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East Sea
Journal of Marine Systems 78 (2009) 226–236
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Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Review on the seasonal variation of the surface circulation in the Japan/East Sea Jong-Hwan Yoon a,⁎, Young-Ju Kim b a b
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, 8168580, Japan Research Institute of Oceanography, Seoul National University, Seoul 151742, South Korea
a r t i c l e
i n f o
Article history: Received 3 September 2007 Received in revised form 18 August 2008 Accepted 2 March 2009 Available online 12 July 2009 Keywords: Japan/East Sea circulation Seasonal variability Mesoscale and submesoscale variability Northwest monsoon Northwest thermal front Surface coastal jet Topical issue on "Observation and Modeling of the Ocean Circulation and Marine Ecosystem for CREAMS/PICES". The issue is compiled and guest-edited by the North Pacific Marine Science Organization (PICES)
a b s t r a c t The seasonal variation of the surface circulation in the Japan/East Sea (JES) and the Tsushima/Korea Straits (TKS) is reviewed and discussed, focusing on mesoscale and submesoscale variabilities. The monsoon modified by coastal geographical features near Vladivostok generates a dipole of vortex off Vladivostok which induces dramatic changes in the surface circulation in the northwest JES, splitting the Subpolar Gyre into two smaller gyres by generating the Vladivostok Dome. Between these two smaller gyres, the Northwest Thermal Front is formed and current reversal is induced along the North Korean coast. The winter monsoon also induces a current reversal along the Sakhalin coast. The volume transport of the surface Subpolar Gyre has two maxima in January and August. The maximum in August is induced by the summer intensification of the Liman-North Korean Cold Current and the shallow and narrow surface coastal jet generated by the sea ice and snow melting. The maximum in January is induced by the northwest monsoon and associated cooling. Salient features in the TKS are the submesoscale variabilities. In the western channel, submesoscale eddies with length scale of about 80 km and time scale of 5–6 days develop in the cold period. On the lee side of the Tsushima Islands, Karman-like vortex pairs are generated in the warm period. Anticyclonic vortices generated at the northern tip of the Tsushima Islands have a time scale of 5 to 8 days, length scale of 35 to 60 km, and propagate toward the JES with a phase speed of 8 cm/s. Cyclonic vortices south of the anticyclonic counter part of the vortex pairs are rather stationary with intermittent occasional propagation toward the east. The development of stratification seems to be necessary for the development of Karman-like vortex pairs. Summarizing the results above, a schematic surface circulation with seasonal change is proposed. © 2009 Published by Elsevier B.V.
1. Introduction One of the most salient features commonly seen in AVHRR SST images of the Japan/East Sea (JES) is that the surface of the JES is filled with very energetic mesoscale and submesoscale eddies. Mesoscale eddies with dominant eddy sizes of 80–90 km in the JES (Toba et al., 1984) are generated mostly by baroclinic instability characterized by the first baroclinic radius deformation. The dominant mesoscale eddy size is roughly estimated to be several times larger than the first baroclinic radius deformation. Submesoscale eddies are defined as eddies with length scale smaller than the dominant mesoscale eddy size in the JES and time scale from several days to a few weeks. The strong signals of these mesoscale and submesoscale variabilities have been obscuring the mean circulation field, especially the mean path of the Tsushima Warm Current (TWC). On account of these strong variabilities and lack of direct current measurements, various flow patterns on the mean path of the TWC have been proposed (Kawai, 1974; Naganuma, 1977; Kim and Yoon, 1996), although a great number of hydrographic surveys had been ⁎ Corresponding author. Tel.: +82 81358414288. E-mail address: [email protected] (J.-H. Yoon). 0924-7963/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jmarsys.2009.03.003
performed in the Tsushima Warm Water region. On the other hand, since hydrographic surveys in the Subpolar region had been restricted for a long time by the “Iron Curtain” until the beginning of 1990s, the study of the circulation had stagnated. Since the end of the Cold War in 1991, oceanic circulation of the JES has been investigated through international cooperative studies such as the CREAMS (Circulation Research of the East Asian Marginal Seas) (Danchenkov et al., 2006). These studies have uncovered many new findings on the circulation of the JES, worth mentioning among these are the rise in temperature and decrease in dissolved oxygen in the deep layer associated with the cessation or decrease in bottom water formation since the 1950s (Kim et al., 2002a, 2004), the strong mean deep current in the Japan Basin with energetic mesoscale variabilities (Takematsu et al., 1999; Senjyu et al., 2005), the new formation of bottom water in the 2000–2001 winter (Kim et al., 2002b; Senjyu et al., 2002; Talley et al., 2003), the extensive measurements of the circulation of the Ulleung Basin (Chang et al., 2004), and the PALACE experiment for the deep circulation of the JES (Danchenkov and Riser, 2000; Danchenkov et al., 2003a,b). Many observations in various seasons during the past 15 years and some earlier studies illuminated the seasonal variation of the surface
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Fig. 1. Schematic of summer circulation (Yarichin, 1980). JB is Japan Basin and EKB is East Korean Bay.
circulation with strong mesoscale signals in the Subpolar region associated with the dipole of the monsoon wind stress curl south of Vladivostok (Yoon et al., 2005) such as the coastal current reversal in November along the Korean coast and the development of the Northwest Thermal Front (NWTF) in winter south of Vladivostok (Danchenkov et al., 2003a). In the Tsushima/Korea Straits (TKS), the monitoring of the TWC structure has been monitored since 1997 using an ADCP (Acoustic Doppler Current Profiler) mounted on a ferry boat traveling between Pusan and Hakata (Takikawa et al., 2003, 2005a,b), revealing that the transport of the TWC shows a remarkable seasonal variability with a minimum in winter and maximum (or maxima) in summer, being accompanied by energetic submesoscale variabilities with strong seasonal dependence (Seung et al., 2007). Thus, recent studies on the circulation of the JES including the TKS suggest that mesoscale and submesoscale variabilities are important aspects in the seasonal variation of the JES circulation. During the past few decades, many numerical model studies have tried to simulate the JES circulation. However, to simulate these mesoscale and submesoscale variabilities in the JES and the TKS, the horizontal grid resolution of numerical models must be at least smaller than a few kilometers. Models that meet this condition are the four layer model by Hogan and Hurlburt (2000) and the 3D primitive OGCM by Kim (2007).
The advent of the Earth Simulator (JAMSTEC) in 2002 enabled the use of a horizontal grid resolution finer than 1/32° in a 3D primitive OGCM. Kim (2007) tried to simulate the circulation of the JES, mainly focusing on the mesoscale, submesoscale and associated phenomena, such as submesoscale fluctuations appearing in the ADCP monitoring of the TWC (Takikawa et al., 2003, 2005a,b) and the cyclonic deep mean flow of the JES with a 3D primitive OGCM (RIAMOM, RIAM Ocean Model) with a horizontal grid resolution of 1/36° and 46 vertical levels. Details of the basic structure of this model can be found in Seung et al. (2007). In this study, we briefly survey recent progress in studies on the surface circulation in the JES. This review concentrates on the seasonal variation of the surface circulation of the JES with special focus on mesoscale and submesoscale variabilities, which were observed or successfully simulated in extra fine resolution models. The intermediate and deep circulation of the JES will be reviewed at another time. 2. Seasonal variation of the surface circulation in the Japan/East Sea (JES) Major features of the surface mean circulation of the JES consist of the Subpolar Front along about 40°N, the cyclonic Subpolar Gyre in the cold water region north of the Supolar Front with the southwestward
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Fig. 2. Schematic of surface circulation of the JES. Warm period (left) and cold period (right). TWC (Tsushima Warm Current), LCC–NKCC (Liman Cold Current–North Korean Cold Current), EKWC (East Korean Warm Current), SPF (Subpolar Front) (Kim, 2007).
