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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, C10003, doi:10.1029/2005JC003402, 2006

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ArticleQuasi-stationary jets transporting surface warm waters across thetransition zone between the subtropical and the subarctic gyres in the
North PacificOsamu Isoguchi,1 Hiroshi Kawamura,1 and Eitarou Oka2,3Received 18 November 2005; revised 1 May 2006; accepted 15 June 2006; published 3 October 2006.

[1] Surface flow jets and associated sea surface temperature (SST) distribution areinvestigated in the northwestern North Pacific, using satellite-derived surface currents and
SST data that can resolve fine spatial scale structure. The combined use of these data
reveals warm tongue phenomena driven by surface geostrophic jets, which extend
northeastward in the Kuroshio-Oyashio transition area. They roughly coincide with the
Subarctic Front (SAF) defined as the 4°C isotherm at 100 m depth. The phenomena appear
throughout the year with seasonal cycles, which do not correspond with that of thesubarctic North Pacific. Their positions are affected by bottom topography so that their
quasi-stationary and consistent features are suggested. Thus it is implied that these jets
play an important role in transporting warm waters toward the subarctic region. The
time series of SST and current fields describe the jets’ year-to-year variability involving
some changes in strength and connections with the Kuroshio Extension (KE), anddemonstrate the effects of advection by the jets on SST fields. When KE extends
northward at its crests for 1999–2002, one of the jets along SAF simultaneously
strengthens, resulting in high SST region directly over and along the south side of the jet.
Hydrographic data show warm, saline water intrusions along the jets. Their mean fields
also reveal that these jets are vertically well-developed, forming a boundary between thesubtropical and subarctic gyres in the North Pacific.
Citation: Isoguchi, O., H. Kawamura, and E. Oka (2006), Quasi-stationary jets transporting surface warm waters across the transitionzone between the subtropical and the subarctic gyres in the North Pacific, J. Geophys. Res., 111, C10003,
doi:10.1029/2005JC003402.

1. Introduction[2] The Kuroshio and the Oyashio are the westernboundary currents of the subtropical and subarctic gyres
in the North Pacific. The Kuroshio-Oyashio transition areaeast of Japan is an ocean area where they converge.
Complex oceanic features such as complicated frontal
structures appear and various water masses are formed[e.g., Yasuda, 2003]. In addition, this area is known as
one of the most important fishing grounds. Thus theKuroshio-Oyashio transition area is thought to be an important region for the understanding of climate and ecosystem variations. This region is sometimes referred as theMixed Water Region (MWR). MWR is defined as the area
between the northern edge of the Kuroshio Extension (KE)and the Subarctic Front (SAF), defined as the 4°C isotherm
at 100 m depth [Favorite et al., 1976].

1
Center for Atmospheric and Oceanic Studies, Graduate School of
Science, Tohoku University, Aoba, Sendai, Miyagi, Japan.
2
Institute of Observational Research for Global Change, Japan Agency
for Marine-Earth Science and Technology, Yokosuka, Kanagawa, Japan.
3
Ocean Research Institute, University of Tokyo, Tokyo, Japan.

Copyright 2006 by the American Geophysical Union.
0148-0227/06/2005JC003402$09.00[3] Several major fronts such as SAF, the Subarctic
Boundary (SAB), and the Kuroshio Bifurcation Front(KBF) have been defined in this region, on the basis ofhydrographic observations [e.g., Favorite et al., 1976;
Mizuno and White, 1983]. Zhang and Hanawa [1993] andYuan and Talley [1996] have given comprehensive discussions about frontal structures and their variability in the
North Pacific by using climatological data and synoptic
surveys. They gave better understanding about frontal
structure. However, the detailed description of their horizontal distribution and connection was not researchedenough due to limitations on the temporal/spatial resolutionof the hydrographic data. In addition, although oceanic jet isone of the principal factors in the formation of these fronts,
distinct relation between the fronts and the flow fields was
not adequately described. Recently, the accumulation ofhistorical observations, new observation technology, andhigh-resolution numerical simulations have made it possible
for us to investigate the detailed structure related to flow
fields. Qu et al. [2001] have constructed new hydrographic
climatologies in a 0.5°  0.5° grid to investigate timeaverage structure of the Kuroshio/Oyashio system east ofJapan, and have revealed the detailed structure associated with the narrow western boundary currents. Iwao et
al. [2003] have derived intermediate flow fields in the

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Table 1. Summary of the SST Products Used in This Study
SpatialResolution

TemporalCoverage

Temporal
Resolution

Other

Reference

13 – 63°N,
116 – 166°E
OI SST
Global4 km AVHRR Pathfinder
Global

0.05°

Feb 2003 – Dec 2004

Daily

Cloud-free

Guan and Kawamura [2004]

0.1°
4 km

Jan 1993 – Jun 2003
1993 – 2004

Daily
5 day

Cloud-free

9 km AVHRR Pathfinder
Climatology (CSST)

9 km

Products

SpatialCoverage

NGSST

Global

5 day

Kuroshio-Oyashio transition area from the trajectories ofsubsurface floats. They have investigated the formation,
distribution, and synoptic scale circulation structure of theNorth Pacific Intermediate Water (NPIW), which is characterized as a salinity minimum distributed in the subtropicalNorth Pacific [e.g., Talley, 1993].
[4] The KE’s bifurcation has been studied based on
observations and numerical simulations, because its northern branch connects to the SAF and gives a significant
impact on heat transports into the subarctic region. Mizuno
and White [1983] have revealed from the synoptic thermal
fields at a 300 m depth that KE often bifurcated at 150 –165°E. Levine and White [1983] have demonstrated thatmean strong thermal fronts divided into two separate bands
over the Shatsky Rise and that the northern band kept
heading northeastward along the Shatsky Rise, eventuallyconnecting to SAF. Sainz-Trapaga et al. [2001] haveestimated by combining altimeter and hydrographic datawith a two-layer reduced gravity model that KE’s bifurcations occurred for the range between 147°E and 160°E.
This bifurcation over the Shatsky Rise has been simulatedwith a numerical model [e.g., Hurlburt and Metzger, 1998].
They pointed out that the high-resolution model andrealistic bottom topography are essential to reproducing
the bifurcation.
[5] The advance of satellite observations allows us toinvestigate spatially/temporally high-frequency phenomena
on an ocean surface and has given improved knowledge
about the Kuroshio-Oyashio transition area. Using satellite
infrared (IR) images and intensive hydrographic observations, the detachment process of eddies from the Kuroshio
Extension (KE) and an eddy-eddy interaction have been
described in detail as a short-term variation [Kawamura etal., 1986; Yasuda et al., 1992]. In addition, geostrophic
warm streamers and warm tongues, separating from KE,
which are sometimes called the secondary Kuroshio Front[Kawai, 1972], have been investigated [e.g., Kawai and
Saitoh, 1986]. These phenomena are directly related to not
only the supply of warm water from KE into MWR but also
the northward migration of pelagic fish and fishing grounds[e.g., Kawai and Saitoh, 1986; Sugimoto and Tameishi,
1992]. However, the connection between the secondary
Kuroshio Front, SAF, and the Subarctic Current has notbeen elucidated clearly [Kawai, 1972].
[6] The accumulation of satellite SST observations over
20 years makes it possible to analyze phenomena with a finespatial scale, such as fronts and streamers, at climatologicalpoints of view. Moreover, altimeter-derived sea surface
height (SSH) measurements over 10 years can extract
surface current fields on a scale of 100 km. As forhydrographic structure, almost all climatologies in the North

