J Oceanogr (2012) 68:63–77
DOI 10.1007/s10872-011-0049-9

SPECIAL SECTION: ORIGINAL ARTICLE

New developments in mode-water research: Dynamic
and climatic effects

The role of meso-scale eddies in mixed layer deepening and mode
water formation in the western North Pacific
Shinya Kouketsu • Hiroyuki Tomita •Eitarou Oka • Shigeki Hosoda • Taiyo Kobayashi •Kanako Sato

Received: 25 January 2011 / Revised: 31 May 2011 / Accepted: 6 June 2011 / Published online: 2 July 2011Ó The Oceanographic Society of Japan and Springer 2011

Abstract Distributions of mixed layer depths around thecenters of anti-cyclonic and cyclonic eddies in the North
Pacific Ocean were composited by using satellite-derived
sea surface height anomaly data and Argo profiling float
data. The composite distributions showed that in late
winter, deeper mixed layers were more (less) frequently
observed inside the cores of the anti-cyclonic (cyclonic)
eddies than outside. This relationship was the clearest in
the region of 140°E–160°W and 35°N–40°N, where thetemperature and salinity of the deep mixed layers weresimilar to those of the lighter variety of central mode water(L-CMW). A simple one-dimensional bulk mixed layer
model showed that both strong sea-surface heat andmomentum fluxes and weak preexisting stratification contributed to formation of the deep mixed layer. These conditions were associated with the anti-cyclonic eddies,

S. Kouketsu (&)  H. Tomita  S. Hosoda  T. Kobayashi 
K. SatoResearch Institute for Global Change,
Japan Agency for Marine-Earth Science and Technology,
2-15 Natsushima-cho, Yokosuka 237-0061, Japane-mail: skouketsu@jamstec.go.jp
H. Tomitae-mail: tomitah@jamstec.go.jp
S. Hosodae-mail: hosodas@jamstec.go.jp
T. Kobayashi
e-mail: taiyok@jamstec.go.jp
K. Satoe-mail: k_sato@jamstec.go.jp
E. OkaAtmosphere and Ocean Research Institute,
The University of Tokyo, Kashiwa, Japane-mail: eoka@aori.u-tokyo.ac.jp

suggesting that these eddies are important in the formationof mode waters, particularly L-CMW.
Keywords Mixed layer  Mode water  Meso-scale eddy Sea-surface flux

1 Introduction
Ocean surface mixed layers, which are the layers at theocean surface with vertically uniform water properties,
develop as a result of momentum and buoyancy forcing,
especially in winter. Because heat, freshwater and chemicalcomponents (e.g., CO2) are exchanged between the ocean
and the atmosphere through the mixed layers (Takahashi
et al. 2009; Hosoda et al. 2010), mixed layer depths(MLDs) and properties are strongly related to climate andenvironmental variability. Furthermore, deep mixed layersthat develop in winter are separated from surface forcing
by surface re-stratification, mainly because of heating in
spring, and remain in the subsurface as the frequently
observed thick, vertically homogeneous layers called‘‘mode waters’’ (Hanawa and Talley 2001). In association
with mode water formation and subduction, the heat,
freshwater, and chemical components that are exchanged
with the atmosphere and the materials produced by the
biological processes near the sea surface are transported tothe subsurface layers in the ocean. The part of mode waters
remains under the seasonal thermocline, and is entrained
again in the deep mixed layer during the next winter. This
process affects the late-winter sea surface temperature
anomaly in some regions (Hanawa and Sugimoto 2004;
Sugimoto and Hanawa 2005). The other part is subductedto the permanent pycnocline. The mode water formationand subduction are crucially involved in long-term

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subsurface temperature or other environmental changes(Talley and Raymer 1982; Deser et al. 1996; Emerson
et al. 2004), and might have an important effect on decadal
climate variability (Latif and Barnett 1994; Gu and Philander 1997).
On the western side of the North Pacific Ocean, subtropical mode water (STMW; Masuzawa 1969), centralmode water (CMW; Nakamura 1996; Suga et al. 1997),
and transition region mode water (TRMW; Saito et al.
2007) are frequently observed as the subsurface verticallyhomogeneous layers. STMW and CMW are formed as
deep winter mixed layers in the region south and north,respectively, of the Kuroshio Extension (Suga et al. 2004),
and TRMW is formed south of the subarctic front (Saito
et al. 2007). Investigations into the formation and seasonalevolution of the deep mixed layers in the mode-water
formation regions have mainly used climatology or spatially sparse observations (Suga et al. 2004; Ohno et al.
2009). Recently, profiling float data became available forthe global ocean (Roemmich 2001), and the distribution
and structure of the deep mixed layers and mode waters
have been investigated in more detail (Oka et al. 2011;
Toyama and Suga 2010).
Oka et al. (2011) showed the detailed distribution of thelate-winter deep mixed layer in the North Pacific and its
relationships with mode waters. They divided the mode
waters into four types—STMW, the lighter and denser
varieties of CMW (L-CMW and D-CMW; Tsujino and
Yasuda 2004; Oka and Suga 2005), and TRMW—by using
the temperature and salinity relationships of deep mixed
layers as observed by profiling floats. They found that
L-CMW, D-CMW, and TRMW become lighter to the eastand subduct into the permanent thermocline from theeastern part of the formation region where the deep mixed
layers were observed.
In the western North Pacific, the warm (cold) and saline(fresh) anti-cyclonic (cyclonic) eddies known as warm-core(cold-core) rings, which originate from the Kuroshio
Extension, are frequently observed (Mizuno and White
1983; Kawamura et al. 1986; Itoh and Yasuda 2010), andtheir relatively weak (strong) preexisting stratificationaccompanied by deep (shallow) thermocline can create
favorable (unfavorable) conditions for mixed-layer deepening. The spatial changes in preexisting stratification
structure associated with the eddies may cause small-scalespatial changes in MLDs. Indeed, there is small-scalespatial variability in the mixed-layer distribution map ofOka et al. (2011), and deep mixed layers have frequently
been observed in the ridges and anti-cyclonic eddies along
the Kuroshio Extension (Uehara et al. 2003; Qiu et al.
2006; Oka 2009).
Furthermore, small-scale spatial changes in wind andbuoyancy corresponding to ocean fronts or eddies have123

