JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, C04003, doi:10.1029/2005JC003142, 2006

Effects of tidal mixing at the Kuril Straits on North
Pacific ventilation: Adjustment of the intermediate
layer revealed from numerical experiments
Tomohiro Nakamura,1,2 Takahiro Toyoda,1,3 Yoichi Ishikawa,4 and Toshiyuki Awaji1,5Received 4 July 2005; revised 30 November 2005; accepted 27 December 2005; published 8 April 2006.

[1] The effects of tidal mixing at the Kuril Straits on the North Pacific intermediate layer

are investigated using an ocean general circulation model. A comparison of numericalexperiments with and without a tidal mixing effect suggests that tidal mixing at the Kuril
Straits enhances the ventilation of the North Pacific intermediate layer. The enhanced
ventilation results in both freshening and cooling down to 27.6 sq. In particular, thesimulated North Pacific Intermediate Water (NPIW) becomes fresher and denser (by0.3 psu and 0.1 sq at the maximum) and hence more realistic. The enhanced ventilation
is caused both through the supply of the ventilated water from the Kuril Straits,
which subsequently spreads along the subarctic and subtropical gyres, and through a
modification of the circulation there. The ventilation of the supplied water in turnoriginates from a combination of tidally enhanced convection in the Okhotsk Sea anddownward diffusion at the Kuril Straits as discussed in a previous paper. The former
affects the upper part of the NPIW, while the latter is dominant in the denser layers.
The circulation is modified through a dynamical adjustment to the mass input into anintermediate layer that is produced by the convergence of diapycnal transport due tothe tidally enhanced convection and diffusion. The dynamical adjustment is conducted
mainly through Kelvin waves which have the ability to induce intergyre flow along
the western boundary and also by eastward moving long Rossby waves. The lattercan be present under the influence of the wind-driven gyres for the second and higher
baroclinic modes and act to spread information into the interior directly fromthe western boundary. As a direct consequence of this adjustment, transport of theventilated water from the Kuril Straits to the subtropical gyre is enhanced by theintergyre flow along the western boundary, which appears as the southward intrusionof the Oyashio Current. The transported water leaves the coast to encircle the interior,and returns to the western boundary, eventually flowing into the equatorial region.
Such equatorward transport associated with mass convergence in the intermediate layer
is compensated by transport toward the Kuril Straits in the shallower and deeper layers,
thereby enhancing both shallow and deep meridional overturning cells by 2–3 Sv.
The above dynamical adjustment is thus central to our understanding of the ventilationof the intermediate layer and provides a basis for the analytical model developed in an
accompanying paper.
Citation: Nakamura, T., T. Toyoda, Y. Ishikawa, and T. Awaji (2006), Effects of tidal mixing at the Kuril Straits on North Pacificventilation: Adjustment of the intermediate layer revealed from numerical experiments, J. Geophys. Res., 111, C04003,
doi:10.1029/2005JC003142.

1. Introduction
1

Frontier Research Center for Global Change, JAMSTEC, Yokohama,Japan.
2
Now at Institute of Low Temperature Science, Hokkaido University,
Sapporo, Japan.
3Formerly at Department of Geophysics, Graduate School of Science,
Kyoto University, Kyoto, Japan.
4
Department of Geophysics, Graduate School of Science, Kyoto
University, Kyoto, Japan.
5
Also at Department of Geophysics, Graduate School of Science, Kyoto
University, Kyoto, Japan.
Copyright 2006 by the American Geophysical Union.
0148-0227/06/2005JC003142$09.00[2] The ventilation originating in and around the subarctic Pacific Ocean carries fresh, subarctic surface water into
the intermediate layer from the subarctic to the subtropicalNorth Pacific. One prominent indication of this ventilation/
freshening is the North Pacific Intermediate Water (NPIW),
which is characterized by a well defined salinity minimum
around 26.8 sq [e.g., Sverdrup et al., 1942]. The NPIW
spreads over most of the subtropical gyre, and intrudes eveninto the tropics [e.g., Talley, 1993]. Since the above
ventilation process involves the storage of large amounts

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Figure 1. Schematic view of the two roles of tidal mixing
in the Kuril Straits in ventilation of the Okhotsk Sea.
of heat, fresh water, and greenhouse gases such as CO2 in
the ocean [e.g., Yamanaka and Tajika, 1996], clarification of
the intermediate water ventilation process is important inunderstanding, modeling, and predicting the potential
response of the North Pacific intermediate layer to longterm variability.
[3] The main source of low-salinity water in the NPIW
is considered to be the Okhotsk Sea [e.g., Talley, 1991;
Yasuda et al., 1996], although the low salinity sources at
the top of the NPIW (say, 26.7– 26.8 sq) are also thought
to lie in the Alaskan Gyre [e.g., Van Scoy et al., 1991; You etal., 2000] and in the subarctic, winter mixed layer [Talley,
1993; Talley et al., 1995; Talley and Yun, 2001]. In the
Okhotsk Sea, convection associated with sea ice formationin winter produces Dense Shelf Water (DSW), whose
maximum density reaches 27.0 sq [Kitani, 1973; Alfultis
and Martin, 1987; Martin et al., 1998; Gladyshev et al.,2000]. The DSW transforms Okhotsk Sea intermediatewater, which subsequently enters the Pacific, thereby
modifying the subarctic Pacific water [e.g., Yasuda, 1997;
Kono and Kawasaki, 1997a; Watanabe and Wakatsuchi,
1998]. While the main part of the modified subarctic
water remains in the subarctic gyre, a portion intrudes into
the subtropical gyre mainly near the western boundary,leading to the formation of new NPIW [e.g., Yasuda etal., 1996].
[4] While this DSW is the densest water subducted inand around the North Pacific [Talley, 1991], the ventilation occurring in the North Pacific reaches much denser
layers (27.4  27.6 sq) as indicated by Chlorofluorocarbons (CFCs) observations [Watanabe et al., 1994; Warner etal., 1996; Fine et al., 2001] (convection in the Japan Sea is
not considered here since this sea is separated by very
shallow straits). What processes ventilate the lower part

C04003(say, 27.0  27.6 sq) of the North Pacific intermediate
layer?
[5] The cabbeling effect is also unlikely because thedensity increase it causes during mixing of subarctic andsubtropical waters is less than 0.02 sq below the 27.0 sq
density layer [Talley and Yun, 2001]. In addition, thenecessary conditions for double diffusion are not met below
the density layer of 26.8 sq in the North Pacific or 26.8 
27.2 sq in the Okhotsk Sea (where the NPIW salinity
minimum in the subtropics or the temperature maximum
layer in the subarctic is present), and hence mechanicalmixing is more likely. This expectation is consistent withthe observed nutrients distributions which suggest theoccurrence of enhanced vertical mixing somewhere in the
North Pacific so that nutrient rich water of the deep ocean isbrought up into the intermediate layer [Sarmiento et al.,2004].
[6] Among mechanical mixing mechanisms, one likely
candidate is vertical mixing at the Kuril Straits [e.g., Talley,
1991], that is induced by tides [e.g., Nakamura et al.,2000a]. The Kuril Straits are located between the North
Pacific and the Okhotsk Sea. The ventilated water in thelower intermediate layer also originates around the Okhotsk
Sea, according to the observed distributions of CFCs
[Warner et al., 1996] and numerical simulations [Yamanaka
et al., 1998a, 1998b]. In particular, the Kuril Straits have thelowest salinity and highest oxygen on the deep density
layers (27.0  27.6 sq), indicating that ventilation reachingthese deep layers occurs locally in the Kuril Straits [e.g.,
Yasuoka, 1968; Kitani, 1973; Kawasaki and Kono, 1994;
Freeland et al., 1998; Aramaki et al., 2001]. This isprobably due to downward diffusion by locally enhanced
vertical mixing, since the outcropping of such dense
isopycnal surfaces (^27.1 sq) have never been observedin either the Okhotsk Sea or the open North Pacific(including the Bering Sea) [e.g., Kitani, 1973; Talley, 1991].
[7] The enhanced vertical mixing is in fact made possibleby the subinertial, diurnal tides [Nakamura et al., 2000a,
2000b; Nakamura and Awaji, 2001, 2004]. This is because
the tidal flow is considerably intensified around the Kuril
Straits (2 m s1) due to the effective amplification of
topographically trapped waves generated by the subinertial
diurnal tides, in addition to the effect of topographic
contraction, so that internal lee waves of large amplitudes(100 m) are thought to be generated over topographic
features and break, eventually leading to the enhanced
vertical mixing. The mixing occurs from the surface tothe sill bottom and all along the Kuril Island Chain, and isparticularly intense over shallow sills or banks where themaximum diffusivity may exceed 1000 cm2 s1.
[8] However, although the above studies suggest theimportance of tidally induced vertical mixing (hereafter,
tidal mixing) at the Kuril Straits, its effect is still not wellunderstood. For example, the recent results of numericalexperiments performed by Nakamura et al. [2006] havesuggested that tidal mixing at the Kuril Straits is fundamental for the formation of the fresh and cold Okhotsk
Sea water even at the NPIW salinity minimum density
layer. According to their study, tidal mixing exerts its
influence through two processes, as shown schematicallyin Figure 1. First, tidal mixing directly modifies water
properties in the Kuril Straits. This modification process

