Journal of Oceanography, Vol. 55, pp. 111 to 122. 1999

111Copyright The Oceanographic Society of Japan.

Keywords:⋅ Japan Sea,⋅ salinity minimumlayer,

⋅ intermediate water,⋅mid-depth circula-tion,

⋅ subduction area,⋅ isopycnal analysis.

The Japan Sea Intermediate Water; Its Characteristics andCirculation

TOMOHARU SENJYU

Department of Fishery Science and Technology, National Fisheries University,2-7-1, Nagata-honmachi, Shimonoseki, Yamaguchi 759-6595, Japan

(Received 18 September 1998; in revised form 2 November 1998; accepted 5 November 1998)

In the southern Japan Sea there is a salinity minimum layer between the TsushimaCurrent Water and the Japan Sea Proper Water. Since the salinity minimum correspondsto the North Pacific Intermediate Water, it is named the Japan Sea Intermediate Water(JIW). To examine the source and circulation of JIW, the basin-wide salinity minimumdistribution was investigated on the basis of hydrographic data obtained in 1969. Theyoung JIW, showing the highest oxygen concentration and the lowest salinity, is seen inthe southwestern Japan Sea west of 133°E, while another JIW with lower oxygen andhigher salinity occupies the southeastern Japan Sea south of the subpolar front. Since theyoung JIW shows high oxygen concentrations, high temperatures and low densities, thesource of the water is probably in the surface layer. It is inferred that the most probableregion of subduction is the subarctic front west of 132°E with the highest oxygen and thelowest salinity at shallow salinity minimum. In addition, property distributions suggestthat JIW takes two flow paths: a eastward flow along the subarctic front and ansouthward flow toward the Ulleung Basin. On the other hand, a different salinityminimum from JIW occupies the northern Japan Sea north of the subarctic front, whichshows an apparently higher salinity and high oxygen concentration than JIW. However,this salinity minimum is considered not to be a water mass but to be a boundary betweenoverlying and underlying water masses.

1. IntroductionThe Japan Sea is one of the marginal seas on the

western North Pacific, but it has subtropical and subarcticcirculations bounded by the subarctic front, similar to theopen oceans. The Tsushima Current flowing from the EastChina Sea branches into two or three flows after passing theTsushima Strait (Fig. 1). The westernmost flow is called theEast Korean Warm Current, which flows northward alongthe east coast of Korea. This flow is considered to be thewestern boundary current in the subtropical circulation inthe Japan Sea (Yoon, 1982). The counterpart in the subarcticcirculation is the Liman Current, which flows south tosouthwestward along the Russian and North Korean coasts.Both currents contact each other around 38°–40°N west of132°E and flow eastward along the subarctic front at about40°N. This situation is very similar to the Kuroshio-Oyashiocurrents system in the North Pacific east of Japan (Kawai,1974).

Another important feature of the Japan Sea similar tothe open oceans is deep water formation; the Japan Sea hasa peculiar deep water called the Japan Sea Proper Water(Uda, 1934). Sudo (1986) and Senjyu and Sudo (1993,

1994) revealed that the Proper Water consists of at least twowater masses, the upper portion and the deep water, and theformer is produced by the wintertime deep convection southof Vladivostok. The deep convection occurring in the JapanSea is the so-called open-ocean convection (Senjyu andSudo, 1993, 1994; Seung and Yoon, 1995; Choi, 1996); thistype of convection has been observed in the Mediterranean,the Labrador Sea, the Greenland Sea, and the AntarcticOcean (Killworth, 1983; Gascard, 1991).

