Benthic Front and the Yamato Basin Bottom Water … Front and the Yamato Basin Bottom Water in the...

1047

Journal of Oceanography, Vol. 61, pp. 1047 to 1058, 2005

Keywords:⋅⋅⋅⋅⋅ Japan Sea,⋅⋅⋅⋅⋅ Yamato Basin,⋅⋅⋅⋅⋅ benthic front,⋅⋅⋅⋅⋅ Bottom Water,⋅⋅⋅⋅⋅ vertical diffusivity,⋅⋅⋅⋅⋅ geothermalheating,

⋅⋅⋅⋅⋅ oxygen consump-tion,

⋅⋅⋅⋅⋅ water massmodification,

⋅⋅⋅⋅⋅ deep circulation,⋅⋅⋅⋅⋅ box model.

* Corresponding author. E-mail: [email protected]

Copyright © The Oceanographic Society of Japan.

Benthic Front and the Yamato Basin Bottom Water inthe Japan Sea

TOMOHARU SENJYU1*, YUTAKA ISODA2, TAKAFUMI ARAMAKI3,4, SHIGEYOSHI OTOSAKA3,SHINZO FUJIO5, DAIGO YANAGIMOTO5, TAKASHI SUZUKI3, KENSHI KUMA2 and KOSUKE MORI6

1Research Institute for Applied Mechanics, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan2Graduate School of Fisheries Science, Hokkaido University, Minato-cho, Hakodate 041-8611, Japan3Marine Research Laboratory, Japan Atomic Energy Research Institute, Minato-machi, Mutsu, Aomori 035-0064, Japan4National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan5Ocean Research Institute, the University of Tokyo, Minami-dai, Nakano-ku, Tokyo 164-8639, Japan6Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

(Received 21 October 2004; in revised form 25 March 2005; accepted 6 April 2005)

Hydrographic observations have revealed detailed structure of the Bottom Water inthe Japan Sea. The Yamato Basin Bottom Water (YBBW) exhibits higher tempera-tures and lower dissolved oxygen concentrations than those found in the Japan BasinBottom Water (JBBW). Both Bottom Waters meet around the boundary region be-tween the Yamato and the Japan Basins, forming a clear benthic front. The structureof the benthic front suggests an estuary-like water exchange between both Basins,with the inflow from the Japan Basin passing under the outflow from the YamatoBasin. It is inferred from the property distributions that the JBBW flowing into theYamato Basin is entrained by the cyclonic circulation in the basin, and modified tobecome the YBBW. Vertical diffusion and thermal balance in the YBBW are exam-ined using a box model. The results show that the effect of geothermal heating hasabout 70% of the magnitude of the vertical thermal diffusion and both terms cancelthe advection term of the cold JBBW from the Japan Basin. The box model also esti-mates the turnover time and vertical diffusivity for the YBBW as 9.1 years and 3.4 ×××××10–3 m2s–1, respectively.

tions (Nitani, 1972; Sudo, 1986; Senjyu and Sudo, 1993,1994). Because of the very narrow temperature and sa-linity ranges, the Proper Water had been recognized as asingle water mass (Worthington, 1981). However, recentstudies based on very accurate data have revealed thatthe Proper Water consists of more than one water mass:the upper portion (Sudo, 1986; Senjyu and Sudo, 1993,1994), the Deep Water and the Bottom Water (Gamo andHoribe, 1983).

The Bottom Water, which lies in the layer deeper thanabout 2000 m, is characterized by an extreme homogene-ity in the vertical distribution of water properties (Nitani,1972; Gamo and Horibe, 1983). In spite of the verticalhomogeneity, Gamo et al. (1986) revealed that the Bot-tom Water shows a horizontal variability; the BottomWater in the Yamato Basin exhibits higher temperatures

1. IntroductionThe Japan Sea is a semi-closed marginal sea in the

western North Pacific (Fig. 1). It is known that the deepwater in the Japan Sea exhibits unique characteristicswhich are never seen in the open oceans in themidlatitudes: exceptionally narrow ranges of temperatureand salinity of 0.0–1.0°C and 34.06–34.08 in practicalsalinity scale, respectively, and high oxygen concentra-tions of more than 200 µmol/kg. This is the reason forthe name “the Japan Sea Proper Water” (Uda, 1934). Sucha peculiarity is attributed to the fact that the Proper Wa-ter is formed within the Japan Sea itself by deep convec-

1048 T. Senjyu et al.

and lower oxygen concentrations than those in the Japanand Ulleung Basins. Sudo’s (1986) statistical analysisbased on historical bottle data reached the same conclu-sion. These studies indicate a distinctive Bottom Waterin the Yamato Basin which we call the Yamato BasinBottom Water (YBBW).

The structural water mass distribution suggests anorganized circulation in the abyssal Japan Sea. Thus, thefirst purpose of this study is to show the detailed struc-ture of the Bottom Waters in the Yamato and Japan Ba-sins on the basis of hydrographic observations, and dis-cuss the deep circulation in the Japan Sea. In the courseof the description, we reveal a benthic front between bothof the Bottom Waters and suggest an abyssal water massexchange between the Basins.

