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Journal of Oceanography, Vol. 63, pp. 243 to 253, 2007

Keywords:⋅⋅⋅⋅⋅ Annual formationrate,

⋅⋅⋅⋅⋅ turnover time,⋅⋅⋅⋅⋅ Upper portion ofthe Japan SeaProper Water,

⋅⋅⋅⋅⋅ Japan Sea Interme-diate Water,

⋅⋅⋅⋅⋅ particle trajectoryexperiment.

* Corresponding author. E-mail: kawamura.hideyuki@jaea.go.jp

Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

Formation Rate of Water Masses in the Japan Sea

HIDEYUKI KAWAMURA1*, JONG-HWAN YOON2 and TOSHIMICHI ITO1

1Research Group for Environmental Science, Division of Environment and Radiation Sciences, Japan Atomic Energy Agency, Shirakata-shirane, Tokai-mura, Ibaraki 319-1195, Japan2Dynamics Simulation Research Center, Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

(Received 19 May 2006; in revised form 26 October 2006; accepted 26 October 2006)

Water masses in the subsurface and the intermediate layer are actively formed due tostrong winter convection in the Japan Sea. It is probable that some fraction of pollu-tion is carried into the layer below the sea surface together with these water masses,so it is important to estimate the formation rate and turnover time of water masses tostudy the fate of pollutants. The present study estimates the annual formation rateand the turnover time of water masses using a three-dimensional ocean circulationmodel and a particle chasing method. The total annual formation rate of water massesbelow the sea surface amounted to about 3.53 ± 0.55 Sv in the Japan Sea. Regardingrepresentative intermediate water masses, the annual formation rate of the Upperportion of the Japan Sea Proper Water (UJSPW) and the Japan Sea IntermediateWater (JSIW) were estimated to be about 0.38 ± 0.11 and 1.43 ± 0.16 Sv, respectively,although there was little evidence of the formation of deeper water masses below adepth of about 1500 m in a numerical experiment. An estimate of turnover time showsthat the UJSPW and the JSIW circulate in the intermediate layer of the Japan Seawith timescales of about 22.1 and 2.2 years, respectively.

through the Soya Strait and the Mamiya Strait. The meandepths of the Tsushima Straits are about 100 and 50 m inthe west channel and the east channel, and those of theTsugaru Strait, the Soya Strait and the Mamiya Strait areabout 114, 44 and 8 m, respectively. These straits are shal-low and thus prevent direct exchange of water massesbelow the sill depth of the straits between the Japan Seaand other seas, which means that water masses belowabout 100 m are formed inside the Japan Sea.

Water in the Japan Sea below the pycnocline is veryhomogeneous with temperature between 0 and 1°C, andsalinity between 33.96 and 34.14 (Yasui et al., 1967); thisis called the Japan Sea Proper Water (Uda, 1934). Later,Sudo (1986) clearly showed discontinuity at a depth ofabout 1000 m in vertical profiles of salinity and DO, anddivided the water below pycnocline to two parts: theUpper portion of the Japan Sea Proper Water (UJSPW)above the discontinuity and the deeper water below it.The potential density (σθ) referred to the sea surface ofthe UJSPW ranges from 27.31 to 27.34 kg/m3 and thesalinity is very homogeneous, ranging from 34.06 to34.07. Above the UJSPW, one finds another remarkablewater mass, characterized by the salinity minimum layer(SML) centered at about 200 m (Miyazaki, 1952, 1953;Kim and Chung, 1984). Since this water mass is charac-

1. IntroductionThe Japan Sea is experiencing changes suggestive

of impacts of global warming. During the second half ofthe 20th century the temperature of the deep layer (belowabout 1000 m) of the Japan Sea was increasing at a rateof 0.002°C/yr and the dissolved oxygen (DO) was de-creasing at a rate of 0.55 µmol/(kg·yr) (Kim et al., 1996;Minami et al., 1997). Based on tracer data and a simplemodel, Riser et al. (1999) suggested that deep ventilationin the Japan Sea was weak during the same period. Onthe other hand, it has been suggested that intermediatewater masses existing between about 100 and 1000 m areactively being formed at present (Kim et al., 2001).

