Journal of Oceanography, Vol. 60, pp. 411 to 423, 2004

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean
TOMOHIRO NAKAMURA*, TAKAHIRO TOYODA, Y OICHI ISHIKAWA and TOSHIYUKI AWAJI
Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan(Received 19 February 2003; in revised form 30 August 2003; accepted 28 September 2003)

A numerical study using a 3-D nonhydrostatic model has been applied to baroclinic
processes generated by the K1 tidal flow in and around the Kuril Straits. The resultshows that large-amplitude unsteady lee waves are generated and cause intense
diapycnal mixing all along the Kuril Island Chain to levels of a maximum diapycnal
diffusivity exceeding 103 cm 2s –1. Significant water transformation by the vigorous
mixing in shallow regions produces the distinct density and potential vorticity (PV)fronts along the Island Chain. The pinched-off eddies that arise and move away fromthe fronts have the ability to transport a large amount of mixed water (~14 Sv) to theoffshore regions, roughly half being directed to the North Pacific. These features are
consistent with recent satellite imagery and in-situ observations, suggesting that
diapycnal mixing within the vicinity of the Kuril Islands has a greater impact than
was previously supposed on the Okhotsk Sea and the North Pacific. To examine thisinfluence of tidal processes at the Kurils on circulations in the neighboring two basins, another numerical experiment was conducted using an ocean general circulation model with inclusion of tidal mixing along the islands, which gives a better representation of the Okhotsk Sea Mode Water than in the case without the tidal mixing.
This is mainly attributed to the added effect of a significant upward salt flux into thesurface layer due to tidal mixing in the Kuril Straits, which is subsequently transported to the interior region of the Okhotsk Sea. With a saline flux into the surface
layer, cooling in winter in the northern part of the Okhotsk Sea can produce heavier
water and thus enhance subduction, which is capable of reproducing a realistic
Okhotsk Sea Mode Water. The associated low PV flux from the Kuril Straits to theopen North Pacific excites the 2nd baroclinic-mode Kelvin and Rossby waves in addition to the 1st mode. Interestingly, the meridional overturning in the North Pacific isstrengthened as a result of the dynamical adjustment caused by these waves, leading
to a more realistic reproduction of the North Pacific Intermediate Water (NPIW)
than in the case without tidal mixing. Accordingly, the joint effect of tidally-induced
transport and transformation dominating in the Kuril Straits and subsequent eddytransport is considered to play an important role in the ventilation of both the Okhotsk
Sea and the North Pacific Ocean.

Keywords:⋅ Tidal mixing,⋅ ventilation,⋅ eddy transport,⋅ Okhotsk Sea,
⋅ North Pacific
Ocean.

which is characterized by a salinity minimum around26.8σθ and spreads out over most of the subtropical gyre
(Talley, 1993). Accordingly, the ventilation in thesubarctic region may largely affect the subsurface circulation, particularly at mid-depth, in both the subarctic andsubtropic gyres of the North Pacific. Since the ventilation process also creates a body of water capable of storing large amounts of greenhouse gases, such as CO 2
(Tsunogai et al., 1993; Yamanaka and Tajika, 1996), itsstudy is basic to our understanding of the potential response of the ocean to increased greenhouse effects.

1. Introduction
The ventilation taking place in the North Pacificsubarctic gyre reaches down to the 27.4σθ density layer,
and its influence extends to the subtropics (e.g., Watanabe
et al., 1994; Fine et al., 2001). One product of this ventilation is the North Pacific Intermediate Water (NPIW),* Corresponding author. E-mail: nakamura@kugi.kyoto-u.ac.jp
Copyright © The Oceanographic Society of Japan.

411

A prominent feature of the subarctic region of theNorth Pacific is a shallow, strong halocline around 26.6σθ
due to a combination of upward Ekman pumping and apositive freshwater flux (e.g., Ohtani, 1989; Miura et al.,2002). Although the low salinity water in the upper partof the NPIW is thought to originate from the subarctic
mixed-layer water above this halocline (e.g., Talley et al.,1995), the freshwater supply to the lower part of the NPIW
requires a mechanism to drive the downward transfer
across the strong halocline. Previous studies have identified subduction in the Okhotsk Sea and vertical diffusionin the Gulf of Alaska as likely candidate driving mechanisms. Recent field observations (Talley, 1991; Yasuda,
1997; Kono, 1998; Watanabe and Wakatsuchi, 1998) haveunderlined the importance of the subduction associatedwith sea ice formation in the Okhotsk Sea, forming theheavy Dense Shelf Water (DSW) (Kitani, 1973; Alfultis
and Martin, 1987; Martin et al., 1998; Gladyshev et al.,2000). The mixing of DSW with the surroundings can
produce a low potential vorticity water centered at 26.8σθ(the Okhotsk Sea Mode Water, OSMW, Yasuda, 1997).
This feature is favorable for the realistic production of
the NPIW and is regarded as the most likely origin of the
NPIW.
However, it is difficult to explain the ventilation of
the lower part of the NPIW (say, below 27.0σ θ) in termsof the above processes. This is because the direct sinkingof surface water in the Okhotsk Sea influences the 27.0σθ
level at most (Gladyshev et al., 2000). In addition, vertical diffusion in the Alaskan Gyre extends only to 26.8σθ
(Van Scoy et al., 1991; You et al., 2000). Regarding thecabbeling effect, though it is inevitable in mixing processes of the subarctic and subtropical waters, the density
increase of the mixture is less than 0.02 σ θ below the27.0 σθ density layer (Talley and Yun, 2001). These facts
imply that there should be other processes causing ventilation in the lower part of the intermediate layer.
One possible mechanism for this process is verticalmixing in the Kuril Straits, which connect the Okhotsk
Sea and the North Pacific. The analysis of observed
chlorofluorocarbons (CFCs) concentrations indicates that
the Okhotsk Sea is an important location for the ventilation of the intermediate water (Warner et al., 1996) andthat diapycnal mixing is necessary for the formation of
the Okhotsk Sea water below the 27.2σθ level (Wong etal., 1998). Many observations have revealed the presenceof vertically mixed water in the Kuril Straits and haveshown that the influence of the mixing extends to 2200 m
depth or more (Yasuoka, 1968; Kitani, 1973; Kawasaki
and Kono, 1994; Gladyshev, 1995; Kawasaki, 1996;
Freeland et al., 1998; Aramaki et al., 2001).
Recent detailed studies on the process in the Kuril
Straits have suggested that the tides, especially the K1
tide, are mainly responsible for the vertical mixing de-

