Seasonal axis migration of the upstream Kuroshio Extension associated with standing oscillations

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C8, PAGES 16,685-16,692, AUGUST 15, 2001 Seasonal axis migration of the upstream Kuroshio Extension associated with standing oscillations Hiroaki Tatebe and Ichiro Yasuda Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo Tokyo, Japan Abstract. The TOPEX/Poseidon altimeter data from October 1992 to October 1998 are analyzed to investigate the seasonal cycle of the sea surface height (SSH) in the upstream region of the Kuroshio Extension. After removing steric height changes caused by surface buoyancy fluxes, two maxima of the seasonal SSH amplitude are found around the crest and trough of the quasi-stationary meanders as standing waves, accompanied with nodes of amplitude minima. Seasonal variations are identified in the subsurface temperature at 400 m depth similar to those of SSH in amplitude and phase. Associated with the standing waves, the current follows a straight path in fall and then changes to a meandering path in spring. From a composite map of the seasonal Kuroshio Extension axes, nodes of minimum displacement are found. One node is located over the Izu Ridge, while the others are located in the areas where meridional acceleration is large. The seasonal axis migration of the upstream Kuroshio Extension is regulated by these nodes. 1. Introduction Once the Kuroshio leaves the coast of Japan, flowing eastward into the North Pacific, it is known as the Kuroshio Extension. Quasi-stationary meanders are known to be present near 144øE and 150øE in the Kuroshio Extension [e.g., Kawai, 1972; Mizuno and White, 1983]. Warm and cold core rings are spawned from the unstable meanders in this highly variable current [e.g., Bernstein and White, 1977; Tai and White, 1990; Nishida and White, 1982]. Mizuno and White [1983] first investigated annual and interannual variability over the entire Kuroshio Current system using temperature data at 300 m depth from June 1976 to May Several maxima of root-mean-squared (rms) amplitude are located near the crests of the quasi-stationary meander path around (38øN, 144øE) and (36øN, 150øE) and the Shatsky Rise. The positions of these maxima are regulated by nodes of rms amplitude minima (Figure 1).The two maxima of the quasi-stationary meander behave as standing waves accompanying the meridional shifts of the current axes. They suggested that these standing waves result from dynamical process within the Kuroshio Current system: (1) the wavelength change of the Rossby-lee wave ex- Copyright 2001 by the American Geophysical Union. Paper number 2000JC /01/2000JC cited by the interaction of the Kuroshio with the Izu Ridge, and (2) the bifurcation of the Kuroshio Extension around the Shatsky Rise. Baroclinic Rossby waves explain the westward propagation observed east of 155øE. No dynamical balance explains eastward prop- agation west of 155øE. Two studies [Qiu et al., 1991; Qiu, 1995] using the TOPEX/Poseidon (T/P) altimeter estimate the mean and instantaneousea surface height (SSH) in the Kuroshio Extension with the method of Kelly and Gille [1990]. The axis position of the Kuroshio Extension averaged between 140 ø E and 180 ø E shifts northward in fall, which is coincident with the surface transport maximum in fall. Significant variations of the surface transport are detected in the region from 141øE to 150øE. Qiu et al. [1991] and Qiu [1995] also found standing waves or a slow eastward propagating wave in the Kuroshio Extension west of 154øE and between 33øN and 37øN. In the region east of 154øE, westward propagating signals are reported. Similar to Mizuno and White [1983], they explained these waves as baroclinic Rossby waves. On the other hand, standing waves in the SSH cannot be explained immediately by the dynamics of the Kuroshio Current system because standing oscillations of the SSH contain steric height changes due to surface buoyancy flux. Wang and Koblinsky [1995, 1996] reported that the steric height is at a maximum in fall and minimum in spring with horizontally homogeneous phase. Large-amplitude standing oscillations are ob- 16,685