The topography of the Japan Sea is characterized bydeep basins and a rise. The Japan Sea contains three ma-jor basins—the Japan Basin, the Yamato Basin and theTsushima Basin—and a seamount called the Yamato Rise(Fig. 1). The Japan Sea is connected to the East ChinaSea through the Tsushima Straits, to the North PacificOcean through the Tsugaru Strait, and to the Okhotsk Sea

244 H. Kawamura et al.

terized by a DO maximum as well as a salinity minimumin vertical profiles, it is useful as a tracer of circulationin the intermediate layer. This oxygen-rich water masswith salinity minimum is called the Japan Sea Intermedi-ate Water (JSIW), which is described as having potentialdensity between 26.90 and 27.30 kg/m3 and salinity lessthan 34.06.

Movement of these water masses is closely associ-ated with that of pollutants in the Japan Sea. In fact, Itoet al. (2003) reported maxima in vertical profiles of rep-resentative anthropogenic radionuclides, 137Cs and 90Sr,at about 200 m, corresponding to the depth of SML in theJapan Sea. Furthermore, it was also found that the sub-surface chlorofluorocarbon (CFC) maximum was centeredwithin the typical JSIW layer (Min and Warner, 2005).Since these pollutants and the JSIW originate from thesea surface water, some fraction of the pollution in theJapan Sea can be considered to move below the surfacelayer together with the JSIW. An understanding of themovement of these water masses would therefore be help-ful to study the fate of pollutants. One aim of our researchis to estimate the annual formation rate and turnover time

of water masses using a numerical model. The Euler-Lagrangian technique (Awaji et al., 1991; Yoshikawa etal., 1999) was used to calculate the movement of waterparticles in a particle trajectory experiment.

This paper is organized as follows. Section 2 de-scribes the features of an ocean circulation model andexplains a method to calculate the annual formation rateof water masses. Section 3 gives the results of numericalexperiments. Finally, a summary is presented in Section4.

2. Numerical Model

2.1 Description of the ocean circulation modelThe ocean circulation model used in this study is the

same as that used by Yoon and Kawamura (2002), whichis a multi-level primitive equation model, the GFDLMOM, under the Boussinesq approximation, the hydro-static assumption and the rigid-lid surface on a sphericalcoordinate. The model domain extends from 33 to 52°Nin latitude and from 126.5 to 142.5°E in longitude, cov-ering the entire Japan Sea. The horizontal grid resolutionis 1/6° in both latitude and longitude with 19 levels in thedeepest part (Table 1).

The coefficients of horizontal and vertical eddy vis-cosity are set to 2.0 × 106 and 1.0 cm2/s, respectively.Instead of constant horizontal eddy diffusivity, anisopycnal mixing scheme is adopted for diffusion of tem-perature and salinity (Kim and Yoon, 1999). The coeffi-cient of vertical eddy diffusivity increases linearly fromthe top 10 layers (0.1 cm2/s) toward the bottom layer (0.4cm2/s), as shown in Table 1. In regions of static instabil-ity, the convective adjustment process is implicitly solvedwith an enhanced coefficient of vertical eddy diffusivityof 104 cm2/s.

Monthly mean wind stress applied at the sea surfaceis given by the European Centre for Medium-RangeWeather Forecasts (ECMWF). The original daily windstress with horizontal resolution of about 1/2° from Janu-ary 1992 to December 1997 was reanalyzed to calculatethe monthly mean wind stress. Imposed heat flux at thesea surface (QT) is specified by the Haney type heat flux(Haney, 1971) expressed as below:

Q Q Q T TT a= + −( )1 2 1 ,

where Q1 and Q2 are the heat flux coefficients modifiedby Hirose et al. (1996), Ta the monthly mean air tempera-ture obtained from the Comprehensive Ocean-AtmosphereData Set (COADS), and T1 the simulated water tempera-ture at the first level in the ocean circulation model. Saltflux at the sea surface (QS) is given by the Newtoniantype restoring boundary condition expressed as:

Fig. 1. Bottom topography in the Japan Sea with C.I. = 1000m. Dotted line shows the bathymetric line for the depth of200 m. Abbreviations: TS: Tsushima Straits, TG: TsugaruStrait, SS: Soya Strait, MS: Mamiya Strait, JB: Japan Ba-sin, YB: Yamato Basin, TB: Tsushima Basin and YR: YamatoRise.