In addition, a vertical salinity minimum is found betweenthe Tsushima Current Water and the Japan Sea Proper Waterin the subtropical circulation, which seems to correspond tothe North Pacific Intermediate Water in the North Pacificsubtropical gyre (Sverdrup et al., 1942; Reid, 1965; Talley,1993). Miyazaki (1952, 1953) was the first to point out theexistence of the salinity minimum, and named it the Inter-mediate Water. This water is the same as “the forth water”referred to by Kajiura et al. (1958) and Moriyasu (1972).Since the salinity minimum is often accompanied by thedissolved oxygen maximum, Miyazaki (1952, 1953) andMiyazaki and Abe (1960) speculated that the origin of theIntermediate Water is the sea surface water having sunk

112 T. Senjyu

around the subarctic front.On the other hand, Korean oceanographers have stud-

ied the salinity minimum layer in the southwestern JapanSea in relation to the cold water appearing along the Koreancoast. A low salinity and high dissolved oxygen water isfound along the Korean coast at depths of 100–200 m (Kimand Kim, 1983). They suggested that the water is brought tothe southwestern Japan Sea by the North Korean ColdCurrent (Uda, 1934). Kim and Chung (1984) showed thatthe salinity minimum coincides with the dissolved oxygenmaximum east of the Korean coast on the basis of thehydrographic data as far as 130°30′ E. They also showed thatthe characteristics of salinity minimum and dissolved oxy-gen maximum near the Korean coast are emphasized com-pared to Miyazaki’s (1952, 1953) Intermediate Water. Theycalled the water the East Sea Intermediate Water. Recentstudies using CTD data suggested that the salinity minimumwater east of the Korean coast has two modes: the East SeaIntermediate Water, which is warmer and flows from the

Fig. 1. Bottom topography and main surface currents in the JapanSea. Abbreviations for the main currents are as follows; TC:the Tsushima Current, LC: the Liman Current, EKWC: theEast Korean Warm Current, and NKCC: the North KoreanCold Current.

northeast, and the North Korean Cold Water flowing fromthe north along the Korean coast in summertime (Kim et al.,1991; Cho and Kim, 1994).

Since the East Sea Intermediate Water east of Koreahas a similar character to Miyazaki’s (1952, 1953) Inter-mediate Water, Kim and Chung (1984) thought that both areessentially the same. However, they could not clarify therelationship between the two waters because their work waslimited to the southwestern Japan Sea. Recently, Seung(1997) suggested that there exists a cyclonic circulation ofthe Intermediate Water using a simple numerical modelbased on the ventilated thermocline theory developed byLuyten et al. (1983). His results are notable because shallowsalinity minima are formed by the same mechanism in theNorth Pacific (Talley, 1985; Yuan and Talley, 1992). How-ever, to confirm his speculations, it is necessary to investigatethe basin-wide structure of the salinity minimum layer.

As the salinity minimum in the southern Japan Sea isconsidered to the counterpart of the North Pacific Interme-diate Water in the North Pacific, in this paper, we call thesalinity minimum water the Japan Sea Intermediate Water(JIW). The Japan Sea can be regarded as a “miniature ocean”because of its open ocean like characteristics (Ichiye, 1984).Thus, the mechanism of JIW formation and circulation inthe Japan Sea may be applied to other oceans.

In this paper, JIW is defined and its formation andcirculation are inferred based on the careful examination ofsalinity minimum structure in most of the Japan Sea. Thefollowing basinwide maps of the salinity minimum willprovide useful information not only about the source andmodification of JIW but also about the mid-depth circula-tion in the Japan Sea.

2. DataComprehensive hydrographic surveys in the Japan Sea

have been carried out by the Japan Meteorological Agency(JMA), the Maizuru Marine Observatory, and the Hydro-graphic Department (HD) of the Japan Maritime SafetyAgency since 1965. Among them, surveys made by R/VTakuyo of HD in the period from July 1 to 22, 1969 and themulti-ship observation by JMA (Ryofu-Maru, Kofu-Maru,Chofu-Maru, and Seifu-Maru) during the period from Sep-tember 29 to October 18 are used for the present study,because these surveys were carried out in almost all of theJapan Sea area, except for regions of north of 45°N and offNorth Korea (Fig. 2).

The hydrographic data used in this study were obtainedby serial observations with Nansen bottles, and thus tem-perature, salinity and dissolved oxygen concentration wereobserved at standard depths. Temperature and salinity ac-curacies are considered to be 0.02 deg and 0.01 psu, re-spectively. Dissolved oxygen concentrations were deter-mined by Winkler’s method, and its error is considered to be0.03 ml l–1.