412

T. Nakamura et al.

scribed above. In fact, the diurnal tidal currents are dominant in and around the Kuril Straits, and their speeds reacha few ms–1 over shallow sills and 0.3 ms –1 even in deep
regions (Thomson et al., 1997; Ohshima et al., 2002).
Such strong tidal currents are likely to cause intense vertical mixing due to interactions with the topographic features in the Kuril Straits. Modelling and theoretical studies of the tidal process have revealed that: (1) since theK 1 tide is subinertial in the Kurils, it generates
topographically trapped waves; (2) these waves are amplified while circulating around the islands and produce
a significant water exchange through the Kuril Straits;and (3) as the amplified currents flow over sills in thestraits, large-amplitude unsteady lee waves are generated,
eventually leading to enhanced diapycnal mixing due towave breaking (Nakamura et al., 2000a, b; Nakamura andAwaji, 2001, 2003).
Such vigorous mixing takes place at the exit of the
OSMW to the North Pacific and the entrance of the North
Pacific water, which is in turn the main source of the
Okhotsk Sea water. It could thus affect the water properties of the neighboring two basins. From a dynamical view
point, the vertical mixing implies the supply of potential
vorticity (PV) from the lateral boundary (i.e., the Kuril
Island Chain) to both the North Pacific and the Okhotsk
Sea. According to the ventilated thermocline theory(Pedlosky, 1996), the subsurface information travels
roughly along the Sverdrup flow, thus propagating northward in a region of upward Ekman pumping and progressing eastward in the southern part of the subarctic gyre in
the North Pacific. Since the Kuril Straits is located in thearea corresponding to the south of the Okhotsk Sea andto the west of the North Pacific subarctic gyre, respectively, the PV flux from the Kurils could affect the circulations in the Okhotsk Sea and the subarctic gyre of theNorth Pacific. Moreover, since the layer of the NPIW
density range corresponds to the pool zone (Huang andRussell, 1994), in which the circulation is determined by
the PV flux from the western boundary region, it may bepossible that the PV flux from the Kurils affects the circulation of the NPIW.
A clarification of the tidal processes and their effecton the water mass structures and the circulation in both
the Okhotsk Sea and the North Pacific is therefore a key
to a better understanding of the ventilation of these twobasins. Moreover, examination of the thermohaline adjustment to such mixing in the western boundary region
may contribute to progress in the ventilated thermocline
theory.
In this paper we therefore investigate the water transformation and transport processes associated with the K1
tide around the Kuril Straits, using a 3-dimensional
nonhydrostatic model (Section 2). The impacts of the tidal
mixing at the Kuril Straits on the basin scale phenomena

are then examined in Section 3 using an ocean generalcirculation model (OGCM). The results are summarized
in Section 4.
2. Tidal Mixing in the Kuril Straits
2.1 Model and experimental design
In order to simulate baroclinic processes induced by
the tides and the associated vertical mixing in and aroundthe Kuril Straits, we use a three-dimensional
nonhydrostatic model, which has been constructed on thebasis of the work of Tanaka and Akitomo (2001). As
shown below, intense vertical mixing around the straits
will give rise to tidal fronts and associated eddies. We
thus take into account the effect of a planetary-vorticity
gradient by adopting the β -plane approximation. A
barotropic tidal flow is calculated separately from abarotropic tide model and is given in the advection terms
which work as the main forcing. The values of the eddy
diffusivities (and viscosities) are chosen from parametersweeping tests such that the diapycnal mixing is of the

Urup St.

Bussol St. Simusir St.

OkhotskSea

same order as those estimated in the next section.
The model topography (Fig. 1) is set up from the
DBDB5 (U.S. National Geophysical Data Center). The
grid sizes are about 700 m horizontally and 30 m vertically. The model domain covers most of the straits in the
Kuril Island Chain so as to involve the two deep passages,
the Bussol and Kruzenshterna Straits, through which the
major water flows are considered to take place. In addition, the propagation of topographically trapped waves
and the tidally-induced mean circulation around islands
and/or sills can be reproduced. Near the open boundaries, a sponge zone is adopted to prevent artificial wave
reflection, where the viscosities and the horizontal
diffusivities are gradually increased. In addition,
baroclinic velocity, potential temperature and salinity are
gradually restored to their initial values close to the open
boundaries, which, in effect, acts as an artificial buoyancy flux and thereby maintains stratification in the interior.
The initial potential temperature and salinity fields
are set to be horizontally uniform using the dataset of the
U.S. National Oceanographic Data Center. A summertime
climatology in the Kuril Basin is used from the sea surface to 1500 m depth in order to examine the modification of the Okhotsk Sea water entering the North Pacific.
Since the Kuril Basin (~3200 m depth) is shallower thanthe deeper parts of the Pacific in this region, the initial
vertical profiles between 1500 m and 3000 m depth aregradually changed to the summertime climatology of theEast Kamchatka Current Water (EKCW), which acts as
the local origin of the water. The baroclinic velocity is
set to zero at the beginning of the calculation.
In this experiment, we direct our attention to the K1
tide alone since the K1 current is the largest constituent
around the Kuril Island Chain (e.g., Thomson et al., 1997;
Nakamura et al., 2000a). The barotropic tide model usedhere is the same as that of Nakamura et al. (2000a). The
horizontal resolution, however, is taken to be the same asin this 3-dimensional model. The basic features of thecalculated barotropic field are essentially the same and

in
ha
C
d
an
Isl
l
ri
Ku

North PacificFig. 1. Model topography of the tidal processes (upper panel).
Lower panel shows the locations of the 3-D model domain(solid) and the barotropic model domain (dashed). Black
colored areas indicate land grids.

Fig. 2. Vertical velocity after 4 periods at 390 m depth.

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean

413

a

a

b

Bussol StraitbcFig. 4. (a) Potential temperature at the model sea surface (15m depth) after 25 periods. White circles indicate eddy-likestructures also seen in AVHRR thermal infrared imagery(Fig. 5). (b) Planetary part of potential vorticity after 25
periods at the model sea surface.