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C04003Figure 2. (a) Bottom topography around the Okhotsk Sea. Contours are drawn at 200, 3000, and 6000 m
depth. (b) Model topography around the Okhotsk Sea. Thick white lines indicate the region of enhanced
vertical diffusion in the Tmix case, while thick black lines indicate land boundaries.
involves downward diffusion, which enhances the ventilation and freshening of the intermediate and deeper layers(the freshening occurs because salinity generally increases
with depth in the subarctic Pacific and the Okhotsk Sea).
Second, tidal mixing in the Kuril Straits preconditions
the DSW production in the northern Okhotsk Sea, as
follows. Tidal mixing makes water in the upper layer
saltier because of the salinity stratification. This saltier
water is carried northward to the DSW production region
by the wind-driven basin-scale cyclonic circulation in the
Okhotsk Sea. Accordingly, the DSW produced in amixed environment is relatively saltier and hence denser
and there is an increase in production rate, when DSW isdefined by density and potential temperature in the traditional fashion. We call this second process ‘‘tidally enhanced
convection’’. The tidally enhanced convection strengthens
the ventilation and freshening of the intermediate layer(the latter occurs since DSW is so cold that it is less saline
when compared on an isopycnal surface). The above
results imply that the Okhotsk Sea water that is freshened
and ventilated through the above two processes flows intoand spreads over the North Pacific. In parallel with this,tidal mixing at the Kuril Straits also affects density layers
around the NPIW salinity minimum layer as well as thelower intermediate layer. The tidal mixing should thus
have a greater impact on the North Pacific than previously
thought.
[9] From a dynamical point of view, both the diffusionand enhanced convection induced by tidal mixing willchange potential vorticity (PV) supplied to the North
Pacific. Such a change in PV is expected to travel
eastward roughly along the Sverdrup flow lines andthereby affect the circulation en route, on the basis of
the ventilated thermocline theory [e.g., Pedlosky, 1996].
The investigation of such dynamical adjustments andassociated changes in circulation will benefit our under-

standing of the North Pacific intermediate layer circulation. In addition, two important factors need to beconsidered. First, the PV input is given from the west(of the subpolar gyre), instead of the east or the interior (ofthe subtropical gyre), where responses have been extensively investigated [e.g., Luyten and Stommel, 1986; Liu,
1993; 1999; Sirven and Frankignoul, 2000; Stephens etal., 2001; Dewar and Huang, 2001; Samelson, 1998].
Secondly, the convection in the Okhotsk Sea does notreach great depths in contrast to the deep convection in the
North Atlantic, where the response can be understood on
the basis of the work of Kawase [1987]. Thus an investigation of the adjustment mechanism is of theoreticalinterest.
[10] In this study, we therefore investigate the effects of
tidally enhanced vertical mixing at the Kuril Straits on theventilation of the North Pacific intermediate layer. To thisend, numerical experiments with and without tidally enhanced vertical mixing at the Kuril Straits are conducted,using an ocean general circulation model (OGCM). On thebasis of the results obtained here, an analytical model of theintermediate layer ventilated from the west is developed inan accompanying paper (T. Nakamura et al., manuscript in
preparation, 2006) (hereafter Part 2).
[11] The rest of the paper is organized as follows. The
numerical model and experimental design are described in
section 2. By comparing the cases with and without a tidal
mixing effect, the effects on water mass structures are
examined in section 3, where we focus on the differencesin salinity and PV distributions in the intermediate layer andon the temperature profile of the mesothermal water in thewestern subarctic. Dynamical adjustments due to tidal
mixing at the Kuril Straits in the intermediate layer are
analyzed in section 4, and the resulting differences in theintermediate layer circulation and meridional overturning
are shown in section 5. The sensitivity of the results to tidal-

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Table 1. List of Experiments
caseincrease in Kz at the Kuril Straits
Main Experiments

Ctrl
Tmix (Kz200)

none
200 cm2s1
Sensitivity Experiments

Kz10
Kz20
Kz30
Kz50
Kz100
Kz200 (Tmix)
Kz300
Kz500
Kz1000noDD

10 cm2s1
20 cm2s1
30 cm2s1
50 cm2s1
100 cm2s1
200 cm2s1
300 cm2s1
500 cm2s11000 cm2s1
200 cm2s1

Vertical diffusivity coefficient, Kz, is increased over the sills in the Kuril
Straits except for the Ctrl case. In the noDD case, double-diffusion
parameterization is excluded.

mixing strength is discussed in section 6, and a summaryand discussion is presented in section 7.

2. Model and Experimental Design[12] The model and experimental design are the same as
those of Nakamura et al. [2006] and are summarized in thefollowing. The model used is an OGCM developed at Kyoto
University [Toyoda et al., 2004; Nakamura et al., 2006],
which solves the primitive equations in spherical coordinates. For a realistic reproduction of the subduction process,this model adopts a third-order advection scheme for the
tracer equation (UTOPIA and QUICKEST [Hasumi and
Suginohara, 1999]), in addition to isopycnal diffusion witheddy parameterization [Redi, 1982; Gent and McWilliams,
1990] (hereafter GM90) and a turbulence closure mixed
layer scheme [Noh and Kim, 1999]. In the latter schemes,
both the isopycnal and GM90 diffusivity coefficients are set
to 107 cm2 s1, and the background vertical and diapycnal
diffusivity and viscosity coefficients are set to 0.01 cm2 s1.
To accurately simulate the convection associated with sea
ice formation in the Okhotsk Sea, a sea ice model isincorporated on the basis of the works of Ikeda [1989a,
1989b] and Ikeda et al. [1988]. This model is a Hibler[1979] two-category ice model and includes both dynamic
and thermodynamic processes (the simulated ice production and distribution in the Okhotsk Sea are basicallysimilar to those observed [Nakamura et al., 2006]). The
effect of double diffusion is also included [Merryfield etal., 1999; Schmitt, 1981; Fedorov, 1988], since it isconsidered to affect the top of the NPIW [Talley andYun, 2001], although it turns out to be insignificant in the
deeper layers. For the momentum equation, an enstrophy
conserving scheme (in addition to momentum and kinetic
energy) is incorporated [Ishizaki and Motoi, 1999] to yieldan accurate simulation of the expected circulation change.
The partial cell method is used for a better representationof bottom topography.
[13] The model domain is virtually global (75S –75N)with a horizontal resolution of 1  1. In the vertical, the34 levels used are spaced from 20m at the sea surface to400m in the abyss. The model topography is based on ahorizontal average of DBDB5 (U.S. National Geophysical

