Progress in Oceanography 135 (2015) 64–76

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Progress in Oceanography
journal homepage: www.elsevier.com/locate/pocean

Effects of the STCC eddies on the Kuroshio based on the 20-year JCOPE2
reanalysis results
Yu-Lin Chang a,⇑, Yasumasa Miyazawa b, Xinyu Guo caNational Taiwan Normal University, Taiwan
Japan Agency for Marine-Earth Science and Technology, Japanc
Ehime University, Japanba r t i c l e

i n f oArticle history:
Received 14 October 2014
Received in revised form 12 March 2015
Accepted 20 April 2015
Available online 25 April 2015a b s t r a c t
In this study, 20 years of model reanalysis data are analyzed to study the effects of the subtropical countercurrent (STCC) eddies on the upstream Kuroshio, from east of Luzon to east of Taiwan. The effects areassessed from individual events to interannual time scales. The wind-driven Kuroshio is modiﬁed by the
STCC eddies, with high spatiotemporal variations. The mass balance in the composite eddy events indicates that the strengthening and weakening of the Kuroshio transport are locally caused by the mass convergence and divergence produced by the eddies. The same analogy applies to the interannual time scale.
In the eddy-rich years, the upstream Kuroshio is generally stronger because of wind forcing, yet thestrengthening is nonuniform because of modiﬁcation by the eddies. The larger number of warm eddies
to the east of Taiwan and Luzon Island further strengthen the jet, whereas the larger number of cold
eddies to the east of the Luzon Strait weaken the Kuroshio in the Luzon Strait. Drifter trajectories show
larger Luzon Strait intrusion during the occurrence of cold eddies. The local weakening of the Kuroshio bycold eddies leads to a weaker potential vorticity jump, producing favorable conditions for the intrusion ofa water mass into the South China Sea.
Ó 2015 Elsevier Ltd. All rights reserved.

Introduction
To the east of Taiwan and Luzon, a weak (a few cm s1) andshallow (100 m) zonal current—the North Paciﬁc subtropicalcountercurrent (STCC)—penetrates thousands of kilometers into
the Paciﬁc Ocean. The STCC was ﬁrst observed by Uda andHasunuma (1969) and later conﬁrmed by White et al. (1978).
The STCC is supported by subsurface density fronts produced by
low potential vorticity mode water formed in the Kuroshio andKuroshio Extension (Kobashi and Kubokawa, 2012). The STCC
region is an enhanced eddy variability zone (Fig. 1), populated by
eddies with typical diameters of 150–300 km and sea surface
height (SSH) anomalies of ±0.1 m (Chelton et al., 2011). The STCC
eddies are spawned through baroclinic instability of the STCC
and the North Equatorial Current system (Qiu, 1999). The sea surface temperature front in northern STCC also contributes to thegrowth of instability (Kobashi and Xie, 2012; Chang and Oey,
2014a). The eddies are distributed from the east of Taiwan to thedateline and from 17°N to 27°N (Chang and Oey, 2014b). Hwanget al. (2004) demonstrated that the movement of mesoscale eddies⇑ Corresponding author at: Institute of Marine Environmental Science andTechnology, 88, Sec 4, Tingzhou Rd., Taipei 10677, Taiwan.
E-mail address: ychang@ntnu.edu.tw (Y.-L. Chang).
http://dx.doi.org/10.1016/j.pocean.2015.04.006
0079-6611/Ó 2015 Elsevier Ltd. All rights reserved.

near 22°N was 8 km/day, based on TOPEX/Poseidon altimeterdata. The STCC eddies propagate westward and approach the
Kuroshio east of Taiwan and Luzon in a month to a year, dependingon the locations at which the eddies are formed. The propagation
direction can change when eddies encounter topography or interact with other eddies.
The Kuroshio is the western boundary current in the North
Paciﬁc Ocean. It ﬂows through the area east of Luzon Island andTaiwan with a mean speed of 1–1.5 m/s (Nitani, 1972). The impactof eddies on the Kuroshio has been addressed in previous studies.
Waseda et al. (2002) were one of the ﬁrst to study the Kuroshio–
eddy interaction using a high-resolution regional general circulation model. They revealed that mesoscale eddies to the southeastof Kyushu were associated with the Kuroshio meander to the southof Japan. Ebuchi and Hanawa (2003) found that the interaction
between mesoscale eddies, for both anti-cyclonic and cycloniceddies and the Kuroshio east of Kyushu, can sometimes trigger
the Kuroshio meander south of Japan. Miyazawa et al. (2004) suggested that eddies further to the south (east of Taiwan and thePhilippines) can affect the Kuroshio meander. Recent work by
Soeyanto et al. (2014) indicated that the correlation between the
Paciﬁc Decadal Oscillation (PDO) index and the Kuroshio dropped
signiﬁcantly after 2002 because of the enhanced mesoscale eddies
southeast of Taiwan.