Kawai et al. [2006](http://www.nodc.noaa.gov/sog/pathfinder4km/
userguide.html)
Climatology
Armstrong and Vazquez-Cuervo [2001]

Pacific have been constructed by applying some smoothing
procedures due to sparse observations, especially in the
open ocean. Thus they don’t necessarily have enoughinformation to describe the fine spatial structure related tofronts and jets. Hydrographic measurements have recently
increased dramatically, even in such a gap region, with theaid of the global deployment of profiling floats, known as
Argo [e.g., Argo Science Team, 2001]. These data lead tothe construction of the hydrographic mean fields, which
preserve structure as finely as possible.
[7] In this study, by using satellite-based SST and surface
current data, and historical hydrographic observations, surface jet streams and associated SST phenomena are investigated in the Kuroshio-Oyashio transition area. They reveal
the existence of warm tongues driven by quasi-stationary
jets, which suggests that these jet streams can be a possible
consistent mechanism to transport the subtropical water into
the northern region. In section 2 the data used are described.
The analyzed results including the extraction of the warm
tongues driven by quasi-stationary jets, their temporalvariability, and hydrographic structure related to the jets
are presented in section 3. Section 4 gives summary anddiscussions.

2. Data
2.1. Satellite-Based Observations[8] Several satellite-based SST products, which are summarized in Table 1, are used in this study. The New
Generation Sea Surface Temperature (NGSST) data
(http://www.ocean.caos.tohoku.ac.jp/merge/sstbinary/actvalbm.cgi?eng=1) are quality-controlled, cloud-free, andhigh-spatial resolution (0.05°-grided) products. They are
SST generated daily by objectively merging the satellite
SST observations from infrared radiometers and a microwave radiometer [Guan and Kawamura, 2004]. The satellite-based global daily 0.1°-grid SST data [Kawai et al.,2006], which are produced by applying the optimum
interpolation (OI) method (hereinafter OI SST), are also
used. They have a relatively longer time period fromJanuary 1993 to June 2003.
[9] The 4 km Advanced Very High Resolution Radiometer (AVHRR) Pathfinder Version 5.0 SST Project (Pathfinder V5) is a new reanalysis of the AVHRR data stream
developed by the University of Miami’s Rosenstiel Schoolof Marine and Atmospheric Science (RSMAS) and the
NOAA National Oceanographic Data Center (NODC),
distributed in partnership with the NASA Physical Oceanography Distributed Active Archive Center (PO.DAAC)(http://www.nodc.noaa.gov/sog/pathfinder4km/userguide.
html). In this 4 km Pathfinder project, the AVHRR data has

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C10003Figure 1. The distribution of the number of hydrographic observations at each 0.5°  0.5° grid. The
values are expressed in a common logarithm. The white grids mean no data. Pure Kuroshio and Oyashio
water profiles, which are depicted in Figure 13, are derived at the grids surrounded with white and blacklines, respectively.
been reprocessed at the 4 km Global Area Coverage (GAC)
level, the highest resolution possible. We use the 5-day
average product (hereinafter, 5-day SST) for 12 years,
1993 – 2004. The data for 2002 is, however, still an interim
version.
[10] We also use the AVHRR Pathfinder Global 9 km
Pentad SST Climatology (hereinafter, CSST) [Armstrong
and Vazquez-Cuervo, 2001] as a global high resolution
climatology. The CSST pentad climatology consists of 73
fields representing the mean annual cycle of SST with 5-day
resolution. It was derived from the daily Pathfinder SST
time series from 1985 – 1999. Gaussian interpolation wasapplied to interpolate the satellite data into 5-day pentads
onto a 9 km grid to create a global high resolution
climatology.
[11] Altimeter data is used in this study to estimate
surface flow fields. They are maps of sea level anomaly(SLA) obtained from ‘‘Segment Sol multimissions d’ALTimétrie, d’Orbitographie et de localisation précise/Data Unification and Altimeter Combination System (SSALTO/DUACS) Delayed Time Sea Level Anomalies (DTMSLA)’’. These were constructed for the period from
October 1992 to January 2005 with a 7-day interval, by
merging the SLA observations of the ocean TOPography
Experiment (TOPEX)/Poseidon and European Remote
Sensing satellite (ERS)-1/2 or those of Jason-1 and ENVISAT. No ERS data are used between January 1994 andMarch 1995. The grid interval is a MERCATOR 1/3°,
which means that resolutions in kilometers in latitude andlongitude are identical and vary with the cosine of latitude(from 37 km at the equator to 18.5 km at 60°N/S). The
detail of data processing has been described by Ducet et al.
[2000].
[12] In this study, we estimate upper ocean steric signals
due to the seasonal cycle of heating and cooling within theoceanic mixed layer and remove them from SSHA. Following Stammer [1997], the thermosteric signals (h0heat) are
estimated using net surface heat flux data as
h0heat ðt Þ  h0heat ðt0 Þ þ

