ARTICLE IN PRESS

Deep-Sea Research I 53 (2006) 425–448
www.elsevier.com/locate/dsr

Enhanced ventilation in the Okhotsk Sea through tidal mixing at
the Kuril Straits
Tomohiro Nakamuraa,, Takahiro Toyodab, Yoichi Ishikawab, Toshiyuki Awajib,aaFrontier Research Center for Global Change, JAMSTEC, Yokohama, 236-0001, Japan
Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japanb

Received 25 October 2004; received in revised form 2 September 2005; accepted 27 December 2005

Abstract
The role played by tide-induced diapycnal mixing at the Kuril Straits in the formation of the Okhotsk Sea water is
examined using an ocean general circulation model. The result shows that tidal mixing causes a freshening of intermediatewater down to 27:6sy . This implies an enhancement of ventilation by tidal mixing, since a freshening ﬂux can come only
from above in the Okhotsk Sea. In fact, when compared with the case without tidal mixing, the production rate of Dense
Shelf Water (DSW) increases from 0.01 to 0.13 Sv ð106 m3 s1 Þ with maximum DSW density increased from 26.6 to 26:8sy .
This enhanced production of DSW allows us to successfully reproduce a layer of local potential vorticity minimum in the
Okhotsk Sea, which characterizes the Okhotsk Sea Mode Water. The analysis reveals that tidal mixing at the Kuril Straits
has two roles in enhancing ventilation. First, it preconditions the water properties of the DSW because tidal mixing at the
Kuril Straits induces an upward salt ﬂux from the saltier lower layer. The subsequent transport of the saline water from theStraits to regions of sea-ice generation increases the density of subducting water, resulting in more realistic densities andproduction rates of DSW. Second, tidal mixing directly modiﬁes water properties throughout the water column. The
ventilation due to this mechanical mixing further enhances the intermediate layer ventilation, and is dominant in the Kuril
Straits and in the layer denser than the DSW. Although the effect of double diffusion on ventilation in the Okhotsk Sea isalso examined, it is much less effective than the tidal mixing. Since the Okhotsk Sea is a key location for the ventilation of
the North Paciﬁc intermediate layer, the important implication here is that tidal mixing at the Kuril Straits enhances themeridional overturning in the entire North Paciﬁc Ocean.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Tidal mixing; Subduction; Okhotsk Sea; North Paciﬁc intermediate water

1. Introduction
The Okhotsk Sea, which is a marginal sea
connected to the North Paciﬁc Ocean through theCorresponding author. Present address: Institute of Low
Temperature Science, Hokkaido University, Sapporo 060–0819,
Japan.
E-mail address: nakamura@lowtem.hokudai.ac.jp(T. Nakamura).

0967-0637/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr.2005.12.006

Kuril Straits (Fig. 1a), has received much attention
in association with ventilation of the North Paciﬁc(e.g., Kitani, 1973; Favorite et al., 1976; Talley,
1991; Yasuda et al., 1996).
Observations show that Okhotsk Sea water isconsiderably different from the subarctic water of
the North Paciﬁc. In particular, the intermediatewater in the Okhotsk Sea is considerably fresher,colder and more oxygenated than water at the same

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

426

Model topography

DBDB5BeringSea
Kamchatka
lin
Sakha

TartarSt.

Soya St.

Okhotsk
Sea

sin

il Ba

Kur

its

tra

lS

ri
Ku

Hokkaido
Japan
Sea

North Pacific Ocean

(b)

(a)

Fig. 1. (a) Bottom topography around the Okhotsk Sea. Contours are drawn at 200, 3000, and 6000 m depth. (b) Model topographyaround the Okhotsk Sea. Thick white lines indicate the region of enhanced vertical diffusion in the Tmix case, while thick black lines
indicate land boundaries.

density in the subarctic Paciﬁc and the difference
extends to about 2000 m or 27:6sy (Moroshkin,1966; Yasuoka, 1968; Talley, 1991; Watanabe andWakatsuchi, 1998; Freeland et al., 1998). Anotherimportant difference is the presence of a local
maximum in the thickness of isopycnal layers
around 26:8sy , which is termed ‘Okhotsk Sea Mode
Water’ (OSMW) (Yasuda, 1997).
These properties imply that ventilation signals insuch layers are stronger in the Okhotsk Sea than in
the subarctic Paciﬁc. This fact means that ventilation in the Okhotsk Sea reaches the 27:6sy density
layer, and that it does not originate from the Paciﬁc.
Note that in the Okhotsk Sea, freshening below thesurface layer is a good indicator of ventilation inmuch the same way as elevated oxygen concentrations. This is because salinity increases almost
monotonically with depth around the Okhotsk Sea
and because salinity values are lower in the Okhotsk
Sea than in the North Paciﬁc at the same potential
density, and thus a freshening ﬂux can come only
from above. Also, a maximum in layer thickness
usually indicates the inﬂuence of subduction as in
the case of mode waters.
Supply of the Okhotsk Sea water modiﬁes thesubarctic water which ﬂows as a western boundary

current (East Kamchatka Current Water, EKCW).
The modiﬁed water, called ‘Oyashio water’, spreads
both to the subarctic and subtropic North Paciﬁc(e.g., Favorite et al., 1976; Talley, 1991; Yasuda
et al., 1996). In particular, the OSMW is considered
to be a major source of the North Paciﬁc Intermediate Water (NPIW) (Yasuda, 1997), which ischaracterized by a salinity minimum around 26:8sy(Sverdrup et al., 1942). Because the NPIW spreads
out over most of the subtropical gyre (Talley, 1993),
the Okhotsk Sea is regarded as an important locationfor ventilation of the North Paciﬁc intermediate
layer. This view is also supported by observed
distributions of Chloroﬂuorocarbons (CFCs) (Warner et al., 1996). Since the above ventilation processcreates a body of water capable of storing large
amounts of greenhouse gases such as CO2 (e.g.,
Yamanaka and Tajika, 1996) as well as heat andfresh water, the clariﬁcation of the water-massformation and subsequent transformation processes
in the Okhotsk Sea is important for understanding,modeling, and predicting the potential response of
the ocean to increased greenhouse gases and to longterm variability in the North Paciﬁc.
Within the Okhotsk Sea, the most important
process in the transformation of the incoming

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

North Paciﬁc water is considered to be sea ice
production (e.g., Watanabe and Wakatsuchi, 1998).
The associated brine rejection contributes to the
production of highly saline water near freezing
point observed over the shelf of the northern
Okhotsk Sea (Kitani, 1973; Gladyshev et al.,2000). Note that this water is fresher than surrounding water at the same density since the water is close
to the freezing point. The deeper component of thiswater, with a density greater than the winter mixed
layer in the interior Okhotsk Sea (i.e., 426:6sy ), iscalled ‘Dense Shelf Water’ (DSW). Its maximum
observed density reaches 27:03sy (Kitani, 1973).
The production of the DSW freshens the intermediate layer, as the fresh DSW (and/or components derived from it through entrainment of theambient water) is transported southward along theeast coast of Sakhalin and into the Kuril Basin (e.g.,
Fujii and Abe, 1976; Talley, 1991). In the Kuril
Basin, the fresh water originating from the DSW
mixes widely with the water in the basin and thus
ventilates the intermediate water of the Okhotsk
Sea. This process includes the OSMW formation,
which was traditionally thought to occur throughisopycnal mixing of the DSW and the EKCW with asmall contribution of Soya Warm Current Water(SWCW) (Yasuda, 1997; Watanabe and Wakatsuchi, 1998).
However, when compared with the DSW density(p27:05sy ), the ventilation signals in the Okhotsk
Sea (i.e., the relatively fresh and oxygenated water)
extends to much denser layers ð27:6sy Þ. Since the
DSW is the densest water that is directly formed bysurface forcing in both the North Paciﬁc and the
Okhotsk Sea (Talley, 1991; Gladyshev et al., 2000),
diapycnal mixing of low salinity and high oxygencontent water from above must be working (thoughdeeper subduction occurs in the Japan Sea, theshallow sills in the straits prevent it from leakingout). In fact, a result from a box model using CFCs
in addition to potential temperature and salinity hassuggested that diapycnal mixing is required for the
production of the Okhotsk Sea water below 27:2sy(Wong et al., 1998). Also, an analysis of oxygenisotopes revealed that vertical mixing from above isalso important in freshening of the Okhotsk Sea
water even at the density of the DSW (Yamamoto
et al., 2002). Hence, in order to better understand thewater modiﬁcation in the Okhotsk Sea, it isimportant to clarify the process responsible for suchdiapycnal mixing and the mechanism through which
the mixing process affects the water mass structure.

