upper bound:

24T2| ^ dh9 &(v) | # , (9)

2 2p L

which decreases with decreasing T. On the other hand,in the low-frequency forcing limit (i.e., when T is muchlonger than the transit time by baroclinic Rossby wavesover the distances C and L), Eq. (8) simplifies to

22 24p C 1 12| ^ dh9 &(v) | . 1 1 2 , (10)

21 21 2c c T c cRB RA RB RA

which decreases with increasing T. Clearly, there is anoptimum value of T between these two limits for which| ^ & | 2 is a maximum.dh9

In Fig. 14, we plot the power spectra of Eq. (8) as afunction of the forcing period T. Solid lines (identicalin Figs. 14a–c) show the result when the parameter val-ues are chosen appropriate for the midlatitude NorthPacific: L 5 408 (from 1408E to 1808), C 5 508 (at1608E relative to the maximum forcing at 1508W), A5 32.48N, and B 5 38.18N. With these nominal values,the maximum modulation in ^dh9& occurs when T . 10yr. Modulation in ^dh9& is ineffective when T , 5 yr,and the spectral power also decreases rapidly when Texceeds 20 yr.

Notice that this ‘‘preferred’’ forcing period is not verysensitive to the detailed chosen parameter values. Forexample, Fig. 14a compares the results when the nom-inal zonal length of the jet, L 5 408, is changed to either508 or 308 (the other parameters are kept at their nominalvalues). Although the maximum responding periodshifts with changing L values, the range of this shift isrelatively small, about 62 yr. That the preferred forcingperiod lengthens with increasing L is because averagingover progressively longer values of L cancels out thephase extremes of progressively longer waves. At thelow-frequency forcing limit, Fig. 14a shows that| ^ & | 2 becomes independent of L, a result consistentdh9with Eq. (10). The preferred forcing period, as shownin Fig. 14b, is also relatively insensitive to deviationsfrom the chosen value of C. This is so because for theparameter values of our interest, phase lines of the SSHanomalies in the west are nearly zonal, making changesin C inconsequential. For T $ 10 yr, Fig. 14b showsthat the spectral power increases with increasing C [seeEq. (10)], suggesting that forcing farther to the east ismore effective in inducing large ^dh9& because the phaselines become increasingly better aligned with the zonalaveraging box. Figure 14c compares the cases when thelatitudes bounding the jet (A, B) are changed from(32.48, 38.18N) to either (34.38, 38.18N) or (32.48,36.28N). The preferred forcing period remains near 10yr in the (34.38, 38.18N) case, whereas it shifts to ;8yr in the (32.48, 36.28N) case. This shift to a shorterpreferred period is due to the fact that the zonal jet inthis latter case is located at a more southerly latitude,

which shortens the required baroclinic adjustment time.Under the low-frequency forcing, the two cases with anarrower jet width have a smaller spectral power thanthat in the nominal case. As shown in Eq. (10), this isbecause | ^ & | 2 in this limit is proportional to cRA 2dh9cRB, which is smaller for a more narrowly defined jet.

For tractability of analytical expressions, we have inthe above assumed the spatial function of the forcingin Eq. (5) to be a d function. While centered over the


FIG. 15. Power spectra under the forcing with the spatial functionF(x) 5 d(x 2 xo) vs F(x) 5 exp[2(x 2 xo)2/a2]. In the latter case,a 5 158 and a 5 258 are used, respectively. For comparison, thespectral power in each case has been normalized by its respectivemaximum value. In all cases, L 5 408, C 5 508, A 5 32.48N, andB 5 38.18N are used.

