ARTICLE IN PRESS
Continental Shelf Research 30 (2010) 598–607

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Continental Shelf Research
journal homepage: www.elsevier.com/locate/csr

Large-amplitude internal waves observed in the Kruzenshtern Strait of theKuril Island Chain and possible water transport and mixing
Tomohiro Nakamura a,, Yasuhiro Kawasaki b, Tokihiro Kono c, Toshiyuki Awaji daPan-Okhotsk Research Center, Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Kushiro 085-0802, Japanc
Department of Marine Biology and Sciences, School of Biological Science and Engineering, Tokai University, Sapporo 005-8601, Japand
Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japanba r t i c l e in f o

a b s t r a c t

Article history:
Received 17 December 2008
Received in revised form
17 July 2009
Accepted 20 July 2009

Vertical sections of temperature and current velocity across the sill in the Kruzenshtern Strait wereobserved four times in one day in September 1997. The observations captured nonlinear internal waves
near the sea surface, which are likely to be tidally generated. Two of these waves had maximum
amplitudes near the surface and each contained a trapped core, suggesting that a signiﬁcant volumetricﬂux was induced. At mid depths, isotherms moved vertically more than 50 m, which is consistent withthe generation of internal lee waves. From water-mass movements, the currents are estimated to ﬂow
from the North Paciﬁc to the Sea of Okhotsk in the temperature-minimum layer at a mean speed of
0.2–0:3 m s1 . The mean ﬂow was weaker and the oscillatory component was stronger at mid depths,
with the amplitude of the latter around 0:4 m s1 , leading to the above lee wave generation. Active
temporal variations in potential temperature, particularly those associated with small-scale patches,suggest the occurrence of vigorous water-mass transformation. Vertical and horizontal diffusivities are
estimated to be Oð102 m2 s1 Þ and Oð102 2103 m2 s1 Þ, respectively. Because the tidal amplitude wasclose to the annual minimum at the time of observation, larger-amplitude internal waves and morevigorous water-mass transformation could take place at other times of the year.
& 2009 Elsevier Ltd. All rights reserved.

Topical issue on ‘‘Tides in Marginal Seas’’(in memory of Professor Alexei V. Nekrasov).
The issue is published with support of theNorth Paciﬁc Marine Science
Organization (PICES).
Available online 28 July 2009
Keywords:
Internal wave
Water-mass transformation
Tidal mixing
Kuril Straits
Sea of Okhotsk

1. Introduction
The Sea of Okhotsk and the North Paciﬁc Ocean are connected
through the straits in the Kuril Island Chain (Fig. 1). Water
exchange between these two basins is considered to occur mainly
through the two deep straits, the Kruzenshtern Strait and the
Bussol Strait. This water exchange has an important role in
the formation of water masses and circulation in both basins. In
the Sea of Okhotsk, winter cooling and brine rejection associatedwith sea ice production lead to the formation of saline water nearfreezing temperature over the northern shelf, which is called
Dense Shelf Water (e.g., Kitani, 1973; Shcherbina et al., 2003). Thiswater is fresher than the surrounding water at the same density
because of its low temperature and is considered to be one of the
major sources of fresh water in the North Paciﬁc intermediate
layer (e.g., Talley, 1991; Yasuda, 1997; Kono, 1998; Watanabe andWakatsuchi, 1998; Itoh et al., 2003).
Besides the water exchange, strong tidal mixing in the Kuril
Straits is also considered to affect both the Sea of Okhotsk and the Corresponding author.

E-mail address: nakamura@lowtem.hokudai.ac.jp (T. Nakamura).
0278-4343/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.csr.2009.07.010

North Paciﬁc (Nakamura et al., 2004, 2006). The tidal mixing not
only modiﬁes water properties directly in the Kuril Straits, but it
also induces an upward salt ﬂux into the surface layer from thesaltier lower layer. The subsequent transport of the saline waterfrom the Straits to regions of sea-ice generation increases thedensity of the Dense Shelf Water. Both of these effects enhance theventilation in the Sea of Okhotsk. The inﬂuence of these processes
spreads over almost all of the North Paciﬁc as the water supplied
from the Sea of Okhotsk circulates in the intermediate layer. Also,
Nakamura et al. suggested that the dynamical adjustment
processes conducted mainly through coastally trapped waves
and eastward advected long-Rossby waves induce intergyretransport and modify the interior circulation, respectively.
Further, Yasuda et al. (2006) suggested that with the 18.6-year
period nodal tidal cycle, tidal mixing in the Kuril Straits causes
bi-decadal variation in the North Paciﬁc through the above
process. (There could also be a variation with a period of about
six months associated with the reduction of diurnal tides nearequinoxes. Such a variation, however, has not been paid muchattention, which may be partly because it should be contaminatedby the seasonal variation.)
Numerical modeling and theoretical studies have shown thatthe diurnal tides, especially the K1 tide, have the ability to induce