427

Previous studies have underlined two possible
mixing mechanisms. The ﬁrst of these is tidal mixing
in the Kuril Straits. Observations have foundvertically mixed water in the Kuril Straits, down
to 27:1sy or more (Kitani, 1973; Kawasaki and
Kono, 1994; Gladyshev, 1995; Kawasaki, 1996).
Locally enhanced ventilation is also indicated by thepresence of a high oxygen anomaly in the Kuril
Straits, which is observed from the surface toaround 2200 m or 427:4sy (Talley, 1991; Freeland
et al., 1998). The high oxygen anomaly appears to
intrude into the Okhotsk Sea on a deep 27:4st
surface (Yasuoka, 1968). By combining these facts,
Kitani (1973) and Talley and Nagata (1995)
suggested that diapycnal mixing in the Kuril Straits
is responsible for water transformation in the
Okhotsk Sea deeper than the DSW density layer.
They further hypothesized that tidal mixing is thelikely cause of the mixing. In fact, the tidal currents,
especially the diurnal components, are very swift
around the Kuril Straits with speeds exceeding
2 ms1 in the relatively shallow straits (100–500 m
depth) (e.g., Thomson et al., 1997; Talley andNagata, 1995). Even in the deep Bussol Strait(2000 m depth), the maximum speed exceeds
1 ms1 (Katsumata et al., 2004; Nakamura andAwaji, 2004; Riser, 2001).
Recent modeling and theoretical studies (Nakamura et al., 2000a,b; Nakamura and Awaji, 2001,
2004) showed that the diurnal tides (especially theK 1 tide) have the ability to induce vigorous mixing(with a maximum diffusivity of over 1000 cm2 s1 ).
The mixing occurs from the surface to the sillbottom and across a wide area, thus causing large
diapycnal transport to the deep layers. In thesestudies, the locally enhanced mixing is associatedwith tidally generated ‘unsteady lee waves’ rather
than internal tides. This is because the diurnal tides
are subinertial at the Kurils (so that internal-tides
generated by them cannot be internal gravity waves)and because unsteady lee waves are more likely togrow by superposition than internal tides. Energetic
internal waves are then formed with amplitudes
exceeding 100 m in their simulations.
The second mechanism for enhanced diapycnal
mixing is double diffusion (You et al., 2000). A
situation favorable for diffusive double diffusion isfound in the subarctic Paciﬁc and the Okhotsk Sea.
In these regions, salinity generally increases withdepth, while temperature has a local minimum(called the dichothermal layer) just below thesummer surface layer and a local maximum (theARTICLE IN PRESS
428

T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

mesothermal layer) below (e.g., Uda, 1963; Favorite
et al., 1976). Thus, between the dichothermal andmesothermal layers, both temperature and salinity
increase with depth, so that a necessary condition
for diffusive double diffusion is satisﬁed. Thissituation develops at greater depth in the Okhotsk
Sea (26:8–27:2sy ) than in the North Paciﬁc(p26:8sy ) (You et al., 2000), since the mesothermal
layer in the Okhotsk Sea is much deeper than in the
North Paciﬁc (e.g., Yasuoka, 1968). Hence thismechanism could explain the deep ventilation in the
Okhotsk Sea. However, it is not clear whether it is
an important process for diapycnal mixing.
In this study, we investigate the effects of these
two mixing processes on the ventilation of the
Okhotsk Sea. To this end, a set of experiments using
an OGCM is conducted with an emphasis on therole of tidal mixing at the Kuril Straits in modifying
the Okhotsk Sea water, particularly in the production of the DSW. Our experiments suggest that tidal
mixing is much more effective than double diffusion.
2. Model and experimental design
The model used is an OGCM developed at Kyoto
University (Ishikawa et al., 2002; Nakamura et al.,2003a; Toyoda et al., 2004), which solves theprimitive equations in spherical coordinates. For arealistic reproduction of subduction processes, thismodel adopts a third-order advection scheme forthe tracer equation (UTOPIA and QUICKEST;
Hasumi and Suginohara, 1999). In addition, themodel incorporates isopycnal diffusion with eddy
parameterization (Redi, 1982; Gent and McWilliams, 1990), together with a turbulence closure
mixed layer scheme (Noh and Kim, 1999). In theseschemes, both the isopycnal and the eddy-parameterization diffusivity coefﬁcients are set to
107 cm2 s1 , and the background vertical anddiapycnal diffusivity and viscosity coefﬁcients are
set to 0:01 cm2 s1 . To accurately simulate thesubduction associated with brine rejection, a sea
ice model is incorporated based on Ikeda (1989a,b).
The sea ice model is a Hibler (1979) two-category ice
model which solves ice momentum equations andthe equations for ice concentration and ice volume(i.e., thickness times concentration). The effect of
double diffusion is parameterized on the basis of
Schmitt (1981) for the salt ﬁngering condition andFedorov (1988) for the diffusive convection condition, following Merryﬁeld et al. (1999). Also, thepartial cell method is used for a better representation of bottom topography. A more detailed
description is given in Appendix A.
The model domain is nearly global (75 S–75 N),
in order to simultaneously investigate the effect on
the North Paciﬁc (Nakamura et al., 2006). The
horizontal resolution is 1  1 , and 34 levels areused in the vertical spaced from 20 m at the sea
surface to 400 m, as shown in Table 1. The model
topography is based on DBDB5 (U.S. National
Geophysical Data Center) but care is taken not tosmooth out the sills in the Kuril Straits (Fig. 1b), as
discussed in Section 7. The initial values of potential
temperature and salinity are taken from the World
Ocean Atlas 1994 Monthly Data compilation(WOA94, Levitus and Boyer, 1994; Levitus et al.,1994). Sea surface ﬂuxes are calculated with the bulk
formulae (Röske, 2001; Ikeda, 1989b; da Silva et al.,1994), but the commonly used ﬂux correction
method is adopted for heat and fresh water ﬂuxes
with a relaxation time scale of 60 days (e.g., Barnier
et al., 1995; Weaver and Hughes, 1996). The
sensitivity to this method is discussed in Section 6.
The atmospheric and river runoff data used are
based on the climatological monthly mean datacompiled by Röske (2001), which is in turn based on
the European Centre for Medium-Range Forecasts
reanalysis data. The ice and oceanic data used in
calculating the surface ﬂuxes are taken fromsimulated values, as is usual for an ice-ocean
coupled model. In addition, restoration of potential
temperature and salinity to those of WOA94 is
Table 1Model vertical resolution
Level

Thickness (m)Depth range (m)

1–3
4
5–6
7
8
9–10
11
12
13
14–16
17
18
19
20
21–22
23–27
28–34

20
25
30
35
45
50
60
75
90
100
125
150
175
200
250
300
400

0260
60285
8521451452180
1802225
225232532523853852460
4602550
5502850
8502975
97521125112521300
130021500
150022000
200023500
350026300

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applied at the northern and southern boundaries, atthe exits of the Mediterranean and Red Seas, and inlayers deeper than 2000 m with the same time scale
for ﬂux correction. The Bering Strait is closed since
water from the Arctic Ocean through the strait is
not considered to affect the water mass formation in
the North Paciﬁc (e.g., Overland et al., 1994).
Three main experiments were conducted (Table 2)
after a 40-year spinup by which time the model
ocean had reached a state of approximate equilibrium. The ﬁrst of these was a control experimentobtained by a further 40-year integration (hereafterthe ‘Ctrl’ case). The second was an experiment forthe case with tidal mixing (the ‘Tmix’ case). Thiscase was obtained by a 40-year integration after the
spinup, but with the vertical diffusivity coefﬁcient
over the sills at the Kuril Straits increased by
200 cm2 s1 (Fig. 1b), based on the results ofNakamura et al. (2000b) and Nakamura and Awaji(2004). Note that both the Ctrl and Tmix cases
include the effect of double diffusion. To estimate
its relative importance, a third experiment was
conducted, which excludes double diffusion fromthe scenario simulated by the Tmix case (the‘noDD’ case). In the Tmix and noDD cases, theﬂux correction was not applied around the Kuril
Straits in order to avoid artiﬁcial heat and fresh
Table 2
List of experiments
Case

Increase in Kz
at the Kuril Straits

Main experiments
Ctrl
None
Tmix (Kz200)
200 cm2 s1
noDD
200 cm2 s1
Sensitivity experiments
Kz10
10 cm2 s1
Kz20
20 cm2 s1
Kz30
30 cm2 s1
Kz50
50 cm2 s1
Kz100
100 cm2 s1
Kz200 (Tmix)
200 cm2 s1
Kz300
300 cm2 s1
Kz500
500 cm2 s1
Kz10001000 cm2 s1
FC30
Same as Tmix case

DoublediffusionFlux
correction

Included
IncludedNone

60 daysSame as Ctrl case

30 days

Vertical diffusivity coefﬁcient, Kz, is speciﬁed over the sills in the
Kuril Straits except for the Ctrl case. Double-diffusion parameterization is excluded only in the noDD case. The relaxation
time of ﬂux correction is set to be 60 days except for the FC30
case.