Formation Rate of Water Masses in the Japan Sea 245

Q S SS s= −( )−γ 11 ,

where γ is a restoring time scale (10 days), Ss the ob-served sea surface salinity, and S1 the simulated watersalinity at the first level in the ocean circulation model.For the observed sea surface salinity we used the long-term monthly mean sea surface salinity data obtained fromthe Japan Oceanographic Data Center (JODC), the Fish-eries Research and Development Agency of Korea(FRDAK) and the Far Eastern Hydrometeorological In-stitute of Russia (FEHMI).

We assume a simple form of sinusoidal variation forseasonal change of volume transport through the TsushimaStraits. The annual mean volume transport is approxi-mated as 2.2 Sv (1 Sv = 106 m3/s) and the amplitude ofseasonal variation is set to 0.35 Sv according to ADCPmeasurement by Isobe (1994). The volume transportthrough the Tsugaru Strait is fixed at 1.4 Sv without sea-sonal variation considering the ADCP measurement(Shikama, 1994), and the rest of volume transport flowsout through the Soya Strait.

The initial conditions for temperature and salinityfield are given by observed long-term climatologic meanvalues in January (JODC and FRDAK) at every grid pointin the ocean circulation model after interpolating andextrapolating the original data. Time integration was car-ried out for 350 years and we performed model analysesduring the last year when the interannual variation mostlydisappears. This long-term integration is expected to re-

solve a problem associated with the original data for ini-tial conditions, which is a little sparse in the northern partin comparison with the southern part of the Japan Sea.This study focuses mainly on the annual formation rateand the turnover time of water masses in the Japan Sea.

2.2 Method for calculating the annual formation rate ofwater massesThe annual formation rate of water masses is esti-

mated as the sum of the volumes of the water particlessubducted from the mixed layer to the deeper layer in theJapan Sea. We performed a particle-tracking simulationusing velocity fields of the ocean circulation model fol-lowing Hofmann et al. (1991) to select the subductedwater particles. The velocities are provided every 6 hoursfrom numerical results of the ocean circulation model andare interpolated by bicubic interpolation from 16neighboring grid points. Three-dimensional advection ofwater particles is calculated using the fourth-order Runge-Kutta scheme. In addition to basic movement due toadvection by ocean currents, the water particles experi-ence random diffusion caused by horizontal and verticalturbulence. As a result, trajectories of the water particleswith time interval of ∆t are represented on the sphericalcoordinate as

x x u t xt t+ = + +1 ∆ δ ,

y y v t yt t+ = + +1 ∆ δ ,

Level Thickness(m)

Distance from the sea surfaceto the center of level (m)

Coefficient of verticaleddy diffusivity (cm2/s)

1 15 7.5 0.1002 15 22.5 0.1003 20 40.0 0.1004 25 62.5 0.1005 30 90.0 0.1006 35 122.5 0.1007 35 157.5 0.1008 35 192.5 0.1009 50 235.0 0.100

10 75 297.5 0.10011 100 385.0 0.10312 150 510.0 0.11013 200 685.0 0.12014 300 935.0 0.14515 450 1310.0 0.18016 600 1835.0 0.23017 600 2435.0 0.29018 600 3035.0 0.34519 600 3635.0 0.400

Table 1. Level thickness, total distance from the sea surface to the center of level and coefficient of vertical eddy diffusivity ateach level in the ocean circulation model.

246 H. Kawamura et al.

offshore area between 41 and 42°N, and 132 and 137°E(Figs. 3(b) and (c)), since the fresh water prevents thewinter convection along the Primorye coast, as pointedout by Seung and Yoon (1995). Therefore, the initial wa-ter particles are set in the mixed layer of the entire JapanSea on 1 March and the trajectories of water particles aresubsequently calculated from March to the following Feb-ruary in the particle trajectory experiment. Supposing thatthe water particles are assigned to the volume of theirexisting grid box, the annual formation rate of watermasses is estimated as the net mass flux from the mixedlayer to an interior in the particle trajectory experiment.For more details of the method of calculating the forma-tion rate of water masses, the reader is referred toYoshikawa et al. (1999).