Liman Cold Current (LCC) along the Russian coast, the North Korean Cold Current (NKCC) along the North Korean coast, and two major currents in the warm water region south of the Polar Front, the East Korean Warm Current (EKWC) along the Korean coast and the TWC along the Japanese coast. The EKWC flows eastward along the Subpolar Front after separating from the Korean coast at around 38°N, mixing with the Subpolar Water and joins the TWC west of the Tsugaru Strait. The mixed water feeds the current recirculating off the LCC along the Russian coast, which supplies salinity as well as heat to the region of winter convection off Vladivostok. The salinity supplied by this recirculating current contributes much to the initiation of deep convection accompanied by the formation of the Upper Japan Sea Proper Water. (Yoon and Kawamura, 2002) The TWC bifurcates into two main branches near the TKS, one of which flows northward along the Korean coast as the EKWC and the other is the Nearshore Branch of the TWC along the Japanese coast. Between these two main currents, another current (the second branch or offshore branch) has been suggested to exist on the offshore side of the Nearshore Branch of the TWC (Shimomaru and Miyata, 1957; Naganuma, 1977, 1985; Kawabe, 1982; Moriyasu, 1972; Ohwada and Tanioka, 1972), whereas others have proposed one meandering current instead of branching currents (Moriyasu, 1972; Ohwada and Tanioka, 1972). A recent study by Hase et al. (1999) on the branching of the TWC concluded that the Nearshore Branch (the first branch) starts from the eastern channel of the TKS follows the isobaths shallower than 200 m along the Japanese coast as a topographically steered current, and flows out through the Tsugaru Strait in all seasons. On the other hand,
the second branch of the TWC develops from spring to fall flowing along the continental shelf break with a strong baroclinicity as a topographically steered current west of Noto Peninsula. East of Noto Peninsula, the second branch flows northeastward toward the Tsugaru Strait being relieved from the topographic steering. The seasonality of the second branch was also suggested by Kawabe (1982). Referring to the schematic summer circulation of Yarichin (1980) in Fig. 1, the surface circulation in the subpolar region consists of a large cyclonic gyre in the Japan Basin (JB in Fig. 1) and two small cyclonic gyres in the East Korean Bay (EKB in Fig. 1) and west of Sakhalin. Along the Russian coast and North Korean coast, the LCC and NKCC flow southwestward, respectively. The large cyclonic gyre in the Japan Basin is driven by the positive wind stress curl in winter (Kim and Yoon, 1996) and thermohaline forcing which has not been investigated in great deal. Many observations in various seasons during the past 15 years and some earlier studies provided information on many new aspects of seasonal variability of the surface circulation in the Subpolar region such as a dramatic seasonal variability of the coastal current along the North Korean coast (Yoon et al., 2005), the summertime southward current along the east coast of Korea (Lie and Byun, 1985) and the development of the NWTF in winter (Danchenkov et al., 2003a). Some of these mesoscale aspects are closely related to atmospheric submesoscale disturbance (dipole of wind stress curl) south of Vladivostok (Yoon et al., 2005). Many numerical model studies have been done in recent years to simulate the oceanic circulation of the JES (Seung and Kim, 1993; Kim and Yoon, 1996; Kim and Seung, 1999; Seung and Yoon, 1995;
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Fig. 3. Seasonal variation of monthly mean eastward velocity for the upper 500 m on the vertical section along 133°E. Contour interval is 2.5 cm/s and color bar unit is cm/s (Kim, 2007).
Yoshikawa et al., 1999; Yoon and Kawamura, 2002; Yoon et al., 2005; Hogan and Hurlburt, 2000; Kim, 2007). However, most of these numerical models except Hogan and Hurlburt (2000) and Kim (2007) did not have sufficient horizontal resolution to resolve mesoscale and submesoscale variabilities although many major features of the surface circulation of the JES were well reproduced. Hogan and Hurlburt (2000) demonstrated the importance of mesoscale variabilities in the formation of the mean JES circulation, revealing that the mean pathway of surface circulation is strongly influenced by abyssal circulation via the eddy–mean flow interaction which generates a realistic cyclonic deep mean flow in the JES with the horizontal resolution finer than 1/32°. The eddy–mean flow interaction in Hogan and Hurlburt (2000) can be interpreted as the eddy– topography interaction (Holloway et al., 1995; Greatbatch and Li, 2000). The realistic separation of the EKWC in numerical models requires that mesoscale eddies be very well resolved to give rise to realistic eddy–topography interaction as well as the presence of realistic bottom topography. However, Hogan and Hurlburt (2000) concluded little about the seasonal variation of the surface JES circulation at all. The model study by Kim (2007) with an extra fine horizontal resolution described well the seasonal variation of the surface circulation in the JES and the TWC in the TKS as shown schematically in Fig. 2 (Kim, 2007) where the cold period indicates the season of winter monsoons, and the warm period the remaining season. The schematic surface circulation in the warm period in Fig. 2 compares well with that of Yarichin (1980) in Fig. 1 except for a narrow and shallow coastal jet along the Russian and North Korean coast and the western extent of the Subpolar Gyre in the Japan Basin. In the cold period, the northwest monsoon modified by the
geographical geometry near Vladivostok generates a dipole of vortex south of Vladivostok which induces dramatic seasonal changes in circulation of the northwestern JES. Salient features in the TKS are the submesoscale variability generated on the lee side of the TKS in the warm period and in the eastern and western channels of the TKS in the cold period. In the subsequent sections, the seasonal changes in the northwestern JES, the Tatar Strait, the TKS and the Nearshore Branch of the TWC are described and discussed in more detail, mainly referring to Kim (2007).
2.1. Northwest Japan/East Sea (JES) and Tatar Strait The eastward current along the Subpolar Front at around 40°N and the Liman Cold Current–North Korea Cold Current (LCC–NKCC) along the RussianNorth Korean coast are the two main constituents of the cyclonic Subpolar Gyre (Isobe and Isoda, 1997) in the JES as seen in Fig. 2. The hydrographic survey of the CREAMS in summer of 1995 suggested an additional constituent of the Subpolar Gyre, which is a narrow and shallow coastal current with low salinity developed between the LCC and the Russian coast. The salinity vertical section along the C section (136°E) in Kim et al. (2004) clearly shows a low salinity area with narrow horizontal (50 km) and shallow vertical extent (100 m) adjacent to the Russian coast, suggesting a westward surface coastal current, which distinctly differs distinctly from the LCC. Along the Korean coast a southward surface coastal jet with the same horizontal and vertical extent was observed by Lie and Byun (1985) in summer at around 38°N.
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Fig. 4. Climatological mean wind stress vector (left) and wind stress curl (right) in February, averaged for the period from 1997 to 2000, using the ECMWF reanalysis data. Shaded area in the right panel indicates negative values.
The aforementioned westward surface coastal jet with low salinity along the Russian coast in summer has been successfully reproduced by Kim (2007) showing that a strong southwestward coastal jet develops in a shallow and narrow belt along the Russian and North Korean coast in the warm period from May to September as shown in monthly vertical sections of eastward velocity along 133°E in Fig. 3. The development of a distinct core of westward velocity can be seen in the surface layer between the Russian coast and the LCC from May to September, whereas the LCC has a deeper, broader structure. The surface coastal jet and the LCC are eventually combined into one current in October. This surface coastal jet with low salinity starts to develop in the northern Primorye coast in May and progresses southwestward and reaches 38°N in August as reported by Lie and Byun (1985). The melting of snow and sea ice along the Primorye coast and in the Tatar Strait seems to generate this fresh coastal jet as a density current between the Russia–Korea coast and the LCC–NKCC, forming a double core structure of velocity along the Russia– Korea coast. The fresh water supplied from the Amur River (Yakunin, 1975) is responsible for the sea ice formation in the Tatar Strait. Another notable feature is the intensification of the LCC in summer as well as in winter as seen in Fig. 3.
In the cold period from late fall to early spring, the circulation in the northwest JES changes its features dramatically. This change is caused by the northwest monsoon modified by the coastal geographical features near Vladivostok, which prevails from late fall to the end of February as shown by Yoon et al. (2005). The northwest monsoon converges into a narrow jet when it passes through the narrow valley around Vladivostok, being accompanied by a cyclonic vortex at its eastern side and an anticyclonic one at the western side (a dipole of wind vortex) off Vladivostok as seen in Fig. 4. This wind vortex dipole induces a dramatic change in the circulation of the northwest JES, generating a dipole of vortex (Vladivostok dipole) in the ocean off Vladivostok (Yoon et al., 2005). The positive vortex of the dipole develops into the Vladivostok Dome which plays important roles in the formation of the Upper Japan Sea Proper Water (Yoon and Kawamura, 2002; Kawamura and Yoon, 2007). The negative anomaly of SST associated with the Vladivostok Dome is clearly seen in both the model (Kim, 2007) and the AVHRR image (Yoon et al., 2005) from the end of October to December, although a positive anomaly of SST associated with the negative vortex of the
Fig. 5. Horizontal distribution of sea surface temperature (left, °C) and vertical section of temperature (right, °C) along the dotted line in the left panel in January 1986. The Northwest Thermal Front (NWTF) and Subpolar Front (SPF) are clearly seen at around 41°N and 39.5°N, respectively (right) (Danchenkov et al., 2003a).