429

water ﬂuxes which would arise due to the fact that
the WOA94 data does not resolve the vertically
mixed water in the Kuril Straits.
It should be noted that tides were not directly
simulated and their effect was parameterizedthrough local increase of vertical diffusion, which
is important particularly on the scales underconsideration (e.g., Shiller et al., 1998; Hibiya
et al., 1998). Although the tidal motion has otherimportant effects, the present study focuses only on
the effects of enhanced vertical mixing to gain abasic insight into one of the major tidal effects
before exploring more comprehensive and thus
more complicated effects. Also, the value of vertical
diffusion is increased at all depths in the Kuril
Straits as a ﬁrst approximation. Strong tidal mixing
reaches the surface layer from the bottom in the
Kuril Straits, according to ship and satellite
observations and tidal simulations (e.g., Kawasaki,
1996; Gladyshev, 1995; Kawasaki and Kono, 1994;
Nakamura and Awaji, 2004). The speciﬁed valueð200 cm2 s1 Þ roughly corresponds to twice thatinduced by the K 1 tide (Nakamura and Awaji,2004). The reason for this selection is that the total
tidal ﬂow speed in the Kuril Straits is roughly twiceas large as that of the K 1 component alone,according to the tidal simulations reported byNakamura et al. (2000a). Taking this fact into
account, the above rough estimate of diapycnal
diffusivity was used for a basic investigation of the
impacts of tidal mixing on the Okhotsk Sea.
In order to examine the sensitivity of the water
modiﬁcation in the Okhotsk Sea to the strength of
the tidal mixing, eight experiments were additionallyconducted, in which the increase in vertical diffusivity coefﬁcient was varied from 10 to 1000 cm2 s1(Kz10–Kz1000 cases, respectively). Also, for the
discussion on the sensitivity to the ﬂux correction
method, another experiment was performed, inwhich the relaxation time scale was changed to 30
days while the other settings were the same as in theTmix case (FC30 case). The settings used in all of
our experiments are summarized in Table 2.
In the analysis below, the ﬁnal year of theintegration (i.e. the 80th year) is used. The sensitivity
experiments for both the tidal mixing and the ﬂux
correction will be discussed in Section 6.
3. Circulation and sea ice in the Ctrl case
Prior to a comparison between the main experiments made in Section 4, the performance of the

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

OGCM in the Ctrl case is discussed in this section interms of water circulation and sea ice formation in
and around the Okhotsk Sea.
3.1. Current field
Fig. 2a shows near surface current ﬁelds insummer and winter around the Okhotsk Sea in the
Ctrl case. In general, the simulated large-scale
circulations in both basins reasonably reproduce
the familiar gross features. For example, the largescale cyclonic circulation in the Okhotsk Sea, thesubarctic and subtropic gyres in the North Paciﬁc,and the western boundary currents (i.e., the East
Kamchatka Current (EKC), the Oyashio, and theKuroshio) simulated here are basically consistent
with both past observational studies (e.g., Favorite
et al., 1976) and a recent state estimation by data
assimilation (Awaji et al., 2003).
A detailed examination of Fig. 2a in the Okhotsk
Sea shows that the simulated current ﬁeld qualitatively reproduces the following important featuresof the intermediate water formation. Based on
temperature and salinity observations it is inferredthat relatively warm water ﬂows into the Okhotsk
Sea from the North Paciﬁc mainly through thenorthern passages of the Kuril Islands (e.g., Kitani,
1973; Favorite et al., 1976). The inﬂowing waterﬂows northward, forming the eastern part of thecyclonic circulation. On the other hand, the westernpart of the cyclonic circulation consists of the East
Sakhalin Current (ESC), whose presence has been
recently conﬁrmed from mooring observations(Ohshima et al., 2002; Mizuta et al., 2003). This
southward part of the circulation carries the watertransformed in the northern Okhotsk Sea to the
Kuril Basin, and eventually to the Paciﬁc mainly
through the central part of the Kuril Islands. In
addition, an inﬂow from the Japan Sea through theshallow Soya Strait (40 m) brings the SWCW, which
ﬂows southwestward along the coast of Hokkaido
with part of it diffusing into the Kuril Basin before
ﬂowing out of the Okhotsk Sea. These current
features are also seen in Fig. 2a, though the Soya
Warm Current (SWC) is difﬁcult to identify in the
March period due to its seasonal variation (discussed in the next section).
3.2. Volume transport
Given the qualitative similarity between thesimulated and observed current ﬁeld, we made aquantitative comparison of the volume transports ofthe main currents around the Okhotsk Sea—
namely, the EKC and Oyashio in the North Paciﬁcand the SWC and ESC in the Okhotsk Sea. (Volumetransport is chosen here because model results
represent spatially averaged ﬁelds on a grid scale,and hence observed values should be also averaged
or integrated for a proper comparison. Observations in the other regions of the Okhotsk Sea are too
limited for this purpose.)
The calculated monthly volume transports areshown in Fig. 3. The phases of the transports ofthese currents are consistent with observational
studies and their amplitudes are also reasonable(e.g., Talley and Nagata, 1995; Kono and Kawasaki, 1997; Aota and Kawamura, 1978; Mizuta et al.,2003). In particular, the range of the calculated
SWC transport (0.3 Sv in winter and 1.2 Sv insummer) is in good agreement with observational
estimates of 0.1 Sv in winter and 1.2 Sv in summer(Aota and Kawamura, 1978; Matsuyama et al.,
1999; Itoh et al., 2003). The reason for such a good
agreement is that the driving force of the SWC is
associated with the difference in sea surface height
between the Okhotsk Sea and the Japan Sea, which
in turn is associated with a large scale wind and thus
is reasonably reproduced.
Most of the other transports are also within therange of observational estimates, as summarized inTable 3 (e.g., Reid, 1973; Talley and Nagata, 1995;
Yasuda et al., 2001, 2002; Kono and Kawasaki,
1997; Shimizu and Ohshima, 2002; Ohshima et al.,
2002; Mizuta et al., 2003). The simulated EKC
transport is in good agreement with most observational estimates (Table 3). Although the summertime Oyashio transport in the Ctrl case is somewhat
small, this is partly due to the absence of a tidal
mixing effect (the transport increases by 1–3 Sv in
the Tmix case) and because of the somewhat coarse
resolution. The latter makes the simulated Oyashio
more closely subject to the Sverdrup transport ascompared with observations, which in turn results
in a northward shift of its separation latitude whencompared with the observations and thus leads to asmaller summertime transport off Hokkaido Island.
The simulated ESC transport (1.0–2.8 Sv) is considerably smaller than the estimated transport basedon mooring observations (1.2–12.3 Sv), which isconsiderably larger than the Sverdrup transport(and/or that expected from the wind stress and thelinear wave dynamics, Shimizu and Ohshima, 2002)
in winter. Because the volume transport due toARTICLE IN PRESS
T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

431

Ctrl case (without tidal mixing)

Mar.

Sep.

(a)
Tmix case (with tidal mixing)

Mar.

Sep.

(b)

Difference (Tmix - Ctrl)

Mar.

Sep.

(c)

Fig. 2. Velocity vectors at 50 m depth of (a) Ctrl and (b) Tmix cases and (c) their difference in winter (left) and summer (right). The circles
in panels (b) and (c) indicate the anticyclonic gyre strengthened by tidal mixing, and squares in panels (a) and (b) indicate the southward
shift of the separation latitude of both the Oyashio and Kuroshio due to tidal mixing.

thermohaline forcing is not signiﬁcant (Mizuta
et al., 2003), the difference between the Sverdruptransport and the estimate from the mooring

observation could arise from effects of recirculation
or eddies. Since these effects are not resolved well in
our model, the simulated volume transport is closer

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432

to the Sverdrup transport. Nevertheless, qualitative
similarities suggest that our results would bereasonable in a qualitative sense.
The above qualitative and quantitative similarities suggest that the OGCM reasonably reproduced
the observed ﬂow ﬁeld at least on a basin scale (theOkhotsk Sea scale), although some aspects of theocean circulation, particularly those driven by
thermohaline forcing, are not well presented in the
Ctrl case due to the lack of tidal mixing, as discussed
in Section 4.1.

Vol. Transport (thick: Ctrl, thin: Tmix)
25

20

transport (Sv)

EKC

15

3.3. Sea ice
10

Oyashio
5
ESC

SWC
0

1

2

3

4

5

6 7
month

8

9

10 11 12Fig. 3. Monthly volume transports of the Soya Warm Current(SWC) and the East Sakhalin Current (ESC) in the Okhotsk Sea,and the East Kamchatka Current (EKC) and the Oyashio in the
North Paciﬁc in the Ctrl (thick) and Tmix (thin) cases. Thesetransports are calculated as follows; the SWC is calculated as an
eastward transport through the Soya Strait, the ESC as asouthward transport at 51:5 N in the western part of the Okhotsk
Sea, the EKC as a westward transport at 158:5 E between the
Kamchatka Peninsula and 42:5 N, and the Oyashio as asouthward transport at 43:5 N between the Hokkaido Islandand 155:5 E.

Table 3Comparison of volume transport
Observational estimates EKC

Oyashio SWC

ESC

Summer or fall
Winter or spring

10
2–10
20–25 10–21

1.2 (max) 0:2–4
0.1 (min) 1.2–12.3

Ctrl case (max or min)
Summer or fall
Winter or spring

9
22

1.2
0.3

2
15

1.0
2.8

The references for observational estimates are provided in the
text. For acronyms and the calculation method of simulated
vales, please refer to the caption of Fig. 3.