The Japan Sea Intermediate Water; Its Characteristics and Circulation 113

Fig. 2. Locations of traces and hydrographic stations used in the study. (a) R/V Takuyo survey carried out in the period of July 1–22,1969 and (b) JMA multi-ship survey in September 29–October 18, 1969.

3. Classification of Salinity MinimumTo confirm the JIW distribution, first, all salinity mini-

mum depth data are extracted from the dataset. Then, thedepth data with more than 27.32 of potential density (σθ) areexcluded because the dense water above 27.31σθ is con-sidered to be the upper portion of the Japan Sea Proper Water(Senjyu and Sudo, 1994).

Meridional distributions of potential density and salinityfor the salinity minimum are shown in Fig. 3. There are twomodes of potential density for the salinity minimum (Fig.3(a)). The lower density mode lies in the range 22.0–26.0σθshowing a density increase northward; the higher densitymode lies at 27.0σθ or more, showing a slight density in-crease northward. Both modes exist over the latitude rangefrom 36° to 45°N. Salinity also shows two modes (Fig. 3(b)):the lower salinity mode (less than 34.00 psu) showing anincrease with latitude; the higher salinity mode has salinity34.00 psu or more. Figure 4 shows a typical example ofpotential temperature (θ), salinity, dissolved oxygen and σθprofiles in the southern Japan Sea. The shallow salinityminimum in the surface layer corresponds to the lower

density and lower salinity modes in Figs. 3(a) and 3(b),respectively. Since salinity in the shallow minimum showslow values of up to about 32.20 psu in the southern Japan Seaand increases with latitude (Fig. 3(b)), the shallow salinityminimum is considered to be formed by evaporation at thesea surface of the coastal water or the East China Sea Water.On the other hand, the higher density mode corresponds tothe deeper salinity minimum lying in about 230 m. Thedeeper salinity minimum is found just below the mainthermocline, and between a salinity maximum of theTsushima Current Water at about 70 m and relatively salinewater of the upper portion of the Japan Sea Proper Waterbelow. The salinity minimum layer corresponds to a dissolvedoxygen maximum layer; this is a general characteristic ofthe Intermediate Water described by Miyazaki (1952, 1953).A similar salinity minimum is recognizable at most stationsin the southern Japan Sea. Thus, this study treats the salinityminimum of the higher density mode.

The meridional distribution of the salinity minimum ina density range of 27.00–27.32σθ is shown in Fig. 5. Most ofthe salinity minimum north of 40°N shows more than 34.05

114 T. Senjyu

psu. By contrast, south of 40°N it is mostly less than 34.05psu and seems to be subdivided into two groups by adiscontinuity around 34.03 psu. Thus, three groups of thesalinity minimum are formed: a group of more than 34.05psu of salinity north of 40°N (Group A; squares in Fig. 5),a higher salinity group south of 40°N having 34.025 psu or

more (Group B; triangles), and a lower salinity group of lessthan 34.025 psu south of 40°N (Group C; circles). Rela-tionships between θ-S and O2-S for the salinity minimum areshown in Fig. 6. Group C shows higher temperatures (1.0–3.2°C) and a wider density range (mostly 27.15–27.25σθ) thanthe other two groups (Fig. 6(a)). Though Group A shows

Fig. 3. Meridional distributions of (a) potential density (σθ) and (b) salinity for the salinity minimum.

Fig. 4. Typical profiles of potential temperature (θ), salinity (S), dissolved oxygen (O2), and potential density (σθ) in the southern JapanSea (Sta. TA40, 36°49.0′ N 132°16.0′ E). Solid arrows indicate salinity minimum depths.

The Japan Sea Intermediate Water; Its Characteristics and Circulation 115

somewhat higher salinities than Group B, as mentionedabove, its temperatures are mostly in the same range (0.3–1.5°C) as those of Group B. However, dissolved oxygenconcentrations of Group A are higher than those of Group B(6.1 ml l–1 or more with two exceptions, Fig. 6(b)); Group Bshows a wider oxygen range of 5.1–6.6 ml l–1 but mostly ina range of 5.4–6.1 ml l–1. High oxygen concentrations ofmore than 6.1 ml l–1 are also found in Group C. This indi-cates that the waters of Groups A and C are younger than thatof Group B, having left the sea surface later.