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Fig. 6. Seasonal variation of volume transport stream function for the upper 210 m. Contour interval is 0.2 Sv (Kim, 2007).
dipole is not well recognized. From October to December, since the vertical temperature gradient in the region north of the Subpolar Front is relatively significant, the upwelling induced by positive wind stress curl off Vladivostok can effectively generate negative SST anomaly. However, continuous strong heat loss from the surface in the northern JES during winter continues to weaken not only the vertical temperature gradient but also horizontal temperature gradient. By February, the vertical and horizontal temperature gradient will become so weak that the upwelling associated with the positive wind stress curl off Vladivostok cannot generate strong negative SST anomaly in accordance with the disappearance of negative SST anomaly in the positive wind stress curl area in mid and late winter. As the dipole of vortex develops in the ocean, a front called the NWTF (Danchenkov et al., 2003a) extending southeastward from the Tuman River mouth is formed along the boundary between the cold and warm eddy from late fall to spring as seen in Fig. 5. The position of the NWTF shifts slightly westward from the boundary line between
the positive and negative vortex of the wind vortex dipole due to the β effect. Consequently, the LCC bifurcates south of Vladivostok in winter. One branch separating from the LCC flows along the NWTF. The other continues to flow southwestward along the North Korean coast as the NKCC forming a western limit of the Subpolar Gyre. The former contributes to split the interior of the Subpolar Gyre into two smaller cyclonic circulations in the cold period north of the Subpolar Front as seen in the cold period in Fig. 2. Another salient feature generated by the wind vortex dipole is the coastal current reversal along the Vladivostok–North Korean coast. The anti-cyclonic wind vortex generates clockwise circulation in the Peter's Great Bay south of Vladivostok and along the North Korean coast which propagates southwestward as topographic Rossby waves, generating a northeastward current along the Vladivostok–North Korean coast, where the NKCC and surface coastal jet flow southwestward during late spring to early fall as mentioned above. This seasonal current reversal was captured dramatically by an ARGOS
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Fig. 7. Seasonal variations of volume transport between the Russian coast and two points, A (42°N, 134°E) and B (41.5°N, 134°E) as shown in Fig. 6 for the upper 210 m (left). Seasonal variation of volume transport between B and A in the upper 210 m (right).
buoy deployed south of Vladivostok in July 1994 during the CREAMS'94 summer cruise (Danchenkov et al., 2003a; Yoon et al., 2005). The buoy flowed southwestward along the North Korean coast until the end of October and then turned back suddenly northeastward along the North Korean coast up to 42°N, and then separated from the coast in the middle of December, forming an anticyclonic trajectory with a rotational radius of 150 km. This seasonal coastal current reversal starting from late fall along the North Korean coast was also confirmed by a numerical model by Yoon et al. (2005) and Kim (2007). Another example of the circulation that seems to be generated by the coastal geographical feature is a small cyclonic circulation around the East Korean Bay (38.5°–40.5°) as seen in Fig. 2. This small circulation is also depicted in a schematic picture of the surface circulation of the JES by Yarichin (1980) in Fig. 1. The circulation seems to be generated by a positive wind stress curl in the East Korean Bay as seen in Fig. 4. This positive wind stress curl is generated by the convergence of the northwest winter monsoon passing through the valley around the East Korean Bay. The positive wind stress curl west of Hokkaido and Sakhalin may also generate a small cyclonic circulation north of 45°N as seen in Figs. 1 and 2. As seen in Fig. 2 in Yoon et al. (2005), the track of the ARGOS buoy deployed off the Russian coast at 43.5°N on 12 July 1994 suggests the existence of a cyclonic gyre north of 44°N. The buoy was trapped by a small cyclonic gyre, circulated twice in nearly 1 year with a speed of over 10 cm/s, and eventually flowed southwestward along the Russian and North Korean coast, suggesting the existence of a southwestward surface coastal jet in summer. The location and scale of the gyre suggested by the ARGOS buoy correspond well with model results by Yoon et al. (2005). The circulation in the Tatar Strait also shows a dramatic seasonal change. As shown in Figs. 1 and 2 the strait is occupied by a cyclonic circulation that is a continuation of the TWC in the warm period until the northwest winter monsoon starts to blow. The northwest monsoon generates a southward coastal current along the Sakhalin and Primorye coasts and a northward compensating current in the central part of the Tatar Strait as suggested by Park (1986) in the study for the Yellow and East China Seas. 2.2. Volume transport in the Subpolar Gyre The seasonal variations of the surface circulation described in the previous section are reflected in the volume transport stream functions for the upper 210 m in the Subpolar Gyre in Fig. 6. A remarkable feature in the stream function fields is that the interior of the Subpolar Gyre in
the Japan Basin splits into two small cyclonic gyres in the cold period. One is a small cyclonic gyre in the central part (roughly, 40.5°N–42.5°N, 131°E–137°E) of the Japan Basin associated with the Vladivostok Dome in the previous section and the other is a small cyclonic gyre in the East Korean Bay (roughly, 38.5°N–40.5°N, 128.5°E–130.5°E). These gyres develop in the cold period and spin down toward autumn. These small cyclonic gyres are caused by the positive wind stress curls in winter as suggested in the previous section. Along the western rim of the small gyre in the central part of the Japan Basin, the NWTF develops as discussed before. Another remarkable feature is the intensification of the current along the Russian and North Korean coast twice a year, in summer and winter. The intensification twice a year can be seen more clearly in the volume transport of the current. Fig. 7 (left) shows the volume transport for the upper 210 m between the Russian coast and points A and B shown in Fig. 6. The volume transport between the Russian coast and point A (42°N, 134°E) (hereafter, Transport A) is contributed by the LCC and the surface coastal jet along the Russian coast. That between the Russian coast and point B (41.5°N, 134°E) (hereafter, Transport B) corresponds to the transport of the entire Subpolar Gyre. Transport A for the surface layer shows two maxima in summer and winter. The intensification of the LCC–NKCC due to the winter monsoon with positive wind stress curl over the JES (Kim and Yoon, 1996; Yoon et al., 2005) and related cooling is responsible for the increase in winter. The development of the surface coastal jet along the Russian coast is partly responsible for the increase in Transport A from April toward August as inferred from Fig. 3. The remaining part of the increase is contributed by the intensification of the LCC–NKCC in summer. The mechanism of the summer intensification of the LCC–NKCC should be clarified in future. The difference between Transports A and B for the surface layer in Fig. 7 (right) shows two maxima in February and August. The maximum in February may correspond to the development of the small gyre in the central part of the JES. The same seasonal variation can be seen in the volume transport along the North Korean coast and in a small cyclonic gyre in the East Korean Bay (not shown here). Since the difference between Transports A and B shows a relatively small seasonal variation and the same tendency can be seen along the North Korean coast, the main player in the seasonal variation of the volume transport of the Subpolar Gyre in the surface layer is the current along the Russian and North Korean coast. The summer intensification of the surface circulation of the Subpolar Gyre was also pointed out by Morimoto and Yanagi (2001) from the seasonal variation of the first empirical orthogonal function (EOF) mode of the surface circulation, analyzing satellite altimetry
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data. However, the strength of the first EOF mode, explaining 87% of the variance, has a single maximum in summer and a minimum in winter. Considering the spatial distribution of the first EOF mode (see Fig. 10 in Morimoto and Yanagi, 2001), the seasonal variation of the TWC with a maximum in summer and minimum in winter seems to be strongly reflected in the first EOF mode. Therefore, the first EOF mode does not seem to describe the seasonal variation of the surface circulation of the Subpolar Gyre properly. Trusenkova et al. (accepted for publication) also suggested the weakening (intensification) of the surface Subpolar Gyre in winter (summer) form the analysis of their model SSH and surface velocity fields. However, the calculation of volume transport in upper 200 m layer in their model might show a maximum in winter due to the winter monsoon. 2.3. Bifurcation of the Nearshore Branch of the Tsushima Warm Current (TWC) Although many studies have investigated the path of the TWC, its strong mesoscale variability and the lack of resolution of previous numerical models have been obscuring the mean path, especially the Nearshore Branch of the TWC along the Japanese coast. As seen in Fig. 2, the model results show that the Nearshore Branch consists of two branches, the first and second branches in the warm period (roughly from April to November). In the cold period (roughly from December to March), the second branch becomes indiscernible whereas the first branch continues to exist along the Japanese coast. The first branch of the TWC, which flows into the JES through the eastern channel of the TKS is trapped within an area shallower than 200 m along the Japanese coast during a year with maximum transport in summer and minimum in winter. This current is very difficult to detect by the traditional geostrophic calculation due to its relatively strong barotropy and shallowness of the bottom. In recent years, many direct current measurements using such as a moored current meter (Hase et al., 1999), an ADCP, or an ARGOS buoy have shown the strong coastal trapped current within the area shallower than 200 m along the Japanese coast. The track of an ARGOS buoy, which was deployed at the TKS in summer (Ishii and Michida, 1996) and reached the Tsugaru Strait in a month, shows the existence of a very strong current of mean speed of 50 cm/s along the Japanese coast in the summer season, corresponding well with the model results. The second branch, which flows into the JES through the eastern channel of the TKS, flows along the offshore side of the first branch, following the isobath of 500 m–1000 m in the warm period. In the cold period, the second branch, originating from the eastern channel of the TKS, joins the first branch west of the Oki Islands. The current (dotted line) branching from the third branch might feed the second branch. However, the mesoscale variability in the model is so strong that the mean path of the second branch is not identifiable. 2.4. Submesoscale variabilities in the Tsushima/Korea Straits (TKS) In the TKS, the TWC structure has been monitored since February 1997 using an ADCP mounted on a ferry boat that made three (six times from July 2004) round trips between Pusan and Hakata (Takikawa et al., 2003, 2005a,b). Takikawa et al. (2005a) showed that the monthly volume transport of the TWC averaged from February 1997 to August 2002 has a remarkable seasonal change with a minimum (1.75Sv) in winter and two maxima (2.8 Sv in June and 3.2 Sv in October) with an average volume transport of 2.64 Sv. The ADCP measurements showed another salient feature of the TWC, submesoscale variaibilities that show strong seasonal dependence (Seung et al., 2007). The image of chlorophylla/MODIS on 12 January 2006 (NPEC) in Fig. 8 is an example of submesoscale variability with
Fig. 8. Chlorophyl-a/MODIS on January 2, 2006 (NPEC, Nothwest Pacific Region Environmental Cooperation Center). Color bar unit is mg/m3.