C04003

Data Center), whose original resolution is 1/12  1/12.
However, the averaged depths along the Kuril Island Chainthen become around 2000 m, much deeper than the observations. Actually, sills in the Kuril Straits are shallow(]200 m), except for two main deep straits, so that the
Kuril Island Chain effectively blocks the western boundary
current of the subarctic Pacific from entering the Okhotsk
Sea. Thus we have decreased depths in the Kuril Straits
from the averaged values, except for the two main straits(Figure 2). This modification enables us to reproduce thebasic features of the western boundary current and thecirculation in the Okhotsk Sea.
[14] 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 withthe bulk formulae [Röske, 2001; Ikeda, 1989b; da Silva etal., 1994]. The atmospheric and river runoff data used are
based on the climatological monthly mean data compiled byRöske [2001], which is in turn based on the European
Centre for Medium-Range Forecasts (ECMWF) reanalysis
data. The ice and oceanic data used in calculating thesurface fluxes are taken from simulated values, as is often
the case in ice-ocean coupled models. Note that the heat andfresh water fluxes calculated with the bulk formulae areadjusted using the flux correction method (i.e., a weakrelaxation term is added) with a relaxation timescale of
60 days, as recommended by, for example, Barnier et al.
[1995] and Weaver and Hughes [1996]. This is because
although the ECMWF reanalysis data is considered as oneof the most realistic data sets, it still has uncertainty due toboth sparse observations over the ocean and the incompatibility between atmosphere and ocean models, and thus
the simultaneous use of both bulk formulae and adequate
relaxation has been recommended. Nevertheless, the fluxes
due to bulk formulae is dominant in our model. In fact, thecomparison with similar experiments but with a relaxation
time of 30 days does not show a qualitative difference, as
will be shown in section 7.
[15] In addition, restoration of potential temperature andsalinity to those of WOA94 is applied at the northernand southern boundaries, at the exits of the Mediterranean
and Red Seas, and in layers deeper than 2000 m with thesame timescale as that of the flux correction. The restorationin the deep layers reduces the spinup time by circumventing
the spinup of the abyssal circulation.
[16] Two main experiments were conducted (Table 1),
after a 40-year spinup by which time the model ocean had
reached a state of approximate equilibrium. The first of thesewas a control experiment obtained by a further 40-year
integration (hereafter the ‘‘Ctrl’’ case). The second was anexperiment for the case with a tidal mixing effect (the‘‘Tmix’’ case). This case 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 (Figure 2), on the basis of the results ofNakamura et al. [2000b] and Nakamura and Awaji [2004].
In this Tmix case, the flux correction was not appliedaround the Kuril Straits in order to avoid artificial heatand fresh water fluxes which would arise owing to thefact that the WOA94 data does not resolve the vertically
mixed water in the Kuril Straits.

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C04003Figure 3. Meridional section of salinity at 165E in the (a) Ctrl and (b) Tmix cases. (c) The difference
between the Ctrl and Tmix cases (Tmix-Ctrl). Dotted lines in Figure 3c indicate 27.1, 27.4, and 27.6 sq
isopycnal surfaces, respectively. (d) Salinity from WOA94.

[17] The final year of the integration (i.e., the 80th year)
is used in the analysis below, except for the investigation of
the temporal evolution after the addition of tidal mixing insection 4.1. The adjustment to tidal mixing is largely
finished in the North Pacific after 40 years, although theadjustment of the temperature and salinity fields in otherbasins associated with the change in the global thermohaline circulation has not yet finished. In fact, the results of a
30-year integration after the addition of tidal mixing andthose of a 40-year integration are similar to each other (notshown).
[18] It should be noted that tides were not directly
simulated and that their effect was parameterized throughlocal increase in vertical diffusion. This parameterization
does not consider the total effect of a tidal process (or tidal
mixing), even though vertical diffusion is a major effect at
the spatial scale under consideration [e.g., Schiller et al.,
1998; Hibiya et al., 1998]. The value of vertical diffusion isincreased at all depths in the Kuril Straits as a first
approximation. The specified value (200 cm2 s1) roughly
corresponds to twice that induced by the K1 tide alone[Nakamura and Awaji, 2004]. The reason for this selectionis that the total tidal flow speed in the Kuril Straits isroughly twice as large as that of the K1 component alone,according to the tidal simulations reported by Nakamura etal. [2000a]. Also, the use of this value resulted in areasonable reproduction of water transformation in the
Okhotsk Sea [Nakamura et al., 2006]. Taking these facts
into account, the above rough estimate of diapycnal diffusivity was used for a basic investigation of the impacts of
tidal mixing on the North Pacific.

[19] In order to examine the sensitivity of the ventilationof the North Pacific intermediate layer to the strength of thetidal mixing, eight additional experiments were conducted,
in which the vertical diffusivity coefficient was varied from10 to 1000 cm2 s1 (Kz10 to Kz1000 cases, respectively;
Table 1). The results of these further experiments are
discussed in section 6.

3. Differences in Water Mass Structures[20] To see how tidal mixing at the Kuril Straits affects
ventilation of the intermediate layer in the North Pacific, wefirst compare the salinity distributions of the Ctrl and Tmix
cases. This is because freshening in these regions indicatesan influence from the surface water of the subarctic Pacificand/or its marginal seas since a freshening influx can only
come from this layer. The associated changes in temperatureand PV are also examined.
3.1. Salinity Distribution in a Meridional Section[21] Salinity distributions derived from the Ctrl and Tmix
cases and from the WOA94 data set at 165E from thesubarctic to subtropical gyres are shown in Figure 3.
Comparison of the two simulations shows that tidal mixing
at the Kuril Straits causes a significant freshening of theintermediate water, as freshened water is supplied from theKuril Straits and subsequently spreads out. This indicatesthat the addition of tidal mixing there leads to a strengthening and deepening of the ventilation in the North Pacific.
[22] On closer examination, the freshening consists of
two features. First, the salinity minimum layer extending

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mainly by the tidally induced diapycnal mixing at the Kuril
Straits.
[24] On the basis of the above difference in the driving
mechanism of downward conduction of a freshening flux,
we tentatively categorize the North Pacific intermediate
layer into upper and lower parts. The upper part is reachedby ventilation due to both convection in the northern partof the Okhotsk Sea and tidal mixing at the Kuril Straits,
and corresponds to the top and salinity-minimum layers of
the NPIW. The lower part is ventilated almost solely bytidal mixing at the Kuril Straits. This categorization isconvenient in discussing the effects of tidal mixing at the
Kuril Straits.

Figure 4. Maps of salinity difference between the Ctrl andTmix cases on (a) 27.1 sq and (b) 27.4 sq isopycnal
surfaces.
from the subarctic to subtropics, which is the feature of the
NPIW, is effectively freshened and deepened. As a result,salinity there decreases by up to 0.3 psu and its density
increases by about 0.1 sq (Figure 3c), leading to a better
reproduction of the NPIW. Secondly, the deeper layers arealso significantly freshened. This deep freshening is still
appreciable around 1500 m depth or the 27.6 sq density
layer (Figure 3c). The occurrence of freshening or
ventilation in the North Pacific down to such deep layers
is consistent with the CFCs observations [e.g., Warner et al.,1996].
[23] Such difference in freshening intensity between theshallower and deeper layers is associated with the two
freshening processes in the Okhotsk Sea described in the
introduction. The shallower, well-freshened water component concentrates on the 27.1 sq surface (Figure 3c),suggesting that it mainly originates from the tidally
enhanced convection in the Okhotsk Sea. In fact, its density
roughly corresponds to the DSW density (although density
increases somewhat owing to effects of tidal mixing andcabbeling). This DSW is produced through the tidally
enhanced convection in the Okhotsk Sea in the Tmix casebut is almost absent in the Ctrl case [Nakamura et al.,2006]. In contrast, the deep freshening extends far beyond
the layer with the DSW density, implying that it is caused