2 16,686 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION (a) 40 N 140' 1 )o- ITO' I " LONGITUOE Figure 1. The total rms amplitude of the temperature anomaly (øc) at 300 m depth. (b) Schematic diagram of the Kuroshio Current variability. The solid circles in this figure represent nodes of rms amplitude minima. Adapted from Mizuno and White [1983]. served near the Kuroshio Extension and its southern recirculation gyre. In the Gulf Stream region the annual cycle of the SSH is mainly explained by the thermodynamics within the mixed layer [Wang and Koblinsky, 1995; Kelly et al., 1999]. In the present study, standing oscillations of the annual SSH associated with the axis migration of the Kuroshio Extension in the upstream region west of 150øE are investigated using T/P altimeter data. To determine that seasonal cycles are not confined to the mixed layer, seasonal variability of the 400 m isotherm is examined. This paper is organized as follows: In section 2 the data processing and analysis methods are described, in section 3 we report the main results, and the discussion is found in section Data Processing We focus on the Kuroshio Extension, extending from 25øN to 40øN and from 136øE to the date-line. SSH data from the T/P altimeter from October 1992 through October 1998 are used. The raw altimeter data are corrected for all the environmental biases [Callahan, 1993], aliased M2 and $2 tidal energy [Mitchum, 1994], and small-scale instrument error [Qiu, 1995]. Residual SSH is computed along the T/P ground tracks, which are then mapped onto a 0.5øx0.5 ø regular grid. We use a Gaussian mapping method with a 2 ø decorrelation scale and a 4 ø cutoff radius in the zonal direction and a 1 ø scale with a 2 ø cutoff in the meridional direction. The mean SSH field used in this study is constructed from the kinematic jet model introduced by Qiu [1995]. We estimate the SSH changes due to the seasonal cycle of the sea surface buoyancy flux [Stammer, 1997] using monthly climatological surface heat flux data Ida Silva et al., 1994] and annual mean temperature and salinity data [Levitus and Boyer, 1994; Levitus et al., 1994]. To derive heat flux data on an individual T/P cycle, the annual harmonic is fitted using a least squares method. The depth of the mixed layer is set at 150 m in the Kuroshio Extension [Wang and Koblinsky, 1996]. In steric height changes due to local heat fluxes, the effect of mixed layer depth appears implicitly through the thermal expansion coeificient. The thermal expansion coeificient above 150 m is evaluated from Table A3.1 of Gill [1982] using Levitus climatology, without pressure dependence. The area-averaged amplitude of the annual heat flux is about 230 Wm -2 in the study region; this induces a SSH change of about 13 cm with a maximum in October and minimum in May. The amplitude and phase of this steric height change are consistent with the results by Stammer [1997]. Next, high-frequency variability (periods shorter than 4 months) and a linear trend of decadal to interdecadal

3 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION 16,687 variations [Wang and Koblinsky, 1998] are removed by a low-pass filter and a least squares fit. Then, to consider propagating wave information, we use complex empirical orthogonal function (CEOF) analysis. Mizuno and White [1983] used EOF analysis to study annual and interannual variability of the subsurface temperature field. However, since EOF analysis can only interpret time series as independent standing modes, it may not be adequate to examine variations which contain both standing and propagating waves. The beginning and final 10% of the temporal T/P cycle are not used in CEOF analysis. After removing steric height changes, the annual variations which have locally distinct amplitude in the vicinity of the upstream Kuroshio Extension axis and around the Shatsky Rise appear in the first CEOF mode (not shown). Tl ese maxima with spatially small scales are not caused by basin-wide surface heat flux and therefore we assume that the annual standing wave of steric origin is largely removed. However, zonally homogeneous variations which peak in spring remain in the annual SSH. Since we use the monthly heat flux climatology from 1945 to 1989 [da Silva et al., 1994], interannual intensification and long-term variations of seasonal steric height changes cannot be included in our analysis. The zonally coherent structure might result from uncertainties in the heat flux data and/or may be induced by large-scale changes of wind stress curl through Ekman pumping. To reduce the estimation error of steric height changes and focus on the dynamical variations of the Kuroshio Extension, the zonal mean SSH is subtracted from the annual SSH represented by the first CEOF mode at each latitude. Comparing the annual SSH anomaly to subsurface variability, we use the temperature data at 400 m depth which are supplied by the Japan Meteorological Agency and were originally obtained in the Subarctic Gyre Experiment (SAGE). These data consist of monthly mean fields from January 1993 to December 1998 with horizontal resolutions of 1 ø longitude and 0.5 ø latitude. 