3. Results

3.1 Annual formation rate of water massesThe ocean circulation model was implemented for

the Japan Sea to simulate seasonal and mesoscale varia-tions in ocean currents and mass fields. It realisticallyreproduced the surface and the intermediate circulationin the Japan Sea, including the Nearshore Branch, the EastKorean Warm Current, the North Korean Cold Currentand the Liman Cold Current (not shown here, please re-fer to figure 10 in Yoon and Kawamura, 2002). Figure 4shows vertical profiles of spatial and annual mean tem-perature, salinity and potential density in numerical re-sults. So far, the past hydrographic data show that theintermediate layer from about 100 to 1000 m is mainlyoccupied by water masses with potential density betweenabout 26.90 and 27.34 kg/m3, as mentioned above, butthe ocean circulation model gives a lower potential den-sity below the surface layer in comparison with the ob-served values, which is caused by the higher temperatureand the lower salinity. The monthly mean surface fluxes(i.e., wind stress, heat flux and salt flux) could be onereason why the ocean circulation model simulated a lowerpotential density than the observed one (Kang andMooers, 2005). Another reason for the lighter watermasses below the surface layer in the numerical experi-ment could be because the model neglects the ice forma-tion/melting effect (Postlethwaite et al., 2005). Althoughthese problems still remain, it is useful to estimate theannual formation rate of water masses using the numeri-cal model, since vertical structures of temperature, salin-ity and potential density in the Japan Sea are qualitativelyreproduced well, as seen in figures 7 and 8 in Yoon andKawamura (2002).

The water masses with potential density between 25.5and 27.5 kg/m3 are driven to the layer below the MLDduring one year in the particle trajectory experiment. Theannual formation rate of water masses is plotted against

Fig. 2. Schematic of the subduction process of water particlesin the mixed layer.

z z w t zt t+ = + +1 ∆ δ ,

where x, y and z are positions of the water particles, andu, v and w are the eastward, northward and upward ve-locities, respectively. The third terms on the right-handsides represent the diffusion effect and are parameterizedby isotropic random motion with a uniform randomnumber (R) between –0.5 and 0.5 as follows.

δ δx y K t Rh= = 24 ∆ ,

δz K t Rv= 24 ∆ ,

where Kh is the coefficient of horizontal eddy diffusivity,set to 4.0 × 105 cm2/s. The coefficient of vertical eddydiffusivity, Kv, largely depends on stratification, which isset to the same values as in the ocean circulation model(Table 1). Moreover, the enhanced coefficient of verticaleddy diffusivity of 104 cm2/s is applied in the case of staticinstability.

The mixed layer depth (MLD) essentially exerts asignificant effect on the subduction process and it variesseasonally, being shallowest in summer and deepest inwinter. Although a large portion of water masses in theshallow mixed layer in summer re-enters into the mixedlayer in the following winter due to deepening of theMLD, water masses in the deep mixed layer in wintercan possibly sink into the deeper layer, remaining belowthe MLD, which results in a net subduction of watermasses in the mixed layer (Fig. 2). In the numerical ex-periment, the base of the mixed layer is defined as thedepth at which the potential density is heavier than thesurface potential density by 0.02 kg/m3. In February andMarch, the deepest mixed layer, reaching a depth of about1000 m, appears due to strong winter convection in an

Formation Rate of Water Masses in the Japan Sea 247

Fig. 3. Mixed layer depth calculated from modeled hydrographic data on 15 January (a), 15 February (b), 15 March (c) and 15April (d).

Fig. 4. Vertical profiles of spatial and annual mean temperature (a), salinity (b) and potential density (c) in the numerical results.

248 H. Kawamura et al.

the potential density in Fig. 5(a) and the total amount ofannual formation rate was calculated to be about 3.51 Svfor water masses with potential density between 25.5 and27.5 kg/m3, suggesting a strong vertical exchange of wa-ter as well as a horizontal exchange through the straitsbetween the Japan Sea and the other seas. In particular,active water mass formation occurs for water masses withpotential density between 27.0 and 27.2 kg/m3, corre-sponding to water masses in the intermediate layer fromabout 200 to 1100 m. On the other hand, heavy watermasses with potential density greater than about 27.2kg/m3 are hardly formed in the numerical experiment.These numerical results coincide with reports indicatingthat the intermediate water masses are actively formedeven now (Kim et al., 2001), and that deep ventilationhas almost stopped in the Japan Sea since the 1950s (Riseret al., 1999).