The second purpose of this study is to examine themodification processes of the Bottom Water in the YamatoBasin. Adequate representations of mixing in the oceaninterior, which occur through three-dimensional advectionand diffusion, are crucial for the numerical modeling ofthe general circulation. In particular, the exact estima-tion of the vertical diffusivity is a key to the modelingbecause it directly associates with the thermohaline cir-culation. Distinctive water masses in semi-closed basins

are favorable for the study of mixing processes in theocean interior. For example, Kaneko and Teramoto (1985)(the results are also summarized in Teramoto (1993)) in-vestigated water mass exchange between the Northwest-ern Pacific and the Shikoku-Philippine Basin on the ba-sis of the distribution of water characteristics and revealeda peculiar water mass lying at 1500–3000 m in the north-ern Philippine Basin, the Shikoku Basin Water. Since thewater shows lower salinity and higher silicate concentra-tions than those of any mixtures between the Northwest-ern Pacific Water and the South Philippine Sea Water onthe surface where potential density (σθ) is 27.70, theyconcluded that the Shikoku Basin Water is formed by thevertical mixing in the basin. Laird (1971) described thedeep and bottom waters in the Panama Basin in the equa-torial Pacific and pointed out the importance of verticalmixing and geothermal heating for the modification ofwater masses.

The decay of the Bottom Water formation and thestagnation of the thermohaline circulation in the JapanSea have been inferred from the gradual warming andoxygen depression in the Deep and Bottom Waters (Gamoet al., 1986, 2001; Kim and Kim, 1996; Minami et al.,1999; Riser et al., 1999; Gamo, 1999; Chen et al., 1999;Kim et al., 2001). Some of these studies have suggesteda relationship with global warming, though this has notbeen verified quantitatively. In order to evaluate the ef-fects of global warming correctly and predict the futureevolution in the Japan Sea, an exact estimation of watermass modification processes is essential.

The remaining part of this paper is organized as fol-low. The observations and data processing are describedin Section 2. The structure of the Bottom Waters and deepcirculation in the Japan Sea are discussed in Section 3 onthe basis of the property distributions. The modificationprocesses of the Bottom Water in the Yamato Basin areexamined in Section 4 using a box model. Though thebox model is a simple one based on the property distribu-tions, useful information is derived about the vertical dif-fusivity and thermal balance in the Yamato Basin. Finally,conclusions and remarks are given in Section 5.

2. ObservationsTwo hydrographic observations by R/V Hakuho

Maru of the Ocean Research Institute, the University ofTokyo were carried out in the Japan Sea during the peri-ods of October 12–22 in 1999 (Leg 2 during the KH99-4cruise) and October 14–19, 2002 (Leg 3 during the KH02-3 cruise). During the KH02-3 cruise, the observation linewas set to extend over the three basins; starting from thesouthern rim of the Ulleung Basin, traversing the YamatoBasin along its longitudinal axis, and running along theeastern Japan Basin to the north (Line-A in Fig. 1). Onthe other hand, observations from Leg 2 during the KH99-

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200m1000m A01

2000m

3000m

3500m

UB

YB

JB

YR

A03

A05

A08

A11

A14

A16

A18

F07B01

B13

Fig. 1. Bottom topography in the Japan Sea and the location ofstations during the KH02-3 (Line-A, solid circles) andKH99-4 (Line-B, solid triangles) cruises. A solid star de-notes observations from Sta. F07 during the KH99-4 cruise.JB, YB, UB, and YR stand for the Japan Basin, the YamatoBasin, the Ulleung Basin, and the Yamato Rise, respectively.

Benthic Front and the Yamato Basin Bottom Water in the Japan Sea 1049

4 cruise were concentrated in the Yamato Basin. Amongthe observation points, we selected the observational linecrossing the basin shown in Fig. 1 (Line-B). It is on thebasis of these two observational lines that we describethe hydrographic structure of the Bottom Waters in andaround the Yamato Basin.

The vertical distributions of temperature, salinity anddissolved oxygen down to the bottom were measured withan SBE911plus CTD system (Sea-Bird Electronics, Inc.)at each of the stations in both of the cruises. The nominalresolution and initial accuracy of the temperature andconductivity sensors are 0.0002°C, ±0.001°C and 0.00004Sm–1, ±0.0003 Sm–1, respectively. The dissolved oxygensensor mounted on the CTD probe is the Beckman po-larographic type (SBE13, Sea-Bird Electronics, Inc.) andthe manufacturer claims a resolution and accuracy of 0.01ml l–1 (0.45 µmol kg–1) and 0.1 ml l–1 (4.47 µmol kg–1),respectively.