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between mixed water in the straits and water in the Sea of
Okhotsk or the North Paciﬁc (Kawasaki and Kono, 1992, 1994;
Gladyshev, 1995). Kawasaki (1996) observed the presence of alarge number of patches in a vertical temperature section throughthe Kruzenshtern Strait, which may be a manifestation of mixing.
Analyses of chemical tracers also suggested the importance ofdiapycnal mixing (Wong et al., 1998; Aramaki et al., 2001;
Yamamoto et al., 2002). Recently, diapycnal diffusivity associatedwith turbulent mixing was estimated around the Bussol Straitfrom extensive Conductivity-Temperature-Depth (CTD) observations (Ono et al., 2007) and direct turbulence microstructure
measurements (Yagi and Yasuda, 2008; Itoh, 2008). The estimateddiffusivity is in fact huge, and there was a site where a daily andvertically averaged value was Oð102 m2 s1 Þ.
Nevertheless, observations of internal waves in the Kuril Straits
are, thus far, very limited. As far as we are aware, only two
examples have been published in international journals: A
hydrographic survey in the Kruzenshtern Strait which showedthat vertical displacements whose horizontal scale corresponds toa lee wave extended from the sill top toward the sea surface(Nakamura et al., 2000a), and satellite imagery of a solitary wave
train near the Bussol Strait (Nakamura and Awaji, 2004). Such alimited number of observations of internal waves is one of the
obstacles to a better understanding of the link between tides anddiapycnal mixing in the Kuril Straits.
In this study, we present internal waves captured in pastobservational data in the Kruzenshtern Strait. The waves are likelyof tidal origin, and thus the observation may provide additional
support for the results of previous simulations on tidal mixing.
Further, some internal waves trapped near the surface may cause
water transport, whose effect has not been considered around thisarea. The remainder of this paper is organized as follows. A summaryof the observations is described in Section 2. The internal waves near
the surface are presented in Section 3, and water-mass movement
and internal wave generation at mid depths are investigated in
Section 4. Finally, the results are summarized in Section 5.

Fig. 1. Observation sites. (a) Location of the Kruzenshtern Strait and the Kuril
Island Chain. (b) Observation sites and topography. Crosses and solid circles
indicate the sites of XBT and CTD measurements, respectively.

vigorous mixing (Nakamura et al., 2000a; Nakamura and Awaji,
2001, 2004). The diurnal tidal ﬂow is considerably intensiﬁedaround the Kuril Straits, with a maximum ﬂow speed of over2 m s1 (e.g., Thomson et al., 1997; Luchin report in Talley andNagata, 1995; Kowalik and Polyakov, 1998; Nakamura et al.,2000b; Ohshima et al., 2002). The intensiﬁcation is due to theeffective ampliﬁcation of topographically trapped waves generated by the subinertial diurnal tides, whose presence wasobserved by Rabinovich and Thomson (2001), in addition to theeffect of topographic contraction. The resulting mixing occurs
from the bottom of topographic features to the sea surface along
the Kuril Island Chain and is particularly intense over sills orbanks of moderate depth, where the maximum vertical diffusivity
may exceed 0:1 m2 s1 . In numerical simulations (Nakamura et al.,2000a; Nakamura and Awaji, 2004), the locally enhanced mixing
is associated with tidally generated ‘unsteady lee waves’ rather
than internal tides of tidal frequency. This is because the diurnal
tides are subinertial at the Kurils (so that internal tides generated
by them cannot be internal gravity waves) and because unsteadylee waves are more likely to grow by superposition than internal
tides. Energetic internal waves are then formed with amplitudes
exceeding 100 m, resulting in wave breaking.
Observations in the Kuril Straits also supported the occurrenceof strong mixing. Density or temperature fronts were observed