Geographical distributions of salinity minimum clas-sified in three groups are shown in Fig. 7. Large symbolsindicate stations of the salinity minimum accompanying thedissolved oxygen maximum. Small solid circles enclosedwith dashed lines denote stations of no salinity minimum atwhich salinity shows a slight increase with depth or ho-mogeneous values. A salinity minimum exists at all of thestations south of 40°N, while no salinity minimum is observedin some regions north or northwest of 40°N. The threegroups of salinity minimum are well organized in both Julyand September–October. Group A occupies the northernJapan Sea north of 40°N; stations of Group C are seen onlyin the southwestern Japan Sea west of 133°E and south of40°N. The Yamato Basin, in the southeastern part of theJapan Sea, is occupied by the stations of Group B. ThoughGroup C shows almost the same oxygen range as Group A(Fig. 6(b)), two group stations are separated geographicallyby stations of no salinity minimum or Group B. Note that

many stations of Group C with the lowest salinity and thehighest oxygen concentration (Fig. 6(b)) show the dissolvedoxygen maximum at the same depths as the salinity mini-mum east of the Korean coast. This agrees with Kim andChung’s (1984) description of the East Sea IntermediateWater.

Fig. 5. Meridional distribution of the higher density mode (27.00–27.32σθ) of salinity minimum. Three groups of salinity mini-mum are discernible: Group A (squares), Group B (triangles),and Group C (circles). Crosses denote salinity minima that donot belong to any of the groups.

Fig. 6. Relationships of θ-S (a) and O2-S (b) for the salinityminimum. Symbols denoting the group of salinity minimumare the same as in Fig. 5. Isopycnals of potential density (thincurved lines) and the typical θ-S range of the upper portion ofthe Japan Sea Proper Water (shaded area) are also shown in theθ-S diagram.

116 T. Senjyu

Fig. 7. Geographical distribution of salinity minimum classified in three groups: (a) in July and (b) in September–October 1969. Largesymbols indicate stations of the salinity minimum accompanying the dissolved oxygen maximum. Small solid circles enclosed withdashed lines denote stations of no salinity minimum. Crossed stations have a salinity minimum that does not belong to any of thethree groups.

From the water characteristics and geographical distri-butions of three salinity minimum groups, the water ofGroup B is considered to be the Intermediate Water de-scribed by Miyazaki (1952, 1953), and Group C is the EastSea Intermediate Water reported by Kim and Chung (1984).JIW consists of two waters of Groups B and C. Group Ashows the highest salinity and the highest oxygen concen-tration and occupies the northern Japan Sea. Thus, thesalinity minimum of Group A must be a of different kindthan JIW.

Groups A and C are considered to be younger water,having been at the sea surface later than Group B. However,Group A is not an early stage of Group B because it requiresmuch more fresh water to make the salinity of Group B.There is no such fresh water around Group A. (Since GroupA is the water of salinity minimum, vertical mixing cannotproduce lower salinity water.) On the other hand, the waterof Group C does seem to be an early stage of that of GroupB because of its low salinity and high dissolved oxygenconcentration.

4. Distribution of the Japan Sea Intermediate WaterThe core-layer method is a helpful technique to analyze

horizontal variations of particular water masses; the core-layer is defined as a depth of property extreme and propertydistributions are then traced along surfaces defined byextremes (Emery and Thomson, 1998). Lateral distributionsof depth and salinity at the salinity minimum are shown inFig. 8. In the northwestern Japan Sea north of 40°N core-layer depths are shallow: less than 200 m in July (Fig. 8(a))and less than 100 m in September–October (Fig. 8(b)).Another shallow core-layer area (less than 200 m) is seen inthe southwestern region west of 133°E and south of 40°N.Note that a steep east-west salinity gradient is formed southof 40°N; it is found at 132°–134°E in July (Fig. 8(c)) and131°–133°E in September–October 1969 (Fig. 8(d)). Lowsalinities (less than 34.03 psu) are found west of the gradient,which coincide with shallow core-layers less than 200 m. Onthe other hand, east of the gradient, a vast area of 34.04–34.05 psu extends eastward to the Japanese coast; this areacorresponds to deep core-layers in the Yamato Basin (deeper