In addition to circulation, a reasonable production of sea ice is also required for realistic
investigations of subduction in the Okhotsk Sea.
Fig. 4 shows the monthly averaged distributions of
ice concentration and thickness simulated by the sea
ice model in the Ctrl case.
The simulated ice distribution was basicallysimilar to the observed ones. Sea ice begins to form
along the northern coast of the Okhotsk Sea usuallyin December and then spreads southward due
mainly to strong winds to cover both the northern
and western coasts. Ice volumes and areas reachmaxima in March, and more than a half of the
Okhotsk Sea is covered (as is observed in usual
winter). Such active ice formation mainly occurs incoastal polynyas along the northern shelf of the
Okhotsk Sea. The ice gradually retreats northward,
and almost disappears around the end of May.
These main features are seen both in simulations
and in observations (e.g., JMA, 2003; Alfultis andMartin, 1987; Martin et al., 1998; Cavalieri et al.,
2002; Fetterer and Knowles, 2002), though thecommencement of ice growth is somewhat delayedin the model.
Quantitatively, the simulated value of ice production in the Ctrl case ð2:1  1011 m3 Þ is nearly equal
to a 12-year average of observational estimatesð2:27  1011 m3 Þ calculated from the reports ofAlfultis and Martin (1987), Martin et al. (1998),
and Gladyshev et al. (2000) and is well within theobserved variability (1.2–3:4  1011 m3 ). Thus, theamount of brine rejection is also reasonably
reproduced. Despite this good agreement, thesimulated value of the maximum ice-covered area
ð4:5  105 km2 Þ is smaller than the observed value ofa usual winter ð11  105 km2 , JMA 2003). This may,in part, be due to the fact that the model Okhotsk
Sea does not include the very shallow part of the

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

50N

65N

A (March)

60N

55N
50N

65N

A (April)

60N

55N
50N

A (May)60Nlatitudelatitude

55N
50N

55N
50N

45N

45N

45N

45N

45N

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

65N

65N

65N

65N

65N

latitude55N
50N

H (March)

60N

55N
50N

H (April)

60N

55N
50N

H (May)60NlatitudeH (February)

60N

latitude

H (Januay)

latitudelatitude

55N

60N
latitude65N

A (February)

60N

latitude65N

A (Januay)

60N

latitude

65N

433

55N
50N

55N
50N

45N

45N

45N

45N

45N

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

40N
140E 150E 160E
longitude

0

0.2

0.4
0.6
concentration

0.8

1

0

50

100 150 200
thickness (cm)

250

300

Fig. 4. Monthly distributions of ice concentration (upper) and thickness (lower) in the Ctrl case from January to May.

Shelikov and Shantarskiy Bays and because the
SWC transport is somewhat large in winter, so thatsome melting takes place around Hokkaido Island.
Such qualitative and quantitative similarities in
sea ice formation as well as water circulation
conﬁrm our basic approach to the investigation of
tidal mixing effects in the Kuril Straits in water mass
modiﬁcation within the Okhotsk Sea.
4. Changes in water mass structure
Effects of tidal mixing and double diffusion on
ventilation are now examined by comparing theCtrl, Tmix, and noDD cases. Fig. 5 shows thesalinity, potential temperature (y), and potential
vorticity (PV) ﬁelds for the three cases and for the
WOA94 ﬁeld in a meridional section across thewestern part of the Okhotsk Sea, where the watertransformed in the Okhotsk Sea is most visible (PVis here calculated from ðf þ zÞ=r  ðqsy =qzÞ, wherer is potential density and z is relative vorticity). The
differences of salinity and potential temperaturefrom the Ctrl case are also shown in Fig. 6.
4.1. Impact of tidal mixing
Comparison of the Ctrl and Tmix cases suggeststhat the addition of tidal mixing signiﬁcantly
changes the simulated water mass structure. When

compared with the Ctrl case, both salinity andpotential temperature values in the Tmix casedecrease considerably in the intermediate layer of
the Okhotsk Sea, down to 27:6sy or 1500 m depth(Fig. 6), and thereby become much closer to theobserved distributions (Fig. 5). The maximum
decrease exceeds 0.4 for salinity and 2  C for
potential temperature in the intermediate layer(Fig. 6).
Since salinity principally determines the density
variation in the subarctic Paciﬁc and the Okhotsk
Sea, an improvement in the representation ofsalinity distribution implies an improved density
structure. Also, the cooling of the intermediate layer
results in the weakening of the local temperature
maximum (the mesothermal layer) in the Okhotsk
Sea as compared with the Ctrl case (and thesubarctic water). This mesothermal layer is amarked feature of the subarctic Paciﬁc water, andits weakening is one of the characteristic features of
the Okhotsk Sea (e.g., Kitani, 1973).
It is noteworthy that a local PV minimum in thevertical direction is produced in the Tmix case in the
Okhotsk Sea around 300–500 m depth (Fig. 5),
whereas it is absent in the Ctrl case (although aweak minimum can be seen, its saline and warm
nature implies that it is not created in the Okhotsk
Sea). This feature is similar to that observed byFreeland et al. (1998), and its presence here is

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

434

Potential Temperature

500

500

500

1000

depth (m)

0

1500

1000

40

45
50
latitude (N)

55

35

60

40

45
50
latitude (N)

55

60

35

0

0

500

500

500

1000

depth (m)0depth (m)depth (m)

2000

2000

1500

1000

45
50
latitude (N)

55

60

35

Tmix

40

45
50
latitude (N)

55

60

35

500

500

500

depth (m)0depth (m)

0

1500

1000

40

45
50
latitude (N)

55

60

noDD

35

40

45
50
latitude (N)

55

60

35

500

500

depth (m)500

depth (m)

0

1500

1000

32

45
50
latitude (N)

33

34

55

45
50
latitude (N)

55

60

1000

WOA94

WOA94
2000

2000
40

40

1500

1500
WOA94

2000

60

2000

0

1000

55

1000

0

35

45
50
latitude (N)

noDD
2000

35

40

1500

1500
noDD

2000

60

1000

0

1000

55

2000

2000
40

45
50
latitude (N)

Tmix

2000
35

40

1500

1500
Tmix

depth (m)

Ctrl

Ctrl

2000
35

1000
1500

1500
Ctrl

depth (m)Potential Vorticity0depth (m)depth (m)Salinity
0

60

35

40

35

0

2

45
50
latitude (N)

4

6
(°C)

55

60

35

8

10

0

40

45
50
latitude (N)

1

2

55

60

3

(10-10m-1s)

Fig. 5. Annual mean salinity, potential temperature and potential vorticity along 145 E for the Ctrl (top), Tmix (upper middle), andnoDD (lower middle) cases and WOA94 (bottom). Circles in the potential vorticity ﬁeld indicate the local minimum mentioned in the text.

remarkable since it is a characteristic feature of the
OSMW. (On these horizontal scales, a local minimum in PV in the vertical direction roughly

corresponds to a local maximum in layer thickness,
which is an important feature of the OSMW.) In
addition, qualitative features of the seasonal varia-

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

435

0

500

500
depth (m)depth (m)difference (Tmix - Ctrl)
0

1000

1000
1500

1500

Potential
Temp.

Salinity
2000
35N

40N

(a)

45N 50N
latitude

55N

2000
35N

60N

45N 50N
latitude55N

60N

difference (noDD - Tmix)
0

0

500
depth (m)500
depth (m)

40N

1000
1500

1000
Potential
Temp.

1500
Salinity

2000
35N

40N

(b)

-0.2

-0.1

45N 50N
latitude

55N

60N

2000
35N

40N

45N 50N
latitude

0

0.1

0.2

-2

-1

0

55N

60N

1

2 °C

Fig. 6. Differences of salinity and potential temperature in Fig. 5 between the Ctrl and Tmix cases (upper), and those between the Tmix
and noDD cases (lower).

tion of the OSMW are basically consistent withthose reported by Gladyshev et al. (2003). For
example, temperature becomes relatively high insummer to fall and low in winter to spring (notshown), although the reproduction of detailed and/or quantitative features needs further improvements.
From the viewpoint of ventilation, the enhanced
freshening below the surface layer implies enhanced
ventilation as noted in the introduction. One reason
for the enhanced ventilation is the enhancement of
subduction, which is suggested by the production of
the PV minimum water. (Subduction here means the
direct sinking of surface water.)
The enhanced subduction is conﬁrmed by anincrease in the production rate of the DSW as
compared with the Ctrl case (Table 4). The DSW isdeﬁned here as yo  1  C, sy X26:6, following
Gladyshev et al. (2000). Using this deﬁnition, anannual mean production rate was estimated as the
maximum amount of the DSW over a 1 year period,
since the DSW disappears before the following
winter (in both the model results and observations).

Table 4
Production rate and density of DSW and ice volume
Case

DSW
production rate

DSW maximum
density

Ice production

Ctrl
Tmix
Obs.

0.01 Sv
0.13 Sv
0:220:67 Sv

26:6sy26:8sy27:0sy2:1  1011 m3
2:3  1011 m3
1:223:4  1011 m3

The DSW is deﬁned as yo  1  C; sy X26:6. The references forobservational estimates are provided in Section 3.3.