3. Results 3.1. Annual Cycle of the SSH Anomaly Previous studies report that in the Kuroshio Extension and the Gulf Stream region, basin-wide standing oscillations due to air-sea heat exchange dominate the annual signal of the SSH [Wang and Koblinsky, ]. These annual signals have relatively large ampli- tudes in the Kuroshio Extension and its southern recir- culation gyre with fall maximum. In the present study, similar results are obtained for the first CEOF mode, if the steric component is not subtracted from the SSH. On the other hand, the spatial amplitude and phase distributions of the first CEOF mode, which also represents an annual cycle (Figire 2), differ in the case where steric height changes are removed. In the upstream region of the Kuroshio Extension, west of 150øE, two maxima of SSH anomaly amplitude are found near the crest (35øN, 143øE) and trough (34øN, 146øE) of the quasi-stationary meanders (Figure 2c). Around the Shatsky Rise (31ø-36øN, øE), another maximum is observed. These maxima are accompanied with nodes of amplitude minima which are located on both sides of each maxima. (c) Spatial Amplitude 38N 30N (a) Temporal Amplitude asn 1... (d) Spatial Phase 0.5 asn..' ---:.:-..' N ' ':. -.;:'.:< - '". 7'"." ' - : - - -' '..'. ovl, -. / E 145E 150E 155E 160E 165E I?OE 175E Time Figure 2. The temporal (a) amplitude and (b)phase and the spatial (c) amplitude and (d) phase of the first complex empirical orthogonal function (CEOF) mode of the sea surface height (SSH) anomalies without steric height changes. This mode accounts for 33% of the total variance. In Figure 2d, waves propagate in the direction from darker to lighter shading.

4 ß 16,688 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION Nov, Sep, Jul. May Mar, Jail 14bs 14'5E (a) i::'.$i:."::':::i:::!-'s!.'.:: -'::-:': :i:.::?&'::': : :i:i:: : -'::..::i:-z-:.%'.'-',.':i: 150E 155E 160E 165E 170E 175E (b) Nov :***'-' ' ""/"'"'" i:i,. :?..!!. : i...--'.-.:.-..':! :: " ::ii ::'[...[: ' '""***'*:..-':::i:i :,'.:i.'.-:'.,.. '..;. "' 140E 145E 150E 155E 160E 165E 170E 175E Figure 3. Time-longitude plots of the monthly climatological SSH anomalies which are meridionally av- and (c) 30øN - 33øN. The monthly anomalies are reconstructed from the first CEOF mode. In all panels the contour intervals are 2 cm, and the negative values are shaded. From the spatial phase distribution in Figure 2d, propagating signals are apparent (note that waves propagate in the direction from darker to lighter shading). In the area east of 155øE and south of 30øN, westward propagation is observed. Wang and Koblinsky [1998] showed SSH variability associated with baroclinic Rossby waves near major topographic features, such as the Shatsky and the Hess Rises. They proposed that local vorticity changes due to external forcing may play an important role in generating annual Rossby waves over topography. The maximum at the Shatsky Rise (31-36øN, øE) observed in the present study may be explained as baroclinic Rossby waves excited near topography as hypothesized by Wang and Kobhnsky [1998]. In the upstream portion of the Kuroshio Extension, the phase distribution is complicated, and the directions of wave propagations are vague. In order to examine wave characteristics, we look at monthly climatological SSH anomalies reconstructed from the first CEOF mode shown in Figure 2. Figure 3 shows time-.longitude plots of monthly SSH anomalies, meridionally averaged in the Kuroshio Extension band (34øN-36øN; Figure 3b) and on both its northern (37ø-39øN; Figure 3a) and southern sides (30ø-33øN; Figure 3c). Two standing waves around 143øE and 146øE are seen in the upstream portion of the Kuroshio Extension band (Figure 3b). These waves correspond to the amplitude maxima near the crest and trough of the quasi-stationary mean- der (Figure 2c). Near 146øE the annual SSH anomaly is maximum in fall, while near 143øE the annual cycle has a spring maximum. In addition, a third standing wave is found around 153øE. This standing wave links to the 140E 145E 150E 155E 160E 165E 170E 175E baroclinic Rossby wave propagating westward from the (½) ß :-:,::-:-:.:? i.:-:- --:.:.1:., ;...:... x.l,l.s :..