Journal of Oceanography, Vol. 55, pp. 111 to 122. 1999 The Japan Sea Intermediate Water; Its Characteristics and Circulation 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 Tsushima Keywords: Current Water and the Japan Sea Proper Water. Since the salinity minimum corresponds ⋅ Japan Sea, to the North Pacific Intermediate Water, it is named the Japan Sea Intermediate Water ⋅ salinity minimum layer, (JIW). To examine the source and circulation of JIW, the basin-wide salinity minimum ⋅ distribution was investigated on the basis of hydrographic data obtained in 1969. The intermediate water, ⋅ mid-depth circula- young JIW, showing the highest oxygen concentration and the lowest salinity, is seen in ° tion, the southwestern Japan Sea west of 133 E, while another JIW with lower oxygen and ⋅ subduction area, higher salinity occupies the southeastern Japan Sea south of the subpolar front. Since the ⋅ isopycnal analysis. young JIW shows high oxygen concentrations, high temperatures and low densities, the source of the water is probably in the surface layer. It is inferred that the most probable region of subduction is the subarctic front west of 132°E with the highest oxygen and the lowest salinity at shallow salinity minimum. In addition, property distributions suggest that JIW takes two flow paths: a eastward flow along the subarctic front and an southward flow toward the Ulleung Basin. On the other hand, a different salinity minimum from JIW occupies the northern Japan Sea north of the subarctic front, which shows 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 between overlying and underlying water masses. 1. Introduction 1994) revealed that the Proper Water consists of at least two The Japan Sea is one of the marginal seas on the water masses, the upper portion and the deep water, and the western North Pacific, but it has subtropical and subarctic former is produced by the wintertime deep convection south circulations bounded by the subarctic front, similar to the of Vladivostok. The deep convection occurring in the Japan open oceans. The Tsushima Current flowing from the East Sea is the so-called open-ocean convection (Senjyu and China Sea branches into two or three flows after passing the Sudo, 1993, 1994; Seung and Yoon, 1995; Choi, 1996); this Tsushima Strait (Fig. 1). The westernmost flow is called the type of convection has been observed in the Mediterranean, East Korean Warm Current, which flows northward along the Labrador Sea, the Greenland Sea, and the Antarctic the east coast of Korea. This flow is considered to be the Ocean (Killworth, 1983; Gascard, 1991). western boundary current in the subtropical circulation in In addition, a vertical salinity minimum is found between the Japan Sea (Yoon, 1982). The counterpart in the subarctic the Tsushima Current Water and the Japan Sea Proper Water circulation is the Liman Current, which flows south to in the subtropical circulation, which seems to correspond to southwestward along the Russian and North Korean coasts. the North Pacific Intermediate Water in the North Pacific Both currents contact each other around 38°–40°N west of subtropical gyre (Sverdrup et al., 1942; Reid, 1965; Talley, 132°E and flow eastward along the subarctic front at about 1993). Miyazaki (1952, 1953) was the first to point out the 40°N. This situation is very similar to the Kuroshio-Oyashio existence of the salinity minimum, and named it the Inter- currents system in the North Pacific east of Japan (Kawai, mediate Water. This water is the same as “the forth water” 1974). referred to by Kajiura et al. (1958) and Moriyasu (1972). Another important feature of the Japan Sea similar to Since the salinity minimum is often accompanied by the the open oceans is deep water formation; the Japan Sea has dissolved oxygen maximum, Miyazaki (1952, 1953) and a peculiar deep water called the Japan Sea Proper Water Miyazaki and Abe (1960) speculated that the origin of the (Uda, 1934). Sudo (1986) and Senjyu and Sudo (1993, Intermediate Water is the sea surface water having sunk 111 Copyright The Oceanographic Society of Japan. northeast, and the North Korean Cold Water flowing from the north along the Korean coast in summertime (Kim et al., 1991; Cho and Kim, 1994). Since the East Sea Intermediate Water east of Korea has a similar character to Miyazaki’s (1952, 1953) Inter- mediate Water, Kim and Chung (1984) thought that both are essentially the same. However, they could not clarify the relationship