2. Observations
The observations were conducted around the Kruzenshtern
Strait (Fig. 1) during an R/V Hokko-Maru cruise in September 1997.
An observation line was set along the strait (i.e., across the sill in thestrait), as shown in Fig. 1b. Temperature measurements on this line
(crosses in Fig. 1b) were repeated four times during one day, using
Expendable BathyThermographs (XBTs). The distance between each
XBT site was set at approximately 4 km to capture internal waves ofrelatively small wavelengths. Horizontal current velocity was alsomeasured with a shipboard Acoustic Doppler Current Proﬁler(ADCP) during the XBT observations. Although CTD measurements
were performed at both ends of the line (solid circles in Fig. 1b), we
focus on the XBT and ADCP measurements here.
The durations of the four XBT observations are shown in Fig. 2cwith predicted tidal elevation at the observation site. In the four
XBT observations, we could see different phases of tidal ﬂow
associated with diurnal tides, which make a major contribution to
the current ﬁeld around the Kuril Straits. However, the amplitudeof tidal elevation in the strait was almost the annual minimum at
the time of the observation (Fig. 2). Further, semi-diurnal tides
were dominant rather than diurnal tides, which usually dominate(Fig. 2). As a result, legs 1 and 3 captured a similar tidal phasecorresponding to the time when the tidal elevation increased toﬂood tides, whereas it decreased to ebb tides during legs 2 and 4.
Fig. 3 shows vertical sections of the observed potential
temperature. Although potential temperature here is calculated
assuming a constant salinity value since salinity was not

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Fig. 2. The time duration of the observation and tidal height during (a) 1997, (b) September 1997, and on (c) 18–19 September 1997. Shaded areas in the bottom panel show
the observation duration. Tidal height is calculated based on Matsumoto et al. (2000) at the location of the sill on the observation line.

measured with the XBTs, the associated errors were less than
0:01  C and are small enough for the discussion in this study.
The ﬁgure shows a typical vertical structure in the subarctic
North Paciﬁc and the Sea of Okhotsk, observed in summer:
Namely, the presence of the warm surface mixed layer above theseasonal thermocline, the temperature minimum layer below(called dichothermal layer), and the local temperature maximum
layer at intermediate depths (called mesothermal water). The
typical difference in water-mass structure between the subarctic
Paciﬁc (the right hand side) and the Sea of Okhotsk (the left handside) is also seen, such as the respective depths of the
dichothermal layer (shallower in the North Paciﬁc) and the North
Paciﬁc’s warmer mesothermal water. These features of the
background potential temperature ﬁeld are similar to thoseobserved previously (Kawasaki and Kono, 1994; Gladyshev,
1995; Kawasaki, 1996).
Between these two basins, large temporal variations occur. The
elevations and depressions of isotherms are seen near the sea

surface and below the mesothermal water, where the horizontal
gradient of potential temperature is relatively small. In thesubsurface layer, it seems that the water-mass structure is being
vigorously transformed in the strait. In the following, we ﬁrst
focus on the disturbances near the surface in Section 3 and will
come back to the subsurface variations in Section 4.

3. Near the sea surface
3.1. Internal waves
Vertical sections of potential temperature and current velocity
near the sea surface are shown in Fig. 4, together with thelocations of the measurements (triangles below each panel). The
current velocity is decomposed into components along and acrossthe observation line. Positive along-line velocity is directed rightward (to the Paciﬁc side across the sill) and positive across-line

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Fig. 3. Vertical sections of potential temperature and the bottom topography along the observation line. Contour intervals are 0:5  C for  3  C and 1  C for 43  C. The North
Paciﬁc Ocean (the Sea of Okhotsk) is on the right (left) hand side.