-:r eastern region. On the southern side of the Kuroshio Extension band, west of 142øE, eastward propagation with relatively small amplitude is observed. This signal peaks in summer and propagates from the Kuroshio south of Japan to the Kuroshio Extension region, similar to Mizuno and White [1983] and Qiu et al. [1991]. On the north- ':"e- --"' 74:'.'.- -,.'".. '..-' i! ii i --,...-' 4!::.':'2..'.:-:.':-.,: ::':.'.':...,....-'.-t: [ ii::'-'"': :..'.-'"iii :.:-'...-:... ern side of the Kuroshio Extension, both standing and propagating waves are observed, but these have smaller amplitude than in the Kuroshio Extension band and to the south. Next we compare the annual SSH variability observed around the quasi-stationary meanders with the subsurface temperature at 400 m depth, which is not affected by mixed layer thermodynamics. Figure 4 shows a time- I'"""' Sep]... '"--'"- Jul.[ ' May[..;.: & Mar... J an. 140E 145E 150E 155E 160E 165E 170E 175E Figure 4. Time-longitude plot of the monthly climatological temperature anomalies at 400 m depth, where the temperature anomalies are meridionally averaged between 34øN and 36øN. The contour interval is 0.1øC and negative values are shaded.

5 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION 16,689 Spring 36N Fall 36N 13õE 140E 142E 144E 146E 14õ 150E 152E 154E 156E 158E 160E 13'8E 14'OE 14'2E 14'4 14'6E 14'õ 15'OE 15'2 15'4 15'õE 15'8E 16'0 - ) Summer Winter 34N 3aN 33N! E 140E 142E 144E 146E 148E 150E 152E 154E 156E 158E 160E 138E 140E 142E 144E 146E 148E 150E 152E 154E 158E 158E 160E ---) Figure 5. Seasonal maps of the SSH fields, along with vectors indicating horizontal accelerations without steric height changes. The geostrophic balance is applied to calculate the velocity. The contour intervals are 5 cm, and the thick curves denote the contour of 238 cm. The unit of arrows is 1.0x 10-6cm s -2. longitude plot of the monthly climatological temperature anomaly at 400 m depth in the Kuroshio Extension band between 34øN and 36øN, similar to Figure 3b. High-frequency variability was removed with a 5 month running-mean filter. In the upstream region of the Kuroshio Extension, the annual cycle appears clearly near 143øE and 146øE, where annual standing 3.2. Seasonal Axis Migration and Transport Variability of the Kuroshio Extension Using the SSH fields, the current axis migration of the Kuroshio Extension west of 150øE is examined. Seasonal maps of the SSH (Figure 5) are obtained as a sum of the mean field from the synthetic method of Qiu [1995] and the monthly anomalies are reconstructed from the first CEOF mode without steric height changes. The path of the Kuroshio Extension meanders in spring, but is straight in fall, corresponding to the temporal evolution of the two standing waves shown in Figure 3b. In Figure 5 the horizontal acceleration vectors have a maxima in summer and form a cyclonic eddy around the waves were observed in the SSH (Figure 3b). Subsurface crest (35øN, 143øE) and an anticyclonic eddy around temperature anomalies are maximum in spring around 143øE and late fall near 146øE with a larger amplitude. Both the annual cycle of SSH (Figure 3b) and subsurface temperature have amplitude maxima near 143øE and 146øE, which behave as standing waves with opposite phase. Around 146øE the annual signal of subsurface temperature lags that of the SSH variation by 2 months. The reason for this lag is unclear. In summary, two maxima of the annual cycle of SSH are found near the crest (35øN, 143øE) and trough (34øN, 146øE) of a quasi-stationary meander of the the trough (34øN, 146øE). Associated with these seasonal variations of the eddy pair, large meridional velocity changes occur in between and near the edges of the eddy pair, around 145øE and 148øE. These seasonal changes act to weaken the mean southward (northward) velocity in summer near 145øE (148øE). As a result, the path of the Kuroshio Extension straightens from spring to fall, and develops meanders from fall to spring. The seasonal intensification of the southern recirculation gyre (30ø-34øN, 149ø-155øE) has a larger (narrower) zonal width in summer and fall (win- Kuroshio Extension, after removing steric height changes. ter and spring). This seasonal width change seems to The