The Japan Sea Intermediate Water; Its Characteristics and Circulation 117

Fig. 8. Depth and salinity distributions at the salinity minimum: depth (a) in July and (b) in September–October, 1969; salinity (c) inJuly and (d) in September–October, 1969. Symbols D, S, H and L denote deeper, shallower, higher and lower values, respectively.Dashed lines are the same as in Fig. 7.

118 T. Senjyu

than 300 m). Another steep salinity gradient exists zonallyat about 40°N, which corresponds to the subarctic front.North of the subarctic front, relatively high salinities (34.06psu or more) are seen. Comparing Fig. 7 to Figs. 8(c) and8(d), one can see areas of three groups of salinity minimum,geographically separated by the subarctic front and the steepsalinity gradient south of 40°N.

Figure 9 shows dissolved oxygen distributions at salin-ity minimum depth. High oxygen areas of more than 6.5ml l–1 are found sporadically in the northern and southwesternJapan Sea. By contrast, the Yamato Basin corresponds to thelowest oxygen area in the Japan Sea (less than 6.0 ml l–1). Theoxygen distribution is similar to that in the upper portion ofthe Japan Sea Proper Water, as shown in Senjyu and Sudo(1993); the formation region of the upper portion of theJapan Sea Proper Water was inferred in the northwesternJapan Sea from the dissolved oxygen distribution. However,as previously stated, the salinity distribution indicates thatthe water north of the subarctic front cannot be a source ofJIW. In the southwestern Japan Sea south of the subarcticfront, higher oxygen stations correspond to lower salinity,while a close relationship between salinity and oxygenconcentrations is not seen in the northern Japan Sea north ofthe subarctic front.

Fig. 9. Dissolved oxygen distributions at salinity minimum depth: (a) in July and (b) in September–October, 1969.

The points of Group C on the θ-S diagram (Fig. 6(a))are within a wide density range of 27.08–27.25σθ. Thissuggests that the diapycnal mixing is dominant in thesouthwestern Japan Sea. In contrast to this, the points ofGroups A and B are not so scattered over a wide density andlie around 27.28σθ except for a few points. Thus, an isopycnalfor the 27.28σθ surface was selected to examine the circu-lation path of JIW. Figure 10 shows maps of salinity anddepth on the 27.28σθ surface in September–October 1969.(Similar distributions are also seen in July 1969 (not shown).)A tongue-shaped area of 34.04–34.05 psu extends eastwardfrom 133° to 138°E along 39°N (Fig. 10(a)). This areacoincides with the area of 34.04–34.05 psu in Fig. 8(d); thisindicates that the salinity minimum lies on the 27.28σθsurface in this area. Low salinities of less than 34.04 psu areseen west of the tongue-shaped area. The low salinity areacoincides with the region of Group C (Fig. 7(b)), but thesalinity minimum lies at shallower depths. Nevertheless,this salinity distribution strongly indicates that the south-western Japan Sea is the upstream region for JIW.

The flow pattern on the isopycnal surface can bededuced from the depth distribution (Fig. 10(b)). Figure10(b) suggests the subtropical and subarctic circulations inthe Japan Sea: a cyclonic subarctic circulation between 41°

The Japan Sea Intermediate Water; Its Characteristics and Circulation 119

Fig. 10. Salinity and depth distributions on the 27.28σθ surface in September–October, 1969.

and 43°N centered on a shallow region (less than 100 m) andan anticyclonic circulation in the southern Japan Sea centeredon a deep region (more than 300 m). Between both circu-lations, a strong eastward flow along the subarctic front canbe deduced; this flow is considered to be the extension of theEast Korean Warm Current. Note that the eastward flow islocated slightly north of the tongue-shaped area of 34.04–34.05 psu in Fig. 10(a). This suggests that the eastward flowtransports a portion of JIW from east off the Korean coasttoward the Japanese coast.