The annual mean production rate in the Ctrl case(0.01 Sv) is negligible, whereas the rate in the Tmix
case (0.13 Sv) is one order of magnitude larger thanthat in the Ctrl case and is around the range of
observational estimates (Martin et al., 1998; Gladyshev et al., 2000; Itoh et al., 2003).
On the other hand, the freshening (and thus
ventilation) extends to deeper layers than the PV
minimum layer (Figs. 5 and 6), as in the observations. Since the ventilation of the layers denser thanthe DSW (or the PV minimum) cannot be caused by

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436

T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

subduction, it must therefore be caused by verticalmixing. In this case, the latter is due to the tidal
mixing. Tidal mixing therefore dictates the water
properties of the upper intermediate layer byenhancing subduction (together with its direct
mixing effect), and inﬂuences the denser layers
through direct mixing only.
Associated with these changes, the circulation is
also modiﬁed in the Tmix case (Figs. 2b and c). For
example, an anticyclonic gyre in the Kuril Basin isstrengthened in summer (circled in Fig. 2) by the PV
supply due to both the enhanced subduction andtidal mixing at the Kuril Straits. In the intermediate
layer, such seasonal variability also appears in theTmix case, whereas it is almost absent in the Ctrl
case (not shown). These features of the Tmix caseare in agreement with the observation reported by
Wakatsuchi and Martin (1991). Thus, we conclude
that tidal mixing in the Kuril Straits is also
important for driving the circulation in the Kuril
Basin, where wind-driven circulation is relatively
weak (Sekine, 1990; Shimizu and Ohshima, 2002).
4.2. Impact of double diffusion
In contrast, water properties in the Tmix and
noDD cases are almost the same, as seen in Figs. 5
and 6. This implies that the diapycnal mixing due todouble diffusion is too weak to modify the watermass structure in the presence of the tidal mixing
effect. Also, the double diffusion effect does notcause deep ventilation and associated freshening in
the absence of the tidal mixing effect, as seen in the
Ctrl case. These results suggest that double diffusion
is ineffective in ventilating the Okhotsk Sea regardless of the presence of the tidal mixing.
This is consistent with an expectation based on
the typical value of the density ratio, ða=bÞðdT=dSÞ,where a and b are the thermal expansion and haline
contraction coefﬁcients of seawater at given temperature and salinity values, and dT and dS are thevertical changes at a given point in the watercolumn. The estimated density ratio is about 0.2 inthe density range between 26:8sy and 27:3sy using
the climatological vertical proﬁle of dT and dS in
the Kuril Basin taken from Watanabe and Wakatsuchi (1998). Thus the vertical diffusivity coefﬁcientdue to double diffusion is estimated to be about
0:3 cm2 s1 (e.g., Zhang et al., 1998) and would thus
be much smaller than that due to tidal mixing.
It should be noted that in the Okhotsk Sea, tidal
currents are strong in both speed and shear, so thesimple double diffusion parameterization used here
probably includes some uncertainty. Nevertheless, if
the double diffusion process is not drasticallyintensiﬁed by such strong currents, the above results
would qualitatively hold. It is thus likely that tidal
mixing in the Kuril Straits is the primary cause of
the enhanced ventilation observed in the OkhotskSea.
5. Mechanism of the enhanced subduction
Although the above results revealed that tidal
mixing enhances the ventilation in the Okhotsk Sea
through both direct mixing and the enhancement of
subduction, the mechanism of the enhanced subduction is not apparent.
To clarify this mechanism, we ﬁrst compared thesea ice production between the two cases (Table 4)since the DSW is traditionally thought to have been
formed by brine rejection. However, the differencein the sea ice production is only 10% and is not
signiﬁcant. This is because the production rate ofsea ice is mainly determined by heat and momentum
ﬂuxes at the sea surface, which are almost the samein the two cases. Thus, the difference in the amountof brine rejection is negligible, as compared with thehuge difference in the DSW production rate. This
fact implies the presence of some other important
controlling factor in the DSW production.
We then examined the seasonal change in the y–S
diagram in the Okhotsk Sea except for the Kuril
Straits (Fig. 7). In the Ctrl case, comparison ofsummer and winter regimes indicates that seasonalvariability due to summer heating and wintercooling is limited to about the 26:4sy density layer,
which becomes the temperature minimum layer insummer (i.e., the bottom of the seasonal thermocline). Although brine rejection augments the
production of denser water near the freezing
temperature, subduction reaches at most the
26:6sy density layer. Accordingly, layers denser
than the subducted water (26:6sy ) show little
seasonal variation (Fig. 7) and are not signiﬁcantly
transformed from the subarctic Paciﬁc water (Fig.
5). This leads to the presence of warmer and saltierintermediate water in the Okhotsk Sea whencompared with observations.
In the Tmix case, diapycnally mixed water
appears below the seasonal thermocline in summer.
The mixed water can be identiﬁed from a comparison of the y–S diagrams of the Ctrl and Tmix cases
in summer (Fig. 8a). In the density range lighter

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

θ-S diagrams in summer (black:Ctrl, red:Tmix)

Ctrl (without tidal mixing)
1010Mar. & Sep.
8

mixed
water

6
4
2
0

}

8
temperature (°C)

temperature (°C)

437

6
4
2
0

-2

-2
31

32

33
salinity

34

35

31

32

33
salinity

(a)

34

35

salinity difference (0-150m depths)

Tmix (with tidal mixing)
10

60N

Mar. & Sep.

650N
latitudetemperature (°C)

8

4
2

40N
0
-2
31

32

33
salinity

34

35

Fig. 7. y–S diagrams in the Okhotsk Sea except near the Kuril
Straits for the Ctrl (upper) and Tmix (lower) cases in winter (bluedots) and summer (red dots).

than 27:0sy , the y–S plots in the Ctrl case (blackdots) show a V-shape distribution with the bottomof the V-shape corresponding to that of the seasonal
thermocline, while those in the Tmix case (red dots)
are also present in the interior of the V-shape. We
can approximately regard the water located in theinterior of the V-shape in Fig. 8a as diapycnally
mixed water for the following reason. The water
warmed by summer heating from above (i.e., raisedin y–S diagram) is lighter than that at the bottom of
the seasonal thermocline (and should be aligned
along the seasonal thermocline). Hence, as the waterin the interior of the V-shape cannot be produced bysummer heating it should be produced by diapycnal
mixing, which is due to tidal mixing at the Kuril
Straits in this case.
The mixed water in the interior of the V-shape(and/or near the surface) is saltier at the same

30N
140E

150E

160E

170E

longitude

(b)
-0.4

-0.2

0

0.2

0.4

Fig. 8. (a) Comparison of the y–S diagrams of the Ctrl (blackdots) and Tmix (red dots) cases in summer. (b) Map of thesalinity difference between the Ctrl and Tmix cases in the summer
surface layer (upper 150 m).

density than the water that is not affected by thetidal mixing (or water in the Ctrl case), that is, thevertical mixing at the Kuril Straits induces upward
salt ﬂux from the saltier lower layer. Such saline
mixed water originating in the Kuril Straits istransported northward by the large-scale cyclonic
circulation. In fact, a map of the salinity differencein the summer surface layer (upper 150 m) showsthat the saltier water in the Kuril Straits extends
along the eastern boundary of the Okhotsk Sea
and to the northern coast, where the DSW forms(Fig. 8b).
Cooling of this saltier water in winter (together
with brine rejection) enables the subduction process

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

to produce the DSW at much more realistic rates
and densities (maximum value 26:8sy ). In otherwords, the extra controlling factor of the DSW
production discussed above is, in effect, a preconditioning, in which salinity values in the (near) surface
layer increase enough to create the DSW. Note that
although the increase in sea surface salinity leads toan increase in the amount of brine rejection, thedifference in salt production would be only a few
percent even when salinity values increase by 1. Thisis because an increase of 1 in sea surface salinity
corresponds to only a few percent increase inproportion to the total values, which are around30 (and because the salt production due to brine
rejection depends on the total value). Hence, thispreconditioning works mainly by increasing water
density (due to salinity change) at the beginning of
winter.
The newly formed DSW is transported southward
to the Kuril Basin. Subsequently, isopycnal mixingof the DSW with its surroundings, especially withvertically well-mixed water in the Kuril Straits,
freshens and cools the intermediate water. The
vigorous tidal mixing at the Kuril Straits diffuses
the subducted water further down, so that a layer
denser than the DSW is freshened and cooled(Fig. 7). As a result, the y–S curve in and below theintermediate layer becomes much ﬂatter than that in
the Ctrl case (Fig. 8a), and lies closer to theobserved curve (Fig. 9). This feature is one of the
major characteristics of Okhotsk Sea water.
6. Sensitivity to tidal mixing and ﬂux correction
In this section, we examine the sensitivity of ourresults to the strength of tidal mixing in the Kuril
Straits and to the restoring time scale of the ﬂux
correction method.
For the former, we focus, particularly, on the
DSW production. Both the production rate and the
maximum density of the DSW are sensitive to thetidal mixing and generally increase with increasing
tidal mixing strength (Fig. 10a,b). The production
rate in the Kz10 to Kz1000 cases becomes 5 to 80
times larger than that in the Ctrl case, and the
maximum DSW density in these cases increases by
0.06 to 0:3sy from that in the Ctrl case. In contrast,
ice production is roughly constant (Fig. 10c). The
difference from the Ctrl case is less than 20% for therange of vertical diffusion values. (The ice production increases slightly with increasing mixing. This ispossibly because sea surface temperature is slightly

WOA94
10
winter & summer
8
temperature (°C)

438

6
4
2
0
-2
31

32

33
salinity

34

35

Fig. 9. y–S diagrams in the Okhotsk Sea except near the Kuril
Straits for WOA94 in winter (blue dots) and summer (red dots).

lowered in summer and fall due to diapycnal mixing
with the cold dichothermal layer, i.e., the remnantof the winter mixed layer.) Hence, in all the cases,
enhanced production of DSW is not driven bychange in sea ice production but by increase insalinity of the surface water caused by increased
diapycnal mixing.
Accordingly, although the strength of tidal
mixing should be estimated more precisely by future
observations, the mechanism identiﬁed in this study
will be valid at least for the above range of tidal
mixing strength. In fact, the mechanism requires
only two features in addition to vertical mixing at
the Kuril Straits and winter cooling. That is (i) the
fresher surface layer due to the large fresh waterﬂux, and (ii) water transport from the Kuril Straits
to the northern shelf of the Okhotsk Sea, which is
conducted by the clockwise circulation in this case(and could be augmented by eddy or wave induced
transport). These features are seen in both themodel results and observations. The mechanism willthus be quite robust, at least qualitatively.
When compared with observations, the change in
ice volume is within 12% of the observed 12-year
average, considerably less than the amplitude of theobserved interannual variation (about 50%). On theother hand, the production rate and maximum
density of the DSW are somewhat less than theobservational estimates. The possible reasons forthe underestimates are two fold. One is the use of acoarse-resolution hydrostatic-model, which could
generally result in weaker convection. This isbecause of insufﬁcient resolution of bottom topography (which lacks for example the canyon effect),ARTICLE IN PRESS
T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