velocity goes from the front side of the ﬁgure to the reverse side
(to the northeast along the sill). The ﬁgure also shows
the horizontal divergence of along-line velocity, the vertical
integration of which gives vertical velocity under the rigid lid
approximation when the divergence of across-line velocity is
negligible.
From internal wave theories, we know that solitary waves near
the sea surface in such deep water form depressions, except forvery high vertical modes (Ostrovsky and Stepanyants, 1989;
Helfrich and Melville, 2006; Apel et al., 2007), and thathorizontally propagating internal waves that are trapped vertically, or that consist of vertical modes, propagate in the directionof the across-trough velocity and have downward (or upward)
vertical velocity on the front (or the rear) side of the trough (e.g.,
Vlasenko et al., 2005).
Such structures are observed in Fig. 4 and are numbered from 1
to 7, with arrows showing the direction of propagation. Depressions 1, 3, 4 and 7 have structures consistent with leftward
propagating waves, whereas the others are consistent withrightward propagating waves. Depressions 2 and 6 exhibit more
typical structures than the others. Depression 4 resembles a kinkfront rather than a depression. Depression 3 seems to have a trainof waves behind the leading trough according to the ADCP
velocity, although the train is not resolved by XBT observations.
Similarly, depression 5 could be associated with a leading wave
train in front of the observed depression at 37 km. The propagation direction of depression 7 is somewhat ambiguous from thepresent data. The current structure suggests that the peak of thedepression is located at around 34 km and is not resolved by XBT
observation. If this is the case, depression 7 is propagating
leftward. The presence of almost symmetric elevations on bothsides of depression 1 is a curious feature. Because solitary wavesof elevation do not easily occur in this situation, one simple
explanation is that one large depression is embedded in therelatively large-scale elevation. Another interpretation is that thisis a part of a nonlinear and non-stationary wave train.
In spite of the ambiguity due to the resolution of the XBT
observations, the relation of the phases of isotherm displacement,

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Fig. 4. Vertical sections of (a) potential temperature, (b) divergence of along-line velocity, (c) along-line velocity, and (d) cross-line velocity observed in legs 2–4 from left toright. Triangles below each panel indicate the locations of measurements.

along-line velocity, and approximated vertical velocity suggeststhat these features are horizontally propagating internal waves. It
is, however, difﬁcult to track these waves since disturbances
which have sufﬁciently similar shapes to the identiﬁed waves inprevious legs are not found in the next legs. They might be nonstationary or might have gone outside the observation section.
Actually, supposing that the phase speed is around 1 m s1 , waves
would propagate about 20 km during the observation timeinterval of 6 h, so that most waves will leave the range of theobservation line before the next observation.

3.2. Bolus-like depression
Among these features, depression 6 has a shape similar to
known soliton solutions, though the shape is partly due to linear
horizontal interpolation in drawing the ﬁgure. In addition, it ismarginally resolved by 5 or 3 XBT sites, which may allow a
qualitative comparison with theoretical models. Fig. 5 shows thesoliton solutions superposed on a close-up of the potential
temperature ﬁeld around depression 6. The solutions compared
are the Korteweg–deVries (KdV) and extended KdV solitons(eKdV; e.g., reviews by Ostrovsky and Stepanyants, 1989;
Helfrich and Melville, 2006; Apel et al., 2007), the Miyata

soliton (Miyata, 1988; Choi and Camassa, 1999), and the JKKD
soliton (Joseph, 1977; Kubota et al., 1978). Note that these are
weakly nonlinear models (shallow water models), except for the
Miyata soliton (the JKKD soliton).
The solutions shown are obtained for a two layer system, inwhich the horizontal length scale of all solitons depends on thedensity and thickness of the upper and lower layers (r1 , r2 , h1 ,and h2 , respectively) and soliton amplitude. When Boussinesq
approximation is valid ðr1  r2 ¼ r0 Þ, r1 and r2 affect thesoliton solutions through the reduced gravity acceleration,
g 0 ¼ ðr2  r1 Þg=r0 . The reduced gravity, however, does not affectthe length scale of these solitons, as indicated in Apel et al. (2006)
for the case of the KdV soliton. (This is because our results were
not obtained from a mooring at a ﬁxed point, so that there is noneed to take the temporal change or the soliton speed into
account.) Further, a soliton amplitude is determined by the depthof the depression at x ¼ 0 km in Fig. 5, and the total depth (i.e.
h1 þ h2 ) is set to be an observed value ð 1700 mÞ. We determined
the remaining parameter ðh1 Þ by doing parameter ﬁtting, using
depths of an isotherm at ﬁve XBT sites, though the outermost two
have relatively little effects.
After the parameter ﬁtting, we obtained a qualitatively similar
shape as compared with observed isotherms for all of the above
soliton solutions. The depths of the isotherm (thin solid line)