a length scale of about 60 km or less along the southern coast of Korea in the western channel of the TKS. Fig. 9 shows a time series of eddy kinetic energy with a period shorter than 10 days calculated from the ADCP measurements between Hakata and Pusan showing the distinct seasonal dependence of eddy activities at the western channel and the lee side of the Tsushima Islands (Kim, 2007). The eddy kinetic energy at station A in Fig. 9 in the western channel shows a strong eddy activity from January to April and a weak one from May to November, whereas that at station C on the lee of the Tsushima Islands, becomes strong from May to November and weak in the other month. Fig. 10 shows snapshots of surface velocity at the TKS on 15 February and 15 June (Kim, 2007), where submesoscale eddy activities are clearly seen in the western channel in February, corresponding to the chlorophylla/ MODIS image in Fig. 8 and on the lee side of the Tsushima Islands in June. Kim (2007) successfully reproduced some features of submesoscale variabilities, which are captured by observations mentioned above. From January to April, submesoscale variabilities with a length scale of about 80 km and time scale of 5–6 days develop in the western channel of the TKS and propagate toward the JES with a phase speed of 16 cm/s, whereas from May to December, the submesoscale variabiility does not appear. The development of the submesoscale variability in the western channel seems to be closely related to the seasonal change in the horizontal shear of the background mean current flowing through the western channel. As shown in Fig. 11, the horizontal shear of the background mean current becomes strong in winter with a maximum at the deepest part of the western channel as the vertical shear of the mean current decreases due to cooling. Then, the horizontal shear gradually decreases toward summer as the vertical shear of the mean current increases. Seung et al. (2007) and Kim (2007) showed that the cross strait ðxÞ = dx profile of the model background potential vorticity (q = f + dV ) H ðxÞ embedded by the submesoscale eddies satisfies the necessary condition for barotropic instability using the depth-averaged model velocity V(x) and the model topography H(x) across the ferry line with an ADCP (the solid line in Fig. 10).
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Fig. 9. Temporal variation of eddy kinetic energy (EKE in cm2/s2) with period shorter than 10 days at the points A (129.3°E, 34.9°N), B (129.9°E, 34.1°N), and C (129.5°E, 34.7°N) shown in Fig. 10 from January 2005 to May 2006, estimated from the Camellia ADCP observations.
On the lee side of the TKS, a Karman-like vortex pair develops in the warm period from May to November as shown in Fig. 10. An anticyclonic vortex generated at the northern tip of the Tsushima Islands has a time scale of 5–8 days, length scale of 35 to 60 km and propagates toward the JES with a phase speed of 8 cm/s. A cyclonic vortex south of the anticyclonic counter part of the vortex pair is rather stationary with intermittent occasional propagation toward the east.
The development of the Karman-like vortex pair in the warm period seems to be closely related to the development of stratification in the warm period. Maruyama et al. (2003) showed that a Karmanlike vortex pair could not be generated in a barotropic model with an idealized model of the Tsushima Islands and a varying bottom topography, whereas a reduced gravity model could produce a Karman-like vortex pair, suggesting that the release from the bottom constraint as the stratification increases in the warm period is
Fig. 10. Snapshots of surface velocity at the Tsushima Straits on 15 February and 15 June (Kim, 2007).
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Fig. 11. Seasonal variations of surface velocity fields at the Tsushima Straits. Left: Camellia ADCP observation. Right: model results of Kim (2007).
necessary for the development of the submesoscale variability on the lee side of the Tsushima Islands. It should be noted that submesoscale eddies are also generated east of the eastern channel and propagate toward Yamaguchi prefecture as seen in the region (34°N–35°N, 130°E–131°E)) in Fig. 10. Although these submesoscale eddies are not captured in the kinetic energy analysis at station C in Fig. 9, the characteristics of the eddy activities listed in Kim (2007) compare well with those of Senjyu and Sugihara (2001). 3. Summary and remarks The seasonal variation in the surface circulation of the JES and the TKS were reviewed with special focus on mesoscale and submesoscale variabilities. The seasonal variability of the JES is summarized as follows. 1. The winter monsoon modified by the coastal geographical feature near Vladivostok generates a dipole of vortex off Vladivostok which induces dramatic seasonal changes in the circulation in the northwest JES, generating the NWTF, the current reversal along the North Korean coast. The monsoon also induces a current reversal along the Sakhalin coast. 2. The sea ice and snow melting in the northern JES as well as the winter monsoon and associated cooling play important roles in the seasonal change in the Subpolar Gyre. A shallow and narrow surface coastal jet as a density current is driven by buoyancy forcing resulting from the sea ice and snow melting along the Russian and North Korean coasts. The volume transport of the surface Subpolar Gyre of the JES has two maxima in January and August. The intensification of the LCC–NKCC due to the winter monsoon with positive wind stress curl over the JES and related cooling is
responsible for the increase in winter. The development of the surface coastal jet along the Russian coast is partly responsible for the increase in Transport A from April toward August. The remaining part of the increase is contributed by the intensification of the LCC–NKCC in summer. 3. A salient feature in the TKS is the submesoscale eddy variability. In the western channel, submesoscale eddies with length scale of about 80 km and time scale of 5–6 days develop in the cold period. On the lee side of the Tsushima Islands, a Karman-like vortex pair is generated in the warm period. The anticyclonic vortex of this vortex pair generated at the northern tip of the Tsushima Islands has a time scale of 5–8 days, length scale of 35 to 60 km, and propagates toward the east with a phase speed of 8 cm/s. The cyclonic vortex south of the anticyclonic counter part of the vortex pair is rather stationary with intermittent occasional propagation toward the east. The development of stratification in the warm period seems to be crucial for the development of the Karman-like vortex pair. A schematic surface circulation with more detailed structures than ever reported previously is shown in Fig. 2, summarizing results above. The modeling of the seasonal variation described here with special attention ton the mesoscale and submesoscale variability is considered to be an initial model. There are many issues to be studied. One of those issues is the role of the interaction between the surface and deep circulation. As suggested by Hogan and Hurlburt (2000), the mean surface circulation can be strongly influenced by abyssal circulation via the eddy–mean flow interaction (eddy–topography interaction). Therefore, it is necessary to reproduce mesoscale eddies as accurately as possible, so that the energy flow from mesoscale eddies to the mean deep circulation through the eddy–topography