FIG. 16. Monthly SST anomalies averaged in the Kuroshio Extension region of 328–388N, 1418E–1808. The SST data arecompiled by Reynolds and Smith (1994).

eastern North Pacific Ocean, Fig. 2a shows that the windstress curl forcing does have a sizable width in longi-tude. To examine how this ‘‘broad width’’ forcing mayalter the spectral pattern of | ^ & | 2, it is useful to as-dh9sume that F(x) has a more realistic Gaussian distribu-tion:

2 2F(x) 5 exp[2(x 2 x ) /a ],o (11)

where a denotes the longitudinal e-folding scale of theforcing. For the North Pacific case (Fig. 2a), a is in therange of 158–258. In Fig. 15, we compare the powerspectrum shapes when a is, respectively, 158 and 258,with that produced by the d-function forcing. In all threecases, A, B, C, and L are fixed at their nominal valuesappropriate for the North Pacific. In general, as a length-ens, the ‘‘preferred’’ timescale tends to shift to a longerperiod and its spectral power relative to the lower-fre-quency signals tends to decrease. (The normalizedcurves in Fig. 15 represent different values of integratedwind stress, and so only relative spectral shapes aresignificant.) It is interesting to note that the spectral

shape with a 5 158 bears a good resemblance to the| ^ & | 2 shape of Fig. 11b, in which the strength of thedh9Kuroshio Extension jet was estimated using the realisticwind stress curl forcing of 1948 to 2001.

This resemblance in spectrum pattern and the ro-bustness of the preferred forcing period with regard tothe detailed parameter values strongly suggest that theKuroshio Extension jet does not respond passively tothe atmospheric forcing over the eastern North Pacific.Rather, through the jet modulation, the differential Ross-by wave lag times preferentially amplify the wind stresscurl forcing at the decadal timescale. Located where thelargest heat loss from the ocean to the atmosphere oc-curs, the modulated Kuroshio Extension can in turnchange the regional SST field and affect the overlyingatmospheric jet stream with the same preferential time-scale. Indeed, the observed SST anomalies in the Ku-roshio Extension region have clear decadal signals (e.g.,Fig. 16; see also Schneider and Miller 2001), and recentstudies by Qiu (2000) and Vivier et al. (2002) haveindicated that these anomalies during the T/P period arein part caused by the low-frequency changes of the Ku-roshio Extension jet.

With its long memory, the ocean has been hypothe-sized by many recent studies to be responsible for settingthe timescales of the midlatitude climate variability.Thus far, the proposed ocean processes include 1) lim-ited resonance of the midlatitude baroclinic Rossbywaves with basinwide stochastic wind forcing (e.g., Jin1997), 2) delayed oscillation with the timescale of delayset by the adjustment of the wind-driven gyre or thethermohaline circulation (e.g., Marshall et al. 2001), 3)combination of Rossby wave delay and advection bythe mean subtropical/subpolar gyre circulations (e.g.,Cessi 2000; Primeau and Cessi 2001), and 4) internalocean dynamics, such as self-sustained oscillations ofwestern boundary current recirculation gyres (e.g.,Dewar 2001). The results presented above suggest thatthe preferred, decadal modulation of the Kuroshio Ex-tension jet responding to the remote atmospheric forcingin the eastern North Pacific can be another potential


process that contributes to set the timescales of the NorthPacific midlatitude climate system. While the baroclinicRossby waves are an important part of this process, theselected timescale is influenced by the characteristics ofthe modulated Kuroshio Extension zonal jet: its latitu-dinal position (which determines cRA and cRB), its co-herent zonal width (L), and its longitudinal position rel-ative to the dominant surface wind forcing (C).

6. Conclusions

Using the available sea surface height data measuredby the TOPEX/Poseidon satellite over the past decade,we investigated the large-scale changes in the KuroshioExtension region of the western North Pacific. The dom-inant low-frequency signal of the region was charac-terized by a modulation in the strength of the zonal meanKuroshio Extension jet: the jet showed a progressivelyweakening trend from 1993 to 1996 and this trend re-versed after 1997. Both the weakening and the re-strengthening of the jet are found to appear first in thedownstream Kuroshio Extension region in the east andto progress westward at the speed, ;0.04 m s21, of thelong baroclinic Rossby waves.