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Fig. 5. Comparison of the observed isotherm depression 6 in Fig. 4 and solutions of Miyata soliton (thick solid line), KdV and eKdV solitons (dotted line), and JKKD soliton(short dashed line), using parameter values selected so as to ﬁt the former three soliton solutions to the shape of the isotherms (h1 ¼ 240 m where h1 corresponds to theupper layer thickness in a two-layer ﬂuid system). The KdV and eKdV soliton solutions are indistinguishable in this ﬁgure. Also shown are a JKKD soliton (dash dot) which is
ﬁtted to the isotherm shape ðh1 ¼ 60 mÞ and a JKKD soliton (long dash) which is obtained by assuming that the pycnocline experienced by the wave lies close to the sea
surface ðh1 ¼ 15 mÞ.

agreed qualitatively with the soliton solutions at the XBT
observation sites, although the shape of the isotherms between
the XBT sites could be an artifact of linear interpolation. The JKKD
soliton (dash dot) has a signiﬁcantly different shape as comparedwith the other soliton solutions, whereas the other three soliton
solutions give very similar shapes for the parameter set used here,
though the Miyata soliton (thick solid line) is slightly wider thanthe KdV and eKdV solitons (two indistinguishable dotted lines). It
should be noted, however, that ambiguity remains in thedetermination of the parameter (h1 or h2 ) due to the absence ofsalinity and hence density distributions with depth. For example,
the shape of the JKKD soliton differs further when the same set ofparameters is used (short dashes). The lack of density proﬁles and/or the number of XBTs prevent us from making a quantitative
comparison.
Interestingly, this depression has the maximum displacement
near the surface and contains warm water enclosed by anisotherm and the sea surface: The maximum displacement occurs
around the 7  C isotherm, which approaches the sea surface awayfrom the depression, whereas the seasonal thermocline is aroundthe 3:5  C isotherm located about at a depth of 40 m (Fig. 4). In
contrast, weakly nonlinear waves are amenable to separation intoa horizontal structure function and vertical modes, which have amaximum vertical displacement around the main pycnocline forthe ﬁrst mode and near the seasonal thermocline for high modes.
The above feature thus suggests strong nonlinearity of the wave orthat the wave is trapped very near the surface. If we assume thatthe latter is the case and that the main pycnocline experienced by
the wave lies near the surface, the JKKD soliton alone provides acurve (long dashes) whose length scale is comparable to that of
the observed isotherms. The other solutions predict much smaller
length scales (not shown). The length scale of this JKKD soliton
solution is, however, still shorter than that observed, whichplausibly suggests that strong nonlinearity also makes a contribution.
Further, if we place it upside down, this feature is similar tointernal boluses on the bottom of continental shelves, which are
generated through the interaction of incident internal waves with
topography and which have been seen in observations (Klymak
and Moum, 2003; Carter et al., 2005) and in numerical experiments (Venayagamoorthy and Fringer, 2006). These boluses are
known to induce transport of water inside them and to provide
signiﬁcant bursts of turbulent kinetic energy that can contributeto diapycnal mixing, suggesting that depression 6 could have
similar effects.
Also, depression 3 observed at the nearly opposite tidal phase(leg 3) has a similar bolus-like feature but with the opposite

propagation direction, suggesting that such boluses are frequently
generated by tides. Assuming that such boluses transport thewater contained inside and that two boluses are generated
in one tidal cycle (with opposite propagating directions), thevolumetric ﬂux is roughly estimated to be of the order of 0.1 Sv
ð1 Sv ¼ 106 m3 s1 Þ, using the estimated area contained in depression 6 of 1:5  105 m2 and assuming that the length scale in theacross-line direction is nearly the width of the strait ð502100 kmÞor that a similar size of soliton is generated over most of the sill in
the Strait. Considering that this volumetric ﬂux occurred throughthe top 30 m depths, the estimated amount is signiﬁcant forsurface water transport when compared with a previous observational estimate of 1.6 Sv taking place in the upper 600 m depths(Katsumata et al., 2001). According to their observation, theresidual along-strait velocity was rather vertically uniform in
the upper layer, so that it would yield a transport of about 0.08 Sv
in the top 30 m. Because the ﬂux due to such boluses takes
place in the surface layer, it might have a signiﬁcant impact on
the supply of nutrients in the euphotic zone around thestrait by spreading out the nutrient-rich water in the strait, whichis in turn expected to be supplied by tidal mixing from subsurface(Freeland et al., 1998; Nakamura and Awaji, 2004; Nishioka et al.,2007).