The conductivity and dissolved oxygen data were

calibrated using water-sample data taken at 25 layers atall of the stations during the KH02-3 cruise and at 16stations during the KH99-4, which correspond to about30% of the total stations visited during the cruise. Theconductivity of the sampled waters was measured with asalinometer (Portasal 8410, Guildline Instruments Ltd.)on board, and the cell factors, which are the ratio of con-ductivity from water samples to that from CTD, weredetermined for each cruise before calculating the salin-ity. Because the nominal accuracy of the salinimeter is±0.003 in practical salinity scale, a similar accuracy isexpected for the calibrated salinity data. On the otherhand, concentrations of dissolved oxygen are calculatedfrom the oxygen sensor electric current and the probe in-ternal temperature, along with the CTD-derived tempera-ture, salinity and pressure according to Owens and Millard(1985). We calibrated the oxygen data following the WorldOcean Circulation Experiment (WOCE) OperationsManual by the WOCE Hydrographic Programme Office

P. Temperature ( )

2010 1.0

0.5

0.3

0.2

0.1

0.09

0.08

0.07

(a)

Salinity > 34.069

34.068

34.06934.07 34.08

34.068

L

L

(b)

P. Density 27.351

27.350

27.345

27.34

27.3227.327.0

(c)

H Dissolved Oxygen ( mol/kg) 210

209

210

206

207

208

208

220

230

250

LL

(d)

Fig. 2. Vertical sections of the water properties along Line-A during the KH02-3 cruise: (a) potential temperature (°C),(b) salinity, (c) potential density, and (d) dissolved oxygen (µmol kg–1).


(1991); five parameters in Owens and Millard’s (1985)algorithm were determined with a nonlinear least squarefitting to the oxygen concentrations in the water samples.The root mean squares of the differences between theWinkler-titrated and CTD-derived oxygen concentrationsare 1.90 µmol kg–1 for the layer below 500 dbar and 4.63µmol kg–1 for that above 1500 dbar in the KH02-3 cruise(see Fig. 3). The same degree of accuracy is expected forthe KH99-4 cruise.

3. Property Distributions and Abyssal Circulation inthe Japan SeaThe sections of potential temperature, salinity, po-

tential density, and dissolved oxygen along Line-A areshown in Fig. 2. A warm, saline, and high oxygen eddy isseen at a depth of 200 m of Sta. A16; the influence of theeddy is recognizable down to 2500 m. A weak salinityminimum of less than 34.068 lies in the depth range of1000–2000 m, which corresponds to the Deep Water (Fig.2(b)). Weak vertical gradients are seen in the water prop-erties below about 2000 and 2500 m deep in the Yamatoand Japan Basins, respectively, which is the characteris-tic of Bottom Water. Figure 3 shows the vertical profilesof potential temperature and dissolved oxygen at Stas.A05 and A06 in the Yamato Basin; the extreme verticalhomogeneity in the YBBW and the boundary between theDeep and Bottom Waters at depths of 2200–2300 m areconfirmed.

The Bottom Water in the Yamato Basin shows a rela-tively high temperature, warmer than 0.085°C comparedto that in the Japan Basin, which is colder than 0.070°C(Fig. 2(a)). Both Bottom Waters meet around the bound-

ary of both basins; as a result, a benthic front is formedbetween Stas. A08 and A12. Vertical profiles of potentialtemperature at Stas. A08–A12 are shown in Fig. 4 (upperpanel). The thermocline (or benthic front) appears nearthe bottom of Sta. A09 and increases its thickness to thenorth.

A more detailed structure of the benthic front can beseen in the dissolved oxygen section (Fig. 2(d)). In thisfigure, oxygen contours have been drawn for every 0.5µmol kg–1 below 210.0 µmol kg–1 to enhance the bottomlayer structure. The Japan Basin Bottom Water (JBBW),having higher oxygen concentrations (>209.0µmol kg–1), intrudes under the YBBW of lower oxygen(<206.0 µmol kg–1), and a steep oxygen front is formednear the bottom of Stas. A08–A12. Vertical profiles ofdissolved oxygen in the frontal region (Fig. 4, lower panel)also show the intrusion of the JBBW along the bottomand the consequent minimum layer associated with theYBBW. The dissolved oxygen section also shows a north-ward extension of lower-oxygen water at a depth of about2000 m, forming another front between Stas. A13 andA14. This northward extension from the Yamato Basincontinues to the oxygen minimum layer in the Japan Ba-sin of less than 208.5 µmol kg–1. Such an oxygen distri-bution implies an estuary-like water exchange at themouth of the Yamato Basin, the outflow from the YamatoBasin lying on top of the inflow from the Japan Basin, assuggested by Gamo et al. (1986).

In the Japan Basin, a slightly dense water of morethan 27.351σθ, which corresponds to the slightly saline(>34.069) and high oxygen (>210.0 µmol kg–1) water, isseen on the southern rim-slope of the Basin (Stas. A13–

1000

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m)

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PT

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θDO A06

Fig. 3. Vertical profiles of the potential temperature (θ) and dissolved oxygen (DO) below 1000 m at Stas. A05 and A06 in theYamato Basin. The open circles denote the dissolved oxygen concentration determined by the Winkler method. Arrows indi-cate the boundary between the Deep and Bottom Waters.


A15 in Figs. 2(b), (c) and (d)); such a density structuresuggests an eastward flow along the rim-slope. Thisdenser, saline and oxygen-rich Bottom Water is possiblynew Bottom Water transported from the western JapanBasin. Senjyu et al. (2002) reported that a new BottomWater with a lower temperature, higher salinity, and higheroxygen concentration was formed during the winter of2000–2001 in the area off the south of Vladivostok. Hattaand Zhang (2003) discussed the new Bottom Wateradvection to the eastern Japan Basin on the basis of thedistribution of dissolved rare earth elements.

