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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C10015, doi:10.1029/2007JC004226, 2008

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ArticleThe Kuroshio large meander formation in 2004 analyzed
by an eddy-resolving ocean forecast system
Yasumasa Miyazawa,1 Takashi Kagimoto,1 Xinyu Guo,1,2 and Hirofumi Sakuma1
Received 20 March 2007; revised 5 July 2008; accepted 24 July 2008; published 18 October 2008.

[1] The recent formation of the Kuroshio large meander southeast of the Kii Peninsula

during the period from May to July 2004 has been simulated successfully in an eddyresolving ocean forecast system (Japanese Coastal Ocean Predictability Experiment). The
formation process was examined from the viewpoint of eddy-Kuroshio interactions
using sensitivity experiments that had various initial conditions for eddies in March and
May 2004. We confirmed that the large meander does not occur when the trigger meander
southeast of the Kyushu Island has small amplitude and that the cyclonic eddy on theoffshore side of the Kuroshio contributes to the large meander formation. Our results showthat the large amplitude of the trigger meander southeast of Kyushu Island in April 2004
was caused by downstream advection of anticyclonic eddies along the Kuroshio in the
East China Sea from east of Taiwan. The results suggest that the successive westward
propagation of strong anticyclonic eddies in the subtropical frontal region from 2003 to2004 was the origin of the formation of the Kuroshio large meander south of Japan insummer 2004.
Citation: Miyazawa, Y., T. Kagimoto, X. Guo, and H. Sakuma (2008), The Kuroshio large meander formation in 2004 analyzed byan eddy-resolving ocean forecast system, J. Geophys. Res., 113, C10015, doi:10.1029/2007JC004226.

1. Introduction[2] The Kuroshio, which is one of most energetic westernboundary currents, is unique in having the bimodality ofstable paths south of Japan (Figure 1): the large meander
and nonlarge meander. The large and nonlarge meander
paths persist from a few years to a decade [Kawabe, 1995].
Recently, the Kuroshio took the large meander path fromsummer 2004 to summer 2005 after a nonlarge meander
period of 13 years. This was the first large meander to occur
since the start of full monitoring of the surface ocean state
started by satellite altimeters. Satellite observations of thelarge meander are quite useful for data assimilation tocapture processes involved in the Kuroshio path variation[e.g., Miyazawa et al., 2005a; Usui et al., 2006, 2008;
Kagimoto et al., 2008]. Thus it is feasible to clarify anddeeply understand the mechanism of the Kuroshio large
meander occurring from 2004 to 2005 by combining thesatellite altimeter data as well as hydrographic observations
with a recently developed numerical ocean forecasting
system.
[3] The bimodality of the Kuroshio path south of Japan
has been investigated by many authors from both theoreticaland observational viewpoints. Theoretical studies [e.g.,
Masuda, 1982] suggested that the Kuroshio may possess
1
Frontier Research Center for Global Change/JAMSTEC, Yokohama,Japan.
2
Also at Center for Marine Environmental Studies, Ehime University,
Matsuyama, Japan.

multiple equilibria under an external condition. Kawabe[1995], using tide gauge observations, showed that either
the large or nonlarge meander state appeared when the
upstream volume transport was larger than, or the sameas its climatological mean. Kurogi and Akitomo [2003]
suggested that the multiple equilibrium states of the Kuroshio
path may have been established by the wind stress field in the
North Pacific over the past 40 years.
[4] Analyses of the satellite data have showed thatmesoscale eddies with a spatial scale of 500 – 800 km anda time scale of 45– 180 days are active both south of Japanand east of Taiwan [e.g., Ebuchi and Hanawa, 2000;
Kobashi and Kawamura, 2001]. Some observational studies
suggested that these mesoscale eddies play important rolesin the Kuroshio path variation south of Japan [Book et al.,
2002; Ebuchi and Hanawa, 2003]. Recent modeling studies
also suggested that an interaction between the Kuroshio andthe mesoscale eddy south of Japan triggers the transition
from the nonlarge meander to the large meander [Akitomo
and Kurogi, 2001; Endoh and Hibiya, 2001; Tsujino et al.,2006]. In fact, our preliminary model experiment showedthat anticyclonic eddies activated in the subtropical frontal
region may affect the Kuroshio path variation south ofJapan through their downstream movement along theKuroshio in the East China Sea [Miyazawa et al., 2004].
In addition, our ensemble forecast experiment indicated thatan anticyclonic eddy southeast of the Kyushu Island playeda key role in selecting the state of the real Kuroshio path in1999 [Miyazawa et al., 2005a]. However, the role of themesoscale eddies in the variations of Kuroshio path has notbeen clarified in detail.

Copyright 2008 by the American Geophysical Union.
0148-0227/08/2007JC004226$09.00

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C10015[7] This paper is organized as follows. In section 2, theconfiguration of the JCOPE ocean forecast system is briefly
described. Section 3 shows results of the model experiment,in particular, the process of the Kuroshio large meander
formation in 2004 is argued. In section 4, conducting
sensitivity experiments to initial states, we discuss rolesof mesoscale eddies east of Taiwan in triggering thelarge meander formation in 2004. Section 5 is devoted tosummary and discussion.

2. The JCOPE Ocean Forecast System

Figure 1. Topography of the Kuroshio region south ofJapan and the stable Kuroshio paths (LM, large meander;
nLM, nonlarge meander). The numbers denote tide gauge
stations (1, Naze; 2, Nakanoshima; 3, Nishinoomote; 4,
Kushimoto; 5, Uragami; 6, Hachijojima), and the abbreviations indicate locations (Ky, Kyushu Island; S, Shikoku
Island; Ki, Kii Peninsula; KS, Koshu Seamount; IOR, Izu
Ogasawara Ridge). Regions deeper than 4000 m are shaded.
[5] Previous observational studies [e.g., Sekine, 1990;
Kawabe, 1995] have suggested that the formation processof the Kuroshio large meander is divided into two sub
processes; the first is the generation of a small meander
southeast of the Kyushu Island (the ‘‘trigger meander’’[Solomon, 1978]), and the second is the abrupt amplification of the trigger meander. Although the first process
occurs frequently, most of the trigger meanders decay
without undergoing the second process. Recently, Usui etal. [2008] investigated the first process of trigger meander
generation in December 2003. We focus on the second
process, the amplification of the trigger meander in April
2004 prior to the final formation process from May to July
2004.
[6] We investigated products of an ocean forecast modelthat was developed as a part of the Japan Coastal Ocean
Predictability Experiment (JCOPE) [Guo et al., 2003;
Miyazawa et al., 2004; Miyazawa et al., 2005a, 2005b;
Kagimoto et al., 2008], with emphasis on the roles ofanticyclonic eddies east of Taiwan in the formation of theKuroshio large meander in 2004. We conducted a number ofsimulation runs using the JCOPE forecast system to analyze
how initial conditions dynamically affect the modeled time
evolution of the Kuroshio path variations. Intensive use of
the state-of-the-art model products allowed us to clarify themechanisms of the real Kuroshio large meander formation,
which previously has not been clearly understood. On thebasis of the previous studies described above, we highlightthe roles of the trigger meander and mesoscale eddies
southeast of the Kyushu Island in the large meander
formation in 2004. In addition, we discuss the possible
relationship of the large meander formation in 2004 to themesoscale eddies in the subtropical frontal region.