S. Kouketsu et al.

been detected (Nonaka and Xie 2003; White and Annis 2003;
Tokinaga et al. 2006), and these changes are important in the
distribution of rainfall near oceanic fronts (Minobe et al.
2008; Tokinaga et al. 2010). As the mixed layer develops asa result of momentum and buoyancy fluxes from the atmosphere, spatial changes in fluxes can also contribute tochanges in MLDs with the distances from the eddy core. The
earlier studies did not describe in detail the effects of smallscale flux variability on the ocean mixed layer; however,more recent studies (Tomita et al. in preparation) havereported that small-scale spatial changes in the forcings
across the subarctic front can contribute to those in MLDs.
Recent high-resolution numerical experiments haveshown the importance of eddies in the formation andtransport of mode waters (especially for STMW) (Tsujino
and Yasuda 2004; Rainville et al. 2007; Nishikawa et al.
2010). There have also been many observational descriptions of deep mixed layers and mode water formation in the
cores of eddies for the STMW formation area (Uehara
et al. 2003; Qiu et al. 2006; Oka 2009), but there were few
for the whole western North Pacific. Here we describe therelationships between the late-winter deep mixed layer andmeso-scale eddies in the whole western North Pacific andclarify the effects of changes in ocean stratification,accompanied by changes in thermocline depths, and of
forcing fields associated with eddies on mixed layer
development and mode water formation.

2 Data and methods
Gridded sea surface height anomaly (SSHA) data were
produced by Segment Sol Multimissions d’Alimétrie
d’Orbitographie et de Localization Precise/Data Unification and Altimeter Combination System (Ssalto/Ducas) anddistributed by Archiving, Validation and Interpretation ofSatellites Oceanographic Data (AVISO) with support fromCentre National d’Etudes Spatiales (CNES). The SSHA
map provided was a 7-day composite with 13   13  grids.
Meso-scale eddies were detected by use of the satellite
SSHA data and essentially the same methods as in previous
studies (Chelton et al. 2007; Itoh and Yasuda 2010), which
defined eddies as areas where the Okubo–Weiss parameter
W (=4 [(qu / qx)2 ? (qv / qx) (qu / qy)]), calculated from an
SSHA map was less than a critical value (Fig. 1). The
center of an eddy is defined as a centroid of the SSHA. In
tracking the eddies detected in each 7-day composite, we
searched for the nearest eddy from the center of an eddy
detected on the previous map (7 days earlier) within a set
search range. In this study, we set the critical value of
the Okubo–Weiss parameter to -0.2 9 rW to allow for
the spatial variability of eddy strength, where rW is theEddies and mixed layers in the North PacificFig. 1 Sea surface heightanomaly (cm) and centers ofdetected anti-cyclonic (whitecircles) and cyclonic (black
circles) eddies in the compositeof the western North Pacific
Ocean for Mar 12, 2008

65

50°N

30
20

40°N

10

30°N

−10

0

−20
−30

20°N
140°E

standard deviation of the Okubo–Weiss parameter in a
5° 9 2.5° box (longitude 9 latitude). The factor of -0.2
was chosen on the basis information presented by IsernFontanet et al. (2004), and the resulting critical value eastof Japan was similar to that used by Itoh and Yasuda(2010) who investigated meso-scale eddies moving throughthe same region. The search range for tracking eddies was
set to 100 km and we used only eddies that could betracked for over 4 weeks, as recommended by Itoh andYasuda (2010).
To determine the MLDs, we used temperature andsalinity profiles obtained by Argo profiling floats in the
North Pacific from 2002 to 2009. These floats drift at aparking pressure (typically 1000 dbar) and take measurements from an intermediate depth (2000 dbar) to near thesea surface at a preset time interval (10 days). In a typicalprofile, temperature and salinity are measured at intervalsof 5 dbar over the upper 200 dbar, 10–25 dbar from 200 to1000 dbar, and 50–100 dbar deeper than 1000 dbar. We
used real-time quality-controlled float data (Wong et al.
2010) and eliminated questionable profiles, for example
those with flagged measurements, by using the same
method as Oka et al. (2007). We then interpolated the
profile data to 1-dbar intervals by using the Akima spline
method (Akima 1970).
MLDs were defined as the depths at which the density
was 0.03 kg m-3 greater than that at the 10-m depth foreach profile (Weller and Plueddemann 1996). Other studies
have used vertical temperature differences as the criterion
for defining MLDs (de Boyer Montégut et al. 2004),
however the difference between MLDs estimated with andwithout the temperature criterion is not large in the North
Pacific (Hosoda et al. 2010). Because large density difference criteria used in previous studies (typically 0.125 kgm-3) are sometimes too large to detect MLDs using Argo
float data (Hosoda et al. 2010), we used the small verticaldensity difference criterion. The mixed layer temperatureand salinity were defined as the vertical mean over theobtained MLDs. To make composites, we set four sub
regions in the region of 140°E–160°W and 25°N–45°N
where almost all of deep mixed layers were observed in the

160°E

180°

160°W

140°W

western North Pacific (Oka et al. 2011). The southernmost
region in the four sub regions is 25°N–30°N. The regions
30°N–35°N and 35°N–40°N approximately correspond tothe main formation regions of STMW and L-CMW,
respectively. In the western and eastern parts of thenorthernmost region of 40°N–45°N, TRMW and D-CMW
are formed (Oka et al. 2011). All MLDs observed in the
North Pacific region were arranged in order of their distance from the center of the eddy nearest to each profile.
We then made composites of the observed MLDs along theradial direction of the eddies in the sub regions in late
winter (February–April), when the mixed layers were thedeepest of the year (Oka et al. 2007). In the composites, weshow 50-km bin averages along the radial direction with
their bootstrapped confidence intervals calculated with thebootstrap sample number of 3000 (Von Storch and Zwiers1999). Note that all the profiles are divided into twogroups: near the anti-cyclonic and cyclonic eddy centers, inthis method. In this study, we especially focus on ‘‘deep
mixed layers’’ deeper than 150 m in the composites,
because the deep mixed layers contribute strongly to theformation of mode waters (Suga et al. 2004; Oka et al.
2011).
To investigate the relationship of the distribution of
surface momentum and heat fluxes to the meso-scaleeddies in the regions, we used a high-resolution satellitederived air-sea heat flux dataset that was developed asversion 2 of the Japanese ocean flux data sets with use of
remote sensing observations (J-OFURO2; Tomita et al.
2010). This dataset provided the daily turbulent heat fluxes
at the sea surface on a 0.25° 9 0.25° grid from 2002 to
2007. We also used the sea surface temperature datathat were used to calculate the turbulent heat fluxes in
J-OFURO2.
To quantify the contributions of ocean stratification andthe sea-surface fluxes to variations in MLD, we used a bulk
one-dimensional mixed layer model, assuming that horizontal advection and diffusion over the mixed-layer
thickness is small in the core of meso-scale eddies. In themodel, we used the following equations to determine MLD
(hm):