Figure 5 shows the cross-basin structure of theYBBW along Line-B. The CTD system detected a weaksalinity minimum of less than 34.064 between 800–1600

m, which corresponds to the Deep Water, though the causeof the lower salinities during the KH99-4 cruise is ob-scure. The potential temperature and density sections(Figs. 5(a) and (c)) show a dome of the Proper Water be-low about 900 m, which suggests a cyclonic circulationin the Yamato Basin. Indeed, the profiles of geostrophicvelocity on Line-B show cyclonic flows above the depthsof 2000 m, though the baroclinic flows are almost zerobelow 2000 m due to the extreme homogeneity in theYBBW (Fig. 6). The dissolved oxygen section shows aminimum layer of less than 209.0 µmol kg–1 in the depthrange of 1800–2500 m (Fig. 5(d)). It is noteworthy thatthe core of the oxygen minimum is biased toward the Japa-nese side and higher oxygen water of more than 209.5

Fig. 4. Vertical profiles of the potential temperature (upper panel) and dissolved oxygen (lower) below 1000 m in the frontalregion between Stas. A08 and A12. Each profile is shifted by 0.04°C or 4.0 µmol kg–1.

Fig. 5. Vertical sections of the water properties along Line-B during the KH99-4 cruise: (a) potential temperature (°C),(b) salinity, (c) potential density, and (d) dissolved oxygen (µmol kg–1).


µmol kg–1 is seen under the minimum layer at the south-eastern base of the Yamato Rise (Stas. B03–B06). Thishigher oxygen water corresponds to the higher salinitynear the bottom at Stas. B03–B05 (Fig. 5(b)). These prop-erty distributions imply the cyclonic intrusion of salineand oxygen-rich Bottom Water from the Japan Basin.

The cyclonic intrusion from the Japan Basin is prob-ably associated with the estuary-like Bottom Water ex-change between the Basins. In the case of estuary circu-lations on the rotating earth, lower (higher) density waterfrom the river (open sea) tends to shift on the right-handside to the flow direction due to the Coriolis force in theNorthern Hemisphere and a cyclonic circulation is formedin the estuary region. A similar flow system is likely tooccur at the mouth of the Yamato Basin.

Recently, Senjyu et al. (2005) compiled direct cur-rent measurements obtained in the Proper Water and de-duced the deep circulation pattern in the Japan Sea. Theproperty distributions mentioned above are consistentwith the deep circulation pattern: the eastward flow onthe southern rim-slope of the Japan Basin, which is partof the cyclonic circulation in the Japan Basin, and thecyclonic circulation in the Yamato Basin.

4. The Yamato Basin Bottom Water and Its Modifi-cation ProcessesA benthic front has been found between the YBBW

and the JBBW at the boundary region of the Yamato andJapan Basins. Benthic fronts are often observed in theopen oceans where water masses with different sources

meet: for example, between the Antarctic Bottom Waterand the North Pacific Deep Water in the South and NorthPacific (Craig et al., 1972; Chung, 1975). However, it isconsidered that the YBBW and the JBBW have the sameorigin because the only source of the Bottom Water is inthe northwestern part of the Japan Sea (Senjyu et al., 2002;Talley et al., 2003).

The YBBW exhibits higher temperature and loweroxygen concentrations than those in the JBBW, whichindicates that the YBBW is older than JBBW. From thisfact, we can imagine qualitatively the formation processof the YBBW as follows. As deduced from the propertydistributions, the JBBW flows into the Yamato Basinthrough the mouth at the northern end of the basin. It isthe only open boundary of the Yamato Basin of deeperthan 2000 m (Fig. 1). Because of the closed cyclonic cir-culation in the Yamato Basin, the flowed JBBW stayswithin the basin and undergoes long-time modificationand as a result, the YBBW having higher temperaturesand lower oxygen concentrations is formed.

In the following subsections, we discuss quantita-tively the modification processes in the Yamato Basin andtry to estimate the timescale for the modification.

4.1 Simple estimation under the assumption of no Bot-tom Water supplyMinami et al. (1999) explained the gradual warming

and oxygen depression in the abyssal Japan Sea using aone-dimensional vertical advection-diffusion model.Thus, as the first step, let us consider the vertical balancemodel with the event-like inflow of the JBBW into theYamato Basin.

Since no Bottom Water is supplied from the JapanBasin after the inflow event, the potential temperatureand oxygen concentration in the poured Bottom Waterchange following the one-dimensional (z) vertical ther-mal and oxygen balances for the YBBW;

∆∆θ κ θt z

Y= ∂∂

( )2

2 1,

∆∆O

t

O

zRY= ∂

∂− ( )κ

2

2 2,

where ∆θ and ∆O denote the typical difference of poten-tial temperature and dissolved oxygen concentration be-tween the YBBW and the JBBW, respectively, θY and OYshow the potential temperature and dissolved oxygen con-centration in the YBBW, κ is the (constant) vertical eddydiffusivity, and R is the oxygen consumption rate in theYamato Basin. From the equations, we can estimate themodification timescale (∆t) and vertical diffusivity asfollows:

Fig. 6. Profiles of geostrophic velocity (cm s–1) referred to2500 dbar on Line-B. Positive (negative) values indicatenortheastward (southwestward) flows. The profile ofgeostrophic velocity between Stas. A and B is referred bythe symbol VA-B.