2.1. Ocean Model[8] The ocean model in the JCOPE system is based on the
Princeton Ocean Model with a generalized coordinate ofsigma (POMgcs) [Mellor et al., 2002]. A high-resolution
regional model with a spatial grid of 1/12° and 45 verticallevels is embedded in a low-resolution model covering theNorth Pacific region (30°S –62°N, 100°E–90°W) with aspatial grid of approximately 1/4° and 21 sigma levels. The
inner model domain covers the western North Pacific (12°–62°N, 117°– 180°E) and its lateral boundary conditions are
determined from the basin-wide model using a one-way
nesting method [Guo et al., 2003]. Details of the model
configuration have been described by Kagimoto et al.
[2008].
[9] The model is driven by wind stresses, and heat andsalt fluxes. The wind stress fields are calculated from the
12-hourly QuikSCAT near-real-time product with 1/2°
resolution [Liu et al., 1998]. The heat flux fields are calculated from 6-hourly National Centers for Environmental
Prediction/National Center for Atmospheric Research(NCEP/NCAR) reanalysis data [Kalnay et al., 1996] combined with the QuikSCAT product. Salinity at the ocean
surface is restored to the monthly mean climatology data
[Levitus et al., 1994] with a time scale of 30 days.
Simulation of the low-resolution model is started from astate of rest with the annual mean temperature and salinity
fields created from 1/4° climatology data [Boyer andLevitus, 1997]. The low-resolution model is spun-up for
20 years by the monthly mean surface forcing from an
initial condition of no motion with the annual mean temperature and salinity fields created from the 1/4° climatology.
The high-resolution model is also run from the same initialconditions. The lateral boundary forcing of the spin-up run
is calculated for the last 5 years using the spin-up results of
the low-resolution model. Both the low- and high-resolution
models are then driven by the 6-hourly surface forcing
during the period from September 1999 to August 2004.
Synoptic variations in the western North Pacific Ocean suchas the Kuroshio large meander and the mesoscale eddies are
simulated in the high-resolution model [Miyazawa et al.,2004].
2.2. Data Assimilation[10] The following observation data are assimilated into
the model: the sea surface height anomaly (SSHA) obtained
from the TOPEX/Poseidon and ERS-1 satellites during
September 1999 to June 2002 and from the Jason-1 andthe Geosat Follow-On during June 2002 to August 2004;
sea surface temperature (SST) obtained from the Advanced
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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

face Temperature (AVHRR/MCSST) Level 2 products; andvertical profile data of temperature and salinity obtained
from the Global Temperature-Salinity Profile Program(GTSPP). The SSHA and temperature/salinity profiles are
assimilated only in the southern region of 12°– 44°N

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because in the subarctic region, the SSHA has weakcorrelation with subsurface temperature/salinity and theobservation density of in situ data is relatively low.
[11] The data assimilation procedure in our systemcomprises three steps: the creation of two-dimensional
uniform grid data by optimum interpolation (OI) [Gandin,
1963] of SSHA, SST, and temperature-salinity profiles; thecreation of temperature/salinity analysis data using themultivariate optimum interpolation (MOI) [e.g., Lorenc,
1981]; and incremental analysis update (IAU) [Bloom etal., 1996] of the analysis data for smooth initialization.
Appendices A, B, and C provide brief explanations of the
OI, MOI, and IAU techniques, respectively.
[12] We conducted the IAU process from September 1999
to August 2004 and stored 2-day mean data, referred to as
the assimilation products. In addition, initial states were
sampled from snapshots stored in approximately 1-monthintervals during the IAU process from October 2002 to May
2004; they were used for hindcast runs without data
assimilation forced by the 6-hourly surface forcing. In
sections 3 and 4, we mainly discuss the hindcast runs toinvestigate the dynamical effects of initial states on the large
meander formation because the hindcast results were completely governed by the model dynamics. The assimilation
products are used for better description of the observed
phenomena, even though they are not always consistent
with the model dynamics.

3. Kuroshio Large Meander Formation in July
2004
3.1. Simulated Large Meander Formation in 2004[13] The Kuroshio south of Japan took the large meander
path during the period from August 2004 to August 2005
after a nonlarge meander period of 13 years. The formation
process of the large meander in 2004 seems to be similar to
the following typical picture of past large meanders[Kawabe, 1995]. The trigger meander formed southeast of
the Kyushu Island. The trigger meander propagated eastward and grew into the large meander east of the Kii
Peninsula. The growth of the trigger meander was wellsimulated by our model (Figure 2).
[14] We compared the time evolution of the Kuroshio
path during May to July 2004 obtained from the hindcast
experiment with observations based on the weekly Quick
Bulletin Ocean Conditions provided by the Hydrographic
and Oceanographic Department of the Japan Coastal Guard(Figure 2, left). We checked the model skill by comparing

Figure 2. (left) The Kuroshio paths, defined as the
positions with maximum kinetic energy in the surface layer
within the Kuroshio region. Thick lines indicate the paths
taken from the hindcast run initialized on 8 May 2004.
Corresponding snapshots of modeled SSH are shown in
Figure 2 (right). Gray lines indicate the biweekly meanobserved Kuroshio path from May to July 2004 as reportedby the Quick Bulletin Ocean Conditions provided by theJapan Coast Guard. (right) Modeled SSH from the 2-day
mean of the hindcast run initialized on 8 May 2004. The
contour interval is 0.1 m. Regions greater than 0.6 m areshaded.
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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015Figure 3. Time series of the (a) RMSE and (b) correlation of the Kuroshio path positions south ofJapan, 130– 140°E, by the observation (gray lines in Figure 2, left). Thick lines indicate the skill of thehindcast run initialized on 8 May 2004 (thick lines in Figure 2, left). Dashed lines with solid circles
indicate that of the persistence. Dashed lines with open squares indicate that of the climatology. The
period of July is shaded.
the simulated and observed Kuroshio path positions. Both themodeled and observed Kuroshio paths were defined as the
positions with the maximum kinetic energy in the surface
layer within the Kuroshio region. Figure 3 shows the time
evolution of root mean square error (RMSE; Figure 3a) andthe correlation (Figure 3b) between the latitudes of themodeled and observed Kuroshio paths at 130° – 140°E withan interval of approximately 1/8°. In July (Figures 3a and3b, shaded period), the skills of the hindcast (thick lines)
were superior to those of the ‘‘persistence’’ (dashed lines
with solid circles) of the initial state on 8 May 2004 (thicklines in Figure 2a (left)) and the ‘‘climatology’’ (dashedlines with squares) calculated from the observed Kuroshio
paths during the period from October 2002 to October 2004.
The model simulated the final stage of the observed transient process from the nonlarge meander to the large
meander in July. It should be noted that the model skill
was worse than both the persistence and the climatology
during the period from 21 May to 21 June. This is because apreceding amplification of the trigger meander simulated in
May occurred farther east than the observed amplification(Figure 2b). The preceding amplification of the trigger

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C10015Figure 4. Gridded SSHA on 8 May 2004 created from satellite altimeter data (Jason-1 and Geosat
Follow-On). The shaded region indicates positive anomaly. Arrows TM, CE, and AE point to anomalies
for which the amplitude was modified in the sensitivity experiments for cases wTM, wCE, and wAE,
respectively.

meander, which may have been associated with a cycloniceddy that was absorbed into and detached from the trigger
meander, was not exactly simulated by the model. In
addition to the trigger meander and the cyclonic eddy, ananticyclonic eddy at 30°N and 134°E on 8 May (Figure 2a,right) appears to have influenced the large meander formation in 2004.
3.2. Perturbed Runs of the Large Meander Formationin 2004[15] Previous modeling and observational studies havesuggested that the trigger meander southeast of the
Kyushu Island [e.g., Solomon, 1978; Endoh and Hibiya,
2001] and mesoscale eddies [e.g., Ebuchi and Hanawa,
2003; Miyazawa et al., 2004, 2005a] affect the Kuroshio
path variation south of Japan. The sea surface height (SSH)
sequences shown in Figure 2 (right) also suggest theimportance of factors such as the trigger meander andmesoscale eddies to the large meander formation in 2004.
A snapshot of the gridded SSHA data used for the initialization on 8 May 2004 (Figure 4) clearly displays threeanomalies corresponding to these factors: the trigger meander
and the cyclonic and anticyclonic eddies on the offshore sideof the Kuroshio southeast of the Kyushu Island.

3.2.1. Identifying Key Factors in the Large Meander
Formation in 2004[16] To identify key factors responsible for the large
meander growth southeast of the Kii Peninsula in July
2004, we conducted three sensitivity experiments with theamplitude of each factor weakened in the initial conditions:weak trigger meander, weak anticyclonic eddy, and weakcyclonic eddy (hereafter wTM, wAE, and wCE, respectively).
Table 1 lists the parameters used in the three sensitivity
experiments. Negative (positive) SSHA data in a target
region on 8 May 2004 were changed to the same spatial
pattern of positive (negative) sign. The perturbed SSHA data
were assimilated into the model on 8 May 2004 through the
MOI and IAU schemes.
[17] Figure 5 shows perturbed initial states (Figure 5, left)
and final states (Figure 5, right) in the three sensitivity experiments. Comparison between Figure 5 (left) and Figure 2a(right) (on 8 May 2004) indicates that the trigger meander
was weakened in the wTM case, that the cyclonic eddy
disappeared in the wCE case, and that the anticyclonic eddy
was weakened in the wAE case. At first glance, the mosteffective factor for the large meander growth seems to havebeen the amplitude of the trigger meander because theresulting amplitude of the meander southeast of the Kii

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Table 1. Parameters Used in the Sensitivity Experiments With the Perturbed Initial State on 8 May 2004a
Experiment

Time

RegionSign Change

Amplitude (Case Name)

Weak trigger meander

8 May 2004

29° – 34°N, 130° – 134°E

Negative!positive

Weak anticyclonic eddy

8 May 2004

26° – 30°N, 130° – 136°E

Positive!negative

No cyclonic eddy

8 May 2004

30° – 34°N, 135° – 138°E

Negative!positive

0.5 (wTM or wt05)
0.4 (wt04)
0.3 (wt03)
0.2 (wt02)
0.1 (wt01)
2.0 (wAE or wae2.0)
1.0 (wae1.0)
1.0 (wCE)a

For example, in the wTM (wt05) case, negative SSHA (e.g., SSHA(i, j)) at 29° – 34°N, 130° – 134°E on 8 May 2004 waschanged to the same spatial pattern of positive sign with half amplitude (e.g., (0.5) SSHA(i, j)). The more negative value of
SSH at 32.5°N, 138°E indicates a stronger meander.