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65

Fig. 1. Root-mean-squared SSH variability in the western North Paciﬁc. SSH variability (m) is based on 20-year JCOPE2 data (left panel). Model bathymetry (m, color shading)is superimposed by the 20-year mean surface current trajectories (gray vectors, right panel). The right panel is a close-up of the domain in the black box in the left panel. (Forinterpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Although there is much evidence for the possible connection
between eddies and the Kuroshio, classical theory suggests thatthe western boundary current obeys the Sverdrup dynamics.
Sverdrup transport, controlled by large-scale wind stress curl, ismainly seasonal in the western Paciﬁc subtropical gyre region.
Based on the 20-month ADCP measurement, Johns et al. (2001)
observed that the Kuroshio transport east of Taiwan ﬂuctuatedover a period of 100 days instead of seasonally. Recently, using
the 29-year tide-gauge sea level difference as a proxy for
Kuroshio transport, Chang and Oey (2011) revealed that theensemble seasonal signal is barely signiﬁcant and suggested a possible contribution of mesoscale eddy signals to the Kuroshio transport. STCC eddies, the origin of which could be associated withbasin-scale water mass variability in the Western North Paciﬁc(Kobashi and Kubokawa, 2012), are thought to be an importantfactor in modulating Kuroshio transport. Yang et al. (1999) demonstrated coherence between the tide gauge sea level difference (as aproxy for the Kuroshio) northeast of Taiwan (25°N) and the SSH
anomaly (SSHA) from satellite altimeter data. Zhang et al. (2001)
concluded that the 100-day Kuroshio transport ﬂuctuation
observed by Johns et al. (2001) is related to STCC eddies. Using
the cruise XBT/XCTD data from south of Taiwan (21–22°N),
Gilson and Roemmich (2002) observed that the Kuroshio geostrophic transport is reduced with the approach of the cycloniceddies, while the warm eddies correspond to the strengtheningof transport. The 8-year modeling simulation of Lee et al. (2013)
showed that the Kuroshio transport variation east of Taiwan(24°N) is related to the impinging eddies. Using observed 1-year
mooring data, a recent study by Lien et al. (2014) suggested that
the Kuroshio transport near the Luzon Strait (18.75°N) is modulated by the eddies. They found that the transport anomaly related
to eddies can vary from 5 to 10 Sv (1 Sv = 106 m3 s1), with a temporal order of 10 days.
The possible link between the STCC eddies and the Kuroshio has
been addressed in previous studies using observations and numerical simulations. In the earlier works, the Kuroshio transport was
taken from a ﬁxed section and a limited time period. Most of theresults lead to the conclusion that warm eddies correspond to anincrease in transport and cold eddies are related to transport
reduction. However, results with the opposite conclusion have also

been reported (Zhang et al., 2001). In this work, we revisit thisproblem by extending the spatial and temporal scale based on
the effective use of a recently produced comprehensive data product. Use of 20-year (1993–2012) eddy-resolving reanalysis data
(JCOPE2; Miyazawa et al., 2009) allows us to perform intensive
analysis of the entire upstream Kuroshio from east of Luzon
Island to east of Taiwan for 1000 km (Fig. 1), together with relevant mesoscale eddy activity. The longer temporal and larger spatial scales encompass more eddy events. By using composites ofinterannual and eddy events, we aim to obtain more general conclusions and to answer the following questions through the principleof mass balance. Does Kuroshio transport change with the presenceof eddies? If so, how does the transport change? Why does thetransport change? Are there spatial and temporal variations?

Data
JCOPE2 is a data-assimilated ocean model based on the
Princeton ocean model, with a generalized coordinate system(Mellor et al., 2002). The domain encompasses the western North
Paciﬁc Ocean (10.5–62°N, 108–180°E) with a horizontal resolutionof 1/12° and 46 vertical layers. The lateral boundary conditions are
determined from the basin-wide model, using a one-way nesting
method (Guo et al., 2003). The model is forced by wind stresses,
heat, and salt ﬂuxes from the six-hourly NCEP/NCAR reanalysis
data. The AVHRR/MCSST, USGODAE project (http://www.usgodae.
org/), CCAR Near Real-Time Altimetry Data (http://eddy.colorado.
edu/ccar/data_viewer/index), and GTSPP in situ temperature andsalinity are assimilated into the model. Details of the model setup
and descriptions can be found in Miyazawa et al. (2009).
The reduced-gravity (RG) model domain includes the North
Paciﬁc Ocean from 15°S to 70°N and from 99°E to 310°E. The horizontal resolution is 0.1°. The coastline is deﬁned as the 200-m isobath. The RG model is forced by the NCEP/NACR 6-hourly
reanalysis wind, as used in JCOPE2. The simulation period is also
the same as in JCOPE2, from 1993 to 2012. Parameters are given
in Table 1. The RG model dynamics are dominated by wind-forced
linear Rossby waves and Sverdrup balance. It does not have STCC
eddies because of the inexistence of baroclinic instability. In this66