∆ ∆ ∆tR

Oz

z

OY

Y

=∂

∂∂

∂

−

( )13

2

2

2

2θ

θ ,

and

κ θθ

=∂

∂

( )∆

∆2

2

4Y

zt

.

Our observational results show that the typical dif-ferences in the potential temperature (∆θ) and dissolvedoxygen concentration (∆O) between the YBBW and theJBBW are 0.017°C and –3.5 µmol kg–1, respectively. Thevertical second derivatives in potential temperature anddissolved oxygen are estimated from the profiles at Stas.A05 and A06 in Fig. 3, and are 8.58 × 10–8 °C m–2 and1.55 × 10–5 µmol kg–1m–2 in the depth range of 2000–2500 m, respectively.

The remaining unknown parameter is the oxygenconsumption rate (R). Table 1 shows the oxygen-decreaserates in the abyssal Japan Sea proposed in the literatures.They lie in the range 0.4–1.0 µmol kg–1yr–1. However, itshould be noted here that the oxygen-decrease rates (∂O2/∂t) listed in Table 1 are different from the oxygen con-sumption rate; the values in Table 1 are estimated fromtime series of the observed oxygen concentration in thebottom layer, except for Gamo and Horibe (1983). If thereare continuous oxygen replenishments through the verti-cal diffusion, as shown in Eq. (2), then these rates shouldbe small compared to R. Hence, in this study we adoptChen et al.’s (1996) oxygen consumption rate (R =1.13µmol kg–1yr–1 = 3.58 × 10–8 µmol kg–1s–1); they appliedthe mass balance model assuming zero initial apparentoxygen utilization (AOU) to the Japan Sea and estimatedthe oxygen consumption rate in the bottom layer.

Substitution of the above parameters yields a modi-fication timescale of 5.8 years, which results in a verticaldiffusivity of 1.08 × 10–3 m2s–1. The oxygen-decrease ratecalculated from the modification timescale (∆O/∆t = 0.60µmol kg–1yr–1) shows a value intermediate betweenMinami et al.’s (1999) and Gamo et al.’s (1986) in theYamato Basin (Table 1). However, the rate of tempera-ture increase (∆θ/∆t = 2.92 × 10–3 °C yr–1) is much largerthan the observed values; for example, Minami et al.(1999) reported the temperature increase at 4.0 × 10–4

°C yr–1 and 6.0 × 10–4 °C yr–1 at the depths of 2500 and2000 m in the Yamato Basin, respectively. This discrep-ancy indicates that the one-dimensional vertical balancemodel is inadequate for the YBBW modification and sug-gests the important role of the lateral processes.

4.2 Box model analysis with horizontal advectionsIn the previous section we suggested an estuary-like

water exchange between the Yamato and Japan Basins.Since the oxygen concentrations in the JBBW are higherthan those in the YBBW, the inflow of the JBBW to theYamato Basin indicates a continuous oxygen replenish-ment into the Yamato Basin. The observational fact thatthe oxygen-decrease rates in the Yamato Basin show lowervalues than those in the Japan Basin, as shown by Gamoet al.’s (1986) and Minami et al.’s (1999) estimations inTable 1, supports the oxygen supply into the Yamato Ba-sin. Thus, we constructed a box model including hori-zontal advections.

The model consists of three boxes, as shown in Fig.7(a). Boxes Y and J denote the Bottom Waters in theYamato and the Japan Basins, respectively, and Box Drepresents the Deep Water. The properties in D and J areassumed to be constant, though recent studies have re-ported gradual changes in the Deep Water and the JBBW(Gamo, 1999; Chen et al., 1999; Minami et al., 1999;Riser et al., 1999). In contrast, the properties in Y arechangeable depending upon the boundary conditions: thehorizontal advections UB to the JBBW and UD to the Deep

References Decreasing rate(µmol kg–1year–1)

Remarks

Gamo and Horibe (1983) 0.4Gamo et al. (1986) 0.7 in the Yamato Basin

1.0 in the eastern Japan BasinMinami et al. (1999) 0.53* in the Yamato Basin at 2500 m

0.66* in the eastern Japan Basin at 2500 mChen et al. (1999) 0.8 in the Japan BasinGamo (1999) 0.76 in the eastern Japan Basin

Table 1. Decreasing rates of dissolved oxygen concentration in the abyssal Japan Sea.

*Assuming σθ = 27.350.


Water, the vertical advection W to the Deep Water, andthe vertical diffusion K. Here, the horizontal diffusionswere assumed to be negligible. The horizontal advectionsUB and UD occur through sections AB and AD, respectively(Fig. 7(b)); they represent the estuary-like water exchangeinferred from the benthic front structure in Fig. 2. On theother hand, the upwelling W and the vertical diffusion Koccur uniformly through the upper surface of the YBBW(area AY). It should be noted that the model provides thesum of UD and W as the net mass flux by the horizontaland vertical advections to the Deep Water, though forconvenience we distinguish between them here. The re-lationship between these physical processes and the wa-ter exchange rates which have been commonly used inthe box models related to the Japan Sea is discussed inthe Appendix.