Fig. 3. Temporally averaged diapycnal diffusivity coefficients(cm2s–1) estimated from the calculated fields in the 4th period. Contour is drawn at 100 cm 2s–1. (a) An example of yz sections across the sill in the Urup Strait. (b) Bussol Strait
section shown along the Island Chain, the vertical axis being potential density. (c) Horizontal distribution in the
isopycnal layer of 26.75~26.8σ θ.

the harmonic constants of both tidal elevation and current field are qualitatively similar to the observed values,
but an increase in horizontal resolution leads to a corresponding improvement in accuracy.
2.2 Diapycnal mixing
As in the vertical 2-dimensional case reported byNakamura et al. (2000b), the main cause of mixing canbe attributed to large-amplitude unsteady lee waves, which
are generated around a sill top and propagate away as the414

T. Nakamura et al.

flow reverses. Figure 2 shows the horizontal distributionof vertical velocity after 4 periods (after an interval oftime corresponding to 4 periods) at 390 m depth, roughly
corresponding to the core density layer of the NPIW. Many
inertial gravity waves are present, indicating that largeamplitude unsteady lee waves are produced all along the
Kuril Island Chain. Our examination of the surfaces of
constant phase reveals that large-amplitude internal waves
originate mainly from (1) the bank located in the northeastern Pacific region (around x = 700 km, y = 150 km),(2) the Urup Strait, and (3) sills in the northeastern partof the island chain.
To estimate the intensity of vertical mixing associated with the wave processes, we calculate diapycnal diffusivity coefficients, Kρ, following the scaling proposedby Gregg (1989), which is based on the internal wave
dynamics and observations. In doing so, due to our limited vertical grid size, a 30-m shear is used instead of the
10-m shear, so that the present calculation provides roughestimates of the diffusivity coefficients which will be discussed in Section 4 in addition to the problem concerning the application of Gregg’s formula in this study. The
estimated Kρ is temporally averaged over the 4th period.

Observed SST (infra red image)
NOAA-12: Orbit 17429: 1994/0921 22:31-22:33

50

48

46
150

152

154

156

Note that the model domain is inclined by 45 degrees.
Circles in the figure indicate similar eddy-like structures seen in the model result.
Arrows emphasize bi-directional current structure explained by tidal residual flow.

Fig. 5. AVHRR thermal infrared image of the Kuril Straits takenfrom the NOAA 12 satellite on September 21, 1994. White
circles indicate eddy-like structures similar to features in
the model SST (Fig. 4) (figure courtesy of Prof. S. Saitoh).

Figure 3(a) shows the distribution in the vertical section
crossing the sill in the Urup Strait. The diapycnal diffusivity is very large over the sill, with a maximum value
exceeding 1000 cm2s –1.
Figure 3(b) shows the Kρ map across the deep Bussol
Strait. On this map, σ θ is used as a vertical axis to identify the density layers of active diapycnal mixing. Although wave-energy generation is much weaker than in
the Urup Strait, considerable mixing (Kρ > 10 cm2s –1)
occurs. This result is consistent with the recently observedcarbon isotopes distributions (Aramaki et al., 2001). The
mixing arises mainly from the clockwise transport of wave
energy originating from the adjacent Urup and Simusir
Straits where energetic wave generation occurs. This significant wave energy flux is attributed to the along-isobath
propagation of subinertial K1 internal tides generated oversill slopes and to the propagation of unsteady lee waves.
It should be noted, then, that the density layer of theneighboring OSMW and NPIW core (~26.8 σθ) is subject
to this mixing. The outflowing Okhotsk Sea water is therefore expected to be considerably modified by tidal mixing to an extent determined by the travel time of water
passing through the Strait.
The distribution of K ρ at an isopycnal layer(26.75~26.8σθ) is shown in Fig. 3(c), which indicates thatthe above features relating to the intensity of diapycnal
mixing are generally present in the Kuril Island Chain.
An intense diapycnal mixing is distributed in shallow
straits, around islands and over the bank, where the tidal
flow is strong and thus energetic unsteady lee waves are
generated. In contrast, the mixing weakens with depth(except near the bottom).
2.3 Eddy-induced offshore transport
Figure 4(a) shows the model sea surface temperature after 25 periods. The mixing with cold subsurface
water produces low-temperature anomalies along the island chain, which manifest as temperature fronts and eddies. The patterns and size of eddies are generally similar
to those seen in infrared imagery (Fig. 5). In particular,
the eddy-like structures marked by white circles take placeat almost the corresponding locations and bear such a resemblance. Such temperature fronts are known to exist
almost throughout the year, although cooling in wintermakes it a warm anomaly (Kitani, 1973) and their positions are quite variable. These facts strongly suggest thattidally-induced baroclinic processes are responsible forthe observed fronts and eddies. It is thus reasonable toexpect that diapycnal mixing induced by tidally generated unsteady lee-waves can contribute to the intense vertical mixing at the Kuril Straits.
Similarly, potential vorticity fronts are formed along
the Island Chain due to the spatial variation in the strengthof vertical mixing. The fronts spawn various instabilities, such as baroclinic instability and associated eddies(e.g., Pedlosky, 1987). Figure 4(b) shows a map of theplanetary component of potential vorticity,−f ∂ρ p = 0ρ 0 ∂z(hereafter PPV), at the model sea surface after 25 periods. The tidally-induced PPV fronts evolve by the formation and release of baroclinic eddies. These eddies
should contain low PPV water in their cores and thus anticyclonic eddies are easier to produce. A recent observation in the Kuril Basin clearly captured such a pinchedoff eddy originating from the Kuril Straits (Itoh et al.,2002).
These results indicate that eddies convey verticallywell-mixed water located around the straits toward theoffshore region. To quantify this transport, the volume
increase of the mixed water in the deep regions is calculated (the mixed water is identified as having a PPV value
either 20% lower or higher than the ambient average, andthe deep region is defined here as >3000 m and/or y >330 km). This corresponds to the offshore transport ofthe mixed water, assuming that diapycnal mixing is not