3.2. Horizontal Distributions[25] Figure 4 shows the horizontal spreading of salinity
difference between the Ctrl and Tmix cases on the 27.1 and27.4 sq isopycnal surfaces: The former corresponds to theintensely freshened, upper intermediate layer and the latterto the lower intermediate layer. On both surfaces, the largest
decrease in salinity occurs around the Kuril Straits. This fact
confirms that the origin of the freshening discussed above islocated here.
[26] On the upper intermediate layer, the freshening
spreads to the entire subarctic region and to virtually thewhole of the subtropics, and even extends to the equatorial
region near the western boundary. This distribution is in
broad agreement with the observed distribution of the
NPIW [Talley, 1993], confirming that tidal mixing at the
Kuril Straits has an important role in the formation of
the NPIW.
[27] On the lower intermediate layer, a similar pattern of
spreading is seen, but its area is reduced considerably. In
particular, the freshened region in the subtropics shrinks
toward the northwest corner of the gyre, as the area of thepool region reduces with increasing density. Such a reduction in the freshened area also occurs in the subarctic but islocated toward the southwest corner of the gyre. These
results suggest that the freshened water supplied from theKuril Straits flows into the pool regions in both subarctic
and subtropical gyres, consistent with a result of an analytical model described in Part 2.
3.3. Temperature in the Subarctic Intermediate Layer[28] In contrast to the subtropics, the intermediate layerof the subarctic Pacific is characterized by a local maximum of potential temperature in the vertical direction,
which is called the ‘‘mesothermal water’’ [e.g., Uda,
1963]. Figure 5 compares vertical profiles of potential
temperature derived from the Ctrl and Tmix cases andfrom the WOA94 data set near the center of the westernpart of the subarctic gyre, where typical mesothermal
water is found [Ueno and Yasuda, 2000].
[29] Potential temperature in the Tmix case decreases by
up to 1C relative to the Ctrl case. Such cooling leads to thereproduction of a more realistic profile of mesothermal
water, particularly below the seasonal thermocline (say,below 150 m) and is associated with the freshening described in the previous section. In fact, since sq depends
solely on salinity and potential temperature, the salinity
decrease on the isopycnal surfaces shown in Figure 4
implies a corresponding decrease in potential temperature.

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C04003enhanced transport of subarctic, lower PV water to thesubtropics. This will be described in the next sections.
[32] The changes taking place in PV distributions areusually accompanied by a modification of the circulation.
Some PV changes themselves are induced by the change incirculation. This in turn implies that the change in watermass structure can be caused by variations in not only theproperties of the source water supplied from the Kuril
Straits but also the paths through which the modified source
water is distributed. We will thus investigate the mechanismof the latter and the resulting change in circulation in thefollowing sections. As for the change in source water
properties, the reader is referred to Nakamura et al. [2006].

4. Thermohaline Adjustment in Intermediate
LayersFigure 5. Vertical profiles of potential temperature at(46N, 160E), which roughly corresponds to the center ofthe western subarctic gyre. Blue, red, and black lines arethose of the Ctrl and Tmix cases and WOA94, respectively.
For each case, four lines are drawn to show those in allseasons.

[33] To facilitate a description of the change in circulation
expected from the change in PV, we identify in advance theimportant dynamical elements responsible for the circulation change in this section. This is done by examining theinitial adjustment process to the forcing arising from theaddition of tidal mixing at the Kuril Straits.
4.1. Forcing From the Kuril Straits[34] First, we examine the vertical structure of the forcing
given to the North Pacific. Figure 7 compares the initial

In other words, the enhanced ventilation brings fresher andcolder water into the intermediate density layer.
3.4. Potential Vorticity[30] Associated with the above change in salinity andtemperature, the PV field is also modified. Maps of PV
difference on the upper and lower intermediate layers areshown in Figure 6 (27.1 and 27.4 sq isopycnal surfaces,
respectively). On the upper intermediate layer, the lower PV
water, that is produced through the tidally enhanced
convection in the Tmix case, is supplied from the Okhotsk
Sea to the southern part of the subarctic gyre and to thesubtropical gyre. The PV increase in the northern part of thesubarctic gyre is associated with the upward Ekman
pumping, which works to reduce the depth of the density
layer as it circulates in the gyre to the north. When tidal
mixing is added at the Kuril Straits, the intermediate layer
becomes thicker (i.e., lower PV). Associated with this, theupper part becomes shallower and progressively shallows
during the circulation and thus it receives more of an
influence from Ekman pumping and/or mixed layer.
[31] On the lower intermediate layer, PV values increasein the subarctic gyre and decrease in the subtropical gyreowing to the tidal mixing effect. In the former case, the PV
increase is consistent with the view that the main cause of
the ventilation of such deep layers is diapycnal mixing at the
Kuril Straits, since convection is generally accompanied by
low PV water. In fact, as will be shown in section 4.1,
vertical mixing at the Kuril Straits generally produces a net
mass flux to the shallower layers from the deeper layers inthis experiment. As a result, the deeper layers becomethinner through enhanced vertical mixing and hence PV
increases. This change spreads over the subarctic gyre,
resulting in the PV increase on this layer. On the other
hand, the PV decrease in the subtropical gyre is due to an

Figure 6. Same as Figure 4, but for potential vorticity.

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C04003Figure 7. Initial evolution of vertical profiles of (top) density (sq) and (bottom) meridional velocity at(149E, 43N), after addition of tidal mixing for the (left) Ctrl and (right) Tmix cases. The site is locatedat the main exit of the Kuril Straits on the North Pacific side. Month 1 is January and corresponds to thefirst month from the addition of tidal mixing. The contour interval is 0.2 sq for density and is 0.5 cm s1
for velocity.
evolution of vertical profiles of density (sq) and meridional
velocity in the Ctrl and Tmix cases on the Pacific side of the
Bussol Strait (the main exit of the Okhotsk Sea water,
located in the central part of the Kuril Straits) after the
addition of tidal mixing (from the first day of the 41st modelyear).
[35] In the first few months, the intermediate density layer
generally becomes deeper and thicker (i.e., PV is lowered)in the Tmix case. Because only vertical diffusion at theStraits can work at this stage, vertical diffusion is responsible for both the deepening and thickening. This continues
until an equilibrium is reached.
[36] The deepening is caused by entrainment from thedeeper layer due to vertical diffusion. Such entrainment
results from the fact that around the Kuril Straits thevertical salinity distribution, which almost determines
density stratification, shows a convex structure as a whole(i.e., @ 2r/@z2 < 0 [e.g., Watanabe and Wakatsuchi, 1998]),
so that a down-gradient type vertical diffusion decreases
salinity and thus density. This density decrease causes a
deepening of the isopycnal surfaces and thus can beinterpreted as ‘‘diapycnal mass transport’’ from the denser
layers to the lighter layers. On the other hand, thethickening is caused by entrainment from the shallower
layers as well as the deeper layers. The former occurs
since the value of the gradient, @r/@z is roughly zeronear the sea surface and hence the inequality @ 2r/@z2 > 0

holds around there. Accordingly, density increases andisopycnal surfaces shallow near the sea surface, so that
diapycnal transport is induced from the lighter, shallower
layers.
[37] After the second winter, further thickening occurs in
the upper intermediate layer (around month 18), as theinfluence of the enhanced convection taking place in the
inner part of the Okhotsk Sea reaches the Kuril Straits. In
fact, as an implication of the seasonality inherent in aconvection process, this further thickening of the upperintermediate layer is accompanied by a strong seasonalvariation with a maximum thickness occurring in spring,which is absent in the Ctrl case.
[38] Overall, the addition of tidal mixing makes theintermediate layer deeper and thicker, which in turn will
induce baroclinic responses. The deepening is associatedwith downward movement of isopycnal surfaces with thepeak at the intermediate layer. Accordingly, the first baroclinic mode response is mainly excited together with thehigher mode responses depending on the vertical distributionof the net diapycnal transport. In contrast, in association withthe thickening centered around 27.1 sq, the isopycnal
surfaces of the upper part of the intermediate layer move
upward while those of the lower part move downward. Sucha vertical movement in the opposite direction excites thesecond and/or higher baroclinic mode response. This issimilar to the case of subduction enhanced by cooling in a

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C04003climatological zero-Sverdrup transport line into the subtropical gyre, as shown by Nakamura et al. [2006]. This featureis also consistent with observations [e.g., Yasuda et al.,2001]. These results suggest the importance of tidal mixing
at the Kuril Straits in driving the Oyashio Current.