3.3. Possible effects of rotation
It is also interesting to note that the across-line velocity
also varies in the region where the horizontally propagatingwaves are present. The variations have similar length scales andsometimes reach signiﬁcant amplitudes compared with thealong-line velocity, suggesting the possible inﬂuence of the Earth’s
rotation.
In fact, if the across-line velocity is caused only by an
inclination of the observation line relative to a wave trough, theacross-line velocity should have the same or opposite phases (i.e.,0 or 180 differences) relative to the phase of the along-line
velocity. This could be the case for depressions 2, 4, and 7.
Nevertheless, the across-line velocity in depression 6 has asymmetric components about the center of the trough, suggesting theinﬂuence of rotation, although the smallness of the acrossvelocity variation suggests that this wave is in the so-called very
weak rotation regime (Grimshaw, 1985), where the rotationinduces transverse velocity but does not affect the wave structure.
In more detail, the effect of Earth’s rotation on surface andinternal wave dynamics is discussed in the review by Grimshaw
et al. (1998).

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4. Subsurface
4.1. Water movement
Active temporal variations also take place below the surface
layer, as shown in Fig. 3. We ﬁrst discuss associated water-mass
movement here and then water-mass transformation and the local
generation of internal waves in the following subsections. Water
movement is seen as the movement of potential temperature
contours or anomalies in Fig. 3. Fig. 6 presents selected isotherms
observed during legs 1–4 (time propagates from black, blue, green
to red) in order to show such movements more clearly.
The movement of the dichothermal layer incoming from theNorth Paciﬁc (the right hand side) is clearest (Fig. 6a) and can beeasily seen also in Fig. 3 as the movement of cold water around
100 m depth. A closer look also reveals the oscillatory movement
at mid depths (Fig. 6b, c), as illustrated by arrows. Thesehorizontal movements are made visible because of the signiﬁcantdifference of water-mass structures of the North Paciﬁc and the
Sea of Okhotsk at these depths. Vertical movement rather thanhorizontal movement is visible below mesothermal water (Fig.
6d), where the horizontal gradient of potential temperature isrelatively small.
The horizontal velocity is estimated from the above water
movements to be 0:220:5 m s1 in the dichothermal layer and
0:1520:5 m s1 at mid depths. The current is always directed fromthe Paciﬁc to the Sea of Okhotsk (leftward in the ﬁgure) in the
dichothermal layer, whereas it reverses direction according to thetidal phase at mid depths, with the average ﬂow directed to the
Sea of Okhotsk. These differences in current velocity with depth
indicate both the presence of vertical shear of the averaged
velocity and the enhanced tidal ﬂow at depths. Similarly, verticalvelocity is estimated to be 2  103 23  103 m s1 in the
dichothermal layer and 4  103 25  103 m s1 at mid depths.
These estimates of horizontal velocity may be conﬁrmed by
ADCP velocity, although the comparison is possible only for the
dichothermal layer due to the limitation of the ADCP data. The
averaged along-line ADCP velocity in the depth range of 25–100 m
is always directed to the Sea of Okhotsk at speeds of
0:07; 0:43; 0:18; 0:33 m s1 during legs 1–4, respectively. The
direction and magnitude of the mean ADCP velocity agree
qualitatively with the estimated along-line velocity in the
dichothermal layer. Also, the movement of the cold water
indicates the current is slow in the time durations between legs
1 and 2 and between legs 3 and 4, whereas it is fast in the
duration between legs 2 and 3, which is roughly consistent withthe time variation of the mean ADCP velocity.
The magnitude of the estimated horizontal velocity is similar to
those observed by lowered ADCP measurements in the same strait(Katsumata et al., 2001). However, the direction in the dichothermal layer (or the direction of the vertical shear) is opposite. Our
observation shows the ﬂow from the Paciﬁc to the Sea of Okhotsk,
which is consistent with past qualitative investigations based onwater-mass analysis (e.g., Kitani, 1973). Considering that theobservation by Katsumata et al. was also conducted in summer(from 31 August to 1 September, 1999), the difference in current
direction suggests the presence of large horizontal variations, suchas the one associated with bi-directional mean currents induced bytides (Nakamura et al., 2000b), or signiﬁcant intra-seasonalvariations in the current ﬁeld.

Fig. 6. Isotherms selected to show the movements of water masses. Black, blue,
green and red contours represent isotherms observed in legs 1–4, respectively. The
solid triangle below the panel (d) indicates the location of the sill top.