To explore the dynamics underlying the observedmodulation of the Kuroshio Extension jet, we examinedthe baroclinic ocean responses under realistic surfacewind forcing. Hindcasting the SSH signals based onlinear vorticity dynamics, we found that the weakeningtrend in the Kuroshio Extension jet in 1993–96 wascaused by westward expansions of negative SSH anom-alies south of the Kuroshio Extension and positive SSHanomalies north of the Kuroshio Extension. Theseanomalies had their origins in the eastern North Pacificand were generated by positive and negative wind stresscurl anomalies before and after 1992, respectively. Incontrast, westward expansions of the positive SSHanomalies to the south of the Kuroshio Extension jet(caused by the negative wind stress curl anomalies after1997) and the negative SSH anomalies to the north ofthe Kuroshio Extension jet (caused by the positive windstress curl anomalies prior to 1997) resulted in astrengthening trend in the zonal mean Kuroshio Exten-sion jet following 1997. Both of the sign changes in thewind stress curl anomaly field in 1992 and 1997 cor-responded to the phase changes of the Pacific decadaloscillations.

The fact that the low-frequency changes in the Ku-roshio Extension jet are forced directly by the interiorwind forcing has several implications. First, it indicatesthat the changes in the zonal mean Kuroshio Extensionjet on the decadal timescales can be independent of theupstream Kuroshio variability in the East China Sea andsouth of Japan. Evidence supporting this notion can beseen in Imawaki et al. (2001), who found no systematicchanges in the Kuroshio transport south of Japan duringthe T/P period. Dynamically, this implies that changesin the wind-driven Sverdrup interior flow, which influ-

ences the upstream Kuroshio along the western bound-ary through mass conservation, are not an importantfactor for the midlatitude North Pacific ocean–atmo-sphere coupling. Thermodynamically, this implies thatSST anomalies in the Kuroshio Extension region, whichthe atmospheric circulation is sensitive to on decadaltimescales (e.g., Peng et al. 1997; Barnett et al. 1999),are more likely influenced by advection of mean tem-perature by the anomalous circulation than by advectionof temperature anomalies by the mean western boundarycurrent. Evidence supporting this notion can be foundin recent upper-ocean heat budget studies for the Ku-roshio Extension region by Qiu (2000), Xie et al. (2001),and Vivier et al. (2002).

By hindcasting the Kuroshio Extension jet strengthover the past 45 years using the available NCEP windstress curl data, we found that the large-amplitude mod-ulation of the Kuroshio Extension jet is not unique tothe recent T/P decade. Low-frequency changes with adominant period at ;12 yr have recurred throughoutthe past 45 years. To clarify the potential roles of theocean dynamics in determining this preferred timescale,we examined the baroclinic adjustment processes understochastic atmospheric forcing. We found that thestrength of the Kuroshio Extension jet modulation de-pends on the frequency, as well as the jet geometry, sothat the response to a white-noise, remote wind forcingis amplified at a preferential timescale. With parametervalues appropriate for the North Pacific Ocean, the max-imum responding amplitude is obtained when the forc-ing period is ;10 yr. At shorter forcing periods, phasesof the wind-induced SSH signals across the KuroshioExtension jet change rapidly along the jet length, de-creasing the coherent modulation of the zonal mean Ku-roshio Extension jet. At longer forcing periods, the smallSSH difference across the jet (because of the broadlength scales of the wind-induced SSH signals) againsuppresses the modulated amplitude of the KuroshioExtension jet. We suggest that this preferred responseby the Kuroshio Extension jet due to the differentialRossby wave delays contributes to an increase in var-iance and persistence of the coupled climate system ofthe North Pacific on the decadal timescale.

Acknowledgments. This study benefited from fruitfuldiscussions with Ted Durland, Lee Fu, Fei-Fei Jin, andNiklas Schneider. Detailed comments made by Art Mill-er and two anonymous reviewers helped to improve anearly version of the manuscript. The surface wind stressand heat flux data were provided by the National Centersfor Environmental Prediction, and the TOPEX/Poseidondata were provided by the Physical OceanographyDAAC at Jet Propulsion Laboratory. Support fromNASA (Contracts 1207881 and 1228847) and NSF(OCE-9911268) is gratefully acknowledged.


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