between the two waters because their work was limited to the southwestern Japan Sea. Recently, Seung (1997) suggested that there exists a cyclonic circulation of the Intermediate Water using a simple numerical model based on the ventilated thermocline theory developed by Luyten et al. (1983). His results are notable because shallow salinity minima are formed by the same mechanism in the North Pacific (Talley, 1985; Yuan and Talley, 1992). How- ever, to confirm his speculations, it is necessary to investigate the basin-wide structure of the salinity minimum layer. As the salinity minimum in the southern Japan Sea is considered to the counterpart of the North Pacific Interme- diate Water in the North Pacific, in this paper, we call the salinity 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 in the Japan Sea may be applied to other oceans. In this paper, JIW is defined and its formation and circulation are inferred based on the careful examination of Fig. 1. Bottom topography and main surface currents in the Japan salinity minimum structure in most of the Japan Sea. The Sea. Abbreviations for the main currents are as follows; TC: following basinwide maps of the salinity minimum will the Tsushima Current, LC: the Liman Current, EKWC: the provide useful information not only about the source and East Korean Warm Current, and NKCC: the North Korean modification of JIW but also about the mid-depth circula- Cold Current. tion in the Japan Sea. 2. Data Comprehensive hydrographic surveys in the Japan Sea around the subarctic front. have been carried out by the Japan Meteorological Agency On the other hand, Korean oceanographers have stud- (JMA), the Maizuru Marine Observatory, and the Hydro- ied the salinity minimum layer in the southwestern Japan graphic Department (HD) of the Japan Maritime Safety Sea in relation to the cold water appearing along the Korean Agency since 1965. Among them, surveys made by R/V coast. A low salinity and high dissolved oxygen water is Takuyo of HD in the period from July 1 to 22, 1969 and the found along the Korean coast at depths of 100–200 m (Kim multi-ship observation by JMA (Ryofu-Maru, Kofu-Maru, and Kim, 1983). They suggested that the water is brought to Chofu-Maru, and Seifu-Maru) during the period from Sep- the southwestern Japan Sea by the North Korean Cold tember 29 to October 18 are used for the present study, Current (Uda, 1934). Kim and Chung (1984) showed that because these surveys were carried out in almost all of the the salinity minimum coincides with the dissolved oxygen Japan Sea area, except for regions of north of 45°N and off maximum east of the Korean coast on the basis of the North Korea (Fig. 2). hydrographic data as far as 130°30′ E. They also showed that The hydrographic data used in this study were obtained the characteristics of salinity minimum and dissolved oxy- by serial observations with Nansen bottles, and thus tem- gen maximum near the Korean coast are emphasized com- perature, salinity and dissolved oxygen concentration were pared to Miyazaki’s (1952, 1953) Intermediate Water. They observed at standard depths. Temperature and salinity ac- called the water the East Sea Intermediate Water. Recent curacies are considered to be 0.02 deg and 0.01 psu, re- studies using CTD data suggested that the salinity minimum spectively. Dissolved oxygen concentrations were deter- water east of the Korean coast has two modes: the East Sea mined by Winkler’s method, and its error is considered to be Intermediate Water, which is warmer and flows from the 0.03 ml l–1. 112 T. Senjyu 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 Minimum density and lower salinity modes in Figs. 3(a) and 3(b), To confirm the JIW distribution, first, all salinity mini- respectively. Since salinity in the shallow minimum shows mum depth data are extracted from the dataset. Then, the low values of up to about 32.20 psu in the southern Japan Sea depth data with more than 27.32 of potential density (σθ) are and increases with latitude (Fig. 3(b)), the shallow salinity excluded because the dense water above 27.31σθ is con- minimum is considered to be formed by evaporation at the sidered to be the upper portion of the Japan Sea Proper Water sea surface of the coastal water or the East China Sea Water. (Senjyu and Sudo, 1994). On the other hand, the higher density mode corresponds to Meridional distributions of potential density and salinity the deeper salinity minimum lying in about 230 m.