SSH and subsurface temperature exhibit similar patterns in the amplitude of the annual cycles, but subsurface temperature may lag the SSH signal. Both curbe induced by the annual standing wave around 153øE (Figure 3b), where westward propagating waves are arrested. rent variability and mixed layer processed play important roles in the upstream Kuroshio Extension. The axis of maximum current is correlated with the 238 cm SSH contour in the upstream region and is used as an indicator of the axis position at each longitude. The current axes are fixed clearly at several nodes (Fig- ure 6). As shown in Figure 5, large horizontal accelera- tions occur near nodes 2 and 3. Node I is located over the Izu Ridge. Around the crest of the quasi-stationary meander (35øN, 143øE) between nodes I and 2, the axis shifts northward in spring. In contrast, near the trough (34øN, 146øE) between nodes 2 and 3, the axis shifts

6 16,690 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION N n 35N 34N 33N...., '8E 14'0E 14'2E 14'4E 14'6E 14'8E 15'0E 15'2E 15'4E Figure 6. Composite of the Kuroshio Extension axes. The arrows in this figure indicate the location of amplitude nodes. The thick solid (dashed) curve denotes the axis in fall (spring), and the thin solid (dashed) curve denotes the axis in summer (winter) Jan Feb Mar Apr May Jun Jul Aug Sep 0cf Nov Dec mont, h Figure 8. Same as Figure 7, but for the surface transport variations. I northward in fall with about 2 times larger amplitude of the migration than the one near the crest. A time series of the zonally averaged (141ø-150øE) axis position of the upstream Kuroshio Extension shows that the current axis is located in the north from sum- mer to fall in the case without the steric component (solid line in Figure 7). With steric height changes, the axis tends to be positioned north of the mean in fall (dashed line in Figure 7). In both cases the axis migration is dominated by the locally large meridional shift around the trough of the quasi-stationary meander near 146øE. Figure 8 shows zonally averaged (141ø-150øE) surface transport variations of the Kuroshio Extension. The transport at each longitude is estimated as a sum of the zonal velocities within the meridional e-folding distance from the current axis. In the case without steric height change (solid line), the surface transport is larger (smaller) in summer and fall (winter and spring). However, this seasonal cycle of the surface transport has a relatively small amplitude, compared with the case including steric height change (dashed line), in which maximum transport occurs in fall , o \ \ \ monlh Figure?. Seasonal variations of the axis positions which are zonally averaged between 141øE and 150øE. The solid line denotes the case without steric height changes, and the dashed curve denotes the case with steric height changes. The seasonal cycle of the zonally averaged (141ø-150 ø E) absolute velocity at the current axis is largely dependent on the steric component (Figure 9). In the case without steric height changes (solid line), the current speed is maximum in spring and minimum from late summer to fall. In contrast, in the case with the steric component (dashed line), the current speed is maximum in fall and minimum in spring. From the seasonal SSH maps (Figure 5), the jet width of the upstream Kuroshio Extension tends to become broader from spring to fall. In the case without the steric component, the effects of zonal velocity changes and seasonal width changes of the jet seem to cancel. As a result, surface transport variations may be less than the ones with steric height changes (Figure 8). Qiu [1995] reported that in the zonally broad extent from 140øE to 180 ø, the surface transport of the Kuroshio Extension is relatively large in fall and winter and the axis shifts northward in fall coincident with the transport. However, in Qiu's [1995] analysis, the contributions of current variability and steric height changes to the SSH signal are not examined separately. In the present study, focusing on the upstream Kuroshio Extension west of 150øE, a relation between the surface transport and the axis migration is found in the case without the steric component (Figures. 7 and 8), indicating thermodynamic effects are not dominant '% '- 46 X 44 f j Jan Feb Mar Apr M y Jt n J l A g 2 p 0'ct N v Dec mon[h Figure 9. Same as Figure 7, but for the current speed at the axis of the Kuroshio Extension.