Peninsula in the wTM case is the smallest among the threeruns shown in Figure 5 (right) (on 31 July 2004). With the
weak cyclonic eddy, the large meander formed faster than in
the unperturbed run (as explained in section 3.2.2). Phenomena such as the cyclonic eddy’s absorption into and detachment from the trigger meander were observed in theunperturbed run (Figures 2b and 2c) and delayed the large
meander formation. Although the wAE case showed theslight weakening of the large meander, the anticyclonic eddy
may not be a major factor in the large meander growth.
3.2.2. Effects of Initial Condition on the Large
Meander Formation in 2004[18] To investigate how the formation process of the large
meander is affected by perturbations in the initial conditions, we examined sea level indices that are closely relatedwith the Kuroshio path variation south of Japan. The first is
the sea level difference between Kushimoto and Uragami(see Figure 1 for the locations), which is negative atapproximately 1 cm and less variable [e.g., Moriyasu,
1958; Kawabe, 1995] when the Kuroshio takes the large
meander path. The second is the Kuroshio Position Index[Kawabe, 1995] defined from sea levels at Naze, Nakanoshima, and Nishinoomote, which indicates the latitude of
the Kuroshio path in the Tokara Strait (see Figure 1 for thelocations). Kawabe [1995] pointed out that the Kuroshio
axis in the Tokara Strait shifts from south to north prior to
the large meander formation.
[19] As a whole, the model captures the dynamics of thelarge meander formation, although the modeled andobserved results have some discrepancies (Figure 6). The
observed sea level difference between Kushimoto and
Uragami decreased by approximately 15 cm during theestablishment of the large meander path in July (Figure 6a,left). The simulated sea level difference decreased fasterthan the observed difference (Figure 6a, right). This discrepancy may be due to the difference between the observations and the initial conditions of the model. Figure 2a
indicates that the initialized trigger meander of the modelslightly moved eastward compared with the observation.
Thus the modeled Kuroshio separated from the Kii Peninsula
faster than did the observed Kuroshio. In the wTM case,because the separation occurred later than in the unperturbed run, the sea level difference between Kushimoto and
Uragami decreased later. In the wCE case, the sea level
difference between Kushimoto and Uragami decreased
earlier than in the other cases, indicating that the large
meander formed earlier than in the other cases. Thus, thewCE case suggests that the cyclonic eddy tends to delay thetiming of the large meander formation.
[20] Both the observed and modeled Kuroshio axes
shifted from south to north before the large meander
formation in 2004 (Figure 6b). The slight earlier northward
shift of the Kuroshio axis in the model than in the observation was not sensitive to the perturbed initial states. In
May 2004, the latitude of the observed Kuroshio axis
reached 30.2°N but those of the model experiments
remained near 29.8°N (also Figures 2a (left) and 2b (left)).
The three factors, which appear in the downstream side of
the Tokara Strait, may not have affected the latitude of theKuroshio path there during the period from May to July
2004.
[21] The sensitivity experiments with three factors (i.e.,
the trigger meander, the cyclonic eddy, and the anticycloniceddy) suggest that both the trigger meander and the anticyclonic eddy may have facilitated the growth of the large
meander in July 2004, whereas the cyclonic eddy delayed
the timing of the large meander formation. To clarify which
events of the trigger meander and anticyclonic eddy
contributed most to the large meander formation, we conducted sensitivity experiments using various perturbation
amplitudes (Table 1). The parameter ranges of the perturbation amplitudes used in the sensitivity experiments were
determined empirically in such a way that the large meander
formation changed significantly but not unrealistically.
When the sea level corresponding to the trigger meander
amplitude in May 2004 was larger than 0.38 m, theamplitude of the resulting Kuroshio meander southeast of
the Kii Peninsula was significantly weaker than that of theunperturbed run (Figure 7a). In contrast, the perturbation of
the anticyclonic eddy may not have affected the amplitudes
of both the trigger and the resulting large meander. The
weakening of the trigger meander amplitude caused theintensification of the anticyclonic eddy, but it did not
facilitate the large meander formation (Figure 7b). We
conclude that the amplitude of the trigger meander in May
is more important for the large meander formation than isthat of the anticyclonic eddy.
3.2.3. Distributions of Vertical Velocity Near the
Kuroshio Path South of Japan[22] Previous modeling studies [Hurlburt et al., 1996;
Endoh and Hibiya, 2001; Miyazawa et al., 2004, 2005a;
Tsujino et al., 2006] have suggested that the Kuroshio large
meander is generated through the baroclinic instability that
occurs southeast of the Kii Peninsula. From this viewpoint,
we examined both the horizontal and vertical distributions

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Figure 5. Sea surface height obtained from three typical sensitivity experiments initialized on 8 May
2004. Left: 8 May 2004. Right: 31 July 2004. The wTM, wCE, and wAE denote the experiments inwhich the trigger meander (TM in Figure 4), the cyclonic eddy (CE in Figure 4), and the anticycloniceddy (AE in Figure 4) were weakened, respectively.

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C10015Figure 6. (a) Time series of the sea level difference between Kushimoto and Uragami from May to July
2004. (left) Observations and (right) unperturbed and sensitivity experiments initialized on 8 May 2004.
(b) Same as Figure 6a except for the Kuroshio Position Index [Kawabe, 1995], which shows the latitudeof the Kuroshio path in the Tokara Strait.
of vertical velocity near the Kuroshio path south of Japan in
the unperturbed run. In the unperturbed run, large anomalous downward vertical velocity occurred at 500 m depth at
the west side of the trough of the large meander, whereas
anomalous upward vertical velocity occurred at the east side
(Figure 8). Vertical profiles of the vertical velocity averaged
over 1°  1° boxes corresponding to anomalous regions
indicate that a downward increase in the downwelling(upwelling) velocity in the upper 500 m and a decrease
below 500 m occurred in the west (east) side of the boxedregion; this means that stretching (shrinking) in the upper
layer and shrinking (stretching) in the lower layer took placeat the west (east) side of the boxed region of the large
meander trough. Profiles of the vertical velocity below
1500 m in a boxed region located slightly north of the westside box on 21 June 2004 (thin line with open circles in
Figure 8a (middle)) show a somewhat different distribution
from the center box, suggesting that the deep verticalvelocity was affected at the west side by the bottom slope
north of the Kuroshio axis when the large meander grows.
In the east side boxes, the distributions of the deep verticalvelocity were sensitive to the location of the boxes, especiallywhen the large meander approached the Izu-Ogasawara
Ridge (Figures 8b (right) and 8c (right)).
[23] Figure 9 shows horizontal distributions of the vertical stretching term at 200 m (Figure 9, left) and 3000 m(Figure 9, right) depths in the unperturbed run. On 21 June
2004, the Kuroshio axis began to separate from the coast. In
the early stages of the large meander formation, althoughboth the stretching and shrinking at 200 m depth occurred at
the west and east sides, respectively, the shrinking andstretching at 3000 m depth was not shown clearly becauseof the effect of the bottom slope. On 11 July 2004, both thestretching (shrinking) at 200 m depth and shrinking (stretching) at 3000 m depth were found at the west (east) side of
the trough of the large meander. In particular, at the westside, the stretching (shrinking) of the upper (lower) layer
supplied the cyclonic (anticyclonic) vorticity for the troughof the large meander (the abyssal anticyclone), suggesting
the joint evolution of the large meander trough and theabyssal anticyclone. On 31 July 2004, the large meander
grew and the trough moved southeastward. The shrinking at