Figure 8. Initial evolution of the velocity difference
between the Ctrl and Tmix cases on the 27.1sq surface
after addition of tidal mixing.
subtropical gyre [e.g., Liu, 1999; Kubokawa and Nagakura,
2002].
[39] Note that in the Tmix case, the thickening anddeepening strengthen the Oyashio Current, which is thesouthward, western boundary current in the subarctic gyre
flowing along the Kuril Islands. This strengthening occurs
predominantly in the intermediate layer and in spring(Figure 7). Such a seasonal variation in the intermediate
layer thickness and current velocity is a prominent featureof the observed Oyashio Current [e.g., Kono and Kawasaki,
1997b, 1997c]. The strengthened Oyashio crosses over the

4.2. Propagation of Kelvin and Rossby Modes[40] The dynamical adjustment to the forcing at the Kuril
Straits is next examined, focusing on the intermediate layer
circulation. Figure 8 shows the temporal evolution of thedifference in velocity between the Ctrl and Tmix cases on
the 27.1 sq isopycnal surface.
[41] Initially, a signal travels rapidly along the westernboundary against the Kuroshio Current, and is split into two
components, one propagating along the equator and theother passing through the Indonesian Archipelago andeventually reaching the Indian Ocean. This disturbanceconsists primarily of Kelvin waves. In fact, the mean speedof the leading front of the signal roughly agrees with that of
the first mode Kelvin wave along the western boundary. The
former is about 1.9 m s1 since the signal travels a distanceof 5200 km from the Kuril Straits (150E, 45N) to theIndonesian Seas (120E, Equator) in a month, while thelatter is about 2 m s1, with the mean buoyancy frequency
and the mean depth approximated as 0.003 rad s1 and2000 m, respectively.
[42] Although disturbances along the western boundary
should also include other topographically trapped waves
such as shelf waves, these trapped waves play a similar
dynamical role in the thermohaline adjustment (i.e., topographically trapped and faster than the local currents). Thus
we hereafter label all the waves responsible for this signal
the ‘‘Kelvin waves’’ for convenience.
[43] In terms of the thermohaline adjustment, the diapycnal transport produced around the Okhotsk Sea due to boththe tidally enhanced diffusion and convection converges in
the intermediate layer, which therefore thickens until theconvergence is compensated by horizontal flow divergencein the corresponding layer (Figure 7). Such horizontal flow
divergence in turn induces mass transport away from theKuril Straits toward the equator in the intermediate layer, amanifestation of which is the strengthening of the Oyashio
Current. Simultaneously, compensating transport is inducedin the shallower and deeper layers. Most of the mass istransported to the other oceans in association with the
spreading of the Kelvin waves shown in Figure 8, as canbe anticipated from the classical thermohaline adjustment
theory of the abyssal layer [e.g., Kawase, 1987; Goodman,
2001].
[44] Interestingly, there is another signal that slowly
extends eastward, moving along the boundary of thesubtropical and subarctic gyres (i.e., along the flow circulation) and decaying very slowly. This behavior is characteristic of long-Rossby waves of the second and/or
higher baroclinic mode under the influence of a background flow [e.g., Rhines, 1986; Schopp, 1993; Liu, 1999;
Thompson and Ladd, 2004]. Some support to this interpretation can be provided by considering the phase velocity of Rossby waves, which we consider here in an
approximate form since a precise form of the phasevelocity is difficult to obtain [e.g., Stephens et al., 2001;
Killworth et al., 1997].

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C04003[45] Using quasi-geostrophic, long wave, and inviscid
approximations and the assumption of a wave solution,
the zonal phase velocity of small-amplitude Rossby waves,cx, becomes



@2U
@ f02 @UL2n ;cx ¼ U  b  2 @y@z N 2 @z

Figure 9. (a) Salinity difference distribution formed
5 years after the addition of tidal mixing, (b) zonal flow
speed and (c) the relative zonal phase velocity estimatedfrom equation (1) when tidal mixing effect is absent (anannual mean of the first year of the Ctrl run is used as the
background), and (d) the same as Figure 9c but after
adjusting to the tidal mixing effect (i.e., an annual mean of
the last year of the Tmix run is used). Red and blue
regions indicate positive and negative values, respectively.
Contour interval is 0.5 cm s1 in Figures 9b– 9d. All ofthese data are calculated on the 27.1 sq surface.

ð1Þ

where a background flow is approximated to be zonal andquasi-steady since the flow around the eastward extending
signal is directed almost to the east. The notation isstandard, U is a zonal background flow, and Ln = N/(f0m)
where m is vertical wave number [cf. Pedlosky, 1987;
2003]. Equation (1) shows that the propagation of Rossby
waves is determined by advection (i.e., the first term) andthe phase velocity relative to the background flow due to an
ambient PV gradient (i.e., the second term). Hence long
Rossby waves are able to move eastward where a strong
eastward flow is present or the relative phase velocity is
directed eastward owing to the ambient PV gradientassociated with a background geostrophic flow.
[46] The equation also implies that relative phase velocity
leads to a difference in speed between a passive tracer,
which moves at the advective speed, and an active tracer(and/or a change in flow associated with Rossby waves),
which propagates at the total phase velocity. Thus, by
estimating this speed difference, we can estimate a relative
phase velocity from the model results to compare it with atheoretical estimate based on equation (1).
[47] Figure 9a shows the isopycnal distribution of salinity
difference, which spreads mostly owing to advection, on
the 27.1 sq surface as defined 5 years after the addition of
tidal mixing. Over this time period, the salinity signal
travels eastward by 30  40 in longitude, while thecorresponding velocity signal travels about 20. Accordingly, in the zonal direction, the mean advection speed isestimated to be 1.5  2 cm s1, and the mean phase speed isabout 1 cm s1, yielding a relative zonal phase speed of
0.5  1 cm s1 (i.e., westward) on average.
[48] Figures 9b and 9c show the distributions of zonal
flow speed and zonal relative phase velocity, respectively,on the corresponding layer. The relative phase velocity is
estimated from equation (1) with a vertical wave numberof m = np/H where n = 2, so that it roughly corresponds
to that of the second baroclinic mode but includes anerror arising from the assumption of the WKB approximation. As the background state, the annual mean of the
Ctrl case after the spinup is chosen so that the difference
between the Ctrl and Tmix cases can be regarded as aperturbation. From these figures, the mean zonal advection speed and relative phase velocity are estimated to beabout 1.5 cm s1 and 0.5 cm s1, respectively, yielding
a total phase velocity of about 1 cm s1. These values
roughly agree with the above estimates based on themodel results.
[49] The agreement of the theoretical estimates and model
results supports the identification that the eastward moving
signal is associated with long-Rossby waves. This in turnsuggests that the eastward moving long Rossby waves ofthe second and higher modes directly affect the circulationof the interior region. It is noteworthy that after adjusting to
the tidal mixing effect, relative phase velocities are also

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C04003lates around the gyre and then reaches the western boundary. It then joins the path of the Kelvin waves, as suggested
from the distribution of salinity difference (Figure 4).

5. Effect on Circulation[52] The adjustment through the Kelvin and eastward
long Rossby waves will lead to differences in the equilibrium states of the Ctrl and Tmix cases. These are investigated here with the focus on intermediate layer circulationand meridional overturning.
5.1. Pseudo-Transport Stream Function ofIntermediate Layers[53] To investigate the difference in intermediate layer
circulation between the Ctrl and Tmix cases, maps of thedifference in ‘‘pseudo transport stream function’’ on the27.1 and 27.4 sq surfaces in the North Pacific are shown in
Figure 10. The pseudo transport stream function ( ) on a sq
surface that we calculate here corresponds to that in thedensity layer within 0.05 to +0.05sq from the central
density. The quantity is obtained from¼