1
ro cp

Z t
t0

aT ðQnet  hQnet iÞdt 0 ;ð1Þ

where ro, cp are the reference density and the specific heat ofseawater, and are defined as constant values. aT(= r1 @r/@T)
is the thermal expansion coefficient which spans 1  104 to
3  104 °C1 for ocean. Qnet is the net surface heat flux,
which is also collected from the NCEP/NCAR Reanalysis.
hQneti means a time averaging value. aT is derived frommonthly temperature and salinity fields from Levitus andBoyer [1994] and Levitus et al. [1994]. Mixed layer depth isdetermined as the depth having the potential density greaterby 0.125 sigma-theta than that at the surface, and then aT is
calculated from the mean temperature and salinity averaged
over the mixed layer depth. Thus monthly thermostericheight signals are calculated on a 1°  1° grid over the North
Pacific and the monthly SSHA relative to the steric signals arecalculated.
2.2. Hydrographic Observations[13] For hydrographic mean fields, the North PacificHydroBase (hereinafter NPHB) [Macdonald et al., 2001]
updated by Suga et al. [2004] is used. The updated NPHB has
been supplemented by some profiles of World Ocean Atlas
1994 [NODC, 1994], the CTD data from several World
Ocean Circulation Experiment (WOCE) sections and preWOCE sections, and the CTD data collected by National
Oceanic and Atmospheric Administration/Pacific Marine
Environmental Laboratory (NOAA/PMEL) [Johnson and
McPhaden, 1999]. We use 27215 profiles for the range,
140°– 180°E, 30°– 50°N. In addition to the NPHB data,
7736 temperature and salinity profiles measured from January 2001 to July 2005 by the Argo profiling floats, which
cover the same range as the NPHB data, are used. These data
were downloaded from Argo’s two Global Data AssemblyCenters in Brest, France (ftp://ftp.ifremer.fr/ifremer/argo/)
and Monterey, California (ftp://usgodae1.fnmoc.navy.mil/
pub/outgoing/argo/). Using analysis tools included in
HydroBase 2 (http://www.whoi.edu/science/PO/hydrobase),
about 35000 potential temperature and salinity profiles from
NPHB and Argo are isopycnally averaged onto 0.5°  0.5°
grids. Figure 1 shows the numbers of the averaged profiles
within each 0.5°  0.5° grid. By the addition of the Argo float
data, substantial blank grids in the NPHB data are filled,
especially in the region east of 160°E. Though the blank grids

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C10003Figure 2. Climatological mean sea surface heights (SSH) relative to 1500 dbar and surface currents
larger than 7 cm/s, which are (a) calculated in this study and (b) derived from Kuragano and Shibata[1997]. Units are meter and a contour interval is 0.05 m. The surface currents larger than 20 cm/s arerescaled and shown with blue arrows. Their references are shown below each figure. J1 and J2 in
Figure 2a denote the jets investigated in this study.
shown in white in Figure 1 still remain, some of them areisopycnally interpolated by an iterative Laplacian/spline
algorithm with a searching radius of 2°. On the other hand,some blank grids, which do not meet the criterion of theradius of 2°, are left as a gap. In this study, the mean property
fields are constructed without a heavy smoothing procedure
to preserve spatial structure as finely as possible, althoughsome gaps and noisier structure are left.
[14] In addition, climatological SSH relative to 1500 dbar
is calculated on the 0.5°  0.5° grid from these potential
temperature and salinity profiles. Before the dynamic computation, these profiles are smoothed along isopycnals
with a 5-point Laplacian filter included in HydroBase 2.
Figure 2a shows the computed SSH superimposed withsurface geostrophic currents larger than 7 cm/s. Largesurface velocities (>20 cm/s) are rescaled and shown ingray. The strongest jet is KE, where the quasi-stationary
meander with the first and second crests at about 144°E and150°E [e.g., Kawai, 1972; Mizuno and White, 1983], isclearly depicted. Along with it, the mean SSH averaged for1993 – 1998 [Kuragano and Shibata, 1997], which is
obtained from the data set CD-ROM of the Subarctic Gyre
Experiment (SAGE) project [Japan Meteorological Agency,
2001], is shown in Figure 2b. Kuragano and Shibata’s[1997] mean SSH has been calculated by combining altim-

eter and hydrographic observations. Dynamic height relative 1500 dbar is first calculated from each observation
profile. Then, the mean SSH for the altimeter observing
period is estimated at each point by subtracting altimeterderived SSHA, which is acquired at near time and location.
This method has an advantage that the mean SSH at thehydrographic observation points can be constructed by atleast one observation during the altimeter observing period.
An error in the constructed SSH has been estimated to beless than 2 cm in most areas [Kuragano and Shibata, 1997].
The SSH constructed in this study shows noisier structure
than Kuragano and Shibata’s [1997] mean SSH and probably retains the effect of instantaneous hydrographic structure, especially in areas without sufficient observations.
Nevertheless, two northeastward surface jets (lettered as
J1 and J2 in Figure 2a) heading into the subarctic region,
which are the main subject in this paper, exist at almost thesame locations in both the mean SSH fields constructedfrom different source data. The mean SSH we computed(Figure 2a) is applied in the following analyses because its
surface current fields related to KE’s bifurcations and theirrelationship with bottom topography, which is shown later,
are closer to the thermal front structure [Levine and White,1983] and the SSH and associated current fields recently
constructed by a combination of altimeter and subsurface

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C10003Figure 3. A snap shot of sea surface temperature (SST) on September 24, 2003. Sea surface height(SSH) contours (contour interval of 0.05 m with 0.25 m thickened) are superimposed. The location of awarm tongue is indicated by ‘‘J1’’.
drifter data [Niiler et al., 2003]. For reference’s sake, we
performed the same analyses with Kuragano and Shibata’s[1997] mean SSH and got essentially the same conclusion
in terms of the above two jets. Based on the mean SSH, weconstruct synthetic SSH every 7 days by adding SSHA, andthen calculate surface geostrophic velocities.
[15] The winter mixed layer climatology of the North
Pacific [Suga et al., 2004] is also obtained from the same
SAGE data set CD-ROM [Japan Meteorological Agency,
2001]. It is also calculated from the NPHB dataset byaveraging original observations within the smallest possiblearea around a grid point to preserve realistic water properties of a mixed layer, especially in frontal regions [Suga etal., 2004].

3. Results
3.1. Warm Tongues Driven by Quasi-Stationary
Surface Jets
[16] Warm streamers or tongues that spread northeastward in the Kuroshio-Oyashio transition area are frequently
confirmed from the animated images of NGSST and OI SST(not shown). These high SST bands are almost parallel with
the iso-lines of the synthetic SSH. Figure 3 shows a snap
shot of the NGSST on September 24, 2003, on which thesynthetic SSH contours are superimposed. A warm waterband, which protrudes northeastward from around 150°E,
40°N, can be seen (indicated by ‘‘J1’’). The warm waterband runs, to some extent, parallel to the SSH contours of
1.45 – 1.55 m with relatively steep horizontal gradients,
which implies the relationship of the warm band withsurface geostrophic flows. Kawai and Saitoh [1986] havedescribed a warm tongue and a geostrophic warm streamer,
based on satellite images and hydrographic observations.
They pointed out that these are unified as a warm waterband transported northward by a geostrophic jet stream