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Figure 7. (a) Relationship between the amplitude of thetrigger meander on 8 May 2004 (abscissa) and theamplitude of the Kuroshio large meander on 31 July 2004
(ordinate). The former index is defined as SSH at 31°N,
133°E, whereas the latter is defined as SSH at 32.5°N,
138°E. Negative or smaller (positive or larger) values
indicate strong (weak) meanders. The unptb indicates thehindcast run initialized on 8 May 2004. The wtm0.1,
wtm0.2, wtm0.3, wtm0.4, and wtm0.5 indicate sensitivity
experiments with the amplitude of the trigger meander;
wtm0.5 is another name for the typical experiment wTM.
The wae1.0 and wae2.0 indicate experiments with a weak
anticyclonic eddy; wae2.0 is another name for the typicalexperiment wAE. (b) Same as Figure 7a except for the
relationship between the amplitude of the anticyclonic eddy
at the offshore side of the Kuroshio on 8 May 2004
(abscissa) and that of the Kuroshio large meander on 31 July
2004 (ordinate). The former index is defined as SSH at
30°N, 134°E. Larger (smaller) values indicate strong (weak)
anticyclonic eddies.
200 m depth was still found in the east side of the trough of
the large meander, but the stretching at 3000 m depth was
not significant there. In the mature stage, the large meander
growth at the east side may have been affected by the IzuOgasawara Ridge. In the unperturbed run, the abyssal

C10015anticyclone developed at the west side of the trough of
the large meander together with the large meander growth(Figure 9), whereas in the ‘‘wTM’’ case, the stretching andshrinking patterns seem to have been significantly distortedby the bottom slope and the abyssal anticyclone was not
generated (not shown).
3.2.4. Abyssal Anticyclone South of the Kii Peninsula[24] The above analysis suggests that the enhanced abyssal
anticyclone is closely related to the large meander growthsoutheast of the Kii Peninsula. The abyssal kinetic energy
averaged in the analysis region (31.5°– 33.5°N, 135°–
138°E) for both the unperturbed run and the wAE caseincreased in July 2004 when the large meander grew
southeast of the Kii Peninsula (Figure 10a). In contrast,
increases in abyssal kinetic energy in the wTM case were
not significant compared with increases in the unperturbed
run. The results calculated in sensitivity experiments for theamplitude of the trigger meander (Figure 10b) also indicatethat the magnitude of the kinetic energy associated with theabyssal anticyclone was small in the nonlarge meandercases of wt03, wt04, and wt05 (wTM) compared with thelarge meander cases of wt01, wt02, and the unperturbed run.
The wCE case had the largest increase in May, associatedwith the strengthened abyssal anticyclone (not shown). In
this run, the amplitude of the large meander was the largest
among all of the runs because the cyclonic eddy neither wasabsorbed into nor detached from the trigger meander andthus probably did not prevent the trigger meander from bothpropagating and growing in May.
[25] The presence of the bottom slope along the coast
generally stabilizes the Kuroshio path variation [e.g.,
Sekine, 1990]. In the simulation, the vertical profiles of
the upwelling/downwelling velocity associated with thelarge meander growth were distorted when the Kuroshio
axis was close to the southern coast of Japan (e.g., Figure 8a(middle)). The wTM case also suggests that the bottom
slope obstructs large meander growth through the distortionof downwelling/upwelling velocity (not shown). Thus, forlarge meander growth southeast of the Kii Peninsula, theKuroshio axis might have to move sufficiently southward
away from the bottom slope in the early stages of growth.
To examine the relationship between the large meander
growth and the distance of the Kuroshio axis from thesouthern coast, we analyze time evolutions of the latitude of
the Kuroshio axis at 135.8°E taken from the sensitivity
experiments (Figure 11). In the unperturbed run (Figure 11a),just after the Kuroshio axis moved southward in the second
half of June, the kinetic energy at 4000 m depth associatedwith the abyssal anticyclone increased as shown in Figure 10a.
The wAE case also illustrates the southward movement of
the Kuroshio axis in the beginning of July and the subsequent increase in the abyssal kinetic energy. The wCE caseshowed significant southward movement of the Kuroshio
axis in the second half of May and subsequent substantial
intensification of the abyssal anticyclone. In contrast, in thewTM case, the Kuroshio axis did not move so far south, andthe abyssal anticyclone was not excited there. Figures 10b
and 11b demonstrate that the time evolutions of both the
Kuroshio axis and kinetic energy associated with theabyssal anticyclone south of the Kii Peninsula depended
on the amplitude of the trigger meander in May 2004,
suggesting that a sufficiently large amplitude of the trigger

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015Figure 8. (left) Snapshots of smoothed vertical velocity (104 m s1) at 500 m depth calculated fromthe unperturbed run initialized on 8 May 2004. The vertical velocity was smoothed by operating a ninepoint smoothing procedure four times. (middle) Vertical profiles of the vertical velocity horizontally
averaged over blue boxes indicated in Figure 8 (left) (thick lines). (right) Same as Figure 8 (middle)
except for the red boxes indicated in Figure 8 (left). Profiles averaged over boxes 0.3° north, south, east,
and west of the red/blue boxes are indicated by thin lines with open circles, thin lines with open squares,
dashed lines with solid circles, and dashed lines with solid squares, respectively.
meander is necessary to move the Kuroshio axis away fromthe bottom slope near the Kii Peninsula and to excite theintense abyssal anticyclone associated with the large meander generation.

4. Amplification of the Trigger Meander in April
2004[26] In section 3, we suggested that the amplitude of thetrigger meander southeast of the Kyushu Island in the
beginning of May is critical for the subsequent large

meander formation. The time evolution of the SSH at
31°N, which indicates the amplitude of the trigger meander
southeast of the Kyushu Island obtained from the assimilation product, suggests that the trigger meander was extremely
amplified in the second half of April 2004 (Figure 12).
Another trigger meander like event with a smaller amplitude
occurred in February 2003. This event caused the weak
amplification of the trigger meander southeast of the Kii
Peninsula during the short period from May to June 2003
[Kagimoto et al., 2008].

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

Figure 9. Snapshots of the smoothed stretching term (1011 s2), f (sw/@z) (left) at 200 m depth and(right) at 3000 m depth calculated from the unperturbed run initialized on 8 May 2004. The stretchingterm was smoothed by operating a nine-point smoothing procedure four times. Green lines indicate
isotherms with a contour interval of 1°C at 200 m depth. The horizontal velocities (m s1) at 4000 m
depth are indicated by vectors. Rectangular boxes indicate the region over which the vertical velocity was
averaged (see Figure 8).

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015[28] Recently, Miyazawa et al. [2005b] and Usui et al.
[2008] suggested that the anticyclonic eddy east of Taiwan
in October 2003 was related to the amplification of thetrigger meander southeast of the Kyushu Island in December 2003. Usui et al. [2008] also suggested that the trigger
meander was initially generated by the collision of theKuroshio and a cyclonic eddy in that region in December
2003. Because the trigger meander developed in December
2003, decayed in February 2004, and redeveloped in
April2004 (Figure 12), it is necessary to clarify the mechanisms associated with the ‘‘second’’ amplification of thetrigger meander southwest of the Kyushu Island in April
2004, which may have acted as the trigger of the large
meander in summer 2004. To investigate the events associatedwith the amplification of the trigger meander in April 2004,
we examined a time-longitude diagram of the SSHA of theassimilation run along 22.5°N (Figure 14). In addition to thewestward propagation of the anticyclonic eddy from May to
October 2003 (dashed line in Figure 14), which was related
to the amplification of the trigger meander southwest of the
Kyushu Island in December 2003 [Miyazawa et al., 2005b;

Figure 10. (a) Time series of kinetic energy (m2 s2) at
4000 m depth averaged over 31.5°– 33.5°N, 135°– 138°E,
calculated from the unperturbed and sensitivity experiments(see Table 1) initialized on 8 May 2004. (b) Same as Figure 10a
except for the sensitivity experiments with the amplitudes of
the trigger meander.
[27] The condition of the amplified trigger meander
seems to have been gradually reproduced by the continuous
data assimilation at 1-week intervals during the period fromMarch to April 2004. To investigate the dynamical effects of
the initial conditions on the Kuroshio path south of Japan,
we conducted long-term hindcast runs using the different
initial conditions produced by the data assimilation during
the period from 13 March to 17 April 2004. Figure 13
shows the relationship between the amplitude of the trigger
meander on 22 April 2004 and that of the resulting large
meander southeast of the Kii Peninsula on 31 July 2004 in
the long-term hindcast runs and sensitivity experiments
initialized on 13 March 2004, which are described later(Table 2). The large meander formation in July 2004
appears to have been related to the trigger meander withlarge amplitude in April 2004 because the initial states withthe larger (smaller) amplitude trigger meander in April
appeared to cause the large (nonlarge) meander states in
July (Figure 13).