Z fe

hvR cos f0 df0 ;fFigure 10. Difference in pseudo transport stream function
between the Ctrl and Tmix cases on (a) 27.1 and (b) 27.4 sq
isopycnal surfaces. The pseudo-transport stream function is
the same as the transport stream function when diapycnaltransport is absent. Details are described in section 5.1.
directed to the east in the western part of the eastward flow
region (Figure 9d). This is caused by the supply of low PV
water from the Kuril Straits, which tends to direct ambient
PV gradient fields there to the south and accelerates thewave speed eastward.
[50] These results suggest that both Kelvin and eastward
moving long Rossby waves are required to understand theadjustment and/or circulation of the intermediate layer. The
requirement of both kinds of waves is an important new
aspect and is peculiar to the intermediate layer. A further
discussion on this point is presented in section 7.
[51] It should be noted that the circulation paths of the
freshened water supplied from the Kuril Straits (or those of
the low salinity signals) are not the same as those of the
Kelvin waves. This is because the Kelvin waves can travel
against the Kuroshio Current, whereas the freshened watercomponent cannot. The water thus deflects to the east into
the interior region. On the contrary, the paths of eastward
long Rossby waves are similar to those of the freshened
water because of their wave dynamics [e.g., Pedlosky, 1996;
Liu, 1999]. After deflecting eastward, the part of the
freshened water that has intruded into the subtropics circu-

where f is longitude and fe is the longitude at the easternboundary of the Pacific, h and v are thickness andmeridional velocity of the density layer, respectively, andR is the Earth radius. (From this definition, a positive
anomaly represents a clockwise circulation as usual.) Notethat
includes a divergent part if a convergence ofdiapycnal transport is present [e.g., Pedlosky, 1996].
Nevertheless, this quantity is a useful indicator of circulation in the interior region where diapycnal transport is small(although it differs from the transport stream function westof the Kuril Straits where vertical mixing is enhanced).
[54] The adjustment through Kelvin waves induces anequatorward flow anomaly along the western boundary.
This feature is clearly seen on the 27.1 sq surface, as apositive anomaly extending from the Kuril Straits to theequator. A similar feature is also seen on the 27.4 sq surface.
[55] Since such a flow anomaly crosses the gyre boundaries, it causes an intergyre flow that leads to a more
efficient transport of the freshened water from the Kuril
Straits to the subtropics, and to the equatorial region. At thesame time, the strengthened intergyre transport carries lower
PV water from the subarctic into the subtropics and leads toa decrease in PV in the subtropical gyre. This occurs evenon the 27.4 sq surface, as is shown in section 3.4, since thePV values are generally smaller in the subarctic than in thesubtropics.
[56] The intergyre flow also results in a latitudinal shift ofthe separation point of a western boundary current from thecoast. In particular, the flow anomaly acts to shift the
separation point of the Oyashio and Kuroshio Currents tothe south by strengthening the former and weakening thelatter.
[57] The eastward long Rossby wave adjustment strengthens the circulation in the northwestern part of the subtropical gyre on both isopycnal surfaces. This area roughly
corresponds to the pool region estimated by Huang and

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Figure 11. Difference in meridional overturning stream
function in the North Pacific between the Ctrl and Tmix
cases. Contour interval is 0.5 Sv.

Russell [1994]. The combination of this flow anomaly andthe intergyre flow creates a water path from the Kuril Straits
to the subtropical interior region, and thus leads to an
enhancement of the ventilation in the subtropical intermediate layer. In addition, the circulation in the subarctic gyreis strengthened on the 27.1 sq surface, but not on the 27.4 sq
surface, suggesting that the responses in the subarctic andsubtropical gyres differ somewhat from each other. The
cause of this difference is discussed in Part 2.
[58 ] The southwestern part of the subtropical gyrecorresponding to the shadow zone is also modified, though
the change is significantly weaker than that in the northwestern region. The change here is induced by the westward
traveling Rossby waves originating from the Kelvin waves
along the eastern boundary. Although the combination ofthese with the Kelvin waves plays an important role in thethermohaline adjustment theory of the abyssal layer[Kawase, 1987] and the adjustment to the IndonesianThroughflow [Hirst and Godfrey, 1993, 1994], its effect isrestricted to the shadow zone in the intermediate layer.
Such a restriction can be explained from the behavior ofhigher mode Rossby waves, as revealed in the ventilated
thermocline theory [e.g., Luyten et al., 1983; Liu, 1999].
Interestingly, the current near the equator is altered significantly on the 27.4 sq surface associated with the equatorial
waves, but this issue is not pursued here.
5.2. Meridional Overturning[59] In addition to the changes in the intermediate layer
flow, the shallower and the deeper layer flows are also
modified in association with the forcing given at the Kuril
Straits. Eventually these processes lead to the modificationof meridional overturning as illustrated in Figure 11. The
meridional overturning is enhanced by 3 Sv at the maximum owing to tidal mixing at the Kuril Straits, as both theshallow, clockwise overturning and deep, anticlockwise
overturning are strengthened (‘‘clockwise’’ and ‘‘anticlockwise’’ should be exchanged if the abscissa is reversed).
[60] The shallow-overturning enhancement occurs at
depths shallower than about 700 m (i.e., above and within
the upper intermediate layer), and is mostly visible aspositive anomalies near the sea surface in the figure. Suchan enhanced shallow overturning is caused mainly by theC04003tidally enhanced convection in the Okhotsk Sea, whichbrings water downward.
[61] The deep-overturning enhancement occurs in the
deeper region, and is somewhat emphasized in this figurecompared with the shallow overturning due to the depth
scale. This enhanced deep overturning is caused by the
upwelling around the Kuril Straits (located around 44 to51N), which is in turn required to compensate the downward diffusion by tidal mixing there.
[62] These enhanced overturning circulations are associated with the strengthening of the flow toward the equatorfrom the Kuril Straits in the intermediate layer and theflow toward the Kuril Straits in the shallower and deeper
layers. Large proportions of these overturning flows leave(or enter) the North Pacific to (or from) the other basins.
Consistent with this, the NPIW is observed to leak into theIndonesian Throughflow where the water properties aresubstantially transformed [e.g., Lukas et al., 1991; Ffieldand Gordon, 1992].

6. Sensitivity to the Tidal-Mixing Strength[63] The comparisons of the Ctrl and Tmix cases haverevealed the important effects of tidal mixing at the Kuril
Straits on the water properties and circulation of the North
Pacific intermediate layer. Nevertheless, the estimate of thetidal mixing strength used in the Tmix case may include
significant error, as commented in section 2. This poses the

Figure 12. Area of significant freshening on isopycnal
surfaces, which is plotted as a function of sq for eachexperiment. Negative values indicate that the salinity
difference is negative (i.e., freshened). Each color corresponds to one experiment (colors closer to red or blue
correspond to experiments with stronger or weaker tidal
mixing, respectively, as indicated by the label in cgs units).
The threshold of significant difference is chosen here as
0.05 psu.

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C04003Figure 13. Changes in the meridional overturning stream function in the North Pacific from the Ctrl
case (DyMOS) for various strength of tidal mixing. (a) Maximum differences. (b) Vertical structure of
DyMOS at 43N. (c) Same as Figure 13b but sq is used for the vertical coordinate.
question as to whether the effects depend on the tidal
mixing strength. The sensitivity to the tidal mixing strength
is therefore investigated in this section on the basis of cases
Kz10 to Kz1000 shown in Table 1. The focus is on theenhancement of freshening and meridional overturning due
to the addition of tidal mixing.

freshening effect of the enhanced convection in the upperintermediate layer.
[67] Last, the upper layer becomes saltier as the tidal
mixing becomes strong. This is because upward diffusion of
the saltier water becomes apparent as the blocking effect of
the tidal front becomes significant.