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S. Kouketsu et al.

ohm¼ we ;ot
oTm Qnet  qd DT
¼

we ;
hm
ot
qo chm
C1 u3  C2 Bhm
;
we ¼
aghDT
Qnet þ qd
B ¼ ag;qo c
DT  Tm  Tðz ¼ hm  20Þ:

fluxes, we integrated the model from November 15 toMarch 15 to obtain estimated MLDs.

ð1Þ

The coefficients and variables in the equations are summarized in Table 1. The coefficients were set to values
similar to those in previous studies (Yasuda et al. 2000;
Kako and Kubota 2009). Qnet was the sum of the turbulent
heat fluxes from J-OFURO2 and the radiative heat fluxes
from the National Centers for Environment Prediction(NCEP (NOAA, USA); Kalnay et al. 1996), and u* wascalculated from daily mean momentum fluxes in J-OFURO2. DT was defined as the temperature difference
between the mixed layer and 20 m below the base of themixed layer. To create a composite view near the center of
meso-scale eddies, we set the initial stratification structuresof 50-km-bin average temperature profiles around thecenter of eddies to the averages of profiles observed from
October to December in 2002–2007. The winter mean(from November to March) momentum and net heat fluxes
were also estimated for each 50-km bin along the radial
direction of the eddies. Because the winter mean fluxes met
the requirements for mixed layer deepening, only theequations for mixed layer deepening (Eq. 1) are used inthis study. Using the mean initial stratification and surface
Table 1 Summary of variables and coefficients for a bulk ocean
mixed layer model (Eq. 1)
Variable

DescriptionhmMixed layer depth

Tm

Mixed layer temperature

we

Entrainment velocityu*Frictional velocity

B

Net buoyancy flux

C1

Tuning coefficients (=1.0)

C2

Tuning coefficients (=0.2)

a

The thermal expansion coefficient of the seawater(=2.5 9 10-4 °-1)gGravitational accelerationDT

The temperature difference between the mixed layer andthe layer below

Qnet

Net surface heat flux

qd

Downward radiative flux at the base of the mixed layerqo

The reference densityc

The specific heat of seawater (=3.99 9 103 kg-1 J-1)

123

3 Results
3.1 Relationship between deep mixed layers
and meso-scale eddies
The composite analysis revealed that deep mixed layers
were more frequently observed in the anti-cyclonic eddies
than in the cyclonic eddies in the late winter from 2002 to2009 (Fig. 2). In the region from 25°N to 30°N, 50-km binaveraged MLDs were approximately 50 m in most of thebins near both the anti-cyclonic and cyclonic eddies, andthere was no clear relationship between MLDs and the
distance from eddies of either type (Fig. 2a).
From 30°N to 35°N, the MLDs in the anti-cyclonic eddies
were deeper than those in the cyclonic eddies, and were
deeper (shallower) closer to the core of the anti-cyclonic(cyclonic) eddies (Fig. 2b). The differences between MLDs
in the anti-cyclonic and cyclonic eddies were mainly associated with the ridges and troughs of a quasi-stationary
meander along the Kuroshio Extension (Mizuno and White
1983; Tatebe and Yasuda 2001), which can be detected as
eddies by this method. The deep mixed layers in the anticyclonic eddies or the ridges of the meander have beenreported previously, and the contribution of the deep mixed
layers to the formation of STMW has been shown (Uehara
et al. 2003; Qiu et al. 2006; Oka 2009). Although there wereclear differences between MLD distributions in the anticyclonic and cyclonic eddies, and the bin-averaged MLDs
were significantly smaller inside 150 km from the cycloniceddy center than outside, the differences between MLDs
inside and outside of 150 km from the anti-cyclonic eddy
center was insignificant in comparison with the 95% confidence intervals for the bin average (Fig. 2b). This region ismostly located south of the Kuroshio Extension, where thepreexisting stratification under the seasonal thermocline wasrelatively weak because of STMW in the subsurface layer.
Such large-horizontal-scale distribution of weak preexisting
stratification resulted in a deepened mixed layer even awayfrom the core of the anti-cyclonic eddies.
From 35°N to 40°N, differences were evident in the
distributions of MLD between inside and outside 100 km
from the anti-cyclonic eddy center and between inside
100 km from the anti-cyclonic and cyclonic eddy centers,
whereas the difference between inside and outside thecyclonic eddies was not clear in comparison with the 95%
confidence intervals (Fig. 2c). Over 70% of all observed
MLDs within 50 km of the anti-cyclonic cores were deep
MLDs ([150 m), and the bin-averaged MLD wasapproximately 200 m, whereas the bin-averaged MLDs

Eddies and mixed layers in the North Pacific(a) 25 N–30 N,140 E–160 W
Cyclonic

Anti-cyclonic
300

100

1.0
0.5

0
0

50

100

150

200

200

100

1.0
0.5

0

0.0
250

0

50

R (km)

100

150

200

Ratio

MLD (m)

200

Ratio

MLD (m)

300

0.0
250

R (km)(b) 30 N–35 N,140 E–160 W
Cyclonic

Anti-cyclonic
300

100

1.0
0.5

0
0

50

100

150

200

200

100

1.0
0.5

0

0.0
250

0

50

R (km)

100

150

200

Ratio

MLD (m)

200

Ratio

MLD (m)

300

0.0
250

R (km)(c) 35 N–40 N,140 E–160 W
Cyclonic

Anti-cyclonic
300

100

1.0
0.5

0
0

50

100

150

200

200

100

1.0
0.5

0.0
250

0
0

50

R (km)