439

assumptions used (e.g., Martin et al., 1998), andaccording to Yamamoto et al. (2002), the inversely
estimated value of Itoh et al. (2003) could include asigniﬁcant error due to the neglect of diapycnal
mixing.
Sensitivity to the restoring time scale of the ﬂux
correction method is next examined. Fig. 11 shows
the differences of salinity and potential temperature
between the Tmix and FC30 cases at the same
section as in Fig. 5. The difference is much smaller
than that between the Ctrl and Tmix cases, although
the decrease in restoring time scale leads to slightheating in the Okhotsk Sea and freshening aroundthe Kuril Straits. Thus the simulation results are notso sensitive to the restoring time scale used, since theforcing is insensitive to the change in restoring timescale, at least in the range recommended by Barnier
et al. (1995).
Although the slight difference between the Tmix
and FC30 cases suggests a robustness of the results
obtained in the previous sections, ﬂux correction
given under sea ice could affect the role of ice
formation. This is particularly so for fresh water
ﬂuxes. The fresh water input due to the ﬂux
correction is thus compared with that due to ice
formation, at the grids where ice is forming. In total,
the input due to the restoring term was 25.5% ofthat due to ice formation and hence was not themain term. Moreover, both values are negative (i.e.,
increasing sea surface salinity), so that the restoring
term did not reduce the effect of brine rejection.
Although heat ﬂux under sea ice could affect sea ice
formation, the good agreement in ice production
suggests that an error originating from this effect isinsigniﬁcant. Hence, ﬂux correction under sea ice
did not lead to underestimate of the role of sea ice in
DSW production.
7. Summary and discussion
Fig. 10. (a) DSW production rate, (b) the maximum DSW
density, and (c) ice production in the Okhotsk Sea as a functionof the vertical diffusivity coefﬁcient (Kz) speciﬁed in the Kuril
Straits. Kz ¼ 0 and Kz ¼ 200 correspond to the Ctrl and Tmix
cases, respectively. The DSW is deﬁned as yo  1  C; sy X26:6.

unresolved patchy convection, or absence of nonhydrostatic effect (which could cause a downdraft).
The other is an underestimate of salination, which
could in turn arise from an underestimate of tidal
mixing and/or errors in fresh water ﬂux. In addition,satellite derived estimates of DSW production
would change by 50–100% depending on the

In order to clarify the mechanisms responsible forthe deep ventilation occurring in the Okhotsk Sea ascompared with that in the North Paciﬁc (e.g.,
Kitani, 1973), the effects of diapycnal mixing due totidal mixing in the Kuril Straits and to double
diffusion on ventilation of the Okhotsk Sea havebeen investigated through a set of experiments using
an OGCM.
The results showed that the addition of (tidallyinduced) diapycnal mixing in the Kuril Straits
considerably enhances the ventilation of the
Okhotsk Sea, whereas that of double diffusion does

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440difference (FC30 - Tmix)
0500

depth (m)depth (m)

0

1000

500
1000
1500

1500

Potential
Temp.

Salinity
2000
35N 40N 45N 50N 55N 60N
latitude

-0.2

-0.1

0

0.1

2000
35N 40N 45N 50N 55N 60N
latitude

-2

0.2

-1

0

1

2 °C

Fig. 11. Same as Fig. 6 but for the differences between the Tmix and FC30 cases.

enhanced subduction
due to preconditioning

direct modification in all layers
by mixing in the Straits
North
Pacific

OkhotskSea

S fluxby tidalmixingventilation ofintermediatelayer

sea iceformation

denser& moreDSW&
breaking internal waves

Okhotsk Sea

Kuril Straits
Kuril Straits

Fig. 12. Schematic view of the two roles of tidal mixing in the Kuril Straits in ventilation of the Okhotsk Sea.

not. Our analysis revealed that the tidal mixing
exerts its inﬂuence through enhanced DSW production in the northern Okhotsk Sea in addition to thedirect diapycnal mixing at the Kuril Straits asshown schematically in Fig. 12.
The mechanism of enhanced DSW production is
identiﬁed as a preconditioning of salinity values in
the northern shelf of the Okhotsk Sea. The
preconditioning is due to the upward salt ﬂux
caused by tidal mixing from the saltier lower layer
to the surface layer. The preconditioned (i.e., salted)
water leads to increase in both density andproduction rate of the DSW when the wintercooling and sea-ice formation work in the northern
Okhotsk Sea. The preconditioning also works to
keep a mixture of subducted and surrounding
waters at high salinity values, by increasing subsurface salinity values over the northern shelf of the
Okhotsk Sea. As a result, the density and production rates of the DSW increase from 26:6sy and0.01 Sv in the case without tidal mixing to 26:8sy
and 0.13 Sv in the case with tidal mixing.

Previous studies of the DSW have so far overlooked the preconditioning due to tidal mixing andhave focused only on the role of brine rejection. To
bring out the importance of the preconditioning, thesalinity increases due both to brine rejection and for
DSW production are estimated. According to
Gladyshev et al. (2000), who used in situ data thatcover most of the shelf in the northern Okhotsk Sea
and were taken in 1996 and 1997, the salinity
increase due to brine rejection was less than 0.3 inmost of the observation sites with the average ofabout 0.2, although the maximum value reached0.8. (Here, the average is calculated from theirestimates of the total salt production and the total
volume of water inﬂuenced by brine.) Their estimate
also suggested that even when the discussion islimited to DSW, a signiﬁcant portion of the DSW
also experienced salinity increases of less than 0.3.
The salinity increase required for DSW production can be estimated by adding a salinity difference
between the DSW (33.3–33.5) and its source water(i.e., the EKCW near the sea surface, 33:0) to a

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possible salinity change in the Okhotsk Sea due toinﬂow of all of the fresh water and salt ﬂuxes during
spring to fall (except for that due to vertical mixing).
Because of the large river runoff and excess
precipitation over evaporation, the total ﬂux would
freshen the surface layer in the northern Okhotsk
Sea by 0.2 on average, if vertical mixing is absent(the details are described in Appendix B and in
Table 5). Thus, the required salinity increase isestimated to be 0.5–0.7. This value is signiﬁcantly
greater than the average salinity increase due tobrine rejection.
Accordingly, if the estimate of Gladyshev et al.
(2000) is appropriate for the climatology of thespatial average of the salinity increase due to brine
rejection contributing to DSW production, there
must be other sources that complement brine
rejection. Our results suggest that one possible
mechanism is the preconditioning of salinity valuesby tidal mixing. (Interestingly, the salt input
through the Soya Strait is much less effective thanriver runoff or excess precipitation over evaporation(Appendix B), although it has been considered to bea major source in the salinity balance in the
Okhotsk Sea (e.g., Talley and Nagata, 1995.)
However, Shcherbina et al. (2003) have recently
shown that a salinity increase due to brine rejection
reached 0.83 at one of the two mooring sites in 2000.
This suggests that some uncertainty may becontained in the above estimates. Thus, an accurate
estimate of the relative importance of the preconditioning by tidal mixing should await the futureaccumulation of observations. Nevertheless, broad
agreement in the important features of the Okhotsk
Sea vertical structure and circulation in the simulation and observations supports the possibility thatthe preconditioning by tidal mixing enhances DSW
production. Also, observed high salinity anomalies
in the upper layer around the Kuril Straits (Reid,
1973; Gladyshev, 1995) suggest that tidal mixing at
Table 5
Estimate of possible salinity change in the northern Okhotsk Sea
Component Runoff E-P
dS
0.17
S
0
0.47
V (103 km3 )Melted ice SWCW Tatar Strait

0.06 0.04
0
3
0.15
0.114

+0.02
34.0
1.5

+0.03
16.5
0.16

dS is the possible salinity change in the northern Okhotsk Sea dueto the corresponding ﬂux component. S and V are respectively
salinity and volume of water supplied from the corresponding
ﬂux components. A detailed description is given in Appendix B.