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Table 1
Parameters used in the North Paciﬁc Ocean reduced-gravity model.
Parameter0g
H
HcoastaN

MeaningValue

gDq /qMean ‘‘upper-layer’’ depth
Isobath where coastline is deﬁned
Newtonian cooling coefﬁcient

0.02 m s2
600 m
200 m
1.25  103 day1

work, the RG model is used to represent the wind-driven Kuroshio
condition.
Results
The Sverdrup transport in the North Paciﬁc Ocean is computedon the basis of the NCEP/NCAR reanalysis wind data. Fig. 2a shows
the Sverdrup transport east of Taiwan at 23.2°N. The signal is led
by seasonal variation with weak interannual variation; however,the seasonal signal is barely visible in the modeled Kuroshio transport (Fig. 2b). The Kuroshio penetrates to a depth of 600–800 m
east of Taiwan (Johns et al., 2001) and is shallower (500 m) eastof Luzon Island (Qu et al., 1998). In this work, the Kuroshio transport is integrated vertically from the surface to z = 600 m, andhorizontally from the deﬁned western to eastern edges. The width(edge) of the Kuroshio is time-dependent. Apart from the ﬁxed
continental boundaries (Taiwan and Luzon Island), the edges of
the Kuroshio are deﬁned using its 0.1 m/s speed contours (Lianget al., 2003). The 20-year mean modeled transport east of Taiwan
at 23.2°N is 20.4 Sv. This agrees with the 20-month observed meantransport of 21.5 Sv with 2.5 Sv uncertainty, as reported by Johns
et al. (2001) using a 20-month moored current meter array eastof Taiwan. The modeled Kuroshio transport shows a higher frequency variation than the seasonal signal from Sverdrup transport.
The leading frequency is 80–100 days, according to the power
spectra analysis. The 100-day variability of the Kuroshio is closely associated with the STCC eddies (Zhang et al., 2001). Taking
the upstream Kuroshio transport at 16.2°N, the correlation was
performed with the downstream (northward) Kuroshio at any
given latitude from 16.2°N to 25.8°N (Fig. 3) on an interannual time

Fig. 3. Correlation coefﬁcient between the Kuroshio transport (0–600 m) at a given
latitude and that at 16.2°N from the model. Black lines indicate where thesigniﬁcance levels exceed 95%; gray lines are insigniﬁcant.

Fig. 2. Sverdrup and modeled Kuroshio transport. (a) NCEP/NCAR reanalysis of wind-derived Sverdrup transport at 23.2°N. (b) Modeled transport at 23.2°N. Superimposed
red lines are the 360-day low-pass for interannual variation. The x-axis represents time in years; the y-axis represents transport in Sv. (For interpretation of the references tocolour in this ﬁgure legend, the reader is referred to the web version of this article.)Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

67

Fig. 4. (a) Eddy area anomaly for the model (solid line) and AVISO (dashed line). Red and blue shading indicate the eddy-rich and eddy-poor years, respectively. Numbers
show the correlation coefﬁcient and 95% signiﬁcance level between the model and AVISO. The x-axis represents time, and the y-axis represents the area anomaly (%).
Composites of eddy-rich years minus eddy-poor years for (b) SSH (m) from the 3D model; (c) 0–600 m mean meridional velocity (m/s) from the 3D model; (d) upper layer
depth (m) from the RG model; and (e) meridional velocity (m/s) from the RG model. (For interpretation of the references to colour in this ﬁgure legend, the reader is referredto the web version of this article.)

Fig. 5. SSHA at 125°E. SSHA are given in m. The x-axis represents time from 1993 to 2012; the y-axis represents latitude from 17°N to 27°N. Magenta lines mark the positionsof the Luzon Strait (18.5–22°N) and Taiwan (22–25.5°N). Solid and dashed black lines denote the chosen warm and cold eddy events, respectively. (For interpretation of thereferences to colour in this ﬁgure legend, the reader is referred to the web version of this article.)68

Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Fig. 6. Composite SSHA (m) based on the chosen eddy events for eddies to the east of the Luzon Strait (a and b) and for eddies to the east of Taiwan (c and d). (a and c) are cold
eddies; (b and d) are warm eddies. From left to right, columns are Days 0, 30, and 60 after the eddy has passed through 125°E. The 20-year mean SSH is removed. Magenta
dots indicate the chosen Kuroshio section that is used in Fig. 7. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version ofthis article.)scale (with a 360-day low-pass ﬁlter). The correlation coefﬁcient
rapidly dropped to 0.2 downstream at 18°N latitude, andremained low (<0.4) further downstream. The subtropical windgyre is on the scale of thousands of kilometers. If the Kuroshio is
mainly wind-driven, a good correlation is expected within a distance of hundreds of kilometers on the interannual time scale.
The Kuroshio transport variations diverge markedly from east of
Luzon Island to east of Taiwan. The faster temporal and spatialchanges of the Kuroshio indicate that the Kuroshio transport is
not only driven by Sverdrup dynamics but is also affected by otherfactors such as eddies.
The Okubo–Weiss method (Okubo, 1970; Weiss, 1991) is used
to detect eddies (Chang and Oey, 2014b). The Okubo–Weissparameter W is deﬁned as