4.2. Water-mass transformation
Fig. 3 also shows the large number of patches of potential
temperature anomalies, with the number increasing near the sill,

and the active formation and disappearance of such patches.
These facts suggest the occurrence of vigorous water-masstransformation in the strait.

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In particular, we may trace two events in which a large
anomaly breaks into smaller patches. As the cold dichothermal
water moves leftward, the cold core initially centered around153 440 E and 120 m depth in leg 1 (Fig. 3a) is separated into twomasses located around (153 380 E, 150 m depth) and (153 360 E,
100 m depth) in leg 4 (Fig. 3d), plausibly due to the combined
effects of vertical displacement and vertical shear. The upward(downward) movement of the cold core during legs 1 and 2(2 and 3) seems consistent with the divergence of the along-line
velocity, and the along-line movement is also roughly consistent
with ADCP velocity, though the vertical shear was not observed
owing to the limitation of the ADCP observation depth. Also,
relatively cold water located around 153 38:50 E and 3002450 m
depths in leg 1 seems to break into three smaller patches centeredat (153 360 E, 300 m depth), (153 360 E, 380 m depth), and(153 400 E, 350 m depth).
Utilizing the above tracing of patches, we made roughestimates of diffusivity from a Lagrangian viewpoint:
KL ¼

1 dF Fðt þ DTÞ  FðtÞ

;
2 dt
2DT

where F is the variance of the spread of water parcels in thehorizontal or vertical direction, FðtÞ and Fðt þ DtÞ are the initial
and the ﬁnal variance, respectively, and DT is the time duration(Awaji, 1982). This parameter represents the apparent diffusivity
due to the distortion of water particle distributions. We estimated
K L by assuming that the patches described above are a manifestation of water particle movement.
The calculated values of horizontal diffusivity are KhL 102 and 103 m2 s1 for the dichothermal layer and at mid depths,
respectively. The calculated vertical diffusivity is KvL 0:01 m2 s1in the dichothermal layer. These rough estimates suggest thatstrong horizontal and vertical mixing (or more precisely, stirring)
was taking place during the observation, leading to energetic
water-mass transformation. It should be noted, however, thatthe estimates relied on the traceability of the patches, and wecannot reject the possibility that the changes were caused bycross-line advection from the available data. Nevertheless, theoccurrence of such strong mixing or stirring is qualitatively
consistent with the result of previous tidal simulations (Nakamura and Awaji, 2004) and is a likely cause of the presence ofmany unsteady patches.
4.3. Internal wave generation over the sill
Vertical displacement of isotherms is seen over the sill below
500 m depth in Fig. 3 and is shown for the selected isotherm in
Fig. 6d. These are suggestive of internal wave generation by theﬂow passing over the sill.
The temporal evolution of the isotherms shown in Fig. 6d
displays elevation on the downstream side of the sill: After thecurrent ﬂows leftward during legs 1 and 2 as estimated from thewater-mass movement (Fig. 6c), the isotherm observed at leg 2(the blue curve in Fig. 6d) is elevated on the left hand side of thesill top (its location is indicated by the solid triangle in Fig. 6d).
Similarly, the rightward (leftward) ﬂow during legs 2 and 3 (3 and4) produced an elevation on the right (left) hand side of the sill
top, as shown by the green (red) colored isotherm. These features
resemble the results of numerical experiments on unsteady lee
waves rather than internal tides.
Internal waves generated by a tidal ﬂow has intrinsic frequency
approximately given by the sum or difference of Doppler shift(or lee wave frequency) and the corresponding tidal frequency,that is kU7o, where k is horizontal wavenumber of generated
waves or the topography, U and o are the tidal ﬂow speed and605