Figure 11. Same as Figure 10 except for the latitude of theKuroshio path at 135.8°E. The Kuroshio path is defined by
the positions with maximum kinetic energy in the surface
layer.

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015

2004. Table 2 lists the parameters used. As shown in
Figure 13, the intensified (weakened) anticyclonic eddies
caused amplification (weakening) of the trigger meander on
22 April 2004, and the amplified trigger meander resulted in
the large meander on 31 July 2004. In particular, a case of
intensified eddies (sAT) demonstrated the comparable magnitude of the trigger meander to the assimilation. Relatively
large parameter values for the modification of the SSHA
data were necessary for effective perturbation because both
the MOI and IAU schemes work to moderate the effects ofanomalous data.
[31] Time sequences of SSH for the sAT case (Figure 16b)
demonstrate the amplification of the trigger meander southeast of the Kyushu Island on 22 April 2004 and thesubsequent formation of the large meander on 31 July
2004. The run with the unperturbed initial state, however,
did not simulate these processes (Figure 16a). In addition,
the intensified anticyclonic eddies were advected by the
Kuroshio mainstream after their collision with the Kuroshio
east of Taiwan, and the eddies passing through the Tokara
Strait were absorbed by the offshore anticyclonic eddy
southeast of the Kyushu Island (Figure 16b).
[32] To clarify how the trigger meander was amplified(weakened) in the sAT (wAT) case, we examined thevorticity balance southeast of the Kyushu Island. The rateof change of the vorticity was determined using the following
equation [e.g., Endoh and Hibiya, 2001; Masumoto, 2004]:
@z
@w
@2z
¼ u  rz þ f bv þ AH r2 z þ Av 2
@t
@z
@z

Figure 12. Time-longitude diagram of SSH at 31°N by theassimilation run, showing the temporal variation of thetrigger meander southeast of the Kyushu Island. The contour
interval is 0.2 m. Positive values are shaded. Eastward
intrusion of the negative value region indicates theamplification of the trigger meander southeast of the Kyushu
Island.
Usui et al., 2008], another anticyclonic eddy propagatedwestward from November 2003 to March 2004 (dashed line
in Figure 14).
[29] Therefore, we investigated sensitivity of the amplitude of the trigger meander in the hindcast run initialized on
13 March 2004 to perturbed initial conditions, focusing on
anticyclonic eddies observed east of Taiwan in March 2004
(Figure 15). We expect that eddies might have caused theamplification of the trigger meander in April 2004 throughtheir collision with the Kuroshio east of Taiwan and theirsubsequent advection, because previous studies have suggested that anticyclonic eddies propagating northeastward
along the Kuroshio path in the East China Sea are closely
related to the generation of the trigger meander southeast of
the Kyushu Island [Ichikawa, 2001; Masumoto, 2004;
Miyazawa et al., 2004].
[30] By assimilating intensified or weakened positive
anomalies corresponding to the eddies shown in the SSHA
data (Figure 15) into the model, we examined the sensitivityof the hindcast run to the perturbed initial states on 13 Marchð1Þ

where z is the vertical component of the relative vorticity;u = (u, v, w) is the velocity; f and b are the Coriolis
parameter and its meridional gradient, respectively; and AH

Figure 13. Relation between the amplitude of the trigger
meander on 22 April 2004 (abscissa) and that of theKuroshio large meander on 31 July 2004 (ordinate). The
former index is defined as SSH at 31°N, 133°E, whereas
the latter is defined as SSH at 32.5°N, 138°E. Open circles
with different initialization dates were obtained fromhindcast runs, that with ‘‘assim’’ was obtained from theassimilation run, and solid circles were obtained from thesensitivity experiments (sAT, sAT2, sAT3, wAT, and wAT2;
see Table 2).

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C10015

C10015

Table 2. Parameters Used in the Sensitivity Experiments With the Perturbed Initial State on 13 March 2004a
Experiment

Time

RegionSign Change

AmplitudesAT

13 March 2004

21° – 24°N, 124° – 130°E

positive!positivewAT

13 March 2004

21° – 24°N, 124° – 130°E

positive!negative

7.0 (sAT)
5.0 (sAT2)
3.0 (sAT3)
2.0 (wAT)
1.0 (wAT2)a
For example, in the sAT case, positive SSHA (e.g., +SSHA (i, j)) at 21° – 24°N, 124° – 130°E on 13 March 2004 waschanged to the same spatial pattern of positive sign with enhanced amplitude (e.g., (+7.0) SSHA (i, j)).

and Av are the horizontal and vertical viscosity coefficients,
respectively. The term on the left-hand side is the tendency
term of the vorticity, and the terms on the right-hand side are
the advection, the vertical stretching (positive sign) or
shrinking (negative sign), the beta, the horizontal diffusion,and the vertical diffusion. Figure 17 (left) display the time
evolution of all terms in vorticity equation (1) averaged over
29°– 31°N, 130°– 132°E (see the box in Figure 17a (right))
from the surface to 400 m depth. Figure 17 (left) indicatethat the stretching term almost balanced the viscosity terms
in the region and that the tendency of the vorticity wascontrolled by the advection. In the sAT case, the positive
vorticity was generated by both the intensified advection
and stretching on 22 April 2004.
[33] The tendency of the vorticity in the surface layer at
the Tokara Strait was controlled mainly by the vorticity
advection (Figure 17, left). In particular, the amplification of
the trigger meander in April 2004 was caused by theadvection of the positive vorticity together with the enhanced stretching. To detect the origin of the positive
vorticity, we examine snapshots of the potential vorticity
on the 26.0 sq isodensity surface in the upstream region of
the Tokara Strait (Figure 18). The high potential vorticity
was clearly enhanced (see the region marked in Figure 18b(top)) along the continental slope of the East China Sea inthe sAT case compared with the unperturbed run. The
enhanced high potential vorticity seems to have moved
downstream along the continental slope (Figure 18, middle).
On 22 April 2004, the enhanced high potential vorticity was
found in the trigger meander region (Figure 18, bottom).
[34] The negative advection term associated with theamplification of the trigger meander was more intensified
on the offshore side of the Kuroshio southeast of the
Kyushu Island in the sAT case (Figure 17b, right) than in
the unperturbed run (Figure 17a, right). The case of weakened
anticyclonic eddies (wAT) shows reduced negative vorticity
at the offshore side (Figure 17c, right). The intensified
negative advection term was related to a strong anticycloniceddy on the offshore side of the Kuroshio southeast of the
Kyushu Island (Figure 16b, middle).
[35] To confirm the downstream advection of high potential
vorticity together with the positive SSHA due to intensified
anticyclonic eddies, we plotted the temporal variation inboth SSHA along the Kuroshio mainstream (regions surrounded by red lines in Figure 18) and potential vorticity
along the continental slope (regions surrounded by bluelines in Figure 18) from the sAT case (Figure 19). The
enhanced high potential vorticity seems to have been
generated at 125°E on 27 March 2004 when the positive
SSHA arrived at 125°E. On 11 April 2004, the anomalous
signals arrived at the Tokara Strait at 130°E.

[36] The existence of the steep slope near the strong
Kuroshio flow may facilitate the production of the positive
vorticity through the bottom friction and/or bottom pressureof the enhanced velocity on the slope. Both Figures 18 and19 suggest that the origin of the positive vorticity is thecontinental slope near 125°E. To confirm this, calculation of
the vorticity budget in that area using the following equation
is useful:

 



@Ay @Ax@ @vD @uD@f uD @f vD

¼


þ
@x
@y
@t @x
@y
@x
@y
þ J ðPb ; DÞ þ curlðt s  t b Þ
ð2Þ

Figure 14. Time-longitude diagram of SSHA along
22.5°N from the assimilation run. The contour interval is
0.1 m, and positive values are shaded. Mean SSH was
calculated from September 1999 to August 2004. Zonal
mean values were excluded to remove large-scale variation
due to thermal effects. Dashed lines indicate the westward
propagation of anticyclonic eddies.