5. Discussion and ConclusionJIW consists of two waters of Groups B and C, and

occupies the subtropical region south of the subarctic front.On the other hand, Group A is a salinity minimum newlyintroduced in this study. What are the physical characteristicsof Group A? Typical profiles in the northern Japan Sea areshown in Fig. 11. The deeper salinity minimum correspond-ing to Group A is situated just below the main thermocline,as other groups, but it coincides with the dissolved oxygenminimum, though its oxygen concentration is higher than6.0 ml l–1. Note that there is an oxygen maximum layer at30–50 m depths above the salinity minimum. This is theintermediate water in the cold-current region reported by

Kajiura et al. (1958) and Moriyasu (1972). Since the oxygenmaximum corresponds to the salinity maximum, it may bea remnant of the Tsushima Current Water cooled in winter.On the other hand, relatively high oxygen water is found justbelow the salinity minimum. This is the upper portion of theJapan Sea Proper Water which shows high oxygen concen-trations of more than 6.0 ml l–1 in the northern Japan Sea(Senjyu and Sudo, 1993, 1994). This oxygen distributionsuggests that the salinity minimum of Group A is not a watermass, but a boundary between these two water masses. Thisnotion is supported by Fig. 6(a), which shows that about halfof Group A points on the θ-S diagram are within the typicaltemperature and salinity ranges of the upper portion of theJapan Sea Proper Water (shaded area).

From property distributions, it can be concluded thatthe southwestern Japan Sea west of 132°E is the upstreamregion of JIW. Since the younger JIW corresponding to theGroup C water shows high dissolved oxygen concentra-tions, high temperatures, and low densities, the source of thewater is probably in the surface layer. The subarctic frontwest of 132°E is a highly probable subduction area. This issupported by the salinity and dissolved oxygen distributionsat the salinity minimum depth; the lowest salinity and thehighest oxygen concentration are found in the 38°–40°N

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Fig. 12. Potential temperature (a) and salinity (b) sections along 39°N in September–October, 1969.

Fig. 11. Typical profiles of potential temperature (θ), salinity (S), dissolved oxygen (O2), and potential density (σθ) in the northern JapanSea (Sta. 3581, 42°00.0′ N 137°00.0′ E). Solid arrows indicate salinity minimum depths.

The Japan Sea Intermediate Water; Its Characteristics and Circulation 121

AcknowledgementsI wish to thank Profs. Hideo Sudo and Masaji

Matsuyama for their valuable comments and discussion.Thanks are also due to Profs. Masaki Takematsu and Jong-Hwan Yoon who gave me a chance to join the CREAMSprogram. The data used in the study were provided from theJapan Oceanographic Data Center. This study was pre-sented in the third CREAMS Workshop at Seoul, Korea onNovember 7–8, 1994.

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minimum layer water in the Ulleung Basin. La mer, 32, 271–278.

Choi, Y.-K. (1996): Open-ocean convection in the Japan (East)Sea. La mer, 34, 259–272.

Emery, W. J. and R. E. Thomson (1998): Data Analysis Methodsin Physical Oceanography. Pergamon Press., Great Britain,634 pp.

Gascard, J.-C. (1991): Open ocean convection and deep waterformation revisited in the Mediterranean, Labrador, Greenlandand Weddell Seas. p. 157–181. In Deep Convection and DeepWater Formation in the Oceans, ed. by P. C. Chu and J.-C.Gascard, Elsevier Oceanography Series, 57, Amsterdam.

Ichiye, T. (1984): Some problems of circulation and hydrographyof the Japan Sea and the Tsushima Current. p. 15–54. In OceanHydrodynamics of the Japan and East China Seas, ed. by T.Ichiye, Elsevier Oceanography Series, 39, Amsterdam.

Kajiura, K., M. Tsuchiya and K. Hidaka (1958): The analysis ofoceanographical condition in the Japan Sea. Rep. Develop.Fisher. Resour. in the Tsushima Warm Current, 1, 158–170(in Japanese).