6.1. Freshening[64] To compare the effect on freshening of the intermediate layer, we have calculated salinity differences fromthe Ctrl case on isopycnal surfaces for each experiment,
and used these values to estimate the area with significant
salinity difference (i.e., the lateral extent of the significantly freshened region), which is plotted as a function of
sq in Figure 12. A threshold value of the significant
difference is chosen here as 0.05 psu. When the salinity
difference becomes negative (i.e., freshened) in a particulararea, this is indicated as a negative value on the figure (andvice versa).
[65] The freshening in the intermediate layer occurs in allcases, though certain quantitative differences are seen as
follows. First, the maximum value of the freshened area
increases with increasing tidal mixing from the Kz10 to theKz50 cases, reaches a maximum around the Kz50 to theKz300 cases, and then decreases with increasing tidal
mixing. The first increase is simply due to increase in boththe tidally enhanced convection and downward diffusion of
fresher water. The latter decrease is mainly due to the factthat intense tidal mixing produces a strong tidal front along
the Kuril Straits. This eventually becomes so strong that itpartially blocks the subducted water in the Okhotsk Sea
from inflowing into the North Pacific (not shown), and this,in turn, leads to the weakening of freshening.
[66] Secondly, the density of the maximum freshened area
generally increases with increasing tidal mixing in association with the above processes. That is, deepening resultsfrom a combination of the increasing downward diffusion
and the strengthening tidal front. The former spreads the
freshening downward, and the latter tends to block the

6.2. Meridional Overturning[68] Sensitivity of meridional overturning in the North
Pacific is examined here with a view to defining its strengthand vertical structure, on the basis of differences in themeridional overturning stream function relative to the Ctrl
case. Changes in the strength of the meridional overturning
are shown in terms of the maximum difference from the Ctrl
case (Figure 13a). The maximum differences are all negative, indicating that these develop in the deep, anticlockwise
meridional overturning cell for all the sensitivity experiments. The differences increase with increasing tidal mixing
strength from 1.6 Sv to 8.2 Sv.
[69] Interestingly, the maximum difference does not linearly increase. Note that the relationship between the
diffusion coefficient and the amount of upwelling (and thus
meridional overturning stream function) should be linear, if
we assume a major balance between downward diffusion
and upward advection in the density equation (i.e., wrz 
Kzrzz, as is usual on a basin scale), and assume no significant change in density stratification. Hence our result
suggests that the above balance was invalid and/or thatthe density structure changed. In fact, the downward transport by the tidally enhanced convection is not considered in
the above balance. In addition, it may alter the stratification,
as implied from the PV difference seen in Figure 6.
[70] To consider the change in vertical structure, thedifferences in meridional overturning at 43N, near thesouthern end of the Kuril Straits, are shown in Figure 13b.
In general, the differences are positive in the shallower layer
and negative beneath it, which is similar to the second mode
structure. This structure indicates that the northward flowin the shallow and deep layers and the southward flow in

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the intermediate layer are induced (i.e., southward when
@DyMOS/@z > 0 and vice versa, where DyMOS is thedifference in meridional overturning stream function).
Such changes in meridional circulation are associated withenhanced downwelling in the upper part where DyMOS > 0
and upwelling in the lower part where DyMOS < 0, northof 43N. These downwelling and upwelling zones arecaused by the enhanced convection in the Okhotsk Sea
and vertical diffusion in the Kuril Straits, as discussed in
section 4.2. Note that these features are common to allcases considered here, though the positions of both positive and negative peaks become deeper and their values
increase, with increasing tidal mixing in the Kuril Straits.
[71] The positive peaks are small and are rather difficult
to identify for the weak tidal mixing cases, owing to the factthat the z-coordinate is used in the definition of themeridional stream function. As a result, density increase
associated with the enhanced convection is not taken into
account unless it is accompanied by vertical movement,
leading to underestimates of the effect of the enhanced
convection. In fact, if sq is used as a vertical coordinate(Figure 13c), the corresponding positive peaks become
significant (12 Sv) even in the weak mixing cases. Thissuggests the importance of the enhancement of the shallow
overturning, or of the tidally enhanced convection, whenviewed in the density coordinate.
[72] Overall, while the response of the North Pacific isquantitatively modified as the tidal mixing strength varies,
the response is generally similar for all the cases. Accordingly, although tidal mixing strength will no doubt be
estimated more precisely by future observations, the basic
response and operative mechanism identified in this study
will be valid within the quoted range of tidal mixing
strength.

7. Summary and Discussion[73] Tidal mixing at the Kuril Straits is likely to cause
ventilation of the layers denser than 27.0 sq in the North
Pacific, which cannot be ventilated by direct convection.
The tidal mixing in the Straits is also considered to enhance
the convection in the Okhotsk Sea, which is regarded as an
origin of the NPIW. In this study, we investigated the effectsof tidal mixing at the Kuril Straits on the North Pacificintermediate layer by performing numerical simulation
experiments with and without tidal mixing effect. The
results obtained can be summarized as follows.
[74] The comparison of the above experiments suggeststhat tidal mixing at the Kuril Straits can enhance theventilation in the North Pacific intermediate layer, resulting
in its freshening (and an associated cooling) down to around27.6 sq. As a result, the simulated NPIW becomes fresher,
deeper (and denser), and thus more realistic than the casewithout tidal mixing. The maximum changes in salinity anddensity of the NPIW core layer exceed 0.3 psu and 0.1 sq,
respectively.
[75] The above enhancement of ventilation can beroughly separated into three factors. (1) Tidal mixing at
the Kuril Straits causes diapycnal transport through tidally
enhanced convection in the Okhotsk Sea and diffusion in
the Kuril Straits and thus ventilates water there, as
discussed by Nakamura et al. [2006]. (2) The ventilated

C04003water flows into and circulates through the North Pacificintermediate layer, leading to enhanced ventilation. The
enhanced ventilation (or freshening) around the NPIW
core density layer is associated mainly with the tidally
enhanced convection in the Okhotsk Sea, while that in thelower part of the intermediate layer is attributable todownward diffusion at the Kuril Straits. (3) The change
in the outflow water in turn modifies the circulation of theintermediate layer, thus enhancing further the ventilationof the subtropical North Pacific intermediate layer.
[76] The circulation change is forced by a diapycnaltransport due to both the tidally enhanced convection anddiapycnal diffusion. The convection brings water from theshallower layer to the intermediate layer, while the diffusion
induces entrainment from the deeper layer (and shallower
layer) to the intermediate layer. Consequently, the induced
diapycnal transport converges in the intermediate layer anddiverges in the shallower and deeper layers, so that theintermediate layer thickens and deepens at the main exit of
the Kuril Straits. The thickening induces the second andhigher baroclinic mode responses, while the deepening
mainly induces the first mode response. In terms of masstransport, the convergence in the intermediate layer works
as a mass source and induces a flow away from the Kuril
Straits, while the divergences in the shallower and deeper
layers work as mass sinks capable of creating a flow
toward the Kuril Straits, eventually leading to the enhancement of both the shallow and deep meridional overturningby 2 –3 Sv.
[77] In the North Pacific intermediate layer, the dynamical adjustment to such mass fluxes from the Kuril Straits
occurs mainly through the agency of coastal Kelvin waves
and eastward moving (or advected) long Rossby waves ofthe second and/or higher baroclinic modes. (Equatorial
Kelvin and westward Rossby waves also have a certain
effect, especially in the shadow zone, but this effect isminor.) The Kelvin waves produce a flow component fromthe Kuril Straits to the equator along the western boundary,while the eastward long Rossby waves enhance the circulation in the pool regions, especially those in the subtropics.
[78] As a result of these flow components, water pathways are modified to enhance ventilation in the subtropicalNorth Pacific. First, the Kelvin-wave induced flow strengthens the Oyashio Current so that it intrudes into the subtropical gyre, and thus enhances the transport of ventilated waterfrom the subarctic to subtropical gyres. The ventilated water
is then carried into the interior, circulates around the pool
region and then encounters the western boundary. Some of
the water leaves the subtropical North Pacific to enter theequatorial region and other basins, in response to a Kelvinwave induced flow. Such a water pathway is similar to thatof the observed NPIW.
[79] The above results illustrate the possible roles oftidal mixing at the Kuril Straits in ventilating the North
Pacific intermediate layer. The results will also be usefulfor understanding the response of the North Pacific intermediate layer to transient fluctuations of the atmosphericand oceanic conditions around the Kuril Straits and in the
Okhotsk Sea. In addition, the results have the following
two important implications.
[80] 1. The results show that key elements in the dynamical adjustment of the intermediate layer to the mass flux