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean415effective in the deep regions. The mean offshore volume
flux of the mixed water during 25 periods reaches 5.7 Sv
and 8.7 Sv, respectively, of which 2.6 Sv and 3.9 Sv enter
the open Pacific. The volume increments are roughly constant around 25 periods, indicating that the offshore transport is roughly in a steady state and hence our estimate ismeaningful. These offshore transports are attributed
mainly to eddies arising and moving away from tidal
fronts (at least in this model) since the net offshore transport induced by tidally-induced residual flow is about 2
Sv for the Okhotsk Sea and 1 Sv for the North Pacific(the latter is not counted in the above estimate since ittakes place over a shelf break shallower than 3000 m
depth).
Diapycnal mixing in the vicinity of the Kuril Islands
therefore has a greater impact than was previously supposed on the Okhotsk Sea and the North Pacific as a result both of modification of in/outflowing water properties and offshore transport of the modified water associated with eddies. For example, the offshore eddy transport of the transformed water in the straits supplies significant PV and salinity fluxes to both the Okhotsk Sea
and the North Pacific. The PV values of such eddies differ on average by 30–40% from the ambient water. The
sea surface salinity is 0.1 psu higher than the ambient
water and this difference leads to a density increase of
0.08 σθ in the offshore regions. Similar high sea-surfacesalinity anomalies of 0.1~0.3 psu greater than the surroundings have been observed in the Kuril Straits (Reid,
1973; Gladyshev, 1995). Such fluxes will affect water
masses and circulation patterns in the two basins. These
aspects form the subject of the next section.
3.

Impacts of Tidal Mixing on the Okhotsk Sea andNorth Pacific

3.1 Model and experimental design
We have performed a series of impact experiments
to investigate the role of the tidal mixing at the Kuril
Straits in the water mass formation and large-scale circulation in both the Okhotsk Sea and the North Pacific.
The model used is an OGCM developed at Kyoto
University (Ishikawa et al., 2002). For a realistic reproduction of the subduction processes, this model adopts athird-order advection scheme (UTOPIA and QUICKEST;
Hasumi and Suginohara (1999)) in the tracer equation. In
addition, an isopycnal diffusion scheme using the eddy
parameterization (Gent and McWilliams, 1990; Griffies
et al., 1998) is incorporated together with a turbulence
closure mixed layer scheme (Noh and Kim, 1999). In order to successfully simulate the subduction associatedwith sea ice formation in the Okhotsk Sea, a sea ice model
based on the work of Ikeda (1989a, b) is added. For more
adequate treatment of a free sea surface process, a σ–z
416

T. Nakamura et al.

hybrid vertical coordinate is adopted. The partial cellmethod is also used for better resolution of the bottom
topography.
The model domain is nearly global (75°S–75°N). The
horizontal resolution is 1° × 1° in both latitude and longitude, with 34 vertical levels spaced from 20 m near thesea surface to 400 m at the bottom. Care is taken not tosmooth out the sills between the North Pacific and the
Okhotsk Sea. The initial values of potential temperatureand salinity are taken from the World Ocean Atlas, 1994
Monthly Data compilation (Levitus and Boyer, 1994;
Levitus et al., 1994), to which the layers deeper than 2000
m, the northern and southern boundaries, and the exits of
the Mediterranean and Red Seas are restored. The sea
surface fluxes are based on the European Centre for Medium-Range Forecasts reanalysis data of a climatologicalseasonal variation (OMIP project; Röske (2001)) together
with the so called “flux correction”, except for regions
around the Kuril Straits. Wind stress and heat fluxes arecalculated using a model sea surface temperature basedon the bulk formulae.
A 30-year spinup was performed, by which time themodel ocean had reached an approximately equilibrium
state. Two experiments were then conducted. In the firstof these, a control situation was created by continuing
the integration for another 30 years. In the second experiment, simulating the case with mixing, the vertical
diffusion coefficient over the sills at the Kuril Straits was
set to a constant value of 200 cm2s –1. This assumed value
roughly corresponds to twice that estimated from the K1case alone (Kρ of ~100 cm2s –1). The reason for this selection is that, according to the simulation experiment of
tidal currents in the Kuril Straits reported by Nakamura
et al. (2000a), the total flow speed of tidal components,
except for the dominant K1 (e.g., O 1 + P1) in the Kuril
Straits, is similar to or larger than the K1 current speed.
Taking this fact into account, we here use such a roughestimate of diapycnal diffusivity for our basic investigation of the impacts of tidal mixing on the Okhotsk Sea
and the North Pacific.
The final year of the integrations (i.e., the 60th year)
is used in the analysis below, except for those investigating temporal evolution after the addition of the tidal mixing.
Prior to the detailed comparison between the control
and the mixing case, some discussion is needed of thequality of the OGCM itself in terms of the circulation ofboth the Okhotsk Sea and the North Pacific (i.e., corresponding to the control case). In general, the features ofthe simulated large-scale circulations in both basins arebasically consistent with previous reports. For example,
the large-scale cyclonic (anti-cyclonic) circulation of the
Okhotsk Sea (the northwestern North Pacific) shown in
Fig. 6(a) is in agreement with the observed temperature/[300m velocity]
(a)

(b)

Fig. 6. Horizontal velocity vectors for (a) the control case and (b) the mixing case in the northwestern part of the North Pacific in
February. See text for circles and squares in the figure.

Fig. 7. Vertical section along 145°E of (top) potential temperature, (middle) salinity, and (bottom) potential vorticity for (left) thecontrol and (right) mixing cases.
Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean

417

flatter -S curv e{
subducted

subducted

}

mixed
water

Fig. 8. θ-S diagrams in the northern part of the Okhotsk Sea in (upper) winter and (lower) summer for (left) the control and (right)
mixing cases.