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015Figure 15. Same as Figure 4 except for on 13 March 2004. Arrows AE1 and AE2 point to anomalies
for which the amplitude was modified in sensitivity experiments: sAT, sAT2, sAT3, wAT, and wAT2.
where D is the depth; u, v are the vertical averaged
velocities; Ax, Ay are the vertically integrated advection plus
diffusion terms in the zonal and meridional momentum
equations, respectively; f is the Coriolis parameter; Pb is thebottom pressure; t s and t b are the wind and bottom stresses,
respectively; and J denotes the Jacobian operator. The termon the left-hand side is the vorticity tendency term, and theterms on the right-hand side are the advection plus diffusionof the vorticity, the planetary geostrophic divergence, thebottom pressure torque, and the curl of wind and bottom
stresses. The second term on the right-hand side of equation (2)is reduced to the planetary beta effect when the timedependence of the surface elevation is negligible [Kagimoto
and Yamagata, 1997].
[37] Figure 20 shows the time series of all terms inequation (2) averaged over 25°– 26.5°N, 123.5°– 125.5°E(see the boxes in Figure 18b) for the unperturbed run(Figure 20a) and for the sAT case (Figure 20b). The positive
bottom friction and bottom pressure torques almost
balanced with the negative advection and beta torques.
The tendency in the variation of the vorticity equation (2)was controlled mainly by the variation in the bottom
friction. Even in the unperturbed run, the positive vorticity
was repeatedly produced by the bottom friction, which wasmore enhanced in the sAT case. The enhanced bottom
friction generated the maximum positive vorticity in thesecond half of March 2004 during the simulation period

from 13 March to 8 May 2004. Downstream of the boxedregion in Figure 20 (see Figure 18b), the advection dominated among all the terms in the right-hand side of thevorticity equation (2) (not shown). Thus, the origin of thepositive vorticity was the continental slope near 125°E in
the East China Sea.
[38] We further investigated the eddy – mean current
interaction simulated in the sensitivity experiments discussed in this section by diagnosing the distribution of theeddy forcing. Using the quasi-geostrophic approximation,
the eddy – time mean flow balances of the momentum
equations are expressed as follows [Cronin, 1996]:"
 1 !#div
@
u
@
u
@
@
r

v0 r0
u þ v¼ byv þ f0~va þ
f0
@x
@y
@z
@z
 0 0

@ ðu u Þ @ ðu0 v0 Þ

þ
@x
@y
"
!#div
 1
@v
@v
@
@
r
0
0

ur
ua 
u þ v ¼ byu  f0 ~
f0
@x
@y
@z
@z
 0 0

@ ðu v Þ @ ðv0 v0 Þ

þ
@x
@y

ð3Þ

where u, v are time-mean quasi-geostrophic horizontal
velocities; u0, v0 are deviation quasi-geostrophic velocities;~ua, ~va are the residual of the mean ageostrophic velocities;

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

Figure 16. Time evolutions of SSH from (a) the unperturbed and (b) sAT case initialized on 13 March
2004. The contour interval is 0.05 m. Regions greater than 0.6 m are shaded.

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

Figure 17. (left) Time series of each term in the vorticity equation (1) calculated from the unperturbed(Figure 17a) and sensitivity experiments (Figures 17b and 17c) initialized on 13 March 2004. Each term
was averaged over 29° –31°N, 130° –132°E (see the box in Figure 17a, right) from the surface to 400 m
depth. (right) Snapshots of the advection term in the vorticity equation (1) at 200 m depth calculated fromthe unperturbed (Figure 17a) and sensitivity experiments (Figures 17b and 17c). The contour interval is
2.0  1010 s2. Regions greater than 2.0  1010 s2 are shaded.
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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

Figure 18. Snapshots of potential vorticity upper 0.6  109 s1 on the 26.0 sq isodensity surface. The
contour interval is 0.1  109 s1. The northern boundary of the shaded region in the East China Sea
corresponds to the continental slope. Regions surrounded by blue lines were used to plot Figure 19 (left);those surrounded by red lines were used to plot Figure 19 (right). (a) The unperturbed run initialized on
13 March 2004. (b) The typical sensitivity experiment with strong anticyclonic eddies east of Taiwan,‘‘sAT.’’ Regions marked by open circles in Figure 18 (right) indicate enhanced high potential vorticity inthe sAT case.
18 of 24

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015Figure 19. (left) Time series of SSHA meridionally averaged in the region surrounded by the blue lines
shown in Figure 18. The contour interval is 0.05 m. (right) Same as Figure 19 (left) except for the
potential vorticity on the 26.0 sq isodensity surface meridionally averaged in the regions surrounded by
red lines shown in Figure 18. The contour interval is 0.1  109 s1.
(z) is time – horizontal mean density profile; r0 is therdeviation density; f0 is the reference value of the Coriolis
parameter; and b is the mean value of the differential along
the north-south (y) direction of the Coriolis parameter. The
third terms of equation (3) are the eddy-forcing components
due to density variation. The horizontal divergence vector(udiv, vdiv) is defined as (udiv, vdiv) = (u, v)  (unodiv, vnodiv),
where nondivergence vectors (unodiv, vnodiv) are obtained bysolving the Poisson equation associated with horizontal
nondivergence of the vectors [e.g., Tsujino et al., 2006]. The
fourth terms of equation (3) are the eddy forcing
components due to momentum variation, which correspondto convergence/divergence of the Reynolds stresses. The
vector sum of the third and fourth terms of equation (3) isthe vector that indicates the total eddy effects associatedwith the variation in both density and momentum.
[39] We calculated the eddy-forcing vector for the meanstate during the period from 16 April to 26 April 2004; thisperiod had the most significant difference in the eddy
forcing components between the unperturbed and sAT cases(Figure 21). Two distinctive regions showed the intensification of the eddy forcing in the sAT case: an area southeastof the Kyushu Island and the continental slope of the East
China Sea. In the former (latter) region corresponding to thetrigger meander (the continental slope of the East China
Sea), the eddy forcing due to the variation in density(momentum) dominated that due to the variation in
momentum (density) (not shown). These features were
almost vertically coherent, although the magnitude of thevector decreased as depth increased (not shown). The
intensified eddy forcing, mainly due to the variation indensity, seems to have been oriented to decelerate and turnthe eastward flow southward in the trigger meander region
west of 133°E (Figure 21b). This is consistent with thevorticity analysis (Figure 17), suggesting that the amplification of the trigger meander was caused by enhanced
stretching at the main thermocline depth (400 – 500 m)
together with the intensified vorticity advection. Even in
the unperturbed run (Figure 21a), the eddy forcing wasoriented to decelerate and turn the eastward flow southward
in the trigger meander region west of 133°E. The perturbation added in the sAT case enhanced the existing feature of
the eddy– mean current interaction in the unperturbed run.

5. Summary and Discussion[40] By analyzing the products of an eddy-resolving
ocean forecast system, JCOPE, we have investigated theKuroshio large meander formation in July 2004, withemphasis on the roles of anticyclonic eddies east of Taiwan
in the preceding amplification of the trigger meander in
April 2004. The JCOPE system initialized on 8 May 2004
successfully simulated the large meander formation in July
2004 (Figure 2). The modeled trigger meander southeast of
the Kyushu Island in May propagated eastward and grew
into the large meander southeast of the Kii Peninsula inJuly, as observed.

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015Figure 20. Time series of torque balance (see equation (2))
in the region 25.0° – 26.5°N, 123.5° – 125.5°E, indicated by
the square in Figure 18b. (a) The unperturbed run initializedon 13 March 2004. (b) The typical sensitivity experimentwith strong anticyclonic eddies east of Taiwan, sAT.

[41] By conducting sensitivity experiments for the initialstate on 8 May 2004 (Table 1), we found that the mostcritical factor in generating the large meander in 2004 was a
trigger meander with sufficiently large amplitude (Figure 7).
In the perturbed runs, in which the amplitude of the trigger
meander was reduced, the Kuroshio axis did not move far
enough southward from the bottom slope south of the Kii
Peninsula (Figure 11), and thus, the large meander did notoccur.
[42] The model products created by the continuous data
assimilation indicated that the trigger meander was extremely
amplified in April 2004 (Figure 12). To detect the maincauses of the amplification of the trigger meander in April,
we conducted other sensitivity experiments for 13 March
2004 with perturbed initial states in which two anticycloniceddies east of Taiwan (Figure 15) were artificially intensified or weakened (Table 2). The intensified (weakened)
anticyclonic eddies east of Taiwan led to the amplification(weakening) of the trigger meander in April 2004 (Figure 13).
Vorticity analysis for the trigger meander region indicatedFigure 21. Eddy forcing vectors (105 m s2), vector sumof the third and fourth terms of equation (3) at 500 m depth
calculated for the mean state from 16 April to 26 April
2004. Contours indicate streamlines calculated from themean geostrophic vectors at 500 m depth (10,000, 15,000,
20,000, and 25,000 m2 s1). (a) The unperturbed run
initialized on 13 March 2004. (b) The typical sensitivity
experiment with strong anticyclonic eddies east of Taiwan,sAT.