Let us consider the YBBW (Box Y) under the coor-dinate system shown in Fig. 7(b). Because of the topo-graphic constraints, the integration of the mass conser-vation equation with respect to x, y, and z gives

A U A U A WB B D D Y+ + = ( )0 5.

As for the potential temperature (θ), using u, v and w asvelocity in x, y and z direction, respectively, we considerthe advection-vertical diffusion equation,

∂∂

+ ∂∂

+ ∂∂

+ ∂∂

= ∂∂

∂∂

θ θ θ θ κ θt

ux

vy

wz z z

.

When we integrate the above equation with respect to x,y, and z, we obtain

Vt

A U

A U A W Az

YY

B B J Y

D D D Y Y D Y YY

∂∂

+ −( )

+ −( ) + −( ) = ∂∂

( )

θ θ θ

θ θ θ θ κ θ, 6

where VY is the volume of the YBBW. Suffixes J, Y, andD denote the Japan Basin, the Yamato Basin, and the DeepWater, respectively. Similarly, the integrated form of theadvection-diffusion equation for the dissolved oxygen (O)is

VO

tA U O O

A U O O A W O O AO

zV R

YY

B B J Y

D D D Y Y D Y YY

Y

∂∂

+ −( )

+ −( ) + −( ) = ∂∂

− ( )κ , 7

where the term VYR denotes the oxygen consumption.

From Eqs. (5), (6), and (7), we obtain the BottomWater advection (ABUB) and the vertical eddy diffusivity(κ) as follows:

A U

V

Oz

zt

O

tR

O O

Oz

z

B B

Y

Y

Y

Y Y

J D

Y

YJ D

=

∂∂

∂∂

∂∂

− ∂∂

−

− −∂

∂∂

∂−( )

( )θ

θ

θ θ θ

, 8

and

κ

θ θ θ

θ=

∂∂

+ −( )∂∂

( )V

tA U

Az

YY

B B J D

YY

. 9

Typical values for θJ, θD, OJ, and OD are 0.07°C, 0.10°C,209.5 µmol kg–1, and 207.5 µmol kg–1, respectively (Fig.2). The vertical gradient terms ∂θY/∂z and ∂OY/∂z areevaluated from the potential temperature and the dissolvedoxygen profiles at Stas. A05 and A06 (Fig. 3) by the leastsquare method, and are 1.02 × 10–5 °C m–1 and 1.76 ×10–3 µmol kg–1m–1, respectively, in the depth range of2000–2500 m. The volume VY (1.83 × 1013 m3) is roughlyestimated from the area AY (3.67 × 1010 m2, estimatingfrom the 2000 m isobath) and the mean thickness of theYBBW (500 m). For ∂θY/∂t and ∂OY/∂t, we adopted theMinami et al.’s (1999) values at a depth of 2500 m in theYamato Basin: 1.27 × 10–11 °C s–1 and –1.69 × 10–8 µmolkg–1s–1, respectively. If we assume Chen et al.’s (1996)

xy

z

0

AD

AB

AY

Y

J

D

UB

UD

WK

VY

(a) (b)

Fig. 7. Configuration of the box model. Boxes Y, J, and D in(a) denote the YBBW, the JBBW, and the Deep Water, re-spectively. VY is the volume of the YBBW. The horizontaladvections UD and UB occur through sections AD and AB in(b), respectively. Both the vertical advection W and the dif-fusion K occur uniformly through the upper surface of theYBBW, AY.


oxygen consumption rate again, the Bottom Water flux(ABUB) is estimated to be –4.26 × 104 m3s–1. The nega-tive value indicates the flow into the Yamato Basin fromthe Japan Basin. Since Section AB is roughly 3.06 × 107

m2, a cross-sectional velocity UB of –1.39 × 10–3 m s–1 isexpected.

Now we can calculate the turnover time of the YBBWabout the influx of the Bottom Water (ABUB) from

τ = ( )V

A UY

B B

, 10

and it is estimated to be 13.6 years.The vertical diffusivity κ from Eq. (9) is 4.03 ×

10–3 m2s–1. This is of the same order of magnitude as thatestimated in the previous subsection, though these val-ues are ten to a hundred times larger than the previouslyreported eddy diffusivities in the abyssal oceans (Tooleand McDougall, 2001).

4.3 Effects of geothermal heatingWe can recognize the boundary between the Deep

and Bottom Waters as a discontinuity in the property pro-files, as shown in Figs. 3 and 4. This is due to the ex-treme vertical homogeneity in the Bottom Water. On theother hand, deepening of the boundary and gradual warm-ing and oxygen depression in the Bottom Water have beenreported (Gamo et al., 1986; Kim and Kim, 1996; Minamiet al., 1999; Gamo, 1999; Chen et al., 1999; Kim et al.,2001). Minami et al. (1999) tried to explain the changesin profiles using a one-dimensional vertical model, andobtained smooth profiles rather than those with a discon-tinuous boundary. The interesting point is that the Bot-tom Water seems to be warming while retaining its verti-cal homogeneity. To maintain the vertical homogeneityagainst the diffusion from the upper layer, some stirringmechanisms are needed, such as vertical convection orbreaking of internal waves.