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interaction. The model by Kim (2007) was successful in reproducing the flow pattern of the deep mean flow, but the amplitude was less than one-fifth of the real amplitude. However, the requirement to have a finer resolution than 1/36° is unrealistic since it requires tremendously large computing power. One of the realistic ways to study the effect of deep circulation on the surface circulation is to incorporate the parameterization of eddy–topography interaction in the model which does not require huge computing power. References Chang, K.I., Teague, W.J., Lyu, S.J., Perkins, H.T., Lee, D.K., Watts, D.R., Kim, Y.B., Mitchell, D.A., Lee, C.M., Kim, K., 2004. Circulation and currents in the southwestern East/ Japan Sea: overview and review. Progress in Oceanography 61 (2–4), 105–156. Danchenkov, M.A., Riser, S.C., 2000. Observations of currents, temperature and salinity in the Japan Sea in 1999–2000 by PALACE floats. Proc. CREAMS'2000, pp. 33–40. Danchenkov, M.A., Aubrey, D.G., Feldman, K.L., 2003a. Oceanography of area close to the Tumannaya River mouth (The Sea of Japan). Pacific Oceanography 1 (1), 61–69. Danchenkov, M.A., Riser, S.C., Yoon, J.H., 2003b. Deep current of the central sea of Japan. Pacific Oceanography 1 (1), 6–11. Danchenkov, M.A., Lobanov, V.B., Riser, S.C., Kim, K., Takematsu, M., Yoon, J.H., 2006. A history of physical oceanographic research in the Japan/East Sea. Oceanography 19 (3), 18–31. Greatbatch, R.J., Li, G., 2000. Alongslope mean flow and an associated upslope bolus flux of tracer in a parameterization of mesoscale turbulence. Deep-sea Research 2000, 709–735. Hase, H., Yoon, J.H., Koterayama, W., 1999. The current structure of the Tsushima Current along the Japanese coast. Journal of Oceanography 55 (2), 217–236. Hogan, P.J., Hurlburt, H.E., 2000. Impact of upper ocean-topographical coupling and isopycnal outcropping in Japan/East Sea models with 1/8° to 1/64° resolution. Journal of Physical Oceanography 30 (10), 2535–2561. Holloway, G., Sou, T., Eby, M., 1995. Dynamics of circulation in the Japan Sea. Journal of Marine Research 53, 539–569. Ishii, H., Michida, Y., 1996. The tracking of the first branch of the Tsushima Warm Current with surface drifter. Report of Hydrographic Research 32, 37–47 (in Japanese with English abstract). Isobe, A., Isoda, Y., 1997. Circulation in the Japan Basin, the northern part of the Japan Sea. Journal of Oceanography 53, 373–381. Kawai, H., (1974): Transition of current images in the Japan Sea. The Tsushima Warm Current-Oceanic-structure and fishery, 7-26, ed. by Fishery Soc. Japan, Koseisha Koseikaku, Tokyo (in Japanese). Kawabe, M., 1982. Branching of the Tsushima Current in the Japan Sea. Part II: Numerical experiment. Journal of the Oceanographic Society of Japan 38, 183–192. Kim, C.H., Yoon, J.H., 1996. Modeling of the wind-driven circulation in the Japan Sea using a reduced gravity model. Journal of Oceanography 52, 359–373. Kim, Y.J., (2007): A study on the Japan/East Sea oceanic circulation using an extra-fine resolution model. Ph.D. thesis, Interdisciplinary graduate school of engineering sciences, Kyushu University. Kim, K.R., Kim, K., Kang, D.J., Volkov, Y., Yoon, J.H., Takematsu, M., 2002a. The changes in the East/Japan Sea found by CREAMS. Oceanography in Japan 11 (3), 419–429. Kim, K., Kim, K.R., Kim, Y.G., Cho, Y.K., Kang, D.J., Takematsu, M., Volkov, Y., 2004. Water masses and decadal variability in the East Sea (Sea of Japan). Progress in Oceanography 61, 157–174. Kim, K.R., Kim, G., Kim, K., Lobanov, V., Ponomarev, V., Salyuk, A., 2002b. A sudden bottom-water formation during the severe winter 2000–2001: the case of the East/ Japan Sea. Geophysical Research letters 29 (8), 75-1. Kim, K.J., Seung, Y.H., 1999. Formation and movement of the ESIW as modeled by MICOM. Journal of Oceanography 55, 369–382. Kawamura, H., Yoon, J.H., 2007. Formation rate of water masses in the Japan sea. J. Oceanography 63, 243–253. Lie, H.J., Byun, S.K., 1985. Summertime southward current along the east coast of Korea. J. Oceanological Society, Korea 20, 22–27. Maruyama, N., Hirose, N., Yoon, J.H., 2003. Numerical experiments on the mechanism of the southwestward countercurrent east of the Tsushima Islands. Engineering science report, vol. 25. Kyushu Univ., pp. 279–283.
Morimoto, A., Yanagi, T., 2001. Variability of sea surface circulation in the Japan Sea. J. Oceanography 57, 1–13. Moriyasu, S., (1972): The Tsushima Current. Kuroshio—Its Physical aspects, 353-369, ed. by H. Stommel and K. Yoshida, Univ. Tokyo Press, Tokyo. Naganuma, K., 1977. The oceanographic fluctuations in the Japan Sea. Marine Science (Kaiyo Kagaku) 9, 137–141 (in Japanese with English abstract). Naganuma, K., 1985. Fishing and oceanographic conditions in the Japan Sea. Umi to Sora 60, 89–103. Ohwada, M., Tanioka, K., 1972. Cruise report on the simultaneous observation of the Japan Sea in October 1969. Oceanographical Magazine 23, 47–58. Park, Y.H., 1986. A simple theoretical model for the upwind flow in the southern Yellow Sea. J. Oceanological Society, Korea 21, 203–210. Senjyu, T., Sugihara, S., 2001. Frontal eddies observed in the eastern channel of the Tsushima Strait. Proceedings of 11th PAMS/JECSS workshop, Cheju, Korea, pp. 421–424. Senjyu, T., Aramaki, T., Otosaka, S., Togawa, O., Danchenkov, M., Karashev, E., Volkov, Y., 2002. Renewal of the bottom water after the winter 2000–2001 may spin-up the thermohaline circulation in the Japan Sea. Geophysical Research Letters 29 (7), 1149. doi:10.1029/2001GL014093. Senjyu, T., Shin, H.R., Yoon, J.H., Nagano, Z., An, H.S., Byun, S.K., Lee, C.K., 2005. Deep flow field in the Japan/East Sea as deduced from direct current measurements. Deep-Sea Research II 52, 1726–1741. Seung, Y.H., Kim, K., 1993. A numerical modeling of the East Sea circulation. J. Oceanological Society, Korea 28, 292–304. Seung, Y.H., Yoon, J.H., 1995. Robust diagnostic modeling of the Japan Sea circulation. Journal of Oceanography 51, 421–440. Seung, Y.H., Kim, Y.J., Yoon, J.H., 2007. Seasonal characteristics of the current in the Tsushima/Korea Strait obtained by a fine-resolution numerical model. Continental Shelf Research 27, 117–133. Shimomaru, T., Miyata, K., 1957. The oceanographical conditions of the Japan Sea and its water systems, laying stress on the summer of 1955. Bulletin of the Japan Sea Regional Fisheries Research Laboratory 6, 23–97 (in Japanese). Takematsu, M., Nagano, Z., Ostrovskii, A.G., Kim, K., Volkov, Y.N., 1999. Direct current measurements of deep currents in the northern Japan Sea. Journal of Oceanography 55, 207–216. Takikawa, T., Yoon, J.H., Cho, K.D., 2003. Tidal current in the Tsushima Straits estimated from ADCP data by Ferryboat. Journal of Oceanography 39, 37–47. Takikawa, T., Yoon, J.H., Cho, K.D., 2005a. The Tsushima Warm Current through Tsushima Straits estimated from ferryboat ADCP data. Journal of Physical Oceanography 35 (6), 1154–1168. Takikawa, T., Yoon, J.H., Cho, K.D., 2005b. Volume transport through the Tsushima Straits from the Sea level difference. Journal of Oceanography 61, 699–708. Talley, L.D., Lobanov, V., Ponomarev, V., Salyuk, A., Tishchenko, P., Zhabin, I., 2003. Deep convection and brine rejection in the Japan Sea. Geophysical Research Letters 30 (4), 1159. doi:10.1029/2002GL016451. Toba, Y., Kawamura, H., Yamashita, F. and Hanawa, K., (1984): Structure of horizontal turbulence in the Japan Sea. pp. 317–332. In Ocean Hydrodynamics of the Japan and East China Seas, 317–332, Elsevier Oceanogr. Ser., ed. by T. Ichie, Elsevier. Trusenkova, O., Nikitin, A., Lobanov, V., (accepted for publication): Circulation features in the Japan/East Sea related to statistically obtained wind patterns (in the warm season). Journal of Marine System, Elsevier. Yakunin, L.P., 1975. On the reliable Amur River discharge through the new channel. Bulletin of Far Eastern Scientific Research. Hydrometeorological Institute, vol. 55, pp. 61–64. Yarichin, V.G., 1980. Steady state of the Japan Sea circulation. In: Pokudov, V. (Ed.), Problems of Oceanography, Hydrometeoizdat, Leningrad 46–61 (in Russian). Yoon, J.H., Kawamura, H., 2002. The formation and circulation of the intermediate water in the Japan Sea. Journal of Oceanography 58, 197–211. Yoon, J.H., Abe, K., Ogata, T., Wakamatsu, Y., 2005. The effects of wind-stress curl on the Japan/East Sea circulation. Deep-Sea Research II 52, 1827–1844. Yoshikawa, Y., Awaji, T., Akitomo, K., 1999. Formation and circulation process of intermediate water in the Japan Sea. Journal of Physical Oceanography 29, 1701–1722.