7 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION 16, Summary and Discussion these questions. For example, on the basis of the time- Seasonal axis migration and the corresponding stand- dependent path equation with 1 l layers, P att [1988] ing waves in the quasi-stationary meander of the upand Cushman-Roisin et al. [1993] proposed that stationary meanders of a narrow jet away from a lateral stream Kuroshio Extension have been investigated using the TOPEX/Poseidon altimeter data from October boundary occur at a threshold wavelength defined as 1992 through October After extracting the anthe wavelength at which westward phase propagation of Rossby waves equals eastward propagation of the jet's nual cycle of SSH with CEOF analysis and removing meander. In the present study we observe a maximum steric height changes due to air-sea heat exchanges aceastward propagating signal in the Kuroshio Extension cording to Stammer [1997], two maxima of seasonal SSH region which peaks in summer but originates earlier variability associated with standing waves are found in the Kuroshio south of Japan (Figure 3c). Similar around the crest (35øN, 143øE) and trough (34øN, 146øE) of the quasi-stationary meander path. The seasonal axis eastward propagation was also reported by Mizuno and migration of the upstream Kuroshio Extension west of White [1983] and Qiu et al. [1991]. This signal may play 150øE is regulated by SSH nodes on either side of the a role in triggering the annual change of the upstream Kuroshio Extension. maxima. Also, the seasonal intensification of the southern re- Comparing this annual SSH with subsurface temper- circulation gyre might be responsible for the upstream ature at 400,n depth, we find similarities in both the current variability through baroclinic Rossby wave dyamplitude and phase. As reported previously in the namics. As noted in section 3.2, the zonal width of the Gulf Stream region, the annual variability of SSH is recirculation gyre located between 149øE and 155øE is mainly explained by the thermodynamics in the mixed largest in summer and fall. In Figure 3b the westward layer [Wang and Koblinsky, 1995; Kelly et al., 1999]. propagating baroclinic Rossby waves become station- On the other hand, in the Kuroshio Extension region, ary around 153øE. These stationary waves appear to the dynamical role of the current variability is signifi- result from the Doppler shift induced by the mean eastcant in addition to the mixed layer process. ward flow. The seasonal changes of the horizontal ve- The temporal evolution of the standing waves around locities around 153øE might be induced through these the quasi-stationary meander indicates relatively large processes. Accordingly, the zonal width of the southern meridional velocity changes from 145øE to 148øE. This recirculation gyre is affected by this velocity change, seasonal change of the meridional velocity acts to weaken and annual variations of the upstream current might be the mean southward (northward) velocity in summer produced. near 145øE (148øE). Large values of temporal rates of Although recent satellite altimeter data with high changes of lateral velocities occur near the two eastern spatial and temporal resolution are available in the nodes of the annual SSH amplitude, straightening the Kuroshio Extension, observations of internal current path of the Kuroshio Extension from spring to fall. structure are limited. To specify the dynamical pro- The zonally averaged surface transport (between 141 ø cesses which induce the seasonal axis migration of the E and 150øE) is larger in summer and fall than in Kuroshio Extension, further observational and modelwinter and spring. When the surface transport is rel- ing studies are required. atively large, the current axis shifts northward. The Finally, we note the differences in the positions of large meridional shift around the trough of the quasi- the amplitude maxima and in the seasonal change of stationary meander near 146øE makes a dominant con- zonal