without any countercurrent components inside the warm
water band. Their typical dimensions are a depth of 50– 170
m and width of 60– 100 km for the warm tongue, and a
depth of 25 – 50 m and width of 35 – 70 km for thegeostrophic warm streamer. In this study, the warm band
and a similar one are, for the sake of convenience, called awarm tongue because of a lack of hydrographic information. Good correspondence in each snap image implies
enough ability of the satellite-derived SST and SSH data
to investigate oceanic phenomena with high-spatial resolution (several tens of kilometers) in the Kuroshio-Oyashiotransition area.
[17] In order to detect the mean picture of the warm
tongues and their relation with surface currents, the climatologies of SST (CSST) and SSH are analyzed. A spatiallyhigh-pass filter is applied for the respective 5-day CSST
data to extract fine spatial structure (several tens of kilometers). First, CSSTs are smoothed with a low-pass filter
with an e-folding scale of 0.75° (hereinafter, LCSST) andthen high-pass filtered CSST anomaly (hereinafter, HCSST)
is derived by subtracting LCSST from the original CSST.
The 73 5-day HCSSTs are then averaged (Figure 4a). The
mean picture of HCSST still shows a northeast-southwest
warm band from 148°E, 39°N to 155°E, 43°N, which islikely to correspond to the warm tongue in Figure 3. In
addition, another warm band is confirmed east of the former
one, lying on the tilt from 165°E, 40°N to 170°E, 42°N.
[18] The surface mean currents (>7 cm/s) shown in
Figure 2a are superimposed on the HCSST map. For
references, the Subarctic Front (SAF), which is defined as
the 4°C isotherm at 100 m depth [Favorite et al., 1976], isestimated from the constructed mean temperature fields andsuperimposed with blue lines (Figure 4a). The abovementioned relatively distinct jet streams with northeastward
flow (J1 and J2 indicated in Figure 2a) are roughly parallelwith SAF north of 40°N. In addition, the position of J1 and

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C10003Figure 4. (a) The climatology of spatially high-pass filtered SST anomalies. Mean surface currents
larger than 7 cm/s are superimposed with arrows, where large velocity vectors (> 20 cm/s) are red andrescaled. Velocity vector scales of 20 cm/s are shown on the right side of the scale bar. Blue lines are 4°C
isotherms at 100 m depth, which is a definition of the Subarctic Front. (b) Bottom topography along withthe strong surface currents (>7 cm/s). The location of J1 and J2 is indicated.
J2 approximately corresponds to the warm bands discussed
above. J1 is generally recognized as the Subarctic Current.
At the same time, Figure 4a indicates that J1 is related towarm water at least in the surface layers.
[19] The existence of the 2 jets discussed above has been
recently pointed out by the direct measurements [e.g., Niiler
et al., 2003; Iwao et al., 2003]. Niiler et al. [2003] haveprovided near-surface current fields in the KE region byusing 657 drifters deployed during 1989 – 2001. They described mean flow fields related to not only KE but also two
prominent jets in the Kuroshio-Oyashio transition area: onealong the eastern slope of the topographic mound (M1; see
Figure 4b) east of the Kuril-Kamchatka (KK) Trench, andthe other along 38° – 40°N that turns to the north at 165°E,
40°N. They seem to correspond to J1 and J2 in this study.
Their current field presents that mean flow speed along boththe jets reaches up to 20– 30 cm/s. Iwao et al. [2003] havepresented intermediate current fields on the 26.7 sq surface
in the northwestern North Pacific by using 21 subsurface
floats deployed for 1998– 2002. They pointed out three
types of strong eastward flows: 1) KE at 32°– 35°N, 2) the
Subarctic Current at 42°– 45°N, and 3) the flow along thesubarctic boundary at 39° – 40°N. Judging from their figure,the second and a part of the third flows correspond to J1 andJ2. Their figure shows that mean velocities on the 26.7 sq
surface, which is equivalent to almost 200 – 300 m depths at
40° –45°N, are almost 15 cm/s and 5 – 7 cm/s for J1 and J2,
respectively.
[20] The positions of J2 as well as J1 are likely to berelated to bottom topography. Figure 4b shows the meansurface current field (>7 cm/s), which is superimposed on
the bottom topography derived from a global digital eleva-

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C10003Figure 5. The distribution of mean eddy kinetic energy (EKE), on which Reynolds’ stress ellipses aresuperimposed. The Reynolds stress ellipses are drawn in two scales and colors. The reference of eachscale is shown on the left side of the scale bar. The location of quasi-stationary jets is indicated.
tion database originally developed by the U. S. Defense
Mapping Agency (ETOPO5). The figure, on the whole,shows the effect of bottom topography on the route of thestrong jets. KE bifurcates over the Shatsky Rise around
160°E and one part of them heads northeastward along theShatsky Rise. This is consistent with the observational
results by Mizuno and White [1983] and Levine and White[1983], who have defined the bifurcation routes withsynoptic thermal maps. The numerical simulation configured with realistic topography has also reproduced thebifurcation over the Shatsky Rise and the mechanism of
the bifurcation has been investigated by Hurlburt andMetzger [1998]. The other strong jets passing through theEmperor Seamounts tend to selectively converge on therelatively deep gaps such as 43°N, 39°N, and 34°N.
[21 ] J1 is, as pointed out by Niiler et al. [2003],
parallel with the eastern slope of M1. J2 seems to have aconnection with a part of the KE bifurcations and is mainly
parallel with the eastern slope of the topographic mound of
the northern part of the Shatsky Rise (M2; see Figure 4b).
Niiler et al. [2003] hypothesized the strong current could
have a strong barotropic component. In a series of numerical experiments, Hurlburt et al. [1996] have successfully
simulated these jets and pointed out that bottom currents
produced by baroclinic instabilities interact with bottom
topography and in turn steer the surface currents. While theavailable data is not sufficient to test these hypotheses, thepoint here is that the quasi-stationary feature of these jets is
attributed to bottom topography.
[22] Each snap shot of SST and SSH shows that warm
water seems to be transported by geostrophic warm streamers or warm tongues into MWR via several anti-cycloniceddies, which distribute in the area, especially west of
155°E [e.g., Kawamura et al., 1986; Yasuda et al., 1992].
One might suspect that the averaging process of theseeddies artificially produces the quasi-stationary jets.
Figure 5 shows the mean eddy kinetic energy (EKE) map
with the Reynolds stress ellipses, which are calculated fromthe altimeter-derived surface current anomalies. EKE isexpressed in a natural logarithm and the Reynolds stressellipses are drawn with two scales. High EKE areas along
KE and Oyashio are prominent. They are probably attributed to the KE jet and mesoscale eddies detached from it,and the Oyashio and eddies propagating on the trench[Isoguchi and Kawamura, 2003], respectively. Meanwhile,
the relatively high EKE streaks appear along J1 and J2. The
principal axes of the Reynolds stress ellipses along thesestreaks are approximately parallel to the directions of thejets. Interestingly, gentle regions can be seen on both sidesof these active streaks. In particular, a lower EKE patch
surrounded with the higher EKE regions, which is locatedon the southeast side of J1 (157 – 164°E, 41 – 43°N), is
prominent. These results support the idea that the highvariability along the two jets north of 40°N is mainly
attributed not to propagating disturbances like eddies, but
to the quasi-stationary feature of surface currents.
3.2. Annual Cycles of the Jets[23] Figure 6 shows seasonal (3 months) mean HCSSTs
with seasonal mean flows larger than 7 cm/s (left figures)and seasonal mean SSHs (right figures). J1 and the associated warm tongues are persistent through the year, while
they, at the same time, show some seasonal features. The
SSH maps show that some SSH contours concerning J1 areconnected with the Western Subarctic Gyre (WSAG), forming return flows as the Subarctic Current, while a part ofthem is also connected with the northern extent of the crestsof the KE meander. In particular, this feature is clearer insummer and fall, as described later. Some effects of thesubtropical gyre on J1 are thus inferred from the SSH maps.
Moreover, it has been reported as a short-term phenomenon
that the secondary Kuroshio Front transports warm waters
northward as geostrophic warm tongues and streamers via
anticyclonic eddies, which exist at any time in MWR [e.g.,
Kawai, 1972; Kawai and Saitoh, 1986]. It is hypothesizedthat these eddies systematically transport the effect of KE to
the quasi-stationary jet, J1.