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Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Review on the seasonal variation of the surface circulation in the Japan/East Sea Jong-Hwan Yoon a,⁎, Young-Ju Kim b a b
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, 8168580, Japan Research Institute of Oceanography, Seoul National University, Seoul 151742, South Korea
a r t i c l e
i n f o
Article history: Received 3 September 2007 Received in revised form 18 August 2008 Accepted 2 March 2009 Available online 12 July 2009 Keywords: Japan/East Sea circulation Seasonal variability Mesoscale and submesoscale variability Northwest monsoon Northwest thermal front Surface coastal jet Topical issue on "Observation and Modeling of the Ocean Circulation and Marine Ecosystem for CREAMS/PICES". The issue is compiled and guest-edited by the North Pacific Marine Science Organization (PICES)
a b s t r a c t The seasonal variation of the surface circulation in the Japan/East Sea (JES) and the Tsushima/Korea Straits (TKS) is reviewed and discussed, focusing on mesoscale and submesoscale variabilities. The monsoon modified by coastal geographical features near Vladivostok generates a dipole of vortex off Vladivostok which induces dramatic changes in the surface circulation in the northwest JES, splitting the Subpolar Gyre into two smaller gyres by generating the Vladivostok Dome. Between these two smaller gyres, the Northwest Thermal Front is formed and current reversal is induced along the North Korean coast. The winter monsoon also induces a current reversal along the Sakhalin coast. The volume transport of the surface Subpolar Gyre has two maxima in January and August. The maximum in August is induced by the summer intensification of the Liman-North Korean Cold Current and the shallow and narrow surface coastal jet generated by the sea ice and snow melting. The maximum in January is induced by the northwest monsoon and associated cooling. Salient features in the TKS are the submesoscale variabilities. In the western channel, submesoscale eddies with length scale of about 80 km and time scale of 5–6 days develop in the cold period. On the lee side of the Tsushima Islands, Karman-like vortex pairs are generated in the warm period. Anticyclonic vortices generated at the northern tip of the Tsushima Islands have a time scale of 5 to 8 days, length scale of 35 to 60 km, and propagate toward the JES with a phase speed of 8 cm/s. Cyclonic vortices south of the anticyclonic counter part of the vortex pairs are rather stationary with intermittent occasional propagation toward the east. The development of stratification seems to be necessary for the development of Karman-like vortex pairs. Summarizing the results above, a schematic surface circulation with seasonal change is proposed. © 2009 Published by Elsevier B.V.
1. Introduction One of the most salient features commonly seen in AVHRR SST images of the Japan/East Sea (JES) is that the surface of the JES is filled with very energetic mesoscale and submesoscale eddies. Mesoscale eddies with dominant eddy sizes of 80–90 km in the JES (Toba et al., 1984) are generated mostly by baroclinic instability characterized by the first baroclinic radius deformation. The dominant mesoscale eddy size is roughly estimated to be several times larger than the first baroclinic radius deformation. Submesoscale eddies are defined as eddies with length scale smaller than the dominant mesoscale eddy size in the JES and time scale from several days to a few weeks. The strong signals of these mesoscale and submesoscale variabilities have been obscuring the mean circulation field, especially the mean path of the Tsushima Warm Current (TWC). On account of these strong variabilities and lack of direct current measurements, various flow patterns on the mean path of the TWC have been proposed (Kawai, 1974; Naganuma, 1977; Kim and Yoon, 1996), although a great number of hydrographic surveys had been ⁎ Corresponding author. Tel.: +82 81358414288. E-mail address: [email protected] (J.-H. Yoon). 0924-7963/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jmarsys.2009.03.003
performed in the Tsushima Warm Water region. On the other hand, since hydrographic surveys in the Subpolar region had been restricted for a long time by the “Iron Curtain” until the beginning of 1990s, the study of the circulation had stagnated. Since the end of the Cold War in 1991, oceanic circulation of the JES has been investigated through international cooperative studies such as the CREAMS (Circulation Research of the East Asian Marginal Seas) (Danchenkov et al., 2006). These studies have uncovered many new findings on the circulation of the JES, worth mentioning among these are the rise in temperature and decrease in dissolved oxygen in the deep layer associated with the cessation or decrease in bottom water formation since the 1950s (Kim et al., 2002a, 2004), the strong mean deep current in the Japan Basin with energetic mesoscale variabilities (Takematsu et al., 1999; Senjyu et al., 2005), the new formation of bottom water in the 2000–2001 winter (Kim et al., 2002b; Senjyu et al., 2002; Talley et al., 2003), the extensive measurements of the circulation of the Ulleung Basin (Chang et al., 2004), and the PALACE experiment for the deep circulation of the JES (Danchenkov and Riser, 2000; Danchenkov et al., 2003a,b). Many observations in various seasons during the past 15 years and some earlier studies illuminated the seasonal variation of the surface
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Fig. 1. Schematic of summer circulation (Yarichin, 1980). JB is Japan Basin and EKB is East Korean Bay.
circulation with strong mesoscale signals in the Subpolar region associated with the dipole of the monsoon wind stress curl south of Vladivostok (Yoon et al., 2005) such as the coastal current reversal in November along the Korean coast and the development of the Northwest Thermal Front (NWTF) in winter south of Vladivostok (Danchenkov et al., 2003a). In the Tsushima/Korea Straits (TKS), the monitoring of the TWC structure has been monitored since 1997 using an ADCP (Acoustic Doppler Current Profiler) mounted on a ferry boat traveling between Pusan and Hakata (Takikawa et al., 2003, 2005a,b), revealing that the transport of the TWC shows a remarkable seasonal variability with a minimum in winter and maximum (or maxima) in summer, being accompanied by energetic submesoscale variabilities with strong seasonal dependence (Seung et al., 2007). Thus, recent studies on the circulation of the JES including the TKS suggest that mesoscale and submesoscale variabilities are important aspects in the seasonal variation of the JES circulation. During the past few decades, many numerical model studies have tried to simulate the JES circulation. However, to simulate these mesoscale and submesoscale variabilities in the JES and the TKS, the horizontal grid resolution of numerical models must be at least smaller than a few kilometers. Models that meet this condition are the four layer model by Hogan and Hurlburt (2000) and the 3D primitive OGCM by Kim (2007).
The advent of the Earth Simulator (JAMSTEC) in 2002 enabled the use of a horizontal grid resolution finer than 1/32° in a 3D primitive OGCM. Kim (2007) tried to simulate the circulation of the JES, mainly focusing on the mesoscale, submesoscale and associated phenomena, such as submesoscale fluctuations appearing in the ADCP monitoring of the TWC (Takikawa et al., 2003, 2005a,b) and the cyclonic deep mean flow of the JES with a 3D primitive OGCM (RIAMOM, RIAM Ocean Model) with a horizontal grid resolution of 1/36° and 46 vertical levels. Details of the basic structure of this model can be found in Seung et al. (2007). In this study, we briefly survey recent progress in studies on the surface circulation in the JES. This review concentrates on the seasonal variation of the surface circulation of the JES with special focus on mesoscale and submesoscale variabilities, which were observed or successfully simulated in extra fine resolution models. The intermediate and deep circulation of the JES will be reviewed at another time. 2. Seasonal variation of the surface circulation in the Japan/East Sea (JES) Major features of the surface mean circulation of the JES consist of the Subpolar Front along about 40°N, the cyclonic Subpolar Gyre in the cold water region north of the Supolar Front with the southwestward
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Fig. 2. Schematic of surface circulation of the JES. Warm period (left) and cold period (right). TWC (Tsushima Warm Current), LCC–NKCC (Liman Cold Current–North Korean Cold Current), EKWC (East Korean Warm Current), SPF (Subpolar Front) (Kim, 2007).