100

150

200

Ratio

MLD (m)

200

Ratio

MLD (m)

300

0.0
250

R (km)(d) 40 N–45 N,140 E–160 W
Cyclonic

Anti-cyclonic
300

100

1.0
0.5

0
0

50

100

150

R (km)

200

0.0
250

200

100

1.0

Ratio

200

0.5
0
0

50

100

150

200

Ratio

MLD (m)300

MLD (m)Fig. 2 Distributions of mixed
layer depths as a function of
distance (R) from the center of
meso-scale eddies in late winter(from February to April). Dotsdenote observed mixed layers.
Open circles and bars denotethe 50-km bin averages andbootstrapped 95% confidence
intervals, respectively, of themixed layer depth. Solid blackbars denote the ratio of deep
mixed layers ([150 m) to all
mixed layer observations in the
50-km bins from 2002 to 2009

67

0.0
250

R (km)

123

68

S. Kouketsu et al.

more than 50 km from the core were less than 120 m. In
this region, warm and saline anti-cyclonic eddies known as
warm-core rings, which originate from the Kuroshio
Extension, are frequently observed (Mizuno and White,
1983; Kawamura et al. 1986; Itoh and Yasuda 2010), andtheir relatively weak preexisting stratification create
favorable conditions for mixed-layer deepening. The
presence of many eddies with clearly different water
properties in this region (Itoh and Yasuda 2010) could
explain the clear MLD differences (Fig. 2c). From 40°N to
45°N, clear differences were also evident between the
distribution of MLDs in the anti-cyclonic and cycloniceddies and between the MLDs inside and outside 100 km
from anti-cyclonic eddy center (Fig. 2d), although there
were not many available observations in this region.
In the composites (Fig. 2), deep MLDs were frequently
observed within 125 km from the eddy centers. Assumingthat 125 km was the scale within which eddies affected
mixed-layer deepening, we calculated the proportions of
the number of deep MLDs ([150 m) inside of the eddies(i.e., an observation within 125 km of an eddy core) to all
observed deep MLDs in each grid box in the horizontal
map (Fig. 3a) to estimate the contribution of the anticyclonic eddies to mixed-layer deepening. Furthermore, totake into account different frequencies of observed anticyclonic eddies in each box, we calculate temporal mean
areas of the anti-cyclonic eddy within each box, and show
the spatial distribution of the ratio of the number ofobserved deep MLDs per unit area within the anti-cyclonic
Fig. 3 The proportion of thenumber of the deep mixed
layers ([150 m) within 125 kmof the center of anti-cycloniceddies in all the observed deep
mixed layers (a), and the ratioof the number of the deep mixed
layers per unit area inside theanti-cyclonic eddies to that in awhole box (b). The proportionand ratio were calculated foreach 5° 9 2.5° box by using
profile data from 2002 to 2009,
and the total areas of the anticyclonic eddies were calculated
assuming the radius of anticyclonic eddies was 125 km.
Gray boxes had fewer than fiveobserved deep mixed layers

eddies to that in a whole box area (Fig. 3b). Note that
125 km is approximately double the typical eddy radius asestimated from the SSHA data (Itoh and Yasuda 2010).
The typical radius of an eddy as defined using the Okubo–
Weiss parameter encompasses the area where vorticity
predominates over strain (i.e., only the inner part of theeddy core), and this radius is about half the radius of theentire vortex (Isern-Fontanet et al. 2006). The distance of
125 km used in this study probably corresponds to theradius of the entire vortex, which includes the areas withstrong currents associated with the eddies. In addition, in
this analysis, we also assume the frequencies of Argo float
observations along the radial directions of the eddies were
uniform, because the ratio of Argo float observations inside
the anti-cyclonic eddies to all the observations was similar
to that of the areas inside the eddies to all the areas (approx.
0.15).
Deep mixed layers were frequently observed from
140°E to 160°W (similar to the observations of Oka et al.
2011), and the contribution of anti-cyclonic eddies to themixed-layer deepening, and thus to mode water formation,varied spatially (Fig. 3). In almost all the high-proportion
areas in Fig. 3a, the ratios of the number of observed deep
MLD per unit area were greater than unity (Fig. 3b). Thus,
many deep MLDs were observed within anti-cycloniceddies in the high-proportion areas (Fig. 3a) not justbecause of the presence of many eddies but also because offavorable conditions for deep MLDs associated with theanti-cyclonic eddies. As the areas with a high proportion of(a) Proportion of the observed deep MLDs inside anti-cyclonic eddies
50°N

0.5
0.4

40°N
0.3
0.2
30°N
0.1
0.0

20°N
140°E

160°E

180°

160°W

140°W

(b) Ratio of the numbers of deep MLDs per unit area inside anti-cyclonic eddies
50°N

3.0
2.5

40°N
2.0
1.5
30°N
1.0
0.0

20°N
140°E

123

160°E

180°

160°W

140°W

Eddies and mixed layers in the North Pacific

69
Salinity

Table 2 Properties of mode waters around the inter-frontal zone in
the North Pacific Ocean (adapted from Oka et al. 2011)
Type

Temperature (°C)