Geothermal heating is one of the candidates havinga potential to cause the vertical mixing in the bottom layer.Yanagi and Shimizu (2003) tried to explain the tempera-ture difference in the Bottom Water between the northernand southern Japan Sea in terms of terrestrial heat flux.Laird (1971) also showed the importance of geothermalheating in the modification of the Panama Basin DeepWater. Thus, tentatively, we incorporate the effect ofgeothermal heating into the box model.

According to Yamano et al. (1996), the heat flowsthrough the bottom crust are relatively uniform in the Ja-pan Sea and the average of the reliable heat flow data inthe Yamato Basin is 97 ± 12 mW m–2, which correspondsto a rate of temperature increase (q) of 4.78 × 10–11

K s–1m–3, assuming uniform heating through the bottomarea AY and a specific heat and in situ density of the YBBW

of 3900 J kg–1K–1 and 1041.0 kg m–3, respectively. If weadd the geothermal term (VYq) to the right-hand side ofEq. (6), the term ∂θY/∂t in Eqs. (8) and (9) is replaced by(∂θY/∂t – q). Substitution of the above q value provides aBottom Water flux (ABUB) of –6.36 × 104 m3s–1, whichresults in a turnover time of 9.1 years, and vertical diffu-sivity of 3.38 × 10–3 m2s–1.

We can evaluate the relative importance of each termin the thermal balance of the YBBW from the modifiedEq. (6),

Vt

A U Az

V qYY

B B J D YY

Y∂∂

+ −( ) = ∂∂

+θ θ θ κ θ.

The approximate values of terms , , , and whenwe substituted the above parameters are 232, 1908, 1265,and 875, respectively. It should be noted that thegeothermal term has about 70% of the magnitude ofthe vertical thermal diffusion term , and both termscancel the cold JBBW advection term . If geothermalheating truly causes a vertical mixing, the Bottom Watershrinks as it retains its vertical homogeneity due to ther-mal diffusion from the upper layer, the largest heat source.

5. Concluding RemarksWe have discussed the deep circulation in the Japan

Sea and the water mass modification in the Yamato Basinon the basis of the Bottom Water structure obtained bythe hydrographic observations. The YBBW exhibitshigher temperatures and lower dissolved oxygen concen-trations than those in the JBBW. A clear benthic front isformed at the boundary of both of the Bottom Waters.The structure of the benthic front suggests an estuary-like water exchange between the Yamato and Japan Ba-sins. The box model analysis based on the property dis-tributions revealed that the JBBW flowing into the YamatoBasin, which is entrained by the cyclonic circulation inthe basin, is modified to the YBBW by vertical diffusion,oxygen consumption, and geothermal heating.

The vertical diffusivity in the deep layer is an im-portant factor for the thermohaline circulation system inthe Japan Sea. The estimated values of κ in the previoussection showed O(10–3 m2s–1). Though the same order ofκ (6.7 × 10–3 m2s–1) has been estimated in the abyssalUlleung Basin in the Japan Sea (Chang et al., 2002), theyare one to two orders of magnitude larger than the previ-ously reported values in the deep oceans. For example,Morris et al. (2001) estimated the space-time mean verti-cal diffusivities, using a box model similar to that in thisstudy, in the range of 1–5 × 10–4 m2s–1 for the AntarcticBottom Water in the Brazil Basin. The verticaldiffusivities estimated from the velocity microstructurealso indicate the values of O(10–5 m2s–1) in the flat ba-


sins and O(10–4 m2s–1) on the slopes of seamounts in theBrazil Basin, though values of O(10–3 m2s–1) have beenreported on the rough flanks of the Mid-Atlantic Ridge(Toole and McDougall, 2001).

The large values of κ in the estimation from Eq. (9)are due to the smallness of the vertical temperature gra-dient in the Bottom Water. Thus, it is considered that theelucidation of cause of the extreme vertical homogeneityin the Japan Sea Bottom Water is a key factor in under-standing the water mass modification. The enhanced in-ternal waves in the bottom layer are possible cause of theabyssal mixing, as well as geothermal heating. Shcherbinaet al. (2003) showed that the mean kinetic energy associ-ated with the internal waves increases with depth below2000 m in the Japan Sea, on the basis of lowered acousticDoppler current profiler (LADCP) observations. In addi-tion, dominant near-inertial flows have been observed inthe abyssal Japan Sea using moored current meters(Takematsu et al., 1999; Mori et al., 2005; Senjyu et al.,2005). This suggests that the internal gravity waves withnear-inertial frequency contribute to maintaining the ver-tical homogeneity in the Bottom Water, probably throughthe mechanisms proposed by Kunze and Sanford (1984).Another possibility is mixing by submesoscale eddies withlarge vertical extension, as found at Sta. A16 in Fig. 2.Gascard et al. (2002) pointed out a potential role of thevertically coherent eddies in the deep ventilation in theGreenland Sea. To answer the question of which mecha-nism is important for the abyssal water mass modifica-tion, more detailed studies are needed, based, for exam-ple, on the estimation of κ from the velocity microstruc-tures.

It should be noted here that the spatial features re-vealed by the observations are not in a steady state. Forexample, the potential temperature in the YBBW in-

Fig. 8. Vertical profiles of the dissolved oxygen below 1000 mat Sta. A11 collected during KH02-3 (bold line) and at Sta.F07 during KH99-4 (thin line).