Liman Cold Current (LCC) along the Russian coast, the North Korean Cold Current (NKCC) along the North Korean coast, and two major currents in the warm water region south of the Polar Front, the East Korean Warm Current (EKWC) along the Korean coast and the TWC along the Japanese coast. The EKWC flows eastward along the Subpolar Front after separating from the Korean coast at around 38°N, mixing with the Subpolar Water and joins the TWC west of the Tsugaru Strait. The mixed water feeds the current recirculating off the LCC along the Russian coast, which supplies salinity as well as heat to the region of winter convection off Vladivostok. The salinity supplied by this recirculating current contributes much to the initiation of deep convection accompanied by the formation of the Upper Japan Sea Proper Water. (Yoon and Kawamura, 2002) The TWC bifurcates into two main branches near the TKS, one of which flows northward along the Korean coast as the EKWC and the other is the Nearshore Branch of the TWC along the Japanese coast. Between these two main currents, another current (the second branch or offshore branch) has been suggested to exist on the offshore side of the Nearshore Branch of the TWC (Shimomaru and Miyata, 1957; Naganuma, 1977, 1985; Kawabe, 1982; Moriyasu, 1972; Ohwada and Tanioka, 1972), whereas others have proposed one meandering current instead of branching currents (Moriyasu, 1972; Ohwada and Tanioka, 1972). A recent study by Hase et al. (1999) on the branching of the TWC concluded that the Nearshore Branch (the first branch) starts from the eastern channel of the TKS follows the isobaths shallower than 200 m along the Japanese coast as a topographically steered current, and flows out through the Tsugaru Strait in all seasons. On the other hand,
the second branch of the TWC develops from spring to fall flowing along the continental shelf break with a strong baroclinicity as a topographically steered current west of Noto Peninsula. East of Noto Peninsula, the second branch flows northeastward toward the Tsugaru Strait being relieved from the topographic steering. The seasonality of the second branch was also suggested by Kawabe (1982). Referring to the schematic summer circulation of Yarichin (1980) in Fig. 1, the surface circulation in the subpolar region consists of a large cyclonic gyre in the Japan Basin (JB in Fig. 1) and two small cyclonic gyres in the East Korean Bay (EKB in Fig. 1) and west of Sakhalin. Along the Russian coast and North Korean coast, the LCC and NKCC flow southwestward, respectively. The large cyclonic gyre in the Japan Basin is driven by the positive wind stress curl in winter (Kim and Yoon, 1996) and thermohaline forcing which has not been investigated in great deal. Many observations in various seasons during the past 15 years and some earlier studies provided information on many new aspects of seasonal variability of the surface circulation in the Subpolar region such as a dramatic seasonal variability of the coastal current along the North Korean coast (Yoon et al., 2005), the summertime southward current along the east coast of Korea (Lie and Byun, 1985) and the development of the NWTF in winter (Danchenkov et al., 2003a). Some of these mesoscale aspects are closely related to atmospheric submesoscale disturbance (dipole of wind stress curl) south of Vladivostok (Yoon et al., 2005). Many numerical model studies have been done in recent years to simulate the oceanic circulation of the JES (Seung and Kim, 1993; Kim and Yoon, 1996; Kim and Seung, 1999; Seung and Yoon, 1995;
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Fig. 3. Seasonal variation of monthly mean eastward velocity for the upper 500 m on the vertical section along 133°E. Contour interval is 2.5 cm/s and color bar unit is cm/s (Kim, 2007).
Yoshikawa et al., 1999; Yoon and Kawamura, 2002; Yoon et al., 2005; Hogan and Hurlburt, 2000; Kim, 2007). However, most of these numerical models except Hogan and Hurlburt (2000) and Kim (2007) did not have sufficient horizontal resolution to resolve mesoscale and submesoscale variabilities although many major features of the surface circulation of the JES were well reproduced. Hogan and Hurlburt (2000) demonstrated the importance of mesoscale variabilities in the formation of the mean JES circulation, revealing that the mean pathway of surface circulation is strongly influenced by abyssal circulation via the eddy–mean flow interaction which generates a realistic cyclonic deep mean flow in the JES with the horizontal resolution finer than 1/32°. The eddy–mean flow interaction in Hogan and Hurlburt (2000) can be interpreted as the eddy– topography interaction (Holloway et al., 1995; Greatbatch and Li, 2000). The realistic separation of the EKWC in numerical models requires that mesoscale eddies be very well resolved to give rise to realistic eddy–topography interaction as well as the presence of realistic bottom topography. However, Hogan and Hurlburt (2000) concluded little about the seasonal variation of the surface JES circulation at all. The model study by Kim (2007) with an extra fine horizontal resolution described well the seasonal variation of the surface circulation in the JES and the TWC in the TKS as shown schematically in Fig. 2 (Kim, 2007) where the cold period indicates the season of winter monsoons, and the warm period the remaining season. The schematic surface circulation in the warm period in Fig. 2 compares well with that of Yarichin (1980) in Fig. 1 except for a narrow and shallow coastal jet along the Russian and North Korean coast and the western extent of the Subpolar Gyre in the Japan Basin. In the cold period, the northwest monsoon modified by the
geographical geometry near Vladivostok generates a dipole of vortex south of Vladivostok which induces dramatic seasonal changes in circulation of the northwestern JES. Salient features in the TKS are the submesoscale variability generated on the lee side of the TKS in the warm period and in the eastern and western channels of the TKS in the cold period. In the subsequent sections, the seasonal changes in the northwestern JES, the Tatar Strait, the TKS and the Nearshore Branch of the TWC are described and discussed in more detail, mainly referring to Kim (2007).
2.1. Northwest Japan/East Sea (JES) and Tatar Strait The eastward current along the Subpolar Front at around 40°N and the Liman Cold Current–North Korea Cold Current (LCC–NKCC) along the RussianNorth Korean coast are the two main constituents of the cyclonic Subpolar Gyre (Isobe and Isoda, 1997) in the JES as seen in Fig. 2. The hydrographic survey of the CREAMS in summer of 1995 suggested an additional constituent of the Subpolar Gyre, which is a narrow and shallow coastal current with low salinity developed between the LCC and the Russian coast. The salinity vertical section along the C section (136°E) in Kim et al. (2004) clearly shows a low salinity area with narrow horizontal (50 km) and shallow vertical extent (100 m) adjacent to the Russian coast, suggesting a westward surface coastal current, which distinctly differs distinctly from the LCC. Along the Korean coast a southward surface coastal jet with the same horizontal and vertical extent was observed by Lie and Byun (1985) in summer at around 38°N.
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Fig. 4. Climatological mean wind stress vector (left) and wind stress curl (right) in February, averaged for the period from 1997 to 2000, using the ECMWF reanalysis data. Shaded area in the right panel indicates negative values.
The aforementioned westward surface coastal jet with low salinity along the Russian coast in summer has been successfully reproduced by Kim (2007) showing that a strong southwestward coastal jet develops in a shallow and narrow belt along the Russian and North Korean coast in the warm period from May to September as shown in monthly vertical sections of eastward velocity along 133°E in Fig. 3. The development of a distinct core of westward velocity can be seen in the surface layer between the Russian coast and the LCC from May to September, whereas the LCC has a deeper, broader structure. The surface coastal jet and the LCC are eventually combined into one current in October. This surface coastal jet with low salinity starts to develop in the northern Primorye coast in May and progresses southwestward and reaches 38°N in August as reported by Lie and Byun (1985). The melting of snow and sea ice along the Primorye coast and in the Tatar Strait seems to generate this fresh coastal jet as a density current between the Russia–Korea coast and the LCC–NKCC, forming a double core structure of velocity along the Russia– Korea coast. The fresh water supplied from the Amur River (Yakunin, 1975) is responsible for the sea ice formation in the Tatar Strait. Another notable feature is the intensification of the LCC in summer as well as in winter as seen in Fig. 3.
In the cold period from late fall to early spring, the circulation in the northwest JES changes its features dramatically. This change is caused by the northwest monsoon modified by the coastal geographical features near Vladivostok, which prevails from late fall to the end of February as shown by Yoon et al. (2005). The northwest monsoon converges into a narrow jet when it passes through the narrow valley around Vladivostok, being accompanied by a cyclonic vortex at its eastern side and an anticyclonic one at the western side (a dipole of wind vortex) off Vladivostok as seen in Fig. 4. This wind vortex dipole induces a dramatic change in the circulation of the northwest JES, generating a dipole of vortex (Vladivostok dipole) in the ocean off Vladivostok (Yoon et al., 2005). The positive vortex of the dipole develops into the Vladivostok Dome which plays important roles in the formation of the Upper Japan Sea Proper Water (Yoon and Kawamura, 2002; Kawamura and Yoon, 2007). The negative anomaly of SST associated with the Vladivostok Dome is clearly seen in both the model (Kim, 2007) and the AVHRR image (Yoon et al., 2005) from the end of October to December, although a positive anomaly of SST associated with the negative vortex of the
Fig. 5. Horizontal distribution of sea surface temperature (left, °C) and vertical section of temperature (right, °C) along the dotted line in the left panel in January 1986. The Northwest Thermal Front (NWTF) and Subpolar Front (SPF) are clearly seen at around 41°N and 39.5°N, respectively (right) (Danchenkov et al., 2003a).