salinity distributions (e.g., Favorite et al., 1976) and a
recent state estimation by data assimilation (e.g., Awaji
et al., 2003). However, some aspects of the ocean circulation, particularly in subsurface layers, are not well presented in the control case, as discussed below.
3.2 Change in the water mass structure in the Okhotsk
Sea
Figure 7 shows the results in a vertical section crossing the Okhotsk Sea to the Kuril Straits. At the Kuril
Straits, fronts are produced in the mixing case in a fashion that is roughly consistent with observations. In the
Okhotsk Sea, both salinity and potential temperature in
the mixing case decrease in the intermediate and deep
layers as compared with the control case, and approach
the observed distributions. The maximum decrease exceeds 0.2 psu for salinity and 2°C for potential temperature in the intermediate layer. The latter leads to a reproduction of the observed weakening of the temperature
maximum (the mesothermal water) in the Okhotsk Sea
from the subarctic gyre (e.g., Kitani, 1973).
Addition of the tidal mixing also results in the lowering of PV in the intermediate layer and a much more
realistic OSMW is thereby formed. As far as we are aware,
this is the first successful reproduction of the OSMW by
an OGCM.
To clarify the reason for the improvement in the simulation quality of the OSMW, we first examine the production of sea ice, since the DSW (the source of the
OSMW) is thought to be formed by brine rejection. Bothmaximum volume and area of sea ice, however, are almost the same for the mixing and control cases. This isbecause the production rate and distribution of sea ice
418

T. Nakamura et al.

are mainly determined by heat and momentum fluxes atthe sea surface, which are almost the same for the twocases. The amount of brine rejection is thus almost thesame in both cases, although the OSMW is produced only
when the tidal mixing is included.
The seasonal change in the θ-S diagram in the northern part of the Okhotsk Sea is examined next (Fig. 8). In
the control case, the effect of winter cooling reaches atmost the 26.5σ θ density layer, which means that subduction does not extend down to the intermediate layer. Thus,
as seen in Fig. 7, the intermediate water in the Okhotsk
Sea does not differ significantly from that in the North
Pacific.
In the mixing case, diapycnally mixed water appears
from the surface to the intermediate layer in summer, witha signal that is most visible in 26.5~26.8σθ density layers. Such mixed water originates in the Kuril Straits andis transported northward by the large-scale cyclonic circulation in the Okhotsk Sea (Fig. 6). Since the verticalmixing at the Kurils induces significant upward salt flux
from the saltier lower layer, the water influenced by thetidal mixing is more saline than the water that is not affected by the tidal mixing (or that in the control case).
The cooling of this more saline water in winter (together with brine rejection) enables the subduction process to involve water with density of around 26.8σ θ, andthus the DSW density is reproduced in a much more realistic fashion. The newly formed DSW is transported to
the Kuril Basin by the southward East Sakhalin Current(Fig. 6). Subsequent isopycnal mixing of the DSW withthe surroundings cools an intermediate water that does
not outcrop and leads to the formation of a more realistic
OSMW. The subducted water is carried further downward

5
4

4
3

3

2

2

34.0
34.6
34.4

34.6
34.4

34.2

34.2
34.4

34.4

34.6

34.6

Fig. 9. Vertical section along 165°E of (upper) potential temperature and (lower) salinity for (left) the control and (right) mixing
cases.

by the tidal mixing and, as a result, the rather flat θ -S
curve unique to the Okhotsk Sea water is reproduced.
The tidal mixing therefore acts on the water mass
structure in the Okhotsk Sea through two important processes. The first of these involves a preconditioning of the
DSW production, in which salt flux by tidal mixing fromthe intermediate layer to the surface layer enhances the
subduction associated with sea ice formation in winter.
This is important in the formation of the intermediatewater, the density of which the direct subduction canreach. The other process involves the direct modificationof water by diapycnal mixing (after subduction) and isdominant in the water mass modification both in the Kuril
Straits and in the deep layer to which the direct subduction does not extend.
In addition, the low PV water from the Kuril Straits
modifies the circulation in the Okhotsk Sea (Fig. 6). For
example, a westward flow in the eastern part of the Kuril
Basin (circled in Fig. 6(a)) almost vanishes in the mixing
case, while the anticyclonic circulation in the Kuril Basin (circled in Fig. 6(b)) is enhanced. These features in
the mixing case are qualitatively consistent with the observed temperature-salinity distributions.
3.3 Thermohaline adjustment of the intermediate layer
in the North PacificTidal mixing in the Kuril Straits also leads to enhancement of the ventilation taking place in the interme-

diate water of the North Pacific. Figure 9 shows potential
temperature and salinity distributions at 165°E from thesubarctic to subtropic gyres. The salinity change is largest on the 27.1σ θ density surface and has a maximum
decrease of over 0.15 psu. This freshening largely originates from the more realistic production of the OSMW.
On the other hand, a significant freshening is also created in deeper layers (say, 1000~1500 m), which is due
to direct tidal mixing. Such changes occur in bothsubarctic and subtropical gyres and lead to a better reproduction of the NPIW. In addition, since these freshening processes mainly occur in the lower part of the NPIW,
where the direct ventilation in the North Pacific does notreach (except in the Japan Sea), the density of the salinity minimum increases by 0.3σ θ. In the subarctic gyre,potential temperature in the mixing case decreases by up
to 1°C and a more realistic profile is reproduced in thelower part (below 400 m) of the mesothermal water ascompared with the climatology shown by Ueno and
Yasuda (2000).
One reason for the improved quality of the simulation is the change in the properties of the water supplied
to the North Pacific. The salinity difference between the
mixing and the control case increases steadily in the intermediate and deep layers on the Pacific side of the
Bussol Strait after the addition of the tidal mixing (Fig.
10). A similar change is also seen on density surfaces since
this is accompanied by a temperature decrease. The fresh-

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean419

(a) without tidal mixing

0.0
-0.05

0.0

-0.10

-0.05

-0.10

-0

.1

5

0
.1

-0

-0

.10

-0.05

Fig. 10. Difference of salinity between the control and mixing
cases on the Pacific side of the Bussol Strait after the addition of the vertical mixing.