1000

1500

2000

2500

DE

PT

H (

m)

205 210 215 220Dissolved Oxygen ( mol/kg)µ

F07 (1999/10/12)

A11 (2002/10/17)

creased from 0.080–0.085°C in 1999 to 0.085–0.090°Cin 2002 (Figs. 2(a) and 5(a)). Gamo et al. (1986) reportedmuch colder Bottom Water in 1984: less than 0.04°C inthe Japan Basin and 0.04–0.05°C in the Yamato Basin.The oxygen concentration of less than 209.0 µmol kg–1

was not found in 1999 in the YBBW, a YBBW oxygenconcentration of less than 206.0 µmol kg–1 was typicalthough for 2002 (Figs. 2(d) and 5(d)). Figure 8 showsdissolved oxygen profiles below 1000 m at Sta. F07 dur-ing the KH99-4 cruise and at A11 during KH02-3, thesestations are at almost the same position (Fig. 1). The de-crease in oxygen concentrations throughout the Deep andBottom Waters is clear. The difference in the oxygen con-centrations tends to be large (>3.0 µmol kg–1) around theminimum layer associated with the northward intrusionof the YBBW (2000–2300 m depths). These temporalchanges in temperature and oxygen concentration areconsistent with previous studies describing the transitionstate of the Japan Sea (Kim and Kim, 1996; Minami etal., 1999; Gamo, 1999; Gamo et al., 2001; Kim et al.,2001).

In such a changing state of the Japan Sea, our boxmodel assuming steady-state boundary conditions and aconstant volume of the YBBW needs to be improved uponin future to include the temporal variability. In particu-lar, the effect of geothermal heating becomes more im-portant under the situation of a reducing volume ofYBBW. However, the basic mechanism of water massmodification revealed in this study remains unchanged,even in the transient state. In addition, it is expected thatthe thermal balance in the YBBW discussed in the boxmodel analysis is also retained, at least over the turnovertime of the YBBW. For an exact evaluation of globalwarming in the Japan Sea, further quantitative studies arerequired.

AcknowledgementsWe wish to thank the officer and crew of the R/V

Hakuho Maru and all of the participants of the KH99-4and KH02-3 cruises for their assistance in collecting theobservations required for this study. Thanks are also dueto Prof. T. Matsuno and colleagues in the Laboratory ofOcean Circulation Dynamics, RIAM, Kyushu Universityfor their useful comments and discussions. The GFDDENNOU and PSPLOT Libraries were used for plottingfigures.

Appendix. Box Model with Rates of Water ExchangeTo clarify the physical processes in the Bottom Wa-

ter modification, we started from the advection-diffusionequations of (6) and (7). However, the advection terms tothe Deep Water in Eq. (6) are combined as (ADUD +AYW)(θD – θY), and furthermore, the vertical diffusionterm in the right-hand side can be also included as


(ADUD + AYW – AYκ /∆z)(θD – θY). The term (ADUD +AYW – AYκ/∆z) has the unit of volume transport and canbe regarded as the rate of water exchange QD betweenthe Deep Water and the YBBW. Similarly, the advectionterm ABUB(θJ – θY) is rewritten as QB(θJ – θY), where QBdenotes the exchange rate between the JBBW and theYBBW. After all, Eqs. (6) and (7) can be represented us-ing the rates of water exchange as follows:

Vt

Q QYY

D Y D B Y J∂∂

= −( ) + −( )θ θ θ θ θ

VO

tQ O O Q O O V RY

YD Y D B Y J Y

∂∂

= −( ) + −( ) − .

This formulation is simple, and one can avoid the con-sideration of unknown parameters such as κ and W. In-stead, individual physical processes become obscure andone cannot evaluate the contribution of each process. Thiskind of model has been applied to the Japan Sea to calcu-late the residence time of water masses (Harada andTsunogai, 1986; Watanabe et al., 1991; Tsunogai et al.,1993).

To check the consistency of the model, we calcu-lated the exchange rates QB and QD from the above equa-tions as follows:

Q

VO

t

O O

tR

O OO OB

YY Y D

Y D

Y

Y JY D

Y DY J

=

∂∂

− −−

∂∂

+

− − −−

−( )θ θ

θ

θ θθ θ

and

Q

VO

t

O O

tR

O OO OD

YY Y J

Y J

Y

Y DY J

Y JY D

=

∂∂

− −−

∂∂

+

− − −−

−( )θ θ

θ

θ θθ θ

.

Substitution of the parameters used in Section 4 providesrates of –5.84 × 104 m3s–1 and –9.43 × 104 m3s–1 for QBand QD, respectively. The estimated value of QB is about1.4 times larger than that of ABUB in Subsection 4.2. Onthe other hand, the value of QD is about one third ofTsunogai et al.’s (1993) water exchange rate (9.3 × 1012

m3yr–1 = 29.49 × 104 m3s–1) between the intermediate(200–1000 m) and deep layers (1000 m–bottom); theyestimated this value using a box model with three layersand four boxes for the concentrations of tritium and 226Ra,in the case of the same areas of warm and cold surfaces.

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