Salinity

33.5

34.0

Density (kg m )34.65–34.95

24.2–25.6

L-CMW

10.0–16.0

34.30–34.65

25.4–26.3

D-CMW

8.0–10.533.80–34.20

26.1–26.5

TRMW

5.0–7.5

33.60–33.90

26.4–26.6

the number of deep MLDs within anti-cyclonic eddies were
distributed in the region from 140°E to 160°W and from
35°N to 45°N (Fig. 3), the relationships evident in thecomposites (Fig. 2c) reflect the characteristics of a wide areawith many eddies. There was also a high proportion of deep
MLDs inside anti-cyclonic eddies around 35°N near the
Kuroshio Extension, whereas the magnitude was relatively
low around 30°N. North of 40°N, there are some regions
where the proportion of the number of deep MLDs within
anti-cyclonic eddies was high west of 160°E. This suggeststhat the relationships in the composites (Fig. 2d) reflect only
a limited area mainly west of 160°E. The region with highproportion of the number of deep mixed layers inside anticyclonic eddies from 145°E to 160°E (Fig. 3) corresponded
to the region along the quasi-stationary jet associated withthe subarctic front (Isoguchi et al. 2006).
Oka et al. (2011) used the temperature and salinity
relationships of mixed layers deeper than 150 m to classify
water property in the deep mixed layers corresponding tomode water types, and described 4 mode water types(Table 2), because the deep mixed layers are the major
source of mode waters in the North Pacific. In a similarmanner, we grouped the observed deep MLDs ([150 m)
by temperature and salinity characteristics and determined
the proportion in each temperature-salinity bin that were
distributed within anti-cyclonic eddies (Fig. 4). This
grouping revealed four types of deep mixed layer waters
similar to the 4 mode water types, separated by bins withfewer observations (4 rectangles surrounded white and grayboxes in Fig. 4). Note that deep MLDs were rarely
observed in the cyclonic eddies (Fig. 2), and proportions of
the number of deep MLDs within cyclonic eddies were lessthan 0.1 in almost all the temperature-salinity bins (notshown). Although deep mixed layers with the properties of
STMW were not always observed in the anti-cycloniceddies (Fig. 2b and around 30°N in Fig. 3), the relatively
cold (\18°C) and fresh (\34.8) deep mixed layers in
STMW (Fig. 4) were frequently found inside the anticyclonic eddies. Water similar to the colder and fresher
range of STMW was often observed in the eastern part of
the STMW formation region south of the Kuroshio
Extension (Oka and Suga 2003), which suggests that theanti-cyclonic eddies contributed to the formation of

Temperature (°C)

16.0–21.5

35.0
0.5

24

20

STMW

34.5

23

-3

0.4

25

STMW

15
0.3

26
10

L−CMW

D−CMW

0.2

27
5

0.1

TRMW

28

0.0

Fig. 4 Proportion of the number of deep mixed layers ([150 m) in
the cores of anti-cyclonic eddies (\125 km) as a function oftemperature and salinity. The boxes used for the summation were inincrements of 0.5° for temperature and 0.05 for salinity. Gray boxes
had fewer than five observed deep mixed layer. The four mode watertypes defined by Oka et al. (2011) (Table 2) are shown by rectangles

STMW, especially in the downstream region of the Kuroshio Extension, which is reflected in the high-proportion
regions around 170°E and 35°N in Fig. 3. In contrast, thehigh proportions of the number of the deep mixed layerswith the properties of L-CMW in the anti-cyclonic eddies(Fig. 4) reflected that the strong contribution of the anticyclonic eddies to deep mixed layers were detected broadly
in the L-CMW formation area (Fig. 2c and from 35°N to
40°N in Fig. 3). As clearly defined large-scale fronts suchas the Kuroshio Extension were not observed in the
L-CMW formation region, the relatively weak preexisting
stratification associated with the anti-cyclonic eddies could
be more important there for mixed-layer deepening. There
were smaller contributions from anti-cyclonic eddies todeep mixed layer formation in the D-CMW region, especially in its cold and fresh part, and relatively large contributions in the TRMW region (Fig. 4).
3.2 Contribution of sea-surface flux and preexisting
stratification to mixed layer deepening
Because warmer (colder) water has frequently been observedin the cores of anti-cyclonic (cyclonic) eddies (Itoh andYasuda 2010), we detected positive (negative) anomalies ofbin-averaged sea surface temperature (SST), which were
calculated from area-weighted averages within 350 km of
the center of each eddy (Fig. 5). The differences between
SST inside and outside eddies were larger north of theKuroshio Extension (Fig. 5c, d) than south (Fig. 5a, b). In
fact strong and large warm-core rings were reported north of

123

70

0.05
0.00
−0.05

1.0
0.5
0.0
−0.5
−1.0

−0.10

0.10

50 100 150 200 250 300 350

0.05
0.00
−0.05

R (km)

0.05
0.00
−0.05

1.0
0.5
0.0
−0.5
−1.0

LHF+SHF (W m −2)

0.10

80
60
40
20
0
−20
−40
−60
−80

−0.10
0

0.5
0.0
−0.5
−1.0

50 100 150 200 250 300 350

R (km)(b) 30 N–35 N,140 E–160 W
Momentum Flux (N m−2)

Cyclonic

1.0

−0.10
0

Anti-cyclonic
0.10

80
60
40
20
0
−20
−40
−60
−80

50 100 150 200 250 300 350

0.05
0.00
−0.05

Momentum Flux (N m−2)0Anti-cyclonic

80
60
40
20
0
−20
−40
−60
−80

Momentum Flux (N m−2)

0.10

LHF+SHF (W m −2)(a) 25 N–30 N,140 E–160 W

80
60
40
20
0
−20
−40
−60
−80

Momentum Flux (N m−2)

LHF+SHF (W m −2)

Cyclonic

LHF+SHF (W m −2)

Fig. 5 Distribution of 50-km
bin averages of sea surface
temperature (SST) anomalies(blue curves) and surface heat
(red curves) and momentum(green curves) flux anomalies
from the ocean to theatmosphere as a function of
distance from the center of
meso-scale eddies. The surface
temperature and flux anomalies
were calculated from areaweighted averages within
350 km of the center of eacheddy. Bars denote bootstrapped
95% confidence intervals for binaverages

S. Kouketsu et al.

1.0
0.5
0.0
−0.5
−1.0

−0.10
0

R (km)

50 100 150 200 250 300 350

R (km)(c) 35 N–40 N,140 E–160 W

0.00
−0.05

0.5
0.0
−0.5
−1.0

−0.10
0

0.10

80
60
40
20
0
−20
−40
−60
−80

0.05
0.00
−0.05

1.0
0.5
0.0
−0.5
−1.0

−0.10
0

50 100 150 200 250 300 350

Momentum Flux (N m−2)

0.05

1.0

LHF+SHF (W m −2)

Anti-cyclonic
0.10

Momentum Flux (N m−2)

LHF+SHF (W m −2)

Cyclonic
80
60
40
20
0
−20
−40
−60
−80

50 100 150 200 250 300 350

R (km)R (km)(d) 40 N–45 N,140 E–160 W

−0.05
−0.10
50 100 150 200 250 300 350

R (km)

the Kuroshio Extension in previous studies, whereas suchclear SST anomalies associated with meso-scale eddies were
not frequently observed to the south (Mizuno and White
1983; Kawamura et al. 1986). This difference between theareas south and north of the Kuroshio Extension is reflectedin the amplitude of composite SST anomalies in eddies.
The anomalies of both turbulent surface heat andmomentum fluxes to the ocean, which were calculated
from area-weighted averages within 350 km of the centerof each eddy, associated with the SST anomalies in