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C10015MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

that advection of the positive vorticity and locally enhanced
stretching were concluded to have been the main cause of
the amplification of the trigger meander (Figure 17). The
strong anticyclonic eddies moved to the downstream part of
the Kuroshio, thereby generating positive vorticity in thecontinental slope region of the East China Sea (Figures 18
and 19) through the enhancement of the bottom friction
together with the bottom pressure torque (Figure 20). The
distribution of the eddy forcing vector estimated using theformulation of the eddy – mean current interaction [Cronin,
1996] suggests that the perturbation associated with theintensified anticyclonic eddies east of Taiwan in March
2004 enhanced the existing features of the eddy-Kuroshio
interaction, leading to the amplification of the trigger
meander in April 2004 (Figure 21).
[43] Previous observational studies [e.g., Sekine, 1990;
Kawabe, 1995] have suggested that the formation process of
the Kuroshio large meander is divided into two sub processes: first, the generation of the trigger meander southeastof the Kyushu Island; and second, the abrupt amplificationof the trigger meander. Although the first process occurs
frequently, most trigger meanders decay without undergoing
the second process. In fact, the trigger meander that developed in December 2003 [Usui et al., 2008] decayed in
February 2004 (Figure 12). Our results emphasize that thesecond process of amplification of the trigger meander
actually occurred in April 2004, prior to the large meander
formation in 2004.
[44] The upstream Kuroshio volume transport has been
considered an important parameter characterizing the dynamical regimes of the Kuroshio paths [e.g., Masuda, 1982;
Kawabe, 1995]. However, the Kuroshio volume transport in
the East China Sea calculated from the assimilation data
showed no significant interannual change during the period
from 1999 to 2004 (not shown). The sensitivity experimentssuggest that the large meander formation in 2004 strongly
depended on the amplitude of the trigger meander, rather
than the interannual variation in upstream volume transport.
The sensitivity experiments demonstrated that a smallamplitude trigger meander resulted in the nonlarge meander
state, and further, that the trigger meander was amplified in
April 2004 through the propagation of the strong anticyclonic eddies from the East China Sea prior to the final
process of large meander formation from May to July 2004.
These results suggest that the Kuroshio large meander
formation in 2004 can be interpreted as a transition between
multiple equilibria (i.e., the large meander and nonlarge
meander states) triggered by strong disturbances east of
Taiwan originating from the subtropical frontal region.
[45] Figures 8 and 9 suggest the joint evolution of thelarge meander trough and the abyssal anticyclone together
with both vertical stretching in the upper layer and shrinking
in the lower layer in the west side of the large meander.
Some type of baroclinic instability may be a possible
mechanism for the large meander growth because the upperand lower layer circulations may couple each other [Endohand Hibiya, 2001]. The process described here is fairly
consistent with those presented by previous studies using
another forecast system with other types of OGCM and data
assimilation schemes [Tsujino et al., 2006; Usui et al.,2006], which have also suggested that baroclinic instability
contributed to the large meander growth southeast of the Kii

C10015Peninsula in 2004. However, more detailed analyses [e.g.,
Tsujino et al., 2006] are needed to determine with certainty
the mechanism that is responsible for the large meander
growth.
[46] Some previous modeling studies have suggested thatthe enhanced abyssal anticyclone around the Koshu Seamount (see Figure 1 for the location) plays a critical role inintensifying the amplification of the large meander southeast of the Kii Peninsula [Hurlburt et al., 1996; Endoh andHibiya, 2001; Miyazawa et al., 2004]. We also found theenhanced abyssal anticyclone around the Koshu Seamount
together with the amplification of the large meander southeast of the Kii Peninsula (Figures 9 and 10). However,Tsujino et al. [2006] conducted an OGCM simulation inwhich the Koshu Seamount was not necessary for theformation of the Kuroshio large meander. More studies
are needed to better understand the roles of local topography
in the formation of the Kuroshio large meander.
[47] The sensitivity experiment initialized on 8 May
2004, in which the cyclonic eddy disappeared, indicatesthat the cyclonic eddy on the offshore side of the Kuroshio
delays the large meander formation through interaction withthe trigger meander. In this way, the transient process of thelarge meander formation is quite sensitive to the initialconditions, including the Kuroshio path and the mesoscaleeddies. We directly perturbed specific parts of the initialconditions based on a priori assumptions of the sensitivityof the Kuroshio path variation south of Japan. For moredetailed evaluation, further studies are needed such as those
involving ensemble forecast experiments [e.g., Miyazawa etal., 2005a] and more sophisticated sensitivity analyses [e.g.,
Ishikawa et al., 2004], including other types of perturbations to initial conditions that are obtained from the model
dynamics.

Appendix A:

Time-Space Optimum Interpolation[48] Using the time-space optimum interpolation technique [Kuragano and Kamachi, 2000], we created data at
7-day intervals on 1/4° grids from the raw data: satellite
SSHA ~h, satellite SST T10, and in situ temperature/salinityoo, S1o. . .TM. In our system, we used six
profiles, T20. . .TM
vertical levels for the gridded observation data M andcorresponding depths of 0, 50, 100, 200, 300, and 400 m.
The sampling period for the satellite data was 7 days (3 days
before and after an analysis day), whereas that for thetemperature/salinity profiles was 31 days. The spatial extent
of data sampling for one grid point was 360 km.
[49] Parameters in the optimum interpolation scheme
were determined by statistical analyses of past observation
data. We assumed minimum spatial and time scales for the
gridded data of 50 km and 10 days, respectively. We also
assumed components of the covariance matrix as follows:"   2  #
Dx  cx Dt 2
Dy
Dt
C Xi ; Xj ¼ C0 ðXi Þ exp 


Lx
Ly
Lt
ðA1Þ

where Xi = (xi, yi, zi, ti) and xj are two observation points; C0
is the autocorrelation at lag 0; Dx = (xi  xj, Dy = yi  yj,
and Dt = ti  tj; cx is a phase speed in the zonal direction;

21 of 24MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C10015and Lx, Ly, and Lt denote the zonal, meridional, and temporaldecorrelation scales, respectively. The parameters, C0, cx,
Lx, Ly, Lt in equation (A1) were statistically estimated fromthe observation data [Kagimoto et al., 2008]. The parametervalues depend on the assumed minimum spatial and timescales.

0

2 f
Eh0

D~h0f DT f ETf

DT f d~h0f Ehf 0

ðDT f Þ2 ETf

DS f D~h0f E~hf 0

DS f DT f ETf
10 f
Eh0
Fh00 SCB
FTS AB
@ 0
1
0

D~h0f

B
B
B
B
@

PH T

0

1
B
¼ @ FT h0

Appendix B: Multivariate Optimum
Interpolation[50] For effective data assimilation of remote sensing data
including SSHA and SST, the vertical projection of thevariation included in the SSHA and SST must be properly
combined with the other in situ observation data of subsurface variables such as temperature and salinity. The SSHA ~ho,and vertical profiles of the temperature/salinity T10. . .TMoo
S1 . . .TM gridded as described in Appendix A, were combined with forecast data to create the analysis data that wereused for the incremental analysis update (IAU) process.
[51] For the vertical projection, we used MOI [e.g.,
Lorenc, 1981]. Before the interpolation, we subtracted
basin-wide seasonal variation such
Pas the thermal expansion
from the SSHA, that is, ~h0 = ~h  i hi, where i is the index
of zonal direction. This allowed us to assimilate the data of
the SSH variations associated with mesoscale eddies effectively into the model [Miyazawa et al., 2005a].
[52] The analysis state of SSH and temperature/salinity
profiles X a = (~h0a, T1a, . . ., TNa, S1a, . . ., SNa) was calculated
from both the model forecast X f = (~h0f, T1f , . . ., TNf, S1f , . . ., SNf)o
, S1o, . . .,SNf) and the observation data yo = (~h0o, T1o, . . ., TMoSM) as follows:
X a ¼ X f þ PH T HPH T þ R