ening is associated with the downward fresh water flux
produced both by vertical mixing in the Kuril Straits andby the enhancement of the direct ventilation in the
Okhotsk Sea. In addition, there is a dynamical forcing,which could change the pathways of the mixed water fromthe Kurils and may thus modify the water mass structure.
The flux of low PV water is generated both by the direct
effect of the vertical mixing given at the Kuril Straits andby the improved OSMW. Figure 11 compares the temporal evolution of density structures in the two cases on the
Pacific side of the Bussol Strait after the addition of thetidal mixing. The main response is the significant change
in the thickness of the intermediate layer, by which thesecond baroclinic mode waves are forced in addition to
the first mode ones. In addition, the seasonal variation in
the thickness is strong, with a maximum in spring (thePV is minimum). Such behavior is a very prominent feature of the Oyashio (e.g., Kono and Kawasaki, 1997).
The required dynamical adjustment to the PV flux
arriving at the western boundary takes place through theagency of Kelvin and Rossby waves. Figure 12 shows
the temporal evolution of the difference in velocity on
the 27.1σθ density surface after the addition of the tidal
mixing. Initially, a signal travels rapidly along the coast
against the Kuroshio Current, and is split into two components, one propagating along the equator and the otherpassing through the Indonesian Archipelago. Thisanomaly consists primarily of Kelvin waves. Another typeof anomaly then slowly moves eastward around theboundary of the subtropic and subarctic gyres. This isassociated with the second mode Rossby waves (to beprecise, the higher modes are included as well). Propagation of the higher mode Rossby waves is affected by themean flow field, so that the waves are forced to move
eastward in such a strong eastward flow region. This feature differs from the classical thermohaline adjustment
theory of the deep layer (e.g., Kawase, 1987; Goodman,
420

T. Nakamura et al.

(b) with tidal mixing

Fig. 11. Temporal evolution of density for (a) the control caseand (b) the mixing case on the Pacific side of Bussol Strait
after the addition of the vertical mixing.

2001), which includes the role of equatorial Kelvin waves
alone in the eastward spreading.
The propagation of both types of waves significantly
modifies the circulation of the North Pacific intermediate layer. Firstly, the Kelvin waves affect the overturning
conveyed through the western boundary region. In particular, water mass exchange between the subarctic andsubtropical gyres is enhanced as the Oyashio is strengthened and extends further south than the zero-Sverdruptransport line (enclosed in Fig. 6). These results suggestthat the reproduction of a realistic Oyashio requires tidal
mixing at the Kuril Straits or at least a realistic OSMW.
Secondly, the second mode Rossby waves modify the circulation (and thus overturning) in the interior region.
Eventually, these processes lead to enhancement of themeridional overturning by 3 Sv at the maximum (Fig. 13).
Accordingly, the effect of tidal mixing in the Kuril Straitsis thought to exert a significant influence on the North
Pacific circulation, mainly through the strengthening ofboth the shallow overturning by enhanced subduction in
the Okhotsk Sea and the deep overturning by diapycnaltransport of the tidally mixed water. The dynamical responses of the North Pacific, in turn, affect the pathwaysof the freshened water supplied from the Kuril Straits.

Kelvin wave

Fig. 13. Difference of the meridional overturning stream function in the North Pacific between the control and mixing
cases. The values are averaged annually.

eastward
Rossby wave
(2nd mode)

Fig. 12. Temporal evolution of velocity difference between
the control and mixing cases on the density layer of 27.1σ θ.

4. Summary and Discussion
Baroclinic processes generated by the K1 tidal flowin the Kuril Straits have been investigated numericallyusing a 3-D nonhydrostatic model. The results revealed
the generation of large-amplitude unsteady lee waves
around sill tops all along the Kuril Island Chain. Thesewaves cause intense diapycnal mixing over the shallow
topographic features and near the bottom and eventuallylead to a maximum diapycnal diffusivity of over 10 3cm2s–1. Vigorous mixing in shallow regions produces low
PV water. Accordingly, PV fronts are formed along theisland chain, which sustain instabilities and in turn release baroclinic eddies with low core PV values. The
amount of eddy-driven offshore transport away from theislands reaches 14 Sv, roughly half being directed to theNorth Pacific. This spreads the effect of the diapycnal
mixing that occurs in the straits over wider areas of theNorth Pacific and the Okhotsk Sea.

To investigate the effect of such tidal processes on
the Okhotsk Sea and North Pacific, we next performed
numerical experiments with and without tidal mixing at
the Kuril Straits using an ocean general circulation model.
The results show that tidal mixing enables us to reproduce a realistic Okhotsk Sea Mode Water through the combined effects of winter cooling and saline flux penetration into the upper layer, which acts to deepen the sinking of surface water in the Okhotsk Sea. In addition, thewater mass modification by tidal mixing in the Kuril
Straits leads to indirect ventilation of deeper layers, assuggested by Talley (1993). As a result of these effects,
the intermediate water supplied to the North Pacific becomes colder and fresher. Moreover, its PV values arelowered, which results in a strengthening of the meridional overturning in the North Pacific by up to 3 Sv. Thischange in the circulation pattern is caused by the dynamical adjustment effected by both the 2nd mode Kelvin andRossby waves in addition to the first mode. The Kelvinwaves are not considered in the ventilated thermocline
theory, while the effect of the eastward Rossby waves is
not included in the classical treatment of thermohaline
adjustment of the deep layer. These modifications eventually lead to the formation of a more realistic North Pacific Intermediate Water (NPIW) for the case including
tidal mixing as compared with the case without tidal mixing. In particular, the density of the corresponding salinity minimum and salinity values of the water below becomes more realistic than in the simulations using thestate-of-the-art OGCMs.
Although these results have revealed the importanceof the tidal mixing process in the ventilation of the North
Pacific, further studies aimed at a quantitative understanding are required to clarify water transformation and transport processes around the Kuril Straits when feedback
processes from wind-driven and thermohaline circulation
to tidal process are included. In addition, a precise estimate of diapycnal mixing must await future intensive
observations and/or improvements of the mixing