W ¼ s2n þ s2s  x2

ð1Þ

where sn = ou/ox  ov/oy and ss = ov/ox + ou/oy are the normal andshear components of strain, respectively, and x = ov/ox  ou/oy is
the relative vorticity. The identiﬁcation criterion for an eddy is anegative value of W, in which vorticity is the leading component
relative to the two strain terms. Following Isern-Fontanet et al.
(2006), an eddy is deﬁned as a region in which the value of W issmaller than a threshold W0 (1.71  1012 s2), whereW0 = 0.2rw, and rw is the spatial standard deviation of W. Afterthe eddies are identiﬁed, the eddy area (m2) in the STCC region(130°E–180°E, 17°N–27°N) is ﬁrst integrated and subsequently
normalized by the sum of the STCC region to represent the eddy
occupation in STCC region (Peddy, %). The eddy occupation is calculated for both AVISO and the model (Fig. 4a). The interannual variation of eddy occupation is well represented by the model, with ahigh correlation between the model and satellite observations of
0.89 (95% signiﬁcance level = 0.39). The model interannual variationof eddy occupation is similar to the leading empirical orthogonal
function mode of the eddy kinetic energy from Qiu and Chen
(2013) computed using satellite altimetry data. The STCC eddies
covered 31.3% and 30.8% of the STCC region from AVISO and themodel, respectively. The eddy-rich and eddy-poor years are deﬁned
as a positive Peddy anomaly and a negative Peddy anomaly, respectively (Fig. 4a). The eddy-rich years were mid-1995 to 1998 and2003 to mid-2008; the eddy-poor years were 1999 to 2002 andmid-2008 to 2012. Weaker variation prior to mid1995 was
excluded from this work. We grouped the model variables according to the deﬁned periods when assessing the difference between
eddy-rich and eddy-poor years. Fig. 4b–d shows the composite difference between the eddy-rich years and the eddy-poor years forY.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

69

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
Fig. 7. Corresponding composite vertical proﬁle of speed change along the Kuroshio path after eddies start to interact with the Kuroshio. Speed is deﬁned as ( u2 þ v 2 , m/s),
and the Kuroshio path is shown by the dotted contour in Fig. 6. After 10 days (left), 30 days (middle), and 50 days (right). Black dotted lines mark the Luzon Strait (a and b),
and Taiwan (c and d). Values that exceed the color scale are contoured. The contour interval is 0.05 m/s. The x-axis represents latitude; the y-axis represents depth in meters.

the three-dimensional (3D) model (JCOPE2) and the RG model. A
positive SSH difference (Dg) with a circular shape is observed in
the STCC region from the 3D model (Fig. 4b). This indicates that
there are more warm eddies during the eddy-rich years than in
the eddy-poor years (17.2% vs 15.2%, with signiﬁcance >99%). The
0–600 m mean meridional velocity difference (Dv) is positive inthe upstream Kuroshio from east of Luzon Island to east of
Taiwan (Fig. 4c). The velocity difference in the upstream Kuroshio
between the eddy-rich and the eddy-poor years accounts for 15–
20% of mean velocity. Strengthening of the jet during the eddy-richyears is consistent with previous ﬁndings (Chang and Oey, 2011).
However, the stronger Kuroshio during the eddy-rich years is notnecessarily controlled by the eddies: the wind-driven dynamic
included in the 3D model should also be considered. The composite
is repeated for the RG model to check the effect of wind. The upper
layer depth difference (DULD) in the STCC region is positive(Fig. 4d). The mesoscale circular shape structure vanished becauseof the inexistence of eddies in the RG model. A stronger Kuroshiowith positive meridional velocity difference (DvRG) is also observed(Fig. 4e). Chang and Oey (2012) suggested that the eddy-rich years
correspond to a positive Philippines–Taiwan Oscillation (PTO) when
the subtropical wind stress curl is highly negative. The stronger
subtropical wind stress curl forces a stronger subtropical gyre
(DULD > 0), leading to a stronger Kuroshio (DvRG > 0) during theeddy-rich years. The wind forcing explains the strengthening of
the Kuroshio during the eddy-rich years.
There is a spatial difference between the 3D model and the RG
model (Fig. 4c and e). The wind-forced strengthening of theKuroshio is strongest near the Luzon Strait (Fig. 4e); however,the strongest enhancement in the 3D model is observed east of
Taiwan (Fig. 4c), and Dv near the Luzon Strait is not as distinct
as that to the east of Taiwan. The discrepancy between the 3D
and RG model suggests that STCC eddies may play a role in modifying the Kuroshio strength.
The Kuroshio and eddies
Fig. 5 plots the SSHA at 125°N with time, in which abundanteddies are visible in the STCC area near the Kuroshio. The eddies
are widely distributed in time and space. To analyze the effect ofeddies on the Kuroshio, we ﬁrst subdivided the domain meridionally into east of the Luzon Strait (18.5°N–22°N) and east of Taiwan(22°N–25.5°N). Grouping eddies geographically can help to avoid
spatial signal cancelation. The composite was based on thoseeddies that passed through 125°E. Stronger eddies were chosen
as a composite for each subdomain: eight warm eddies (solid lines
in Fig. 5) and eight cold eddies (dashed lines in Fig. 5). Thirty-two
eddies in total were sorted into four sets: warm and cold eddies