The initial positions of water particles detrained un-der the mixed layer are plotted against the potential den-sity between 25.5 and 27.5 kg/m3 in Fig. 6. These posi-tions only represent the horizontal positions and they arevertically included in the deep mixed layer in winter. Al-though water mass formation occurs in most areas of theJapan Sea, Fig. 6 clearly shows a spatial distribution offormed water masses, with subsurface water masses aboveabout 100 m mainly originating in the southern Japan Sea(Fig. 6(a)), while sources of the intermediate water massesbelow about 200 m are located in the subpolar region northof 40°N due to the strong convection in winter (Fig. 6(d)).The active formation of water masses with potential den-sity between 27.0 and 27.2 kg/m3 also occurs in the north-ern Japan Sea.

Another particle trajectory experiment was carriedout using the numerical results of the ocean circulationmodel driven without wind forcing to determine howimportant forcing is for the formation of water masses.The annual formation rate of water masses in this parti-

cle trajectory experiment is shown in Fig. 5(b). It is foundthat the total amount of annual formation rate is about3.29 Sv for water masses with potential density between25.5 and 27.5 kg/m3, and that this value is nearly the sameas that derived in the particle trajectory experiment withthe modeled circulation driven by both wind and heatforcing (Fig. 5(a)). The subduction process is contributedby vertical displacement due to Ekman pumping and lat-eral induction due to the combined effect of a slopedmixed layer bottom and the current across it. Two parti-cle trajectory experiments quantitatively verify that thesubduction process is mainly attributed to the lateral in-duction caused by the heat forcing; therefore the heat forc-ing at the sea surface is a main factor in deciding the an-nual formation rate of water masses in the Japan Sea.

3.2 Annual formation rate of the UJSPW and the JSIWThe UJSPW is identified as the water mass with po-

tential density between 27.15 and 27.18 kg/m3 corre-sponding to the water mass in the layer from the depthbelow the SML to the bottom of the convective layer inthe numerical experiment (figures 7 and 8 in Yoon andKawamura, 2002). The JSIW corresponds to the SML withpotential density between 26.75 and 27.15 kg/m3, andsalinity lower than 33.99 in the numerical experiment.As mentioned earlier, although the potential densities ofthese water masses are slightly lighter than the observedones, the ocean circulation model seems to reproduceessential features of water mass formation and circula-tion in the subsurface and the intermediate layer. There-fore, we are able to obtain useful knowledge of the sub-surface and the intermediate circulation in the Japan Seaby estimating the annual formation rate of the UJSPWand the JSIW with the numerical model.

Figure 7(a) shows a bar chart for the annual forma-tion rate of water masses with potential density between27.00 and 27.20 kg/m3, in which black bars indicate the

Fig. 5. Annual formation rate of water masses with potential density between 25.5 and 27.5 kg/m3 in two particle trajectoryexperiments using modeled circulation driven with wind forcing (a) and without wind forcing (b), respectively.

Formation Rate of Water Masses in the Japan Sea 249

Fig. 6. Initial positions of water particles detrained under the mixed layer with potential density between 25.5 and 26.0 kg/m3 (a),26.0 and 26.5 kg/m3 (b), 26.5 and 27.0 kg/m3 (c), and 27.0 and 27.5 kg/m3 (d), respectively.

Fig. 7. Annual formation rate of water masses with potential density between 27.00 and 27.20 kg/m3 (a), and between 26.75 and27.15 kg/m3 (b). Black bars indicate the annual formation rate of the UJSPW (a) and the JSIW (b), respectively.

250 H. Kawamura et al.

annual formation rate of the UJSPW. The UJSPW is to-tally formed at a rate of about 0.34 Sv in the numericalexperiment and the maximum is attained for the watermasses with potential density between 27.15 and 27.16kg/m3, corresponding to the water masses existing in theintermediate layer from about 700 to 800 m. Senjyu andSudo (1996) roughly estimated the formation rate of theUJSPW as 1.5 × 104 km3/year, corresponding to about0.48 Sv, based on hydrographic data analyses. They sup-posed that all outcropping areas of the uppermostisopycnal surfaces corresponding to the UJSPW are theformation area of the UJSPW, and this must be why theirestimate is larger than the annual formation rate of theUJSPW derived in this study. Another possible reasoncomes from the monthly mean surface fluxes that forcethe ocean circulation model. Kang and Mooers (2005)demonstrated quantitatively that without strong synopticatmospheric forcing, diagnostics using the ocean circu-lation model may significantly underestimate buoyancyloss at the sea surface, and, hence, water mass formationas well as mixing and spreading of the formed water mass.