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C10003Figure 6. The seasonal (3 months) mean maps of (left) spatially high-pass filtered SST anomalies, on
which seasonal mean surface currents (>7 cm/s) are superimposed, and (right) the synthetic SSHs. Large
velocity vectors (>20 cm/s) in the left figures are red and rescaled. Velocity vector scales of 20 cm/s areshown on the right side of the scale bars. The contour interval of SSH is 0.05 m, and the location of quasistationary jets is indicated in the upper maps.
[24] The SSHs in WSAG are relatively lower in winterand spring (upper 2 panels) than in summer and fall, whichindicates the wintertime spinning-up of the gyre. Thisentails the relatively stronger northeastward return flowsof the subarctic gyre, widening J1 on the side of thesubarctic gyre, which results in covering over the cold
SST bands (upper 2 left panels). This is reasonable because
the western boundary currents of the subarctic gyre (theEast Kamchatka Current and the Oyashio) are strong in
winter and spring [e.g., Uehara et al., 1997; Isoguchi et al.,1997]. In summer and fall, on the other hand, the SSHs are
relatively higher on the side of the subtropical gyre, which
results in developing a steep SSH gradient along J1, in spiteof the weaker subarctic gyre. This intensified J1 converges
on the warm water bands, implying the northeastward
feeding of warm waters as the geostrophic warm tongue.
J2, on the other hand, forms relatively clear jets in springand summer but less in fall and winter. We calculate thetemporal variations of J1 and J2 using the hydrographic
altimeter-combined surface currents. First, the principal
axes of the jets are defined at grids where the mean currents
are larger than 7 cm/s (arrows in Figure 4a), inside theranges of 150– 162°E, 40.5 –45.5°N and 162– 171°E, 40–
46°N for J1 and J2, respectively. Then the principal axis
components of the altimeter-derived current anomalies arecalculated at the selected grids and their means are derived

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C10003Figure 7. The annual cycles of the J1 (black) and J2 (gray) currents. Error bars denote their standard
deviations. Each annual cycle is shown twice side by side.
as the J1 and J2 variations every month for 1993 –2004. The
annual cycles of the J1 and J2 variations are shown with blackand gray lines in Figure 7. The vertical bars show theirstandard deviations. Although the year-to-year variability,
which will be investigated later, seems to be too large,
especially about J2, to describe the annual cycles, they show
the above mentioned seasonal features, specifically, that J1(J2) is the most intensified in fall (summer). It is emphasizedagain that the seasonal cycle of J1 is not consistent with thatof WSAG. Thus although J1 is involved more by the return
flow of WSAG in winter and spring, the seasonal variation ofits strength is not likely to be controlled by WSAG.

[25] One might suspect that the spatial filtering in
Figures 4a and 6 artificially generates di-pole banded
structure for a strong SST front and the warm bands withcold counterparts in Figures 4a and 6 do not necessarily
correspond to warm tongues or streamers. It might be truethat averaging process in climatoligical fields makes thebanded structure obscure and the di-pole SST bands in
Figures 4a and 6 just represent strong SST fronts, especiallyfor J2. If the di-pole represents the SST front, the axis of thejet is expected to be along its node. Nevertheless, the axis of
J1 is not along the node but just over the warm band insummer and fall, when J1 is relatively strong. In addition,

Figure 8. (a) The Pentad SST Climatology [Armstrong and Vazquez-Cuervo, 2001] for 24–
28 September. The location of J1 is indicated. (b) SST gradient profiles along the black line shown in
Figure 7a for September – October (thin lines) and their average (a thick line). They are calculated fromthe Pentad SST Climatology.
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C10003Figure 9. The time evolution of 2 years mean surface currents (>7 cm/s) and SST anomalies, which arederived by subtracting spatially low-pass filtered SST climatology. Large velocity vectors (>20 cm/s) areblue and rescaled. Velocity vector scales of 20 cm/s are shown on the right side of the scale bar. The
location of quasi-stationary jets is indicated in the upper left panel.
the original 5-day CSST for 24– 28 September (Figure 8a)shows the northeastward intrusions of warm waters at thepositions of J1 and J2, although J2 is not as clear as J1. SST
gradients along the black line in Figure 8a, which cuts
across the warm band concerning J1, are calculated from the
12 CSST fields for September – October and shown (thinlines) along with their average (a thick line) in Figure 8b.
Most of the 12 profiles and the average exhibit negative
gradients around 42.5°N, which indicates warm banded
structure with a center at a zero crossing latitude. Thus
the warm banded structure is apparently retained for September – October even in the climatological data constructedfrom the 15-year data, which indicates the stationary androbust features of J1 and the associated warm tongues.
3.3. Year-to-Year Variability of the Jets and
Its Impact on SST Fields[26] The year-to-year variability of the jets and associated
SST fields including the warm tongues are investigated. The
5-day SST anomalies are first calculated by subtracting the
low-pass filtered CSST (LCSST) from the 5-day SST timeseries. In consequence, anomaly components with a small
spatial scale, which are related to the warm tongue structure,and those with a large scale, which are mainly caused by theinterannual variations of heat fluxes, retain in the 5-day SST
anomalies. By using the 5-day data and climatologies
instead of monthly mean or longer mean data, phenomena
with a small spatial scale are expected to be detected moreclearly. These 5-day anomalies are then averaged for 2
successive years and are shown in Figure 9, on which thesimultaneous mean surface currents (>7 cm/s) are superimposed. Also shown in Figure 10 is the time evolution of
the 2-year mean SSHs. The large-scale SST anomalies over
the whole region in Figure 9 are thought to be mainly
brought about by the interannual variations of heat fluxes.
On the other hand, a notable point is that the year-to-yearchanges of the warm tongues in strength correspond to
those of the jets. J2 is relatively strong during 1993 – 1996,
forming the apparent jet streams and warm tongue structure,
while J1 and associated warm tongues are relatively weak.
After 1997, J1 strengthens and J2 weakens. At the sametime, as seen in Figures 9 and 10, the crests of KE’smeander shift northward for 1999 – 2002, which implies
the some connection between the KE jet and J1. As theresult of advection by J1, positive SST anomalies appear
over and along the south side of J1 in 1999 – 2002, forming
the well-defined SST front along J1. These interannualfeatures are confirmed from the SSH maps (Figure 10):