wavelength of the quasi-stationary meander betribution to the seasonal axis migration. In the up- tween our results and those of Mizuno and White [1983]. stream Kuroshio Extension region, the coincident vari- In the present study, the amplitude maxima of the anation of transport and axis migration is not caused by nual SSH are located near the crest (35øN, 143øE) and thermodynamic effects but by current variability. trough (34øN, 146øE) of the meander path. Mizuno and The zonally averaged horizontal velocity change (be- White [1983] reported that maxima of rms amplitude tween 141øE and 150øE) at the current axis is maximum are located near the two crests of the quasi-stationary in spring and minimum in late summer to fall, corre- meander around (38øN, 144øE)and (36øN, 150øE) (Figsponding to the strength changes in the pair of eddies ure 1). The two maxima of the quasi-stationary meanat 140ø-148øE. In contrast to this variation, the cur- ders are shifted by about 2 ø latitude southward in the rent speed is maximum in fall and minimum in spring present study. For the zonal wavelength change, we when steric components are included. obtain nearly constant values through all seasons (Fig- Owing to the lack of information about internal cur- ure 7a), while Mizuno and White [1983] stated that the rent structure, it is difficult to determine what controls zonal wavelength of the meander becomes progressively the seasonal variability. We also need to clarify the larger from summer to winter. causes of the seasonal axis migration of the upstream Their analysis was during the period from 1976 to Kuroshio Extension. Several past studies may answer 1980, when a large meander of the Kuroshio south of

8 16,692 TATEBE AND YASUDA: AXIS MIGRATION OF THE KUROSHIO EXTENSION Japan was observed. They indicated that the seasonal variation of the Kuroshio Extension is affected by the seasonal intensification of the interannual variability, which is related to the large meander of the Kuroshio south of Japan. In contrast in the present study, no large meander of the Kuroshio is seen. In addition, Wang and Koblinsky [1998] reported on decadal to interdecadal variations of the Kuroshio Extension in T/P altimeter data. Long-term changes of the Kuroshio Current system may account for the different locations of the amplitude maxima between this study and Mizuno and White [1983]. Data processing procedures may be responsible for the zonal wavelength change in the meander. To extract a seasonal cycle, we use CEOF analysis after removing high-frequency variability, while Mizuno and White [1983] only calculated the seasonal mean values. Although the seasonal axis migrations of the Kuroshio Extension reported both in this study and by Mizuno and White [1983] are assumed to be the same kind of current variability, different methods for obtaining seasonal values may account for the discrepancies in the zonal wavelength change in the meander. Acknowledgments. The authors are grateful to Bo Qiu at the University of Hawaii for providing the altimeter data and to the Japan Meteorological Agency for supplying the CD-ROM of the Subarctic Gyre Experiment (SAGE), which contains the recent temperature data set. We also thank two anonymous reviewers and Shaun Johnston at the University of Hawaii for their valuable comments and English grammatical suggestions. Shoshiro Minobe and Takanobu Tokuno at Hokkaido University helped with data analysis techniques. The figures in this paper are prepared with the Grid Analysis and Display System (GRADS). References Bernstein, R.L., and W. 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Koblinsky, Annual and intra-annual sea level variability in the region of the Kuroshio Extension from TOPEX/Poseidon and Geosat altimetry, J. Phys. Oceanogr., 28, , H. Tatebe and I. Yasuda, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, Japan, ( tatebe@aos.eps.s.u-tokyo.ac.jp;ichiro@eps.s.utokyo.ac.jp (Received May 31, 2000; revised February 27, 2001; accepted March 22, 2001.)