70

Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Fig. 8. Corresponding composite change in Kuroshio transport after eddies start to interact with the Kuroshio. Kuroshio transport is for 0–600 m depth, in Sv. Values thatexceed the color scale are contoured. The contour interval is 2 Sv. The x-axis represents time in days; the y-axis represents latitude. (For interpretation of the references tocolour in this ﬁgure legend, the reader is referred to the web version of this article.)near Taiwan and the Luzon Strait. Fig. 6 shows composites of the
SSHA (20-year mean removed) for Days 0, 30, and 60 after eddies
pass through 125°E. On Day 30, the edges of the eddies arrive at122.5°E (Fig. 6a, b, and d). The composite cold eddy east of
Taiwan moves more slowly (Fig. 6c), reaching 123°E on Day 30.
On Day 60, the western edge of the eddies remains at 122°E,
where the eddies weaken and are advected northward by the
Kuroshio. To the east of the Luzon Strait, eddies are carried toward
southern Taiwan (Fig. 6a and b), and eddies east of Taiwan are
moved toward the East China Sea shelf (Fig. 6c and d).
The time axis is redeﬁned in the following analysis. The dates
when the edges of the eddy (0.1-m SSH contour) meet the easternedge of the Kuroshio (0.1-m/s speed contour) are deﬁned as Day 0.
A section along the Kuroshio was chosen (dots in Fig. 6) to illustrate the effect of eddies on the Kuroshio. Fig. 7 shows the correpﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
sponding vertical proﬁle of the speed ( u2 þ v 2 ) anomaly along
the chosen section on Days 10, 30, and 50 after the eddy starts tointeract with the Kuroshio. The Kuroshio speed on Day 0 is subtracted. The Kuroshio responds differently to warm and cold eddies

after the eddies start to interact with the jet. The Kuroshio becomes
weaker when affected by the cold eddies (Fig. 7a and c) and isstrengthened by the warm eddies (Fig. 7b and d). The interaction
time of the eddies and the Kuroshio is around 50 days. The change
in the Kuroshio speed reaches 0.2–0.25 m/s on Day 30 in the upper
200 m. The effect of eddies on the Kuroshio can penetrate toz = 600 m. Notably, a speed anomaly with the opposite sign isalso observed to the north/south of the composite area. In the caseof the cold eddy east of Luzon Strait (Fig. 7a), the decrease in the
Kuroshio speed is sandwiched by a positive anomaly to its northand south. The increase in the Kuroshio speed corresponds to thewarm eddies to the north and south of the Luzon Strait (Fig. 6a).
Similarly, in the case of the warm eddy east of Taiwan (Fig. 7d),the observed negative speed anomaly is associated with the cold
eddy south of Taiwan (Fig. 6d).
A single chosen section is by itself insufﬁcient to represent theoverall Kuroshio. The corresponding composite Kuroshio transport
is demonstrated in Fig. 8. The Kuroshio transport is computed
following the same deﬁnition as used in Fig. 2b. Again, Day 0 of

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71

Fig. 9. Schematic plot for corresponding composite volume transport balance. Units are Sv. Color contours are SSHA: red for positive, blue for negative. The contour interval is0.05 m. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

the anomaly is when the eddy starts to interact with the Kuroshio(similar to that in Fig. 7). The changes in transport are large
between Day 10 and Day 50. The effect of the eddy on the
Kuroshio can last for more than 50 days until the eddy dissipates,
advects, or joins the Kuroshio. The transport anomaly is 3–10 Sv.
Transport variation relating to eddies accounts for 20–50% of
Kuroshio mean transport (20.7 Sv, Fig. 2b). The results show thatthe cold eddies correspond to a decrease in Kuroshio transport,
and warm eddies are associated with an increase in transport. In
the transport composite, the transport anomaly north/south of
the target area can also vary with the opposite sign, as observed
for the speed proﬁle (Fig. 7).