441

the Kuril Straits makes a signiﬁcant contribution asa preconditioning factor.
Associated with the enhanced DSW production,
both freshening and cooling of an intermediate layerof the Okhotsk Sea occur, and thereby improve thesimulated water masses in the Okhotsk Sea. In
particular, a local PV minimum (in the verticaldirection) is produced in the Okhotsk Sea around
300–500 m depth due to the enhanced DSW
production. The reproduction of this feature isremarkable since it is a major characteristic featureof the OSMW. As far as we are aware, this is theﬁrst successful reproduction of such a local PV
minimum in Okhotsk Sea water by an OGCM.
The diapycnal mixing in the Kuril Straits spreads
freshening (or ventilation) to layers denser than theDSW, and down to 27:6sy . This is consistent withthe observational fact that the inﬂuence of thesurface water properties is gradually carried down
to such deep layers through diapycnal mixing(Wong et al., 1998), as mentioned in the introduction.
The direct effect of diapycnal mixing alsoenhances the freshening of the simulated intermediate water, including the DSW density layer. A
quantitative estimate of the relative importance ofthe direct tidal mixing in the Kuril Straits and theDSW formation due to brine rejection in fresheningof the OSMW is, however, beyond the scope of thisstudy and awaits future observational studies.
Nevertheless, because the vertical distribution ofsalinity around the Okhotsk Sea has a convex
structure as a whole (i.e., q2 S=qz2 o0; e.g., Watanabe and Wakatsuchi, 1998), the down-gradienttype vertical diffusion acts to decrease salinity in theintermediate layer. Accordingly, the result that
direct tidal mixing in the Kuril Straits can freshen
the intermediate layer in the Okhotsk Sea should berobust in a qualitative sense. Also, an analysis ofoxygen isotopes suggested that diapycnal mixing isimportant even above the 27.2 density layer(Yamamoto et al., 2002). Thus, it is likely that
direct tidal mixing makes a signiﬁcant contribution
to freshening of the intermediate layer in the
Okhotsk Sea.
Previous attempts to model the water masses in
the Okhotsk Sea, such as the OSMW, and thus the
NPIW have been unsatisfactory even though stateof-the-art OGCMs have been used (e.g, Yamanaka
et al., 1998; Sakuma et al., 2003). Our results
suggest that this is because the effect of diapycnal
mixing process discussed so far has not been taken

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T. Nakamura et al. / Deep-Sea Research I 53 (2006) 425–448

into account through either a parameterization or
explicit reproduction. Thus, attempts on the line ofthe present study could lead to better numericalsimulations of the ventilation of intermediate waterin both the Okhotsk Sea and the North Paciﬁc.
Note that although increase of vertical mixing
everywhere (not only locally) could also lead to asomewhat deeper ventilation due to downward
diffusion, the location of enhanced mixing affects
a ventilation process. This is particularly so for thecase of the Kuril Straits. First, the Kuril Straits are
the entrance of the Paciﬁc water to the Okhotsk Sea.
Such a location allows the effect of Kuril Strait
mixing to spread in the Okhotsk Sea, leading to theenhanced DSW production. Interestingly, thismeans that enhanced mixing in the Kuril Straits
induces enhanced sinking. In contrast, previous
studies on global thermohaline circulation haveshown that globally enhanced mixing induces
enhanced upwelling from abyssal depths (throughenhanced downward diffusion). The difference
originates from the site of diapycnal mixing. The
Kuril Straits are near a subduction region, whereas
most of the global oceans are upwelling regions.
Thus, the present result has an important implication not only for regional oceanography but also for
an understanding of the role of diapycnal mixing inthermohaline circulation.
Second, the Kuril Straits are the exits of the water
modiﬁed in the Okhotsk Sea and are directly
adjacent to the waters of the North Paciﬁc. Thislocation enables the Kuril Strait mixing to affect theNorth Paciﬁc. For example, fresher and colder
water is supplied to the subsurface North Paciﬁc in
the Tmix case (Fig. 5). Also, the PV values in theupper layer are lowered (Fig. 5) by both theenhanced DSW production and the diapycnal
mixing in the Kuril Straits. This lowering in PV
strengthens the western boundary current of thesubarctic gyre (the Oyashio, Figs. 2 and 3), since theisopycnal surfaces of the Oyashio deepen to thewestern boundary. Interestingly, the strengthened
Oyashio crosses over the climatological zero-Sverdrup transport line to the subtropical gyre (Fig. 2).
This cross-gyre boundary ﬂow induces a transportof fresh, ventilated water from the Okhotsk Sea into
the subtropical gyre, and hence affects ventilation of
the subtropical North Paciﬁc. Details of themechanism and overall effect of the above process
will be reported in another paper.
In addition, no observations have shown that it is
the whole Okhotsk Sea where strong diapycnal

mixing occurs. Rather, it is considered to occur
locally. Among such mixing sites in the Okhotsk
Sea, the Kuril Straits are special also in that locallyenhanced diapycnal mixing occurs in such deep
water (mixing occurs down to the 27:6sy density
layer and up to the sea surface) (e.g., Yasuoka,
1968; Nakamura and Awaji, 2004). In contrast,
although the shelf of the northern Okhotsk Sea isalso suggested as a strong mixing site (e.g.,
Rogachev et al., 2001), it is too shallow to induce
the observed deep indirect ventilation reaching tothe 27:6sy density layer since the density over the
shelf is 27:0sy at most (Gladyshev et al., 2000,
2003). Accordingly, diapycnal mixing in the Kuril
Straits will have a different effect from that in otherregions. Nevertheless, vertical mixing over thenorthern shelf may have important effects, forexample, on heat exchange between ocean andatmosphere. Also, the northern shelf has a strong
tidal ﬂow (e.g., Nakamura et al., 2000a), and hence
the effects of horizontal tidal motion may be
important for the spreading and modiﬁcation of
DSW and the distribution and formation of sea ice.
The investigation of these processes is remained for
future works.
It should be noted that the model topography did
not resolve all of the passages in the Kuril Straits
and thus the detailed current ﬁeld in this area.
Nevertheless, the general features are captured in
the simulations as discussed in Section 3. This ispartly because we carefully prepared the model
topography to include the two main straits (theBussol and Kruzenshtern Straits), the shallow sills
in the Kuril Straits, and the large islands near
Hokkaido Island (e.g., Kunashiri Island). Also, thepresence of many eddies and a strong tidal ﬂow arereported around the Kuril Straits (e.g., Yasuda
et al., 2000; Thomson et al., 1997). Some of theeddies arise from a tidal front around the Kuril
Islands (Nakamura and Awaji, 2004). Althoughthese cannot be resolved using the present model
resolution, the effects of the eddies and tides weretaken into consideration through the Gent and
McWilliams’ (1990) parameterization and throughthe increased vertical diffusion term, and lead to thegeneral similarity to observation discussed in Section 3. Accordingly, although a much ﬁner resolution model is needed in the future, the present model
reasonably reproduced the observed ﬂow ﬁeld on abasin scale (the Okhotsk Sea scale) and hence
supports a basic understanding of the role of tidal
mixing in the Kuril Straits.

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Acknowledgements
We appreciate the helpful comments of threeanonymous reviewers. We also wish to acknowledge
Dr. J.P. Matthews for his critical reading. This
study is partly supported by the Category 7 of
MEXT RR2002 Project for Sustainable Coexistenceof Human, Nature and the Earth and by a Grant-inAid for the 21st Century COE Program (Kyoto
University, G3). T.N. was also supported by the
JSPS Research Fellowships for Young Scientists.
Numerical calculations were done on the VPP800 at
the Academic Center for Computing and Media
Studies of Kyoto University.
Appendix A. Model description
The OGCM used in this study has been developed
at Kyoto University (Ishikawa et al., 2002; Nakamura et al., 2003b; Toyoda et al., 2004), based onthat developed in Meteorological Research Instituteof Japan Meteorological Agency (Ishizaki, 1994),
which is in turn based on Bryan (1969). The model
solves the primitive equations with a free surface in
spherical coordinates under Boussinesq and hydrostatic approximations and incorporates a sea ice
model.
The horizontal grid system is a rectangular
Arakawa staggered B grid (Bryan, 1969), and isbasically the same as that of Modular Ocean Model(MOM) 3.0 developed at the Geophysical Fluid
Dynamics Laboratory (GFDL/NOAA Departmentof Commerce). In the vertical, however, a hybrid
system of the so called s and z coordinates is used.
The s coordinate is adopted near the surface (above
60 m depth in the present case), in which the depthsof discrete cells vary with time and space depending
on sea surface height. Below the layer of scoordinate, z coordinate is adopted, in which thepartial cell scheme is used at the bottom in order tomore accurately resolve the bottom topography.
This hybrid system allows a very high resolution in
the vertical near the surface even with a free surface
and is thus useful for the improvement of surface
mixed layer simulations (Toyoda et al., 2004). In
both coordinates, vertical velocity and the othervariables are staggered vertically as in MOM 3.0.
Time integration of the prognostic equations iscarried out by using mainly a centered leap-frogtime stepping scheme with periodic application ofEuler-backward scheme to handle the computational mode characteristic of a leap-frog scheme.