The annual formation rate of water masses with po-

Fig. 8. Initial positions of water particles indicating the UJSPW (a) and the JSIW (b), respectively, detrained under the mixedlayer. Mixed layer depth on the start day of particle trajectory experiment, on 1 March, is overprinted.

Fig. 9. Interannual variation of annual formation rate of thetotal water masses, UJSPW and JSIW.

Formation Rate of Water Masses in the Japan Sea 251

tential density between 26.75 and 27.15 kg/m3 is shownin Fig. 7(b), in which the annual formation rate of theJSIW with salinity lower than 33.99 is emphasized bythe black bars. The total annual formation rate of the JSIWwas calculated to be about 1.42 Sv. For the water masseswith potential density between 27.01 and 27.03 kg/m3

corresponding to the water masses at about 300 m, theannual formation rate attains its maximum. Postlethwaiteet al. (2005) suggested that the subduction of sea-ice meltwater may be a significant ventilation mechanism for theJSIW, based on the argon saturation minimum at the re-cently ventilated salinity minimum in a northern sectorof the Japan Sea. Although the ice formation/melting ef-fect is not included in the ocean circulation model, ournumerical results suggest that the annual formation rateof the JSIW is about four times larger than that of theUJSPW without the ice formation/melting effect.

Figure 8 shows the initial positions of water parti-cles corresponding to the UJSPW and the JSIW over-printed on a MLD map on 1 March, that is, the start dayof the particle trajectory experiment. Figure 8(a) showsthat the UJSPW mainly originates at the sea surface inthe offshore area between 41 and 42°N, and 132 and137°E, where strong winter convection occurs, as de-scribed earlier. According to Senjyu and Sudo (1996), theformation area of the UJSPW lies mainly in the offshorearea of the Primorye coast (between 40 and 43°N, westof 136°E), which corresponds to the “flux center” indi-cating the strong wind and the large heat loss area associ-ated with cold-air outbreak and cyclone described byKawamura and Wu (1998). The particle trajectory experi-ment verifies these observational facts; therefore we canexpect that the estimate of annual formation rate of watermasses in the numerical experiment will be fed back tothe observational study. Figure 8(b) also shows that thesource area of the JSIW extends widely in the northernsection of the Japan Sea in which the fresh water at thesea surface is mainly driven from the Amur River. Minand Warner (2005) studied the distributions of CFC andfound that the subsurface CFC maximum centered within

the typical JSIW layer is widely distributed south of thesubpolar front, and suggested that the JSIW primarilyconsists of the colder and fresher winter surface water inthe northern region that will subduct below the subpolarfront.

The additional time integration was carried out withclimatologic monthly mean forcing at the sea surface andthrough the straits for 4 years to clarify the error range inthe annual formation rate of water masses. As a result,the total annual formation rate of water masses was cal-culated to be about 3.53 ± 0.55 Sv, as seen in Fig. 9. Theannual formation rate of the UJSPW and the JSIW wereestimated to be about 0.38 ± 0.11 and 1.43 ± 0.16 Sv,respectively. Although the ocean circulation model isforced by climatologic monthly mean sea surface fluxes,the annual formation rate of water masses changes sensi-tively to the oceanic conditions.

4. SummaryThe annual formation rate of water masses has been

estimated with the ocean circulation model and the parti-cle chasing method. The method used in this study is use-ful to delineate the newly formed water masses belowthe surface layer in the numerical experiment. However,there are many uncertainties in estimating the amount ofwater mass formation. One derives from the uncertain-ties associated with initial and boundary conditions, whilethe others are associated with physical processes or nu-merical schemes used in the ocean circulation model.These are thus highly model-dependent in terms of areasof deep winter mixing, circulation features and modelparameters, etc. For instance, Yoshikawa et al. (1999)estimated the total formation rate of intermediate waterto lie in the range between 0.48 and 0.69 Sv, according tothe nudging terms and using the diagnostic model. Wedid not use the short-time-period surface fluxes but ratherthe monthly mean surface fluxes, and did not include theice formation/melting effect or the mixed layer schemein the ocean circulation model. However, we expect thatthe estimate of annual formation rate and turnover time

Table 2. Characteristics of the UJSPW and the JSIW suggested by the numerical experiments and the observational studies.