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C10003Figure 10. The time evolution of 2 years mean SSH. Units are meters and a contour interval is 0.05 m.
The contours of 1.5 m are shown with thick lines. The location of quasi-stationary jets is indicated in theupper right panel.
SSH gradients across J1 (J2) are relatively weak (strong)
during 1993 – 1996, being followed by a gradual strengthening (weakening) after 1997. An interesting point again isthat WSAG does not show such clear interannual variations
compared with the subtropical gyre. Thus the interannual
variation of J1 is mainly attributed not to WSAG but to the
SSH change on the southeast side of J1. This is clearlyshown by the north-south migration of the 1.5 m contours(thick black lines in Figure 10). The 1.5 m contours movenorthward after 1997 and are approximately along with J1
after 1999, developing a steep slope across J1. The migration is likely to be consistent with the KE’s meridional shift,which will be investigated later.
[27] We examine the above year-to-year variability, focusing on J1 and J2. The interannual anomalies of J1 and J2
relative to their annual cycles (Figure 7) are shown with red
and blue lines in Figure 11a. This time evolution describes
the low frequency variability shown in Figures 9 and 10. J1
gradually strengthens from 1996 and reaches at its peak for1999 – 2001, being followed by weakening after 2002. On
the other, J2 basically shows the variation with an opposite
phase. The regressed fields of surface current and OI SST
anomalies relative to their annual cycles on the interannual
variation of J1 (Figure 11a) are calculated (Figure 11b). TheOI SST’s field is calculated for the temporal period from Jan

1993 to June 2003. The regressed fields on J2 show, on thewhole, the opposite patterns of those on J1 (not shown). The
regressed current field inevitably leads to a strong and welldefined jet along J1. At the same time, as mentioned above,
the northward shift of the KE’s meander is expected tooccur around its crests east of the Japan coasts. We
conveniently define the Kuroshio axis as the 2.0 m contoursof SSHs, which roughly correspond to the maximum
velocity in the climatological field (Figure 2a), and estimate
its time evolution from each monthly map. The latitudinal
position of the upstream KE, which is averaged for 142–
148°E, is depicted along with the J1 interannual variation(Figure 12). The derived variations of the upstream KE axis
are consistent with those detected by B. Qiu and S. Chen
(Variability of the Kuroshio Extension jet, recirculation gyreand mesoscale eddies on decadal timescales, submitted to
Journal of Physical Oceanography, 2006). The correlation
between the KE position and J1 is 0.61, which is notnecessarily significant to the 95% confidence interval if
their low frequency feature is considered. Nevertheless,both the time series approximately show the above mentioned low frequent variation. The J1 and KE’s northward
shift entail strong positive SST anomalies, spreading northeastward from the crests of the KE’s meander to J1 and its
south region. This describes that when the crests of the KE’s

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C10003Figure 11. (a) The time series of J1 (red) and J2 (blue) relative to their annual cycles. (b) The regressed
maps of SST and surface current anomalies on the J1’s interannaul time series. The regressed current
larger than 10 cm/s are rescaled and shown with white. The location of J1 and J2 is indicated.
meander extend northward after 1999, J1 and the associatedwarm tongue structure simultaneously strengthen, indicating that much heat and substance are transported into thenorthern region. In contrast, J2 tends to weaken (the southwestward arrows) in the same period, entailing the negative
SST anomaly. Figure 11 thus suggests some connections
between the KE jet, J1, and J2. However, it should be notedthat the result just shows the fact that the northward shift of
the KE’s meander, the strengthening of J1, and the weakening of J2 occur concurrently, at least in this period.
Mechanisms controlling the variations of J1 and J2 andtheir interactions with the KE jet are beyond the scopeof this study and should be addressed in the future study,

by using high-resolution numerical simulations configured
with realistic bottom topography.
3.4. Hydrographic Structure[28] In this section, the hydrographic mean fields constructed from the historical observations are used to discuss
the vertical structures of the jets and their water mass type.
Qu et al. [2001] have produced similar climatologies from
NPHB [Macdonald et al., 2001] in the Kuroshio-Oyashiotransition area. In this study, our attention is focused on thephenomena with fine spatial scale that were revealed fromthe satellite observations. Therefore, we created the meanhydrographic fields, trying to hold spatial structure as finely

Figure 12. The time series of J1 (black) and the latitudinal position of the upstream KE axis for 142 –148°E (gray). The KE axis is determined as the 2.0 m contours of the SSH maps.
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C10003Figure 13. q - S diagrams in the (a) J1, (c) intermediate, and (e) J2 regions, on which the pure Kuroshio(a dashed gray line) and Oyashio (a bold gray line) water profiles are superimposed. The histograms of
the Kuroshio mixing rations for the density range from 26.0 sq to 26.5 sq in the (b) J1, (d) intermediate,and (f) J2 regions. Each region is shown in Figure 14c.
as possible. This is the main difference between this study
and the previous analysis.
[29] Following Zhang and Hanawa [1993], the Kuroshio
mixing ratio is estimated from these hydrographic mean
fields to examine the geographical distribution of two
typical water masses of the Kuroshio and Oyashio waters.
The mixing ratio is defined as,rq ð%Þ ¼ 100  ðq  qO Þ=ðqK  qO Þ;ð2Þ
rS ð%Þ ¼ 100  ðS  SO Þ=ðSK  SO Þ;