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean421parameterization. The state-of-the-art scalings, such asthat proposed by Gregg (1989), are not very suitable in aregion where highly energetic wave processes occur, suchas continental shelves. Nevertheless its estimate is thought
to be much better than estimates calculated on the basisof the Fickian formulation (i.e., Kz = ρ ′ w ′ /(∂ ρ /∂z) where( ) and ′ denote a temporal average over one tidal period
and its deviation, respectively). The Fickian-based estimation often adopted in current OGCMs tends to provide
larger values of diffusivity than expected (in this case it
yields values nearly one order of magnitude greater thanthe present estimate). Future studies are also necessary
to understand the ventilation process of the North Pacific.
For example, observations gathered during the Subarctic
Gyre Experiment (SAGE) project revealed that the transport of subarctic water into the intermediate layer of theMixed Water Region is often associated with small-scaleeddies, the effects of which are not sufficiently incorporated in this impact experiment. Consideration of thesefactors is beyond the scope of the present study.
Acknowledgements
We wish to acknowledge Dr. J. P. Matthews for his
critical reading and Prof. S. Saitoh (Hokkaido University, Japan) for the kind offer of NOAA 12 AVHRR imagery. We also thank anonymous reviewers for their useful comments. The authors were supported by the
Subarctic Gyre Experiment (SAGE) project and by theCategory 7 of MEXT RR2002 Project for Sustainable
Coexistence of Human, Nature and the Earth. T.N. is also
supported by the JSPS Research Fellowships for Young
Scientists. Numerical Calculations were done on the
VPP800 at the Data Processing Center of Kyoto University.
References
Alfultis, M. A. and S. Martin (1987): Satellite passive microwave studies of the Sea of Okhotsk Sea ice cover and its
relation to oceanic processes. J. Geophys. Res., 92, 13,013–
13,028.
Aramaki, T., S. Watanabe, T. Kuji and M. Wakatsuchi (2001):The Okhotsk-Pacific seawater exchange in the viewpointof vertical profiles of radiocarbon around the Bussol’ Strait.
Geophys. Res. Lett., 28, 3971–3974.
Awaji, T., S. Masuda, Y. Ishikawa, N. Sugiura, T. Toyoda and
T. Nakamura (2003): State estimation of the North Pacific
Ocean by a four-dimensional variational data assimilation.
J. Oceanogr., 59, 931–943.
Favorite, F., A. J. Dodimead and K. Nasu (1976): Oceanography of the Subarctic Pacific region, 1960–1971. Bull. Int.
North Pac. Fish. Comm., 33, 187 pp.
Fine, R. A., K. A. Maillet, K. F. Sullivan and D. Willey (2001):
Circulation and ventilation flux of the Pacific Ocean. J.
Geophsy. Res., 106, 22,159–22,178.
Freeland, H. J., A. S. Bychkov, F. Whitney, C. Taylor, C. S.
Wong and G. I. Yurasov (1998): WOCE section P1W in the422

T. Nakamura et al.

Sea of Okhotsk 1. Oceanographic data description. J.
Geophsy. Res., 103, 15,613–15,623.
Gent, P. R. and J. C. McWilliams (1990): Isopycnal mixing inocean circulation models. J. Phys. Oceanogr., 20, 150–155.
Gladyshev, S. V. (1995): Fronts in the Kuril Island region. Oceanology, English translation, 34, 452–459.
Gladyshev, S., S. Martin, S. Riser and A. Figurkin (2000): Dense
water production on the northern Okhotsk shelves: Comparison of ship-based spring–summer observations for 1996
and 1997 with satellite observations. J. Geophys. Res., 105,
26,281–26,299.
Goodman, P. J. (2001): Thermohaline adjustment and advection
in an OGCM. J. Phys. Oceanogr., 31, 1477–1497.
Gregg, M. C. (1989): Scaling turbulent dissipation in thethermocline. J. Geophys. Res., 94, 9686–9698.
Griffies, S. M., A. Gnanadesikan, R. C. Pacanowski, V. D.
Larichev, J. K. Dukowicz and R. D. Smith (1998): Isoneutral
diffusion in a z-coordinate ocean model. J. Phys. Oceanogr.,
28, 805–830.
Hasumi, H. and N. Suginohara (1999): Sensitivity of a global
ocean general circulation model to tracer advection schemes.
J. Phys. Oceanogr., 29, 2730–2740.
Huang, R. X. and S. Russell (1994): Ventilation of the Subtropical North Pacific. J. Phys. Oceanogr., 24, 2589–2605.
Ikeda, M. (1989a): A coupled ice-ocean mixed layer model ofthe marginal ice zone to wind forcing. J. Geophys. Res.,94, 9699–9709.
Ikeda, M. (1989b): Snow cover detected by diurnal warming ofsea ice/snow surface off Labrador in NOAA imagery. IEEE
Trans. Geosci. Remote Sens., 27, 552–560.
Ishikawa, Y., T. Awaji, T. Toyoda and N. Komori (2002): Construction of a data assimilation system for ocean generalcirculations—Determination of weight parameters for theadjoint method—. Proc. of Int. Simposium “En route to
GODAE”, B09-21, Biarritz, France.
Itoh, M., Y. Kawasaki, K. I. Ohshima, G. Mizuta, Y. Fukamachi
and M. Wakatsuchi (2002): Anticyclonic eddies in the
Okhotsk Sea. Eos. Trans. AGU, 83(4), Ocean Sciences Meet.
Suppl., Abstract, OS11C-53.
Kawasaki, Y. (1996): The origin of the North Pacific Intermediate Water—from the observations in the Okhotsk Sea—.
Kaiyo Monthly, 28, 545–552 (in Japanese).
Kawasaki, Y. and T. Kono (1994): Distribution and transport ofSubarctic Waters around the middle of Kuril Islands. Umi
to Sora, 70, 71–84 (in Japanese with English abstract andfigure captions).
Kawase, M. (1987): Establishment of deep ocean circulation
driven by deep-water production. J. Phys. Oceanogr., 17,2294–2317.
Kitani, K. (1973): An oceanographic study of the Okhotsk Sea—
particularly in regard to cold waters. Bulletin of the Far
Seas Fisheries Research Laboratory, 9, 45–77.
Kono, T. (1998): Formation of the salinity minimum in the
Mixed Water Region between the Oyashio and Kuroshio
Fronts. Deep-Sea Res. I, 45, 2035–2057.
Kono, T. and Y. Kawasaki (1997): Result of CTD and mooring
observations southeast of Hokkaido. 2, Annual variationsof water mass structure and salt flux of the Oyashio. Bull.
Hokkaido Natl. Fish. Res. Inst., 61, 83–95.