Why does the Kuroshio transport change?
The STCC eddies are nonlinear. The nonlinear feature is determined by the metric U/c > 1, where U is the maximum circum-averaged gyratory speed within the eddy interior and c is thetranslation speed of the eddy (Chelton et al., 2011). The estimatedU/c from the model in the STCC region is 2–4 and can sometimes
reach 6–7. The nonlinearity implies that there is trapped ﬂuid
within the eddy interior (Flierl, 1981). Both warm and cold eddies
carry mass. One may expect that as the eddy approaches theKuroshio the eddy mass is merged into the jet. The Kuroshio transport should increase for both warm and cold eddies. However, the72

Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Fig. 10. Composite SSHA. SSHA are in m for (a) eddy-rich years, and (b) eddy-poor years. Black trajectories indicate the model surface current anomalies. Red dots indicate the20-year mean Kuroshio eastern edge. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 11. Schematic plot for corresponding composite volume transport balance. (a) Eddy-rich years. (b) Eddy-poor years. Contours are SSHAs: black for positive, gray for
negative. The contour interval is 0.01 m.

results reveal that the Kuroshio responds differently to warm andcold eddies. Why does the Kuroshio transport decrease in the caseof the cold eddy? In the case of the warm eddy, is the increase inKuroshio transport maintained entirely in the downstream?
Analysis of mass balance is performed to answer these questions.
The same composite cases are used. The transports are computed
along the given sections (Fig. 9). Two boxes are chosen in eachcomposite case. One box is located southwest of the eddy, andthe other is chosen to be northwest of the eddy. The two boxes
can, therefore, show how the Kuroshio transport changes upstream
and downstream of the eddy. The transport is again integrated for
0–600 m. The box is not completely closed, with an open boundary
at z = 600 m. The transport is averaged from Day 10 to Day 60
after the eddy begins to interact with the Kuroshio. The transport

anomalies are referred to Day 0 (similar to that in Figs. 7 and 8).
Fig. 9 shows a schematic plot of the mass balance. The mass balance shows that the Kuroshio transport changes nonuniformly inspace. In the case of the cold eddy, the Kuroshio transport
decreases west of the eddy; however, the upstream and downstream Kuroshio transport increase or are nearly unchanged(Fig. 9a and c). The condition is reversed in the case of the warm
eddy (Fig. 9b and d). The spatial nonuniform Kuroshio transport
is due to the mass convergence and divergence inﬂuenced by theeddy. The cold eddy produces mass convergence to its north(downstream of the jet) and divergence to its south (upstream of
the jet), leading to a decrease in the Kuroshio transport. The warm
eddy rotates in the opposite direction, resulting in mass convergence in the south and divergence to the north, leading to an

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73

The western sections are located in the Luzon Strait. For theLuzon Strait (Fig. 9a and b), the total transport (south plus north)at the Luzon Strait is similar for both warm and cold eddy cases.
For the individual section, larger mass ﬂuxes are observed in thecase of cold eddies (Fig. 9a) than for warm eddies (Fig. 9b). The cold
eddy weakens the Kuroshio east of Luzon Strait. The weakening
western boundary current tends to form a westward loop whenit meets the gap (Sheremet, 2001). The larger mass ﬂux in the caseof the cold eddy implies that the Kuroshio loops into the South
China Sea more than in the case of the warm eddy. Further details
pertaining to this subject will be discussed in a later section.
How do eddies affect the Kuroshio in the eddy-rich and the eddy-poor
years?

Fig. 12. Schematic plots of (a) how eddies inﬂuence the Kuroshio transport. The
dashed red arrow represents the original state. Pink shading indicates the transport
after eddy inﬂuence on the Kuroshio; and (b) how eddies modify the Kuroshio fromwind-driven dynamics for eddy-rich years minus eddy-poor years. Gray rectangularboxes denote the relative locations of Taiwan (TW) and Luzon Island (LZ Is). The left
panel represents the wind-driven reduced-gravity (RG) Kuroshio (Fig. 4e), themiddle panel is the composite eddy (Fig. 10a minus Fig. 10b), and the right panel
shows the Kuroshio from the 3D model (Fig. 4c). The wider the arrow, the larger the
Kuroshio transport. Cold eddies are drawn in blue; warm eddies in gold. (Forinterpretation of the references to colour in this ﬁgure legend, the reader is referredto the web version of this article.)increase in the Kuroshio transport. A warm eddy is observed to thesouth of the cold eddy (Fig. 9a and c), forming a dipole structure.
The dipole eddies produce 8–9 Sv transport change in 200–
300 km, resulting in strong divergence between the two eddies(southern box in Fig. 9a and c).
In addition, two more sections along the Kuroshio (Fig. 1b) in
the East China Sea are chosen (green lines in Fig. 9) to check theeffect of eddies on the downstream Kuroshio. For eddy cases of
the Luzon Strait (Fig. 9a and b), the Kuroshio in the East China
Sea is affected by separate local events. The increase (decrease)of transport in the southern (northern) section is due to the local
warm (cold) eddies (Figs. 9a, 6a, and 8a). For eddy cases east of
Taiwan, the southern section is affected by the same composite
eddy event east of Taiwan, which results in a response similar to
the eastern section of the northern box (Fig. 9c and d). The northern sections are not inﬂuenced by the same eddies east of Taiwan.
This regional dependence has been addressed by Soeyanto et al.
(2014). They found that the Kuroshio transport in the southern
East China Sea depends on the STCC eddies, while the transport
in the northern East China Sea is related to the eddies from northeast of the Okinawa Islands.