where rq and rS are the mixing ratios for potential
temperature, q, and salinity, S, and qO, qK, SO, and SK are
the potential temperature and salinity of the pure Oyashio
and Kuroshio waters at an isopycnal surface, sq. Although
the two types of the mixing rations are calculated by thedefinition, their average is used in this study. The pure
Kuroshio and Oyashio water properties are selected fromthe mean q and S profiles around 141.25°E, 34.75°N, and147.75°E, 43.25°N, respectively (see Figure 1), where theobservation numbers within each grid are relatively large.
The respective q - S profiles are depicted in Figure 13. The
Kuroshio mixing ratios are then calculated by theequations (2) every 0.1 sq from 25.5 sq to 27.3 sq.
[30] (a) Potential temperature, (b) salinity, (c) acceleration
p o t e n t i a l a n o m a ly ( g e o s t r o p h i c f l o w f u n c t i o n )[Montgomery, 1937] relative to 1500 dbar, and (d) mixing
ratio maps on the 26.5 sq surface are depicted in Figure 14.
Only the acceleration potential anomaly map is smoothedwith a 3  3 median filter to reduce noisy structure. The
mean currents at the sea surface (>7 cm/s) are superimposedon the acceleration potential map. The contours of 50% aredrawn in red on the mixing ratio maps (Figure 14d). Boththe potential temperature and salinity maps on the 26.5 sq
surface (Figures 14a and 14b) show well-defined northeast-

southwest fronts along J1 and J2, due to their compensating
nature on an isopycnal surface. In particular, the J1-related
northeastward protrusions of warm and saline waters areprominent. The acceleration potential map (Figure 14c) also
shows well-defined fronts related to J1 and J2. The 50%
contours in the mixing ratio map are generally along with J1and J2, involving the J1-related protrusion. These results
indicate that J1 and J2 form a boundary between thesubtropical and subarctic waters, at least on the 26.5 sq
surface. In order to investigate the water mass property ofeach temperature, and salinity profiles around and between
J1 and J2, the profiles around J1 and J2 and those between
them are plotted on q - S diagrams (Figures 13a, 13c,and 13e, respectively). Their ranges are shown with red
frames and a green square in Figure 14c. The intermediate
region shown in the green square corresponds to the lower
EKE region on the southeast side of J1 (Figure 5). In
addition to that, mixing ratios are calculated for the plots
between 26.0– 26.5 sq and their histograms with a 5%interval are shown in Figures 13b, 13d, and 13f. For theupper layers lighter than 26.5 sq, which correspond to about
100 m and 150 m around J1 and J2, the plots along J1 andJ2 (Figures 13a and 13e) tend to split and converge aroundboth the pure Oyashio and Kuroshio water references (boldand dashed gray lines, respectively). This water mass
feature around J1 has been clearly pointed out by
simultaneously conducted observations [Kono, 1997].
Meanwhile, those in the green square converge on anintermediate position between the pure Oyashio andKuroshio references (Figure 13c). These tendencies aredescribed well in the histograms of the mixing ratio. The
histograms for J1 and J2 (Figures 13b and 13f) have twoclusters divided into the Kuroshio and Oyashio water type
ranges. The J1’s histogram, in particular, reaches a peak at
relatively higher Kuroshio mixing ratios of 80– 85%. On theother hand, the histogram for the green square (Figure 13d)

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C10003Figure 14. The distributions of (a) potential temperature (°C), (b) salinity (psu), (c) acceleration potential(m2s2) relative to 1500 dbar, and (d) the Kuroshio mixing ratio (%) on the 26.5 sq surface. The acceleration
potential is displayed after applying a 3  3 median filter. The surface mean currents (>7 cm/s) and 155°E
and 169°E sections (white lines) are superimposed on the acceleration potential map. Also shown on theacceleration potential map are regions around J1 and J2 (red frames) and between them (a black frame), eachobservation of which is plotted on the q - S diagrams in Figure 13. The location of J1 and J2 is indicated in
Figure 14a.
just shows a cluster, reaching a peak at 55– 60%. It issuggested from the results that, at least for the layers
shallower than 26.5 sq, the subtropical warm and saline
waters are transported, selectively, by J1 and J2 into thenorthern region in the northwestern North Pacific.
[31] Figure 15 shows mixing ratio sections along (a)155°E and (b) 169°E, which cut across the axes of J1, J2,
and KE (shown with white lines in Figure 14c). The
contours for 10– 90% are drawn every 20% with green
lines on them. Eastward geostrophic velocities relative to1500 dbar (> 5 cm/s) and potential density are superimposed with black and white contours. The velocity
sections describe the vertical structure of KE, J1, and J2.
The eastward flows, corresponding to KE and J1, can beseen at 34°N and 43.5°N in the 155°E section (Figure 15a),while those corresponding to KE and J2 are at 35°N and
43°N in the 169°E section (Figure 15b). J1 and J2 arevertically well developed with the velocity exceeding 5 cm/s
down to about 700 m and 600 m. This result supports theidea that the upper layer warm tongues are driven by theconsistent geostrophic jet along SAF. The mixing ratio maps
show the distinct relationship between J1 and J2 and a watermass boundary. Although the 50% contours shift southward
apart from J1 at the lower levels, the 30% contours run
almost vertically, forming well-defined fronts at the positionof J1 and J2. Thus the contribution of the Oyashio water
becomes larger and larger with depth, on the south sides ofJ1 and J2. These jets are, nevertheless, still a type ofboundary between the subtropical and subarctic waters.

[32] As mentioned above, the warm and saline water
protrusion into the region north of 40°N can been seen on
the 26.5 sq surface (Figures 14a and 14b). According to theacceleration potential map (Figure 14c), intermediate flow
along J1 detours around the brink of the protrusion with anortheastern crest around 166°E, 45°N and eventuallyconnects to J2. This detouring route can be seen in thesurface flow fields for 1999– 2000 and the some SSH maps
in Figures 9 and 10. In addition, even on the 26.7 sq
surface, most intermediate floats have been reported to
depict the detour trajectories with the crest around 166°E,
45°N [see Iwao et al., 2003, Figure 3]. The detour route isroughly consistent with SAF (the 5°C isotherm at 100 m in
Figure 4a). These results suggest the existence of the quasistationary anticyclonic detour circulation, which forms thenorthern extent of the subtropical gyre. In fact, the northern
extent of J1 and J2, or the associated front of theacceleration potential map, reaches around 45°N, which isconsistent with the boundary between the subtropical andsubarctic gyres determined from historical wind stress data
based on the simple linear Sverdrup theory.