0.5
0.0
−0.5
−1.0

0.10

80
60
40
20
0
−20
−40
−60
−80

0.05
0.00
−0.05

Momentum Flux (N m−2)

0.00

1.0

LHF+SHF (W m −2)

0.05

0

123

Anti-cyclonic
0.10

Momentum Flux (N m−2)

LHF+SHF (W m −2)

Cyclonic
80
60
40
20
0
−20
−40
−60
−80

1.0
0.5
0.0
−0.5
−1.0

−0.10
0

50 100 150 200 250 300 350

R (km)

eddies were positive (negative) in the core of anticyclonic (cyclonic) eddies (Fig. 5). The momentum flux
anomalies reflect the tendency for wind near the ocean
surface to be stronger over warmer water, as reported inprevious studies (Nonaka and Xie 2003), and this relationship is also evident in the composite of the eddies
detected from the SSHA (Fig. 5). Note that the 95%
confidence intervals for bin-averaged anomalies of SST
and surface fluxes were generally small, because of thelarge number of samples.

Eddies and mixed layers in the North Pacific71

bulk one-dimensional mixed layer model (Eq. 1). From
30°N to 35°N, the differences between observed MLDs
inside and outside anti-cyclonic eddies were smaller thanthose for the cyclonic eddies (Fig. 7b). Because changes in

To evaluate the effects of the anomalies of turbulent
heat and momentum fluxes (Fig. 5) and the preexisting
stratification along a radial eddy section (Fig. 6) on the
MLD distributions along a radial direction, we used the(a) 25 N–30 N,140 E–160 W
Anti-cyclonic

Cyclonic
0

25
25

20

100

Depth (m)Depth (m)

0

25

25 25

25

100
200

15

300
400

20

200
300

15

400

10

500
0

50

500
100
150
R (km)

200

250

0

50

100
150
R (km)

200

250

200

250

(b)30 N–35 N,140 E–160 W
Cyclonic
0

20

200

15

300

10
400

200

15

300
400

10

500

500
0

50

100
150
R (km)

Cyclonic
0

200

0

250

15

200

10

Anti-cyclonic
15

20
15

100

300
400

100
150
R (km)

0

15

100

50

(c)35 N–40 N,140 E–160 W

Depth (m)Depth (m)20100Depth (m)Depth (m)Anti-cyclonic
0

20

100

200

10

300
400

5

500

500
0

50

100
150
R (km)

Cyclonic
0

200

0

250

50

100
150
R (km)

200

250(d) 40 N–45 N,140 E–160 W

Anti-cyclonic

0

10

10

15

15

10

10
100

Depth (m)100

Depth (m)Fig. 6 Vertical temperaturesections along the radial sectionof eddies averaged from
October to December from 2002
to 2007 (°C). The contour
intervals are 1°C

200

5

300
400

200

5

300
400

500

500
0

50

100
150
R (km)

200

250

0

50

100
150
R (km)

200

250

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72

(a) 25 N–30 N,140 E–160 W
Anti-cyclonic

250

250

200

200

MLD (m)MLD (m)Cyclonic

150
100
50

150
100
50

0

0
0

50

100

150

200

0

250

50

100

150

200

250

200

250

200

250

200

250

R (km)R (km)(b) 30 N–35 N,140 E–160 W
Cyclonic

Anti-cyclonic

250

250

200

200

MLD (m)MLD (m)Fig. 7 Observed mixed layer
depths (MLDs) and thoseestimated using the bulk mixed
layer model as a function of
distance along the radial
direction of eddy. White circles
denote the 50-km bin averagesof observed MLDs similar to
Fig. 2, but calculated using datafrom 2002 to 2007. Green dots
denote MLDs estimated using
the preexisting stratification
associated with the eddies(Fig. 6) and a constant surface
flux over the radial section,
whereas red dots denote MLDs
estimated using both thepreexisting stratification and thesurface flux associated with theeddies (Figs. 5, 6). Bars denotebootstrapped 95% confidence
intervals for bin averages

S. Kouketsu et al.

150
100
50

150
100
50

0

0
0

50

100

150

200

250

0

50

R (km)

100

150

R (km)(c) 35 N–40 N,140 E–160 W
Anti-cyclonic

250

250

200

200

MLD (m)MLD (m)Cyclonic

150
100
50

150
100
50

0

0
0

50

100

150

200

250

0

50

R (km)

100

150

R (km)(d) 40 N–45 N,140 E–160 W
Anti-cyclonic

250

250

200

200

MLD (m)MLD (m)Cyclonic

150
100
50

100
50

0

0
0

50

100

150

R (km)

123

150

200

250

0

50

100

150

R (km)

Eddies and mixed layers in the North Pacificpreexisting stratification along the radial section were larger in the cyclonic eddy than in the anti-cyclonic eddy
(Fig. 6b), there were large differences between MLDs
inside and outside the cyclonic eddies, estimated both withand without the surface flux distribution along the radial
direction (green and red circles in Fig. 7b). On the other
hand, MLDs of the anti-cyclonic eddies estimated without
the surface flux anomalies change little along the radial
direction (green circles in Fig. 7b). This difference ismainly because clearly defined cyclonic eddies are more
frequently observed than anti-cyclonic eddies in this region(Itoh and Yasuda 2010).
In the region from 35°N to 40°N, both the flux anomaly
and preexisting stratification changes along the radial
directions (Figs. 5c, 6c) were obvious, and there were largechanges of MLDs along the radial direction, which were
estimated both with and without the surface flux distribution (Fig. 7c). Thus, both the flux anomaly and the preexisting stratification changes along the radial section
strongly contributed to reproduction of the pattern changes
in MLDs, although the difference of the estimated andobserved MLDs was significantly large except within
50 km of the eddy centers (Fig. 7c). A large portion of the
deep mixed layers observed in this region was located in
the anti-cyclonic eddies (as discussed in the Sect. 3.1).
Thus, the strong cooling and weak preexisting stratification
inside the anti-cyclonic eddies are important in the formation of the deep mixed layers in this region, which
contributed to the L-CMW source region.
The flux anomalies associated with the anti-cycloniceddies were more important for forming the MLD changes
in the radial direction than the preexisting stratification
changes along the radial section in the region from 40°N to
45°N, because the MLD changes estimated using the surface flux distribution along the radial direction were clear,and those without the surface flux distribution were insignificant (Fig. 7d). As shown in the Sect. 3.1, deep mixed
layers in anti-cyclonic eddies have frequently been
observed near the east coast of Japan, where TRMW isformed; the flux anomalies associated with the eddies
might be important for the formation of TRMW (Fig. 7d).
As both the flux (Fig. 5a) and the preexisting stratification
changes along the radial section (Fig. 6a) are small in theregion of 25°N–30°N, the differences between MLDs
inside and outside the eddies were small, and there were noclear differences between the estimated MLD distributionsin the anti-cyclonic and cyclonic eddies (Fig. 7a).