Levitus, S. and T. P. Boyer (1994): World Ocean Atlas 1994,
Vol. 4, Temperature. NOAA Atlas NESDIS 4, U.S. Govt.
Printing Office, Washington, D.C., 117 pp.
Levitus, S., R. Burgett and T. P. Boyer (1994): World Ocean
Atlas 1994, Vol. 3, Salinity. NOAA Atlas NESDIS 3, U.S.
Govt. Printing Office, Washington, D.C., 99 pp.
Martin, S., R. Drucker and K. Yamashita (1998): The production of ice and dense shelf water in the Okhotsk Sea
polynyas. J. Geophys. Res., 103, 27,771–27,782.
Miura T., T. Suga and K. Hanawa (2002): Winter mixed layer
and formation of dichothermal water in the Bering Sea. J.
Oceanogr., 58, 815–823.
Nakamura, T. and T. Awaji (2001): A growth mechanism for
topographic internal waves generated by an oscillatory flow.
J. Phys. Oceanogr., 31, 2511–2524.
Nakamura, T. and T. Awaji (2003): Tidally-induced diapycnal
mixing in the Kuril Straits and its roles in water transformation and transport: A three dimensional nonhydrostatic
model experiment. J. Geophys. Res. (submitted).
Nakamura, T., T. Awaji, T. Hatayama, K. Akitomo and T.
Takizawa (2000a): Tidal exchange through the Kuril Straits.
J. Phys. Oceanogr., 30, 1622–1644.
Nakamura, T., T. Awaji, T. Hatayama, K. Akitomo, T. Takizawa,
T. Kono, Y. Kawasaki and M. Fukasawa (2000b): The generation of large-amplitude unsteady lee waves by subinertial
K1 tidal flow: a possible vertical mixing mechanism in the
Kuril Straits. J. Phys. Oceanogr., 30, 1601–1621.
Noh, Y. and H. J. Kim (1999): Simulations of temperature andturbulence structure of the oceanic boundary layer with the
improved near-surface process. J. Geophys. Res., 104,
15,621–15,634.
Ohshima, K. I., M. Wakatsuchi, Y. Fukamachi and G. Mizuta(2002): Near-surface circulation and tidal currents of the
Okhotsk Sea observed with the satellite-tracked drifters. J.
Geophys. Res., 107, art. no. 3195.
Ohtani, K. (1989): The role of the Sea of Okhotsk on the formation of the Oyashio water. Umi to Sora, 65, 63–83 (inJapanese with English abstract and figure captions).
Pedlosky, J. (1987): Geophysical Fluid Dynamics. 2nd ed.,Springer-Verlag, New York, 710 pp.
Pedlosky, J. (1996): Ocean Circulation Theory. Springer-Verlag,
Berlin, Heidelberg, 453 pp.
Reid, J. L. (1973): North Pacific Ocean Waters in Winter. The
Johns Hopkins Press, Baltimore, 85 pp.
Röske, F. (2001): An atlas of surface fluxes based on the
ECMWF re-analysis—a climatological dataset of force global ocean general circulation models. Max-Planck-Institut
f u̇ Meteorologie, 323, 26 pp.
Talley, L. D. (1991): An Okhotsk Sea water anomaly: implications for ventilation in the North Pacific. Deep-Sea Res.,38, S171–S190.
Talley, L. D. (1993): Distribution and formation of North Pacific Intermediate Water. J. Phys. Oceanogr., 23, 517–537.
Talley, L. D. and J. Y. Yun (2001): The role of cabbeling anddouble diffusion in setting the density of the North Pacificintermediate water salinity minimum. J. Phys. Oceanogr.,
31, 1538–1549.
Talley, L. D., Y. Nagata, M. Fujimura, T. Iwao, T. Kono, D.
Inagake, M. Hirai and K. Okuda (1995): North Pacific Intermediate Water in the Kuroshio/Oyashio Mixed Water
Region. J. Phys. Oceanogr., 25, 475–501.
Tanaka, K. and K. Akitomo (2001): Baroclinic instability ofdensity current along a sloping bottom and the associatedtransport process. J. Geophys. Res., 106, 2621–2638.
Thomson, R. E., P. H. LeBlond and A. B. Rabinovich (1997):
Oceanic odyssey of a satellite-tracked drifter: North Pacificvariability delineated by a single drifter trajectory. J.
Oceanogr., 53, 81–87.
Tsunogai, S., T. Ono and S. Watanabe (1993): Increase in total
carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon. J.
Oceanogr., 49, 305–315.
Ueno, H. and I. Yasuda (2000): Distribution and formation of
the mesothermal structure (temperature inversions) in the
North Pacific subarctic region. J. Geophys. Res., 105,
16,885–16897.
Van Scoy, K. A., D. B. Olson and R. A. Fine (1991): Ventilation of North Pacific Intermediate Waters: The role of the
Alaskan Gyre. J. Geophys. Res., 96, 16,801–16,810.
Warner, M. J., J. L. Bullister, D. P. Wisegraver, R. H. Gammon
and R. F. Weiss (1996): Basin-wide distributions ofchlorofluorocarbons CFC-11 and CFC-12 in the North Pacific. J. Geophys. Res., 101, 20,525–20,542.
Watanabe, T. and M. Wakatsuchi (1998): Formation of 26.8σθwater in the Kuril Basin of the Sea of Okhotsk as a possible
origin of North Pacific Intermediate Water. J. Geophys. Res.,
103, 2849–2865.
Watanabe, Y., K. Harada and K. Ishikawa (1994):
Chlorofluorocarbons in the central North Pacific and southward spreading time of North Pacific intermediate water. J.
Geophys. Res., 99, 25,195–25,213.
Wong, C. S., R. J. Matear, H. J. Freeland, F. A. Whitney and A.
S. Bychkov (1998): WOCE line P1W in the Sea of Okhotsk
2. CFCs and the formation rate of intermediate water. J.
Geophys. Res., 103, 15,625–15,642.
Yamanaka, Y. and E. Tajika (1996): The role of the verticalfluxes of particulate organic matter and calcite in the oceanic carbon cycle: Studies using biogeochemical generalcirculation model. Global Biogeochem. Cycles, 10, 361–
382.
Yasuda, I. (1997): The origin of the North Pacific intermediatewater. J. Geophys. Res., 102, 893–910.
Yasuoka, T. (1968): Hydrography in the Okhotsk Sea-(2). The
Oceanographical Magazine, 20, 55–63.
You, Y., N. Suginohara, M. Fukasawa, I. Yasuda, I. Kaneko, H.
Yoritaka and M. Kawamiya (2000): Roles of the Okhotsk
Sea and Gulf of Alaska in forming the North Pacific Intermediate Water. J. Geophys. Res., 105, 3253–3280.

Tidal Mixing in the Kuril Straits and Its Impact on Ventilation in the North Pacific Ocean423