4. Summary and Discussions[33] In this study, we examined the quasi-stationary jets
and the associated warm tongue phenomena in the Kuroshio-Oyashio transition region, by using satellite-derived
SST, sea level anomaly (SLA), and surface current data.
Then, we discussed the hydrographic structure of the

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C10003Figure 15. The vertical sections of the Kuroshio mixing ratio (%) along (a) 155°E and (b) 169°E withgreen contours every 20% form 10% to 90%. Eastward geostrophic current (> 5 cm/s) and potential
density (sq) contours are superimposed every 5 cm/s and 0.2 sq. The location of J1 and J2 is indicated.

surface jets, based on the hydrographic mean fields constructed from historical observations. The details are as
follows.
[34] The recently constructed cloud-free daily SST datawith high spatial resolution [Guan and Kawamura, 2004;
Kawai et al., 2006] have revealed the warm tongues,
frequently spreading northeastward along SAF. These
cloud-free SST time series were quite useful for understanding of phenomena with fine spatial structure through theconstruction of an animated image and a regressed map of
SST anomaly on certain time series. The consistency of thewarm tongues has been confirmed from the spatially highpass filtered climatological SST, based on the 1985 – 1999
Advanced Very High Resolution Radiometer (AVHRR)
data. The surface current fields derived from the constructedhydrographic mean field indicated that the warm tongues
were driven by the mean geostrophic jets (J1 and J2 shownin Figure 2). J1 and J2 coincided roughly with the surface
and intermediate current fields previously constructed bysurface drifting buoys [Niiler et al., 2003] and subsurface

floats [Iwao et al., 2003]. These geostrophic warm tongues
exist throughout the year with a seasonal tendency, being
strongest in fall (summer) for J1 (J2). The positions of thejets and warm tongues are governed by bottom topography:
they parallel with the eastern slopes of the topographic
mounds, as pointed out by Niiler et al. [2003]. It issuggested that the quasi-stationary warm tongues driven
by the geostrophic jets can be a consistent mechanism
transporting surface warm water toward the subarctic
region.
[35] The time evolution of the 2-year mean maps of thehigh-pass filtered SST anomalies, along with the surface
currents and sea surface height (SSH), demonstrated that thejets underwent some low frequency changes in strength, andthey entailed the corresponding changes of surface thermal
structures. The regressed maps of surface currents and SSTs
on the J1’s interannual time series could describe thefollowing year-to-year variability. When the crests of theKE’s meander extended northward in 1999– 2001, J1 simultaneously strengthened, resulting in the high SST region

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C10003Figure 16. The distribution of late winter (Feb– Mar) mixed layer depth obtained from Suga et al.
[2004], on which the mean surface currents (>7 cm/s) are superimposed. The location of J1 and J2 isindicated.
over and along the south side of J1. J2, in contrast,
weakened after 1999 and mainly changed with an opposite
phase to J1, at least in the analyzed period.
[36] Warm tongues and warm streamers, which spread
northward from the crests of the KE’s meander, are sometimes called the secondary Kuroshio Front [Kawai, 1972]
and are regarded not only as the main source of warm watertransported into MWR but also as a possible mechanism inaccelerating the northward migration of pelagic fish in earlysummer [e.g., Kawai and Saitoh, 1986; Sugimoto andTameishi, 1992]. The behavior of these phenomena has
been investigated as a series of events or short-term phenomena [Kawai and Saitoh, 1986; Sainz-Trapaga andSugimoto, 1998]. The results in this study hypothesize thatthese short-term secondary Kuroshio Front phenomena,
being linked with the KE’s variability, systematically connect to J1, which has quasi-stationary and persistent characteristics. In addition, as shown before, since J1 roughly
parallels with SAF, the warm tongues driven by J1 play
important roles in supplying surface warm water into thesubarctic region and, in addition, they might be direct routes
for the northward migration of pelagic fishes toward thefeeding ground in the subarctic region. The detailed variations, including a mechanism linking them, should bestudied in the future by adding numerical simulations.
[37] In order to discuss the vertical structure related to thejets, the hydrographic mean fields were constructed for
spatial structure to be held as finely as possible. The flow
field relative to 1500 dbar showed the robust structures of J1
and J2 as well as KE, which supports the idea that the warm
tongues were driven by the quasi-stationary geostrophic
jets. The intermediate flow patterns involved the detour
route around the tongue-like protrusion of warm and saline
water, connecting between J1 and J2. These jets roughly
corresponded to SAF and the front of the Kuroshio mixing

ratio (the 30% contours), implying that they probably forma type of boundary between the subtropical and subarctic
gyres in the northwestern North Pacific.
[38] It should be noted that the inside of the anticyclonicdetour route around the tongue-like intrusion corresponds to
the region with the local maximum of late winter mixed
water depth (MLD). Figure 16 shows the mixed layer depth
map in February [Suga et al., 2004], on which the meansurface currents (>7 cm/s) are superimposed. The winter
MLD deeper than 200 m is located south of J1. J1 is roughly
consistent with the sharp MLD fronts. This feature can beseen as analogy with the local MLD maximums in therecirculation gyre south of KE (140– 150°E, 30– 34°N). The
maximum of thickness of density range 26.6 – 27.0 sq has
also been reported to appear south of J1 (around 42°N)[Iwao et al., 2003], which is inferred from the density
section along 155°E (Figure 15a). In fact, potential vorticity
minimum on the 26.6– 26.7 sq surfaces is distributed in thesame region (not shown). The depth at 26.6– 26.7 sq layers
in these regions is about 200 – 250 m (Figure 15a), which isroughly the same depth as the late winter mixed layer. It isthus hypothesized that J1 might play a significant role,
through the supply of saline water, in forming low potential
vorticity water and a deep mixed layer. Although a detailed
discussion is beyond the scope of this study, this is aphenomenon that suggests the possibility for the jets in thisstudy to contribute to water mass formation.
[39] Acknowledgments. The authors thank the editor and the twoanonymous reviewers for constructive comments and English editing,which were helpful in improving our paper. The authors also thankmembers of Center for Atmospheric and Oceanic Studies and Physical
Oceanography Group at Tohoku University for helpful discussions. This
study is supported by Special Coordination Fund for Promoting Scienceand Technology ‘‘New Generation SST’’ of Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan, and the Category 7 of
MEXT RR2002 ‘‘Project for Sustainable Coexistence of Human, Nature

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and the EarthRR2002 Symbiosis Project of MEXT, Japan’’. The altimeterproducts were produced by Segment Sol multimissions d’ALTimétrie,d’Orbitographie et de localisation précise/Data Unification and Altimeter
Combination System (SSALTO/DUACS) as part of the Environment andClimate European Union (EU) ENhanced ocean data Assimilation andClimate prediction (ENACT) project (EVK2-CT2001-00117) and distributed
by Archiving, Validation and Interpretation of Satellites Oceanographic data
(AVISO), with support from Centre National d’Etudes Spatiales (CNES).

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

O. Isoguchi and H. Kawamura, Center for Atmospheric and Oceanic
Studies, Graduate School of Science, Tohoku University, Aoba, Sendai,
Miyagi 980-8578, Japan. (isoguchi.osamu@jaxa.jp)
E. Oka, Ocean Research Institute, University of Tokyo, 1-15-1
Minamidai, Nakano, Tokyo 164-8639, Japan.

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