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Figure 14. Same as Figure 3c, but for the difference
between the Tmix case and the case without double
diffusion. The setting of these two cases are the same,
except for the inclusion/exclusion of a double diffusion
parameterization. (The latitude range in this figure is
reduced from Figure 3.)
from the Kuril Straits are coastal Kelvin and eastward long
Rossby waves. This clue will facilitate development of atheoretical model of intermediate layer ventilation. Previous
theories on ventilation may be categorized into ventilated
thermocline theories along the lines of Luyten et al. [1983]
and Rhines and Young [1982] and those of thermohaline
circulation of an abyssal layer of the type developed by
Stommel and Arons [1960] and Kawase [1987]. However,the former does not include the effect of mass driven Kelvinwaves along the western boundary, while the latter does not
represent the eastward moving long Rossby waves whose
presence is established by wind driven circulation. The lackof the combined effect of these two kinds of waves may be
the reason why a realistic analytical model of the ventilated
intermediate layer has not been developed. Consideration of
the effects of both kinds of waves will allow us toincorporate the important elements from these two major
theories and therefore develop an improved theoreticalmodel of the ventilated intermediate layer. A first step
toward this unified approach is attempted in an accompanying paper (Part 2).
[81] 2. The simulated NPIW becomes fresher, deeper anddenser by adding tidal mixing at the Kuril Straits. Althoughprevious attempts to model the NPIW have succeeded in
reproducing the salinity minimum, simulated NPIWs are
generally saltier and shallower than observed in the ocean[e.g., England, 1993; Hirst and Cai, 1994] even when eddy
permitting OGCMs are used [e.g., Qu et al., 2002]. Our
results suggest that one reason for this is that the effect oftidal mixing has not been taken into account either througha parameterization or through explicit reproduction. Thus
attempts in the direction of the present study would lead tobetter numerical simulations of the ventilation of intermediate water in the North Pacific.
[82] Double diffusion and cabbeling are also consideredas important mechanisms of the downward diapycnal transport required for the formation of the NPIW salinity
minimum [e.g., You et al., 2000; Talley and Yun, 2001].
However, although the effects of both double diffusion and

C04003cabbeling are included in all the above cases, the formationof the NPIW is insufficient in the case without tidal mixing.
This suggests that these two effects are less important than
the tidal mixing effect in the NPIW formation. In particular,when the double diffusion parameterization is excluded
from the Tmix case, the resulting difference in salinity ismuch smaller (0.03 psu at most) and is mainly confined
above the NPIW salinity minimum layer (Figure 14). This isconsistent with the results of Talley and Yun [2001].
[83] It should be noted however that a numerical model
inevitably involves some bias, which may affect a quantitative estimate. Probably the most important bias in thepresent case is the fact that the 26.8 sq density layer isrelatively shallow as compared with the observed depth.
One possible cause is a small production of dichothermal
water in the Bering Sea, which acts to thicken the density
layer around 26.6 sq [e.g., Miura et al., 2002]. In addition,
the simulated Kuroshio slightly overshoots to the north.
Previous studies suggest that when the Kuroshio overshoots
too far to the north, a relaxation boundary correction at thesea surface can cause excessive cooling due to the large
temperature difference between subtropical and subarctic
waters. Such cooling can result in the formation of an overdeep mixed layer in winter and could affect the NPIW
formation [e.g., Kobayashi, 1999]. Thus the separation
point of the Kuroshio is shown using maps of the velocity
field at 50m depth in the Ctrl case (Figure 15a). The figure
indicates that the separation point of the center of theKuroshio is located around 36N in winter and 37N insummer. Even the northern edge of the Kuroshio separates
around 38– 39N in winter and 39– 40N in summer. (In theTmix case, the separation point shifts southward by about
one degree.) These values of the separation latitude (36 –40N) are slightly larger than those observed (around 35–37N). Nevertheless, the simulated Kuroshio does not reachthe subarctic gyre, and hence water masses in the subarctic
gyre or offshore of the Kuril Islands are not directly
affected. Also, the separation in our model occurs within thelatitude range of the Mixed Water Region, where warm corerings arising from the Kuroshio appear very often. Thus therelaxation boundary condition is not considered to cause
severe excessive cooling. In fact, previous studies suggestthat an overshoot of the above extent is permissible for theinvestigation of the NPIW formation [e.g., Kobayashi,
2000]. Further, to see the sensitivity to relaxation, we have
performed similar experiments but with a relaxation timescale of 30 days (i.e., the relaxation strength is doubled).
The results are almost the same as the present results. As an
example, the meridional section of salinity at 165E is shownin Figure 15b. Considering the above, an estimate of therelative importance of double diffusion, cabbeling, and tidal
mixing in the formation of the NPIW salinity minimum inthe actual ocean is beyond the scope of this paper andawaits future observational studies. Nevertheless, since thespreading of the tidal mixing effects is determined mainly
by advection, Kelvin waves and eastward moving/advectedlong Rossby waves, which in turn are controlled mainly by
the large scale flow field reproduced to reasonable accuracy
here, the qualitative features of our results must remain
relevant.
[84] Also, mesoscale eddies, which are present around theKuroshio Extension in particular, are considered to induce a

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NAKAMURA ET AL.: TIDE-ENHANCED NORTH PACIFIC VENTILATION

C04003Figure 15. (a) Velocity vectors at 50 m depth of Ctrl case in (left) winter and (right) summer.
(b) Meridional section of salinity at 165E in the (left) Ctrl and (right) Tmix cases, but with arelaxation timescale of 30 days. (The latitude range is reduced from Figure 3.)
significant intergyre transport in addition to that induced
through the mass-driven Kelvin waves. Such eddies are notresolved with the present model resolution, although theeffect of eddies is considered using the GM90 parameterization. In addition, although the mainstream of the Oyashio
separates at similar latitudes to those observed (about 40N
in winter and 42N in summer, Figure 15a), a narrow,coastal branch of the Oyashio, which often extends further
southward in winter, is not resolved. Since intergyre transport is a key element in the ventilation of the subtropicalNorth Pacific intermediate layer, the limited resolution is
also possibly the reason why the simulated NPIW in thecase with tidal mixing effect is still saltier than is observedin nature. Thus an eddy resolving OGCM with tidal mixing
is desirable in future for reproducing the NPIW more
precisely and for a more complete understanding of intergyre transport.
[85] As discussed in section 2, the presence of a tidal flow
has a number of important consequences [e.g., Nakamura
and Awaji, 2004]. Of these, the present study focused on theeffects of vertical mixing, which is considered to beimportant for basin-scale thermohaline circulation [e.g.,

Munk and Wunsch, 1998]. Although other processes will
inevitably be involved in a complete understanding of therole of tides in North Pacific ventilation, the effect ofvertical mixing must be included as it plays a central role.
[86] Although the occurrence of enhanced vertical mixing
at the Kuril Straits is indicated from both observationaland modeling studies [e.g., Kawasaki and Kono, 1994;
Gladyshev, 1995; Kawasaki, 1996; Aramaki et al., 2001;
Nakamura et al., 2000a; Nakamura and Awaji, 2004],
there may be other regions where tidally induced verticalmixing is strong. Nevertheless, observed CFC distributions
have shown that the most recently ventilated water in thelower part of the North Pacific intermediate layer (say, 27.1
to 27.6 sq) lies around the Kuril Straits [Warner et al., 1996;
Wong et al., 1998]. This observational fact implies thatvertical mixing in the Kuril Straits is more effective thanthat in other regions around the North Pacific, at least in thisdensity range. We thus consider that the investigation of theeffects of vertical mixing around the Kuril Straits (together
with the tidally enhanced convection in the Okhotsk Sea) is
a useful step toward a better understanding of the ventilationof the North Pacific intermediate layer.

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C04003

NAKAMURA ET AL.: TIDE-ENHANCED NORTH PACIFIC VENTILATION[87] Acknowledgments. We thank J. P. Matthews for reading throughthe manuscript. Thanks are also extended to the editor and four anonymous
reviewers for their useful comments. This study is supported by theCategory 7 of MEXT RR2002 Project for Sustainable Coexistence of
Human, Nature and the Earth and by a Grant-in-Aid for the 21st Century
COE Program (Kyoto University, G3). T. N. was partly supported by the
JSPS Research Fellowships for Young Scientists. This study was done
when T. N. was at Department of Geophysics, Kyoto University. Numericalcalculations were done on the VPP800 at the Academic Center for
Computing and Media Studies of Kyoto University.

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T. Awaji and T. Toyoda, Frontier Research Center for Global Change,
JAMSTEC, Yokohama, 236-0001, Japan.
Y. Ishikawa, Department of Geophysics, Kyoto University, Kyoto, 6068502, Japan.
T. Nakamura, Institute of Low Temperature Science, Hokkaido
University, Sapporo, 060-0819, Japan. (nakamura@lowtem.hokudai.ac.jp)

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