Kawai, H. (1974): Transition of current images in the Japan Sea.p. 7–26. In The Tsushima Warm Current—Ocean Structureand Fishery, ed. by Fishery Soc. Japan, Koseisha-Kouseikaku,Tokyo (in Japanese).

Killworth, P. D. (1983): Deep convection in the world ocean. Rev.Geophys. Space Phys., 21, 1–26.

Kim, C. H. and K. Kim (1983): Characteristics and origin of thecold water mass along the east coast of Korea. J. Oceanol. Soc.Korea, 18, 73–83 (in Korean with English abstract).

Kim, C. H., H.-J. Lie and K.-S. Chu (1991): On the IntermediateWater in the southwestern East Sea (Sea of Japan). p. 129–141. In Oceanography of Asian Marginal Seas, ed. by K.Takano, Elsevier Oceanography Series, 54, Amsterdam.

Kim, K. and J. Y. Chung (1984): On the salinity-minimum anddissolved oxygen-maximum layer in the East Sea (Sea ofJapan). p. 55–65. In Ocean Hydrodynamics of the Japan andEast China Seas, ed. by T. Ichiye, Elsevier OceanographySeries, 39, Amsterdam.

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Miyazaki, M. (1952): The heat budget of the Japan Sea. Bull.Hokkaido Reg. Fisher. Res. Lab., 4, 1–54 (in Japanese withEnglish abstract).

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areas west of 132°E (Figs. 8(d) and 9(b)). In addition, thisarea is one of shallow salinity minimum core regions in theJapan Sea (Fig. 8(b)). The potential temperature and salinitysections along 39°N in September–October 1969 are shownin Fig. 12. Indeed, the low salinity water near the sea surface(Stas. F10 and 09) seems to intrude into a sub-thermoclinelayer west of 132°E forming an intermediate salinity mini-mum.

The submerged low salinity water undergoes a diapycnalmixing with the upper portion of the Japan Sea Proper Waterbelow. As a result, the younger JIW shows a fall of tem-perature, increase of salinity and increase of density. Even-tually, the Group C water is modified to become the GroupB water. This mixing process is explained on the θ-Sdiagram (Fig. 6(a)); the Group B water connects the GroupC water with the upper portion of the Japan Sea ProperWater. This indicates that the water of Group B is a mixtureof these two water masses. The temperature and salinitysections (Fig. 12) also imply the diapycnal aspect; thesalinity minimum layer west of 133°E shows a sinkingacross the thermocline.

The property distributions suggest that JIW takes twoflow paths: an eastward flow along the subarctic front and asouthward flow parallel with the Korean coast in the regionwest of 132°E. The eastward flow seems to take an isopycnalprocess because the salinity minimum waters lie on theisopycnal surface of 27.28σθ in the Yamato Basin. A part ofJIW having sunk under the main thermocline is advectedtoward the Japanese coast on the isopycnal surface by theeastward flow along the subarctic front. In the course of thecirculation, JIW probably gradually loses the original char-acteristics through the mixing with the upper portion of theJapan Sea Proper Water, and at the last stage it is entrainedinto overlying or underlying waters.

The southward flow may correspond to the deep west-ern boundary current of the cyclonic gyre suggested bySeung (1997); he simulated the cyclonic circulation in thesubtropical intermediate layer by a simple numerical modelbased on the ventilated theory (Luyten et al., 1983). Thoughsuch a strong southward flow cannot be deduced from thedepth distribution on the 27.28σθ surface (Fig. 10(b)), it isinteresting that the lowest salinity (less than 34.03 psu) at thesouthwestern corner (Fig. 10(a)) coincides with the deeparea of isopycnal depth (more than 300 m) which implies aweak southward flow. The cyclonic circulation in the south-ern Japan Sea is also inferred in the upper portion of theJapan Sea Proper Water below (Senjyu and Sudo, 1993,1994).

The dataset used in the study is restricted in its verticalresolution, as well as in its horizontal coverage. Besides, thedata were taken only in 1969; temporal variations are likelyto exist in both water characteristics and water mass distri-butions. For further discussion of the JIW circulation, fur-ther extensive observations in the Japan Sea are desired.

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