443

The momentum and continuity equations with afree surface are separated into the external andinternal modes and are explicitly solved based on
Killworth et al. (1991). The advection terms in the
momentum equations are discretized using Takano–Oonishi scheme (Ishizaki and Motoi, 1999),
which conserves both energy and pseudo enstrophy.
The subgrid scale eddy viscosity is parameterizedthrough bi-harmonic friction with Smagorinsky-likeviscosity (Grifﬁes and Hallberg, 2000). At the landboundary, no-normal ﬂow is assumed. The bottom
stress of the form r0 C D juh jðu cos y  sgnðf Þv sin y;v cos y þ sgnðf Þu sin yÞ is imposed at the bottom
boundary, where C D is a drag coefﬁcient, y is arotation angle representing the effect of the bottom
Ekman layer, and sgnðf Þ is the sign of the local
Coriolis parameter. A no-slip condition is imposedat the lateral boundary.
The tracer equations, which predict potential
temperature and salinity in the present study, adopt
a third-order advection scheme (UTOPIA andQUICKEST; Hasumi and Suginohara, 1999) andincorporate isopycnal diffusion with eddy parameterization (Redi, 1982; Gent and McWilliams,
1990), together with a turbulence closure mixed
layer scheme (Noh and Kim, 1999). Also, the effectof double diffusion is parameterized on the basis of
Schmitt (1981) for the salt ﬁngering condition andFedorov (1988) for the diffusive convection condition, following Merryﬁeld et al. (1999). In order toallow a large vertical diffusivity for unstably
stratiﬁed water or the turbulence closure scheme,
implicit time stepping is used for vertical mixing. At
the land boundary, no tracer ﬂuxes are assumed.
The equation of state used here is the international equation of state of seawater recommended
by UNESCO (Millero et al., 1980; Millero andPoisson, 1981) but is written in terms of potential
temperature following Jackett and McDougall(1995).
The sea ice model incorporated is a Hibler (1979)
two-category ice model and is based on the works of
Ikeda (1988, 1989a,b) and Ikeda et al. (1988). The
main body of the ice is deﬁned as thick ice. Thin ice
can form in the open water portion of an ice-present
model grid but is transformed into (or merges into)
thick ice immediately. The prognostic variables arethus thickness, concentration, and velocity of the
thick ice.
Ice velocity is calculated from simpliﬁed momentum equations which in the present version, include
air-ice drag, ice-water drag, Coriolis force, gravity

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associated with a sea surface slope, and viscosity
which is retained for numerical stability, along withthe local-acceleration and advection terms. Theinternal stresses are neglected although their effect iscrudely taken into consideration through limiting
the maximum value of ice thickness. Since model ice
concentration is mostly less than 0.9 in the Okhotsk
Sea with the present model resolution (Fig. 4), thissimpliﬁcation does not result in a signiﬁcant change,
although internal stresses should be included in ahigher resolution model.
The thermodynamic equations contain atmospheric heat ﬂuxes (shortwave and longwave radiation and sensible and latent heat ﬂuxes) and a heatbalance with the surface layer of the ocean model.
An excess or shortage of heat in the ocean surface
layer relative to the freezing point is immediately
converted to decay or growth of ice. In thisconversion, heat ﬂux into an open water portion
varies concentration through melting the thick iceor adding the thin ice from its side, while heat ﬂux
into the ice-covered portion varies thickness of the
thick ice. A snow layer and heat storage of ice areneglected. The equations for ice volume (i.e.,
concentration times thickness) and concentration,
which are integrated in time, are obtained from the
combination of these thermodynamic effects andmass conservation for ice, together with kinematicconditions for ice thickness. (Diffusion of ice ofsmall magnitude is added for numerical stability.)

Appendix B. An estimate of possible salinity changes
in the northern Okhotsk Sea due to fresh water andsalt ﬂuxes
In order to evaluate the relative importance of thepreconditioning due to tidal mixing in DSW
formation, possible salinity changes due to theother fresh water and salt ﬂuxes during spring to fallare estimated here.
For the estimate, a box model for the surface
layer in the northern part of the Okhotsk Sea wasconsidered, as shown in Fig. 13. The ﬂuxes
concerning salinity change in the box model are
due to river runoff, excess precipitation overevaporation, melted ice, inﬂows of the SWCW and
EKCW, outﬂows through the ESC and the Tatar
Strait, and vertical mixing from below. (Ice formation does not matter here because of the choice oftime duration.) Quantities associated with these ﬂux
components are hereafter indicated by the sub-river
runoffE-P

meltedice

verticalmixingoutflow via
Tartar St.

} 50m
ESC

EKCW

SWCW

Fig. 13. Schematic of a box model for the surface layer in thenorthern Okhotsk Sea described in Appendix, which is superposed over coastlines around the Okhotsk Sea. (Runoff in reality
enters through the side boundary but is presented by an arrowfrom above for simplicity.)

scripts runoff, e  p, mice, swcw, ekcw, esc, ts, andvmix, respectively.
Suppose that the volume and salinity values ofthe box change from V 0 and S0 to V 0 þ DV andS0 þ DS, respectively, due to the above ﬂux
components. Then the total change in amount ofsalt (DSalt) is,
DSalt ¼ r0 ½ðS 0 þ DSÞðV 0 þ DV Þ  S 0 V 0 ,

(B.1)

where the Boussinesq approximation is used withthe reference density of r0 1:03 g cm3 . Thus, theestimation of DSalt and DV yields the resulting,possible salinity change DS.
The change of the box volume is given by
DV ¼ V runoff þ V ep þ V mice þ V swcw þ V ekcw
 V esc  V ts þ V vmix .

ðB:2Þ

Here V runoff is the volume supplied by runoff during
spring to fall (here, April to December), and theother components are similarly deﬁned. Note thatprecipitation over sea ice is not included because of
the choice of the duration. The vertical volumetransport due to an averaged vertical velocity is
included in the volume change DV so that
V vmix ¼ 0. The terms V esc and V ekcw include Ekman
transport through the southern boundary of the box
model for southward and northward components,
respectively. Using these terms, the change of total

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salt is given by
DSalt=r0 ¼ Srunoff V runoff þ S ep V ep þ S mice V mice
þ S swcw V swcw þ S ekcw V ekcw  S esc V esc
 S ts V ts þ Saltvmix =r0 ,ðB:3Þ

where S is salinity and Saltvmix is a salt input due tovertical mixing.
The initial value of salinity is set as S 0 ¼ S ekcw tofocus on the salinity modiﬁcation in the Okhotsk
Sea. In calculation of the initial box volume, theinitial depth of the box is set to be 50 m to consider
salinity change of a mixed layer just before winter,and the latitude of the southern boundary is set at
51  N. In addition, the salinity of the outﬂowing
ESC is assumed to be the same as the ﬁnal value(Sesc ¼ S0 þ DS) to avoid the difﬁculty of determining its representative value. This may lead to anunderestimate of the possible salinity change since
the outﬂow would ﬁrst export water of the initialsalinity.
Substituting (B.2) and (B.3) into (B.1) and using
the above assumptions, the change in salinity
becomes
DS ¼ dS runoff þ dS ep þ dSmice þ dS swcw

dS f ¼

þ dS ts þ dSvmix ,ðB:4ÞðS f  S0 ÞV f,
V 0 þ DV þ V esc(B.5)

where dS f represents a contribution from each ﬂux
component and the subscript f be replaced by thecorresponding subscript. (The last term in (B.4) isdeﬁned as dS vmix ¼ Saltvmix =½r0 ðV 0 þ DV þ V esc Þ.)
Estimate of each contribution in (B.4) was thenobtained as follows (Table 5). Salinity of river
runoff and excess precipitation over evaporation isneglected (S runoff ¼ S ep ¼ 0), and their volumes
are based on Talley and Nagata (1995) and the
OMIP data set (Röske, 2001), respectively. The
salinity of the water derived from melted ice is set tothat of sea ice, which in turn is set to 3. For thevolume estimate of melted ice, a half of the
produced ice is assumed to melt in the northern
Okhotsk Sea (i.e., the other half is assumed to drift
away to or be produced in the southern region). The
ice production used is the 12-year average value(Alfultis and Martin, 1987; Martin et al., 1998;
Gladyshev et al., 2000). The salinity of the SWCW
is approximated as 34.0 based on Takizawa (1982).
The volume of the SWCW that ﬂows into the
Okhotsk Sea during spring to fall is estimated to be445

0.5 Sv on average (Aota, 1975), and then 18 of thisvolume is assumed to enter the box, on theassumption that a half of the SWCW mixes with
Okhotsk Sea water before ﬂowing out to the Paciﬁc,and that a half of the subsequent mixture reaches
the northern Okhotsk Sea with a half of it present in
the upper 50 m layer. The salinity of the waterﬂowing out through the Tatar Strait is approximated as a half of the initial value in the interior(S0 =2 ¼ 16.5), considering that the sea surface
salinity around the strait is less than 20 (Fujii andAbe, 1976; Gladyshev et al., 2003) and that the true
value would lie somewhere between 0 and S0 . Its
volume is assumed to be a half of the runoff fromthe Amur river because we are not aware of any
available observations, and because the volume
should be less than the Amur river runoff, so thatthe effect of the runoff spreads into the Okhotsk Sea
as in the observations though the Amur river islocated near the Tatar Strait. The initial salinity is
set to be S 0 ¼ 33:0 and the initial box volume isestimated as V 0 4  104 km3 , approximating thearea to 86 km2 . Finally, the other terms in the
denominator in (B.5) become
DV þ V esc ¼ V runoff þ V ep þ V mice þ V swcw
þ V ekcw  V ts .
The right-hand side can be approximated by V ekcw ,which in turn is approximated by V esc . This isbecause the terms V ekcw and V esc are predominantin (B.2) (other terms are smaller than an error inestimating V esc ). To estimate V esc , ESC volumetransport in the surface layer is approximated as
2 Sv. This value corresponds to the total transportof the model ESC and to 12 – 14 of the observed totaltransport (Ohshima et al., 2002), so that it may bean overestimate, leading to an underestimate of thepossible salinity change.
According to the estimates shown in Table 5, thenet ﬂux (except for vertical mixing) works todecrease the surface layer salinity in the northern
Okhotsk Sea by 0.2 during spring to fall. It should
be noted that these estimates may contain errors,
although it is difﬁcult to estimate the magnitudes oferrors because, as far as we are aware, not only true
values but also ranges of errors (or interannual
variations) in the observational values used here are
not available in a climatological sense. Since some
unknown values are chosen so that they would lie
between the maximum and the minimum possible
values, each estimate of dS could have an error of

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100% at the maximum. Nevertheless, as long as
the order of each estimates is relevant, thedominance of a runoff effect would ensure a robust
nature of the results. Thus, the total error depends
mainly on an estimate of runoff.

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