1yo  HX f ;ðB1Þ

where P is a forecast error covariance matrix with (2N + 1) 
(2N + 1) elements, H is an observation matrix with (2M +1) (2N + 1) elements that is a linear operator mapping themodel variables into the observed variables, and R is anobservation error covariance matrix with (2M + 1)  (2M + 1)
elements. The forecast error covariance matrix P is thefunction of time and space that depends on the model
physics. For simplicity, however, we assumed that P wasconstant over time and estimated it in advance of the model
calculation.
[53] Suppose, for a simple explanation, that M = N = 1. In
this case, the observation matrix H becomes a 3  3 unit
matrix, and the matrix, PHT, is expressed as follows:
0D

E
2

d~h0f
B

B
PH T ¼ B dT f d~h0f@
dS f d~h0f

 0f f d~h dT
DE
2
dT f
 f f
dS dT

Fh0 T
1

FSh0
FST
0
2
h0f
0
B D~
B
B
0
ðDT f Þ2
@
0
0

1
D~
h0f DS f ESf CC
DT f DS f ESf C
C
A
2 f
f
ðDS Þ ES

0

0

0

ETf

0

0
1

ESf

C
C
C
A

0
ðDS f Þ2

1
C
C
A

ðB3Þ

where Dx denotes the deviation from the time mean x of avariable x. The nondimensional parameters Ehf 0, ETf, ESf, arethe forecast error variances normalized by the simulation
variances. The regression coefficient between variables x
and y denoted by Fxy is expressed as
Fxy ¼

DxDy
Dy2

ðB4Þ

:

Assuming that an observation error of a variable is notcorrelated to that of another, R becomes a diagonal matrix:
0

Eh00
@ 0
0

R

0
ETo
0

0
1
2
h0f
0 B D~
0 AB
0
@
ESo
0

1
0

0

ðDT f Þ200

ðDS f Þ2C
C; ðB5Þ
A

where Eoh0, EoT, EoS, are the nondimensional observation error
variances normalized by the simulation variances.
[54] By using a scaling matrix,
0

1

1

B
B ðDh0f Þ2
B
B
0
S¼B
B
B
B
@ 0

0

0

1
ðDT f Þ2

C
C
C
C
C;
C
C
C
A

0
1

0

ðDS f Þ2

ðB6Þ

the Kalman gain matrix K = PHT (HPHT + R)1 in equation(B1) is expressed as follows:1
K ¼ PH T S HPH T þ R S ;

where

0

1
PH T S ¼ @ FT h0
FSh0

1

 0f fd~h dS
 f fCC
dT dS C;
D
EA
2
dS f

C10015ðB2Þ

andwhere dx and hxi denote the error and expectation of avariable x, respectively. We assumed that each element of
the matrix PHT can be approximately estimated from thevariance of the variables obtained from the model simulation [Mellor and Ezer, 1991]:
22 of 24

Fh 0 T
1
FST
0

10 f
Eh0
Fh00 SB
FTS A@ 0
1
0

1
Fh0 T
1
HPH T þ R S ¼ @ FT h0
FSh0 FST0 o
Eh0 0
þ @ 0 ETo
0
0

0
ETf
0

10 f
Eh 0
Fh00 SB
FTS A@ 0
1
0
1
0
0 A:
ESo

ðB7Þ

0

1

C
0 A
ESf
0
ETf
0

ðB8Þ

0

1

C
0 A
ESf

ðB9Þ

MIYAZAWA ET AL.: KUROSHIO LARGE MEANDER IN 2004

C100152

2

In this way, the simulation variances (D~h0f)2, DT f , DS f
were excluded from the Kalman gain matrix. To simplify
the assimilation system further, some nondiagonal elementsof the forecast error covariance matrix PHT were assumed to
be zero:Fh0 Tm ¼ Fh0 Sm ¼ FTm1 Sm2 ¼ FSm2 Tm1 ¼ 0ðm; m1; m2 ¼ 1; . . . ; M Þ:ðB10Þ

In our system, the analyzed values of temperature (salinity)
were determined from the observed SSHA and temperature(salinity). Because the variation in salinity was weakcompared with the variation in temperature, especially in
the Kuroshio region, the estimation of salinity from SSHA
was not highly effective. Thus, we used 0.1FSh0 in place of
FSh0 in the forecast error covariance matrix.
[55] We constructed the MOI scheme with N = 23.
Temperature and salinity at depths of 0, 5, 10, 30, 50, 75,
100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800,
900, 1000, 1500, 2000, and 3000 m were analyzed. The
parameters used, namely,
Ehf 0 ¼ 0:036; Eho0 ¼ 0:48; ETf m1 ¼ 0:048; ETom1 0:192; ESf m2¼ 0:024; ESom2 ¼ 0:192ðm1 ¼ 2; . . . ; 6; m2 ¼ 1; . . . ; 6Þ;ðB11Þ

were determined empirically.
[56] If the probability distributions of the observation andforecast errors are independent of each other, we obtain thefollowing innovation covariance matrix:Dyo  HX fyo  HX f

TE¼ HPH T þ R:

"

ðB13Þ

#
2
~h0f  ~h0o ah0 e2i ð~h0o Þ;2
ðD~h0 Þ"#ah0 e2i ð~h0 ÞoE~h0 ¼ G
2
ðD~h0 Þ"#2
T0 f  T0 o aT o e2i T0o
ETf 0 ¼ G
ðDT0 Þ2"#aT o e2i T0oo
ET0 ¼ G;
ðDT0 Þ2

Appendix C: Incremental Analysis Update[58] The analyzed values of temperature and salinity on 1/4°
grids with 23 levels were interpolated to the model gridswith 1/12° resolution and 45 levels. To achieve smooth
initialization, we used the IAU scheme, which introduces
the analyzed values of temperature and salinity into themodel [Bloom et al., 1996]. We chose 7 days as the IAU
period, which is consistent with the minimum time scale of
10 days. Some noise in the data due to dynamical inconsistency between the analyzed temperature and salinity andthe model variables was effectively smoothed out using the
IAU, which acted as a kind of low-pass filter with a cutoff
period of 7 days [Kagimoto et al., 2008]. The horizontal
velocities were geostrophically adjusted in response to thepressure gradient created by the model temperature andsalinity as the IAU gradually changed the model temperatureand salinity to the values close to the analysis temperatureand salinity.
[59] Acknowledgments. This work is part of the Japan Coastal
Ocean Predictability Experiment (JCOPE) promoted by Japan Agency for
Marine-Earth Science and Technology (JAMSTEC). We are very thankful
to two anonymous reviewers for their constructive comments on theoriginal manuscript.

References[57] Thus, the amplitudes of the observation and forecast
errors were not determined independently. On the basis ofequation (B13), we introduced an adaptive assimilation
scheme [Kagimoto et al., 2008] for SSHA in the southwestern region (12°– 44°N, 117°– 141°E) and SST in thewhole domain to estimate the forecast error variances asfollows:
E~hf 0 ¼ G

where G[ ] is the Gaussian filter with a zonal scale of 1.4°
(the equator) to 0.7° (60°N) and a meridional scale of 0.4°
(the equator) to 0.2° (60°N), T0 is the SST, and e2i is aninterpolation error covariance estimated in the uniform grid
mapping (Appendix A). The adjustable parameters ah = 0.6
and aT0 = 1 were found using the sensitivity experiments insuch a way that the predicted Kuroshio large meander was
as close to the observed meander as possible. More detailed
descriptions of the data assimilation scheme, including thequality control and parameter settings, are provided by
Kagimoto et al. [2008].

ðB12Þ

Comparing the diagonal components between both sides of(B11) gives a simple relation of the observation and forecast
errors to the model statistics for each variable:

D
E D
E
2
2
E~hf 0 þ E~ho0 D~hf ¼ ~h0f  ~h0o

D
E D
E
2
2
ETf þ ETo DT f ¼ T f  T o

D
E D
E
2
2
ESf þ ESo DS f ¼ S f  S o:

C10015

ðB14Þ

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X. Guo, Center for Marine Environmental Studies, Ehime University, 2-5
Bunkyo-cho, Matsuyama, Ehime, 790-8577, Japan.
T. Kagimoto, Y. Miyazawa, and H. Sakuma, Frontier Research Center for
Global Change/JAMSTEC, 3173-25 Showamachi, Kanazawa-ku, Yokohama,
Kanagawa 236-0001, Japan. (miyazawa@jamstec.go.jp)

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