A difference between the 3D and RG models in terms ofstrengthening of the Kuroshio is noticed (Fig. 4). The difference
can be explained by the eddies. The interesting tri-pole eddies
are shown in the SSHA composite (Fig. 10). During the eddy-richyears, warm eddies are observed to the east of Taiwan and eastof Luzon Island, while a cold eddy appears to the east of theLuzon Strait (Fig. 10a). This indicates that more warm eddies
appear to the east of Taiwan and Luzon Island, and more cold
eddies arrive east of the Luzon Strait during the eddy-rich years.
The situation is reversed during the eddy-poor years. To demonstrate how eddies modify the Kuroshio transport from the winddriven dynamics, the transport balance analysis is repeated forthe eddy-rich and eddy-poor years (Fig. 11). The Kuroshio transport anomaly is referred to the 20-year mean transport. Threeboxes are constructed to examine the change in the Kuroshiotransport caused by the eddies. The interannual composite SSHA
is weaker than the composite chosen eddies (approximately aquarter of the strength of the stronger cases; Figs. 10 and 6). The
composite transport changes are therefore weaker; however, theresults are consistent with the composite cases (Figs. 11 and 9).
The Kuroshio transport is strengthened by the wind more during
the eddy-rich years than during the eddy-poor years (RG model,
Fig. 4). The Kuroshio transport varies spatially in response to theeddies. During the eddy-rich years, a cold eddy to the east of theLuzon Strait is sandwiched by warm eddies to its north and south.
The convergence and divergence produced by these eddies
enhance the Kuroshio transport to the east of Taiwan and Luzon
Island, and reduce the transport to the east of the Luzon Strait.
The reverse applies during the eddy-poor years (Fig. 11b).
Schematic plots of how the eddies inﬂuence the Kuroshio forindividual cases and how eddies modify the Kuroshio from thewind-driven dynamic for the composite difference between the
eddy-rich and eddy-poor years (Figs. 4 and 10, respectively)
are shown in Fig. 12.
Effect of eddies on the Kuroshio in the Luzon Strait
The effect of the local weakening of the Kuroshio transport in
the Luzon Strait by cold eddies is demonstrated using modeled
drifter trajectories. The same composite events are used. Drifters
are released at 125°E, from 18.5°N to 22°N, at z = [0, 25, 50,
100, 200, 300, 400 m], and tracked for 90 days. Drifters are tracked
horizontally using the scheme of Awaji et al. (1980). Drifters forwarm and cold eddy cases are sorted separately. The drifter trajectories are composited into one map and called the visitation
frequency (VF). The VF represents the frequency of the drifter arrival for each grid. The VF shows both the path and frequency of thedrifters’ trajectories. Fig. 13 shows the composite VF for warm andcold eddies at the surface, z = 100 m, and z = 200 m. Drifters in
the warm eddy cases are unable to travel from the Paciﬁc Ocean
to the South China Sea (Fig. 13, right panel). Conversely, composite

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Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Fig. 13. Composite visitation frequency. Left panels show cold eddy cases, right panels show warm eddy cases, at (a) z = 0 m, (b) z = 100 m, and (c) z = 200 m. The unit istime normalized by 100.

cold eddy cases show clear Luzon Strait intrusion (Fig. 13, left
panel). Similar phenomena are observed at each depth. The cold
eddies weaken the Kuroshio transport east of the Luzon Strait(Fig. 9a). The larger mass ﬂuxes in the Luzon Strait imply loopingof the Kuroshio (Fig. 9a). Sheu et al. (2010) suggested that when
the Kuroshio weakens, it tends to form a westward loop in theLuzon Strait and exhibits a weaker potential vorticity jump acrossthe jet. The weaker jet interacts with the eddies, resulting in a
westward vector due to their mutually induced velocity. In suchcases, the eddies are likely to propagate westward, entering theSouth China Sea. This applies to both warm and cold eddies. The
potential vorticity (PV) is deﬁned as

PV ¼

@q
ðf þ fÞ@zð2Þ

Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Fig. 14. Composite 0–100 m (solid lines) and 0–600 m (dashed lines) mean PV forwarm (red) and cold (blue) eddy events in the Luzon Strait (mean from 19.5°N to22°N) for the mean from Day 10 to Day 60 after the eddy interacts with the
Kuroshio. The x-axis represents longitude. The values shown in table are the PV
jump: the unit is m1 s1. (For interpretation of the references to colour in thisﬁgure legend, the reader is referred to the web version of this article.)