Potential density(kg/m3)

Salinity Volume(km3)

Annual formation rate(Sv)

Turnover time(year)

UJSPWNumerical experiment 27.15−27.18 >33.99 26.1 × 104 0.38 ± 0.11 22.1

Observation 27.31−27.34 >34.06 27.3 × 104 0.48 18.2

JSIWNumerical experiment 26.75−27.15 <33.99 10.0 × 104 1.43 ± 0.16 2.2

Observation 26.90−27.30 <34.06

252 H. Kawamura et al.

of water masses with the monthly mean surface fluxescould provide an useful contribution to understanding thecirculation and fate of pollutants in the Japan Sea, whichwould attract attention from other disciplines.

This study focuses on the annual formation rate ofall newly formed water masses below the surface layeras well as the intermediate water masses. The total an-nual formation rate of water masses below the surfacelayer is about 3.53 ± 0.55 Sv in the Japan Sea. Takinginto account that the mean volume transports throughstraits are 2.2 Sv through the Tsushima Straits and 1.4 Svthrough the Tsugaru Strait, respectively, the water in theJapan Sea is vertically well-ventilated as it is exchangedthrough straits between the Japan Sea and the neighboringseas. Two particle trajectory experiments with themodeled circulation driven by different types of surfaceforcing clearly show that the formation of water massesbelow the surface layer is mainly due to the heat forcingat the sea surface. The numerical experiment showedhardly any evidence of the formation of deep water massesbelow about 1500 m, as suggested by Riser et al. (1999),while on the other hand there is active formation of inter-mediate water masses from about 200 to 1500 m. Thesenumerical results are coincident with the report by Kimet al. (2001) of active occurrence of intermediate waterformation, in spite of climatic change such as globalwarming.

In the numerical experiment, the UJSPW is identi-fied as the water mass with potential density between27.15 and 27.18 kg/m3, corresponding to the water massin the layer from the depth below the SML to the bottomof the deep convective layer. The JSIW existing in theSML is specified as the water mass with potential den-sity from 26.75 to 27.15 kg/m3 and salinity lower than33.99 in the numerical experiment. The annual formationrate of the UJSPW and the JSIW were estimated to beabout 0.38 ± 0.11 and 1.43 ± 0.16 Sv, respectively. Theseestimates suggest that the annual formation rate of theJSIW is about four times more than that of the UJSPW inthe Japan Sea.

Senjyu and Sudo (1996) gave a rough estimate ofthe formation rate of the UJSPW of about 0.48 Sv, basedon hydrographic data analyses. On the basis of ahydrographic dataset they also calculated the existingvolume of the UJSPW as 27.3 × 104 km3. These valuesgive 18.2 years as the turnover time of the UJSPW. Inthis study, the existing volumes of the UJSPW and theJSIW were calculated from the modeled annual meandataset as about 26.1 × 104 and 10.0 × 104 km3, respec-tively, as shown in Table 2. Although during the last 5years the annual mean existing volumes of the UJSPWand the JSIW range from about 25.0 × 104 to 26.5 × 104

km3 and from about 9.1 × 104 to 11.0 × 104 km3, respec-tively, these existing volumes almost reach a steady state,

which is an important reference in evaluating the modelresults. In the numerical experiment, the correspondingturnover times of the UJSPW and the JSIW are about 22.1and 2.2 years, respectively, suggesting that the JSIW cir-culates in the intermediate layer with shorter time scalethan that of the UJSPW. This turnover time of the inter-mediate water masses is much shorter than that for thedeep circulation (100–300 years) as estimated by a chemi-cal tracer study in the Japan Sea (Chen and Wang, 1995).

Although the numerical results are qualitatively ingood agreement with the hydrographic data, they still havesome quantitative mismatches. The modeled potentialdensities are slightly lower than the observed values asseen in Table 2, for instance. One possible reason for thisproblem may be due to the numerical vertical diffusion.These mismatches will be improved by the adoption of ahigher-order advection scheme for tracers, while the short-period surface fluxes (i.e., daily mean wind stress, heatflux and salt flux) and the ice formation/melting effectcould also be helpful in further resolving these problems.

AcknowledgementsHelpful comments by Dr. Y. Yoshikawa at Kyushu

University are deeply appreciated. Special thanks are ex-tended to Dr. T. Senjyu at Kyushu University for his valu-able comments. We would like to thank Dr. C. H. Kim fornumerical techniques. Numerical experiments were per-formed under the framework of a collaborative study be-tween Research Institute for Applied Mechanics atKyushu University and Japan Atomic Energy Agency.

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