frequency, respectively (Nakamura et al., 2000a). Accordingly,
tidally generated internal waves can be classiﬁed using the ratio,kU 0 =o, into internal tides of tidal frequency (when kU 0 =o  1),
unsteady lee waves (when kU 0 =o 1), and waves in theintermediate regime referred to mixed tidal-lee waves (whenkU 0 =o1), where U0 is tidal ﬂow amplitude. This ratio is
estimated to be approximately 2 in the present case, in which
the horizontal scale and the ﬂow speed are about 10 km and
0:4 m s1 , respectively, and semi-diurnal tides are dominant. The
waves generated near the sill top are thus characterized as mixed
tidal-lee waves but are close to unsteady lee waves.
The propagation and growth of unsteady lee waves are deeply
affected by advection (Nakamura and Awaji, 2001). Accordingly,
their co-phase lines and group velocity are not parallel. The wave
phase propagates upstream and downward, forming cophase lines
slanting upstream from the vertical, whereas the wave energy isadvected downstream and thus propagates almost verticallyupward, moving slightly downstream (e.g., reviews by Gill,
1982; Baines, 1995 for steady lee waves and Lott and Teitelbaum,
1993; Nakamura and Awaji, 2001 for unsteady lee waves). As aresult, elevation takes place immediately downstream of the sill
top at a shallower depth. These features of unsteady lee waves are
consistent with the observed isotherm evolution.
The effect of these internal lee waves generated over the sill
would reach the surface layer by emanating wave rays. The lee
wave generation by tidal ﬂow over the sill is thus one candidate of
the origin of the horizontally propagating internal waves near thesea surface.
Note that while unsteady lee waves tend to be generated near asill top, waves generated over sill slopes usually become internal
tides. This is because over sill slopes (near the sill top), the tidal
ﬂow amplitude is small (large) and the horizontal scale of the
topography is large (small), so that the ratio kU 0 =o becomes small(large). The generation of internal tides is thus expected to occur
over the sill slopes; however, it is difﬁcult to extract their signal
from the present data.

5. Summary and discussion
The repeated XBT and ADCP observations in the Kruzenshtern
Strait revealed the presence of internal waves near the sea surface
and the vertical motion of isotherms which are consistent withthe generation of internal lee waves having an amplitude of about
50 m over the sill top in the strait.
Most of the near-surface waves have such large amplitude that
nonlinearity becomes important. In particular, the observation
found nonlinear internal waves which seem to be trapped near
the sea surface and likely carry water inside an area enclosed by
the isotherm and the sea surface as they propagate. A roughestimate of the volume ﬂux induced by such waves is O(0.1 Sv),
which is a signiﬁcant amount considering that it takes place in the
top 30 m depths.
Considering that the propagation direction of these waves is
not one sided (i.e., there were waves propagating to the
Paciﬁc side and those to the Sea of Okhotsk side), the waves areplausibly of tidal origin. However, it is difﬁcult to determine thegeneration sites from the present data. Although lee waves
generated over the sill in the strait are one possible origin, waves
generated in the adjacent straits or the bank located around48:2 N, 154:7 E (Fig. 1b) could come into the observation site, as
seen in a numerical simulation (Fig. 9 of Nakamura and Awaji,2004).
The currents estimated from water-mass movements always
ﬂowed from the North Paciﬁc to the Sea of Okhotsk in the
dichothermal layer at an average speed of 0:220:3 m s1 , whereas

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T. Nakamura et al. / Continental Shelf Research 30 (2010) 598–607

the average ﬂow was weaker and the oscillatory component
was stronger at mid depths, with an amplitude of around
0:4 m s1 . The presence of many small-scale patches in thepotential temperature ﬁeld and their active temporal variations
suggest the occurrence of vigorous water-mass transformation. In
fact, rough estimates yield vertical diffusivity of Oð102 m2 s1 Þ inthe dichothermal layer and horizontal diffusivities ofOð102 m2 s1 Þ in the dichothermal layer and Oð103 m2 s1 Þ at mid
depths.
Lastly, it should be noted that the present observations
were conducted when the tidal amplitude was close to theannual minimum and semi-diurnal tides were dominant. Internal
tides can be internal waves for semi-diurnal tides but cannot
be for diurnal tides, which are subinertial around the Kuril
Straits. The internal-tide response is thus expected to bedifferent from that at the observation time and that in typicalconditions in which diurnal tides dominate. However, unsteadylee waves can be internal waves even when generated bysubinertial diurnal tides. Further, the behavior of unsteady lee
waves is controlled by the ratio kU 0 =o rather than by thecorresponding tidal frequency for the same stratiﬁcation, topography, and inertial frequency. One can therefore expect, atleast theoretically, to see internal lee waves of larger amplitudes
and more intense water-mass transformation at other timesof the year when the tidal ﬂow is swifter and diurnal tides ofsmaller frequency are dominant. This should be especially so near
spring tides.

Acknowledgments
We thank the two reviewers for their valuable comments.
Numerical calculations were performed on the Pan-Okhotsk
Information System of ILTS. One author (T.N.) was partly
supported by a Grant-in-Aid for Young Scientists (KAKENHI).

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