4 Discussion
In this study, we have shown the relationships between
deep mixed layers and spatial changes of ocean preexisting73

stratification and surface fluxes around the centers of mesoscale eddies detected by using satellite SSHA data in thewestern North Pacific Ocean, wherein the weak (strong)preexisting stratification and strong (weak) forcing inside
the core of anti-cyclonic (cyclonic) eddies tend to foster thedevelopment of deep (shallow) mixed layers in the winter.
These relationships were clear, especially in the interfrontal zone between the Kuroshio Extension and subarctic
front. This suggests that meso-scale eddies are important informing mode waters in this area. The small-scale distribution of MLDs associated with the meso-scale eddies
could affect the temporal variability of large-scale MLD
through eddy activity variability, because decadal changesof eddy activity around the Kuroshio Extension werereported after use of SSHA observations and high-resolution numerical models (Qiu and Chen 2005; Taguchi et al.
2010). Because the heat and chemical constituents that areexchanged between the atmosphere and the ocean at the sea
surface can be transported to the sub-surface layer inassociation with CMW subduction (Deser et al. 1996;
Schneider et al. 1999; Emerson et al. 2004), the eddy
activity around the inter-frontal zone might be involved ininter-annual climate variability. Furthermore, many studies
have reported that the distribution of eddies in the interfrontal zone corresponds to the sea surface color pattern(Saitoh et al. 1998); therefore the distribution of MLDs
observed in this study could also affect the biologicalactivity in the eddy cores.
The contributions of the anti-cyclonic eddies to form
deep MLDs, which are important for mode water formation, were different spatially (Figs. 2, 3). In the STMW
formation region, the changes in MLDs along the radial
direction of anti-cyclonic eddies were relatively small(Fig. 2b), and there was a low proportion of the number ofdeep mixed layers inside the anti-cyclonic eddies around
30°N (Fig. 3). This is because both the relatively weakpreexisting stratifications associated with anti-cycloniceddies (near 35°N in Fig. 3) and those observed broadly
south of the Kuroshio Extension (in the region of 30°N),
because of the existence of STMW in the subsurface layer
formed in previous winters, contribute to the formation of
the deep mixed layers there. Furthermore, the small MLD
changes along the radial direction of the anti-cycloniceddies also reflected that the changes in surface flux andpreexisting stratification along the radial directions wererelatively small (Figs. 5b, 6b, 7b), mainly because of thepresence of fewer clearly defined warm-core rings (Itohand Yasuda 2010). In the L-CMW formation region that is
not accompanied by clearly-defined large-scale fronts, forexample the Kuroshio Extension, the relatively weak preexisting stratification associated with the anti-cycloniceddies could be more important there for mixed-layer
deepening. Thus, there was a clear MLD difference along

123

74

the radial direction of the anti-cyclonic eddy (Fig. 2c),
because of the substantially different preexisting stratifications and surface fluxes along the radial direction(Figs. 5c, 6c, 7c), and large contribution of the anticyclonic eddies to deep MLDs were broadly detected
spatially (Fig. 3) and in the temperature–salinity relationships (Fig. 4). The relatively strong contribution of anticyclonic eddies to deep mixed layers with the properties of
TRMW (Fig. 4) might reflect that the anti-cyclonic eddies
along the front (or ridges of the meander) were importantfor forming deep mixed layers with TRMW properties, as
TRMW forms in the narrow area south of the subarctic
front (Saito et al. 2007). Formation of TRMW west of
170°E and its subsequent eastward transportation arethought to set the precondition for formation and subduction of D-CMW east of 170°E (Oka et al. 2011). Thismight explain why there were fewer high-proportion bins
with D-CMW properties in Fig. 4.
Some parts of thick homogeneous layers originatingfrom the deep mixed layers in the anti-cyclonic eddies canbe transported outside the eddies by eddy mixing, and be
subducted into the permanent pycnocline by transportation
along with isopycnal surfaces (Nishikawa et al. 2010). This
process could contribute the broad distribution of mode
waters in the permanent pycnocline. Other parts of thehomogeneous layers seem to remain in the cores of mesoscale eddies during the sequential seasons. Furthermore,
whereas some of these can also be subducted into thepermanent pycnocline with the eddy moving (Nishikawa
et al. 2010), the others remain under the seasonal pycnocline until the next winter, and can affect the deepening
mixed layer. In the L-CMW formation area, while themany anti-cyclonic eddies are moving westward (Itoh andYasuda 2010), the broad distribution of L-CMW in themain thermocline is observed in the downstream region of
the interfrontal zone (Oka et al. 2011). The former process
of eddy mixing and advections in the permanent pycnocline may contribute to the broad distribution of L-CMW in
the permanent pycnocline near the downstream region, andthe latter process of the thick homogeneous layer trappedin the cores may be dominant in the upstream regions,where the thick homogeneous layers were not broadly
observed in the subsurface layer (Oka et al. 2011). However, these subduction processes around eddies were notdirectly shown in this study. To investigate the processes,
we plan to investigate seasonal changes in the subsurface
layers around eddies from future observations.
4.1 Dependency of criteria and large variancesin composites
The results in this study were not strongly dependent on
choices of criteria to determine MLDs (vertical density