Fig. 15. Drifter distribution for (a) cold and (b) warm eddies on Day 0 (black), Day
40 (blue), and Day 140 (red). Gray contours show the SSHA on Day 0. Positive values
are shown by solid contours: negative values are dotted. (For interpretation of thereferences to colour in this ﬁgure legend, the reader is referred to the web version ofthis article.)75

where q is the density, f is the Coriolis parameter, and f is therelative vorticity. The PV jump is the PV change across the jet. The
PV is again composited according to the selected eddy events, using
the mean from 19.5°N to 22°N and from Day 10 to Day 60. The PV isplotted across the Kuroshio in the Luzon Strait for warm and cold
eddy cases (Fig. 14). Vertical averaging was performed over 0–100 m,
where the Kuroshio shows a larger change, and 0–600 m. The PVjump is weaker for cold eddies than for warm eddies, indicating
more favorable conditions for intrusion during the former. The values of the 0–100 m (0–600 m) mean PV jump from Day 10 to Day 60
for warm and cold eddies were 4.7  107 (2.7  107) and 3  107
(1.6  107) m1 s1, respectively. The local weakening of theKuroshio by cold eddies generates favorable conditions for drifters
to penetrate the Kuroshio and enter the South China Sea. The looping of the Kuroshio, along with the stronger westward intrusion,increases water exchange between the Paciﬁc Ocean and theSouth China Sea. The same reasoning can also be applied to theinterannual composite (Figs. 10 and 11).

Summary and discussion
The effect of eddies on the upstream Kuroshio was analyzed
using the 20-year JCOPE2 reanalysis data. The change in Kuroshiotransport is locally inﬂuenced by eddies caused by mass convergence and divergence. The warm eddies enhance the strength of
the Kuroshio because of upstream convergence and downstream
divergence. Conversely, cold eddies tend to weaken the Kuroshio
because of the opposite rotation direction of the eddy. The same
analogy applies to a composite of the eddy-rich and eddy-poor
years. During the eddy-rich years, more warm eddies occur tothe east of Taiwan and Luzon Island, whereas the east of theLuzon Strait is dominated by cold eddies. The convergence anddivergence produced by the triple eddies modify the Kuroshiotransport from wind-driven dynamics. The strongest Kuroshio
observed to the east of the Luzon Strait in the wind-driven RG
model is weakened by cold eddies, while the Kuroshio velocities
to the east of Taiwan and east of Luzon Island are strengthened
by warm eddies. This distribution is reversed during the eddy-poor
years.
Local weakening of the Kuroshio in the Luzon Strait by cold
eddies leads to a weaker PV jump across the jet. The weaker PV
jump in the case of cold eddies allows greater intrusion into theSouth China Sea, increasing the water exchange between it and
the Paciﬁc Ocean.
The effect of the eddy on the Kuroshio transport is local.
However, the eddy signal may be carried downstream. An example
is given in Fig. 15 using drifter tracking analysis (the same tracking
scheme as Fig. 13) to track the path of drifters. The drifters are
released inside the eddy where the absolute SSHA is larger than
0.2 m on Day 0, when the eddy starts to interact with the
Kuroshio. A warm eddy event and a cold eddy event east of
Taiwan are demonstrated. The analysis is repeated in other eddy
cases. The results are similar, as illustrated in Fig. 15. In the caseof the cold eddy on Day 40, the eddy becomes weaker and someof the drifters are still kept inside the eddy; however, others areadvected toward the downstream Kuroshio in the East China Sea(Fig. 15a). On Day 140, the original eddy has disappeared as a resultof dissipation. The distribution of drifters is no longer in circularshape, and some of the drifters are located to the south of Japan.
Generally similar results are obtained in the warm eddy case(Fig. 15b). However, the details of the drifter dispersion appear
complicated and could be affected by the chaotic advection activated by the eddy–Kuroshio interaction (Waseda and Mitsudera,
2002). This result implies that eddies in the upstream Kuroshio
may propagate downstream, thus affecting the downstream

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Y.-L. Chang et al. / Progress in Oceanography 135 (2015) 64–76

Kuroshio. The link between the eddies east of Taiwan and the large
meander south of Japan was noticed by Miyazawa et al. (2004).
Chang and Oey (2012) suggested that the STCC eddies may trigger
the large meander south of Japan during the positive PTO phase(eddy-rich years). Other possible effects on the downstream
Kuroshio are worth exploring.
This study demonstrates the mass ﬂux in the Kuroshio region.
The transport shows mass ﬂux between the Kuroshio and theeddies. However, the water mass exchange between the Kuroshio
and the eddies remains unclear. More detailed analyses including
water mass variability and chaotic transport are needed to investigate the details of the eddy–Kuroshio interaction processes.
Moreover, eddies can affect not only the mass transport but also
the heat ﬂuxes. Further study is required to understand how the
warm and cold eddies change the Kuroshio heat ﬂuxes and how
this may affect the ecosystem.
Acknowledgments
The authors would like to acknowledge the AVHRR/MCSST,USGODAE project, CCAR Near Real-Time Altimetry Data, and the
GTSPP for providing data for model simulations. YLC is supportedby the Ministry of Science and Technology of Taiwan under grant102-2611-M-003-003-MY3. X.G. and Y.M. are supported by JSPS
KAKENHI (26287116). Comments from reviewers were useful for
improvements of the earlier versions of the manuscript.
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