123

S. Kouketsu et al.

difference of 0.03 kg m-3) and deep MLD ([150 m) withstrong contributions to the formation of mode waters.
Using the shallow (deep) criterion of 100 m (200 m) fordeep MLDs, proportions of the number of deep MLDs in
the anti-cyclonic eddies became only approximately 0.1
smaller (larger) than shown in this study. These changes
were not very large, and the difference among the contributions of the anti-cyclonic eddies to mode waters shownin the temperature and salinity relationship were similar to
Fig. 4 (not shown). Although MLDs defined with a large
density difference criterion of (for example) 0.125 kg m-3
were approximately 40-m deeper than those shown in
Fig. 1 (not shown), the proportions of the number of deep
MLDs in the anti-cyclonic eddies became only about 0.1
smaller. This criterion choice for deep MLDs also did notstrongly affect our results. Note that using such a large
density-difference criterion, we might be able to avoid
detecting many shallow mixed layers because of short-termwarming in the early spring (e.g, approximately 10-m
MLDs in Fig. 2), which might persist in the short-term andmay cause underestimation of deep MLDs contributing toform mode waters, but the MLDs determined by this large
density difference criterion tend to be overestimatedbecause they include thermoclines in which water properties are not vertically homogeneous.
While we showed that the different MLDs and fluxes
between in the anti-cyclonic and cyclonic eddies, and thechanges along the radial directions of the eddies, weredetectable in the 50-km bin averages with the bootstrapped
confidence intervals, there were large variances in MLDs
and fluxes (see dots in Fig. 2 for MLDs). Variance of eddy
shapes (assuming axially symmetric shapes in this study)
and uncertainty in the locations of eddy centers can causesuch variances, and the large variances might be mainly
due to short-term or more small-scale variability. Although
warmer waters are much more frequently observed thancolder waters in the anti-cyclonic eddies, anti-cycloniceddies with cold waters, which are sometimes observed(Hosoda and Hanawa 2004; Itoh and Yasuda 2010), canalso affect large variances of MLDs and SSTs. In the
future, the phenomena causing such large variances will be
described in detail by use of higher resolution SSH andlong-term satellite SST observations.
In this study, the distribution of the MLDs along the radial
direction of the eddies was reproduced by using the initialstratification and the surface flux distributions associatedwith the eddies through the bulk formula for the MLDs,
although the variance of observed MLDs for each bin waslarge; this might be because of more small-scale or shortterm variability. The effects of horizontal advection anddiffusion, which were ignored in this study, are presumed tobe small, because the model was integrated over winter only(for 4 months), and strong currents were unlikely to beEddies and mixed layers in the North Pacific(a) 140 E–160 E

FluxandSST

MLD

1.0
0.5

0
100

150

200

20
0.00

0
−20

−0.05

−40
−60
−80
0

R (km)(b) 160 E–180

100

1.0
0.5

0
150

200

LHF+SHF (W m−2 )

200

Ratio

MLD (m)

300

100

0.0
−0.5
−1.0

−0.10
50 100 150 200 250 300 350

FluxandSST

80

50

0.5

R (km)

MLD

0

1.0

0.0
250

R (km)

observed in the core of the eddies. However, there could be
errors in the estimates because of the neglected effects, andthe effect of horizontal advection might cause the difference
between observed and estimated MLDs in Fig. 7.
The composites in this study were produced by using
Argo float data, which provide intensive and uniform
observations of ocean subsurface structure. However, moredetailed observations (e.g., observations focusing on aeddy) are needed to clarify the processes causing deepening of mixed layers and formation of mode waters ineddies, taking into account horizontal advection. Furthermore, although we showed there were some spatial changes
in relationships between meso-scale eddies and deep mixed
layers, by showing the proportions of the number of deep
MLDs inside the anti-cyclonic eddies in the 5° 9 2.5°
boxes (Fig. 3), available observations are not yet sufficient
to enable description of detailed spatial changes in therelationships. For example, along the radial directions of
the anti-cyclonic eddy, there were the differences of MLDs
and flux anomalies between in the upstream (western) anddownstream (eastern) regions of the L-CMW formation
area (Fig. 8). The larger flux anomalies in the anticyclonic eddies might cause the deeper MLDs in the
upstream region than in the downstream region, and thissuggested that flux anomalies corresponding to the mesoscale eddy were more important in the upstream regions.
However, it was difficult to determine preexisting stratification structures along the radial sections (similar to
Fig. 6) because of the small number of available

0.10

60
40

0.05

20
0

0.00

−20
−40

−0.05

−60
−80
0

1.0
0.5
0.0
−0.5

SST (°C)

50

0.05

40

Momentum Flux (N m −2)

0

0.0
250

60

SST (°C)

100

Ratio

200

LHF+SHF (W m−2 )300

Momentum Flux (N m −2)

0.10

80

MLD (m)Fig. 8 Distribution of mixed
layer depths (left) and anomaliesof sea surface temperature andfluxes (right) along the radial
direction of anti-cyclonic eddies
in the upstream of 140°E–160°E(a) and the downstream of
160°E–180° (b) regions of themain L-CMW formation area
(35°N–40°N). The left and rightfigures are same as Figs. 2 and 5
but for zonally narrow sub
regions

75

−1.0

−0.10
50 100 150 200 250 300 350
R (km)

observations in these small sub-regions. It is also difficult
to describe relationships among the inter-annual subsurface temperature changes, eddy activity, and the process
of mode waters subduction, because intense observations
by Argo floats have only been carried out for approximately 5 years in the North Pacific. Continued and more
intense observations by use of Argo floats, fine scaleobservations by ships and gliders, and analysis with highresolution numerical models should help clarify the temporal and spatial changes in detail.
Acknowledgments The Argo float data used in this study werecollected and made freely available by the International Argo Project
and the national programs that contribute to it (http://www.
argo.ucsd.edu, http://argo.jcommops.org). The comments fromanonymous reviewers were useful for improving the manuscript. This
study was supported by the Japan Society for Promotion of Science(KAKENHI, Grant-in-Aid for Young Scientists (B), no. 20740279).
E. Oka is supported by the Japan Society for Promotion of Science(KAKENHI, Grant-in-Aid for Scientific Research (B), no. 21340133)
and the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT; Grant-in-Aid for Scientific Research of
Innovative Areas under grant no. 22106007).

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