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Fig. 6. Seasonal variation of volume transport stream function for the upper 210 m. Contour interval is 0.2 Sv (Kim, 2007).
dipole is not well recognized. From October to December, since the vertical temperature gradient in the region north of the Subpolar Front is relatively significant, the upwelling induced by positive wind stress curl off Vladivostok can effectively generate negative SST anomaly. However, continuous strong heat loss from the surface in the northern JES during winter continues to weaken not only the vertical temperature gradient but also horizontal temperature gradient. By February, the vertical and horizontal temperature gradient will become so weak that the upwelling associated with the positive wind stress curl off Vladivostok cannot generate strong negative SST anomaly in accordance with the disappearance of negative SST anomaly in the positive wind stress curl area in mid and late winter. As the dipole of vortex develops in the ocean, a front called the NWTF (Danchenkov et al., 2003a) extending southeastward from the Tuman River mouth is formed along the boundary between the cold and warm eddy from late fall to spring as seen in Fig. 5. The position of the NWTF shifts slightly westward from the boundary line between
the positive and negative vortex of the wind vortex dipole due to the β effect. Consequently, the LCC bifurcates south of Vladivostok in winter. One branch separating from the LCC flows along the NWTF. The other continues to flow southwestward along the North Korean coast as the NKCC forming a western limit of the Subpolar Gyre. The former contributes to split the interior of the Subpolar Gyre into two smaller cyclonic circulations in the cold period north of the Subpolar Front as seen in the cold period in Fig. 2. Another salient feature generated by the wind vortex dipole is the coastal current reversal along the Vladivostok–North Korean coast. The anti-cyclonic wind vortex generates clockwise circulation in the Peter's Great Bay south of Vladivostok and along the North Korean coast which propagates southwestward as topographic Rossby waves, generating a northeastward current along the Vladivostok–North Korean coast, where the NKCC and surface coastal jet flow southwestward during late spring to early fall as mentioned above. This seasonal current reversal was captured dramatically by an ARGOS
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Fig. 7. Seasonal variations of volume transport between the Russian coast and two points, A (42°N, 134°E) and B (41.5°N, 134°E) as shown in Fig. 6 for the upper 210 m (left). Seasonal variation of volume transport between B and A in the upper 210 m (right).
buoy deployed south of Vladivostok in July 1994 during the CREAMS'94 summer cruise (Danchenkov et al., 2003a; Yoon et al., 2005). The buoy flowed southwestward along the North Korean coast until the end of October and then turned back suddenly northeastward along the North Korean coast up to 42°N, and then separated from the coast in the middle of December, forming an anticyclonic trajectory with a rotational radius of 150 km. This seasonal coastal current reversal starting from late fall along the North Korean coast was also confirmed by a numerical model by Yoon et al. (2005) and Kim (2007). Another example of the circulation that seems to be generated by the coastal geographical feature is a small cyclonic circulation around the East Korean Bay (38.5°–40.5°) as seen in Fig. 2. This small circulation is also depicted in a schematic picture of the surface circulation of the JES by Yarichin (1980) in Fig. 1. The circulation seems to be generated by a positive wind stress curl in the East Korean Bay as seen in Fig. 4. This positive wind stress curl is generated by the convergence of the northwest winter monsoon passing through the valley around the East Korean Bay. The positive wind stress curl west of Hokkaido and Sakhalin may also generate a small cyclonic circulation north of 45°N as seen in Figs. 1 and 2. As seen in Fig. 2 in Yoon et al. (2005), the track of the ARGOS buoy deployed off the Russian coast at 43.5°N on 12 July 1994 suggests the existence of a cyclonic gyre north of 44°N. The buoy was trapped by a small cyclonic gyre, circulated twice in nearly 1 year with a speed of over 10 cm/s, and eventually flowed southwestward along the Russian and North Korean coast, suggesting the existence of a southwestward surface coastal jet in summer. The location and scale of the gyre suggested by the ARGOS buoy correspond well with model results by Yoon et al. (2005). The circulation in the Tatar Strait also shows a dramatic seasonal change. As shown in Figs. 1 and 2 the strait is occupied by a cyclonic circulation that is a continuation of the TWC in the warm period until the northwest winter monsoon starts to blow. The northwest monsoon generates a southward coastal current along the Sakhalin and Primorye coasts and a northward compensating current in the central part of the Tatar Strait as suggested by Park (1986) in the study for the Yellow and East China Seas. 2.2. Volume transport in the Subpolar Gyre The seasonal variations of the surface circulation described in the previous section are reflected in the volume transport stream functions for the upper 210 m in the Subpolar Gyre in Fig. 6. A remarkable feature in the stream function fields is that the interior of the Subpolar Gyre in
the Japan Basin splits into two small cyclonic gyres in the cold period. One is a small cyclonic gyre in the central part (roughly, 40.5°N–42.5°N, 131°E–137°E) of the Japan Basin associated with the Vladivostok Dome in the previous section and the other is a small cyclonic gyre in the East Korean Bay (roughly, 38.5°N–40.5°N, 128.5°E–130.5°E). These gyres develop in the cold period and spin down toward autumn. These small cyclonic gyres are caused by the positive wind stress curls in winter as suggested in the previous section. Along the western rim of the small gyre in the central part of the Japan Basin, the NWTF develops as discussed before. Another remarkable feature is the intensification of the current along the Russian and North Korean coast twice a year, in summer and winter. The intensification twice a year can be seen more clearly in the volume transport of the current. Fig. 7 (left) shows the volume transport for the upper 210 m between the Russian coast and points A and B shown in Fig. 6. The volume transport between the Russian coast and point A (42°N, 134°E) (hereafter, Transport A) is contributed by the LCC and the surface coastal jet along the Russian coast. That between the Russian coast and point B (41.5°N, 134°E) (hereafter, Transport B) corresponds to the transport of the entire Subpolar Gyre. Transport A for the surface layer shows two maxima in summer and winter. The intensification of the LCC–NKCC due to the winter monsoon with positive wind stress curl over the JES (Kim and Yoon, 1996; Yoon et al., 2005) and related cooling is responsible for the increase in winter. The development of the surface coastal jet along the Russian coast is partly responsible for the increase in Transport A from April toward August as inferred from Fig. 3. The remaining part of the increase is contributed by the intensification of the LCC–NKCC in summer. The mechanism of the summer intensification of the LCC–NKCC should be clarified in future. The difference between Transports A and B for the surface layer in Fig. 7 (right) shows two maxima in February and August. The maximum in February may correspond to the development of the small gyre in the central part of the JES. The same seasonal variation can be seen in the volume transport along the North Korean coast and in a small cyclonic gyre in the East Korean Bay (not shown here). Since the difference between Transports A and B shows a relatively small seasonal variation and the same tendency can be seen along the North Korean coast, the main player in the seasonal variation of the volume transport of the Subpolar Gyre in the surface layer is the current along the Russian and North Korean coast. The summer intensification of the surface circulation of the Subpolar Gyre was also pointed out by Morimoto and Yanagi (2001) from the seasonal variation of the first empirical orthogonal function (EOF) mode of the surface circulation, analyzing satellite altimetry
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data. However, the strength of the first EOF mode, explaining 87% of the variance, has a single maximum in summer and a minimum in winter. Considering the spatial distribution of the first EOF mode (see Fig. 10 in Morimoto and Yanagi, 2001), the seasonal variation of the TWC with a maximum in summer and minimum in winter seems to be strongly reflected in the first EOF mode. Therefore, the first EOF mode does not seem to describe the seasonal variation of the surface circulation of the Subpolar Gyre properly. Trusenkova et al. (accepted for publication) also suggested the weakening (intensification) of the surface Subpolar Gyre in winter (summer) form the analysis of their model SSH and surface velocity fields. However, the calculation of volume transport in upper 200 m layer in their model might show a maximum in winter due to the winter monsoon. 2.3. Bifurcation of the Nearshore Branch of the Tsushima Warm Current (TWC) Although many studies have investigated the path of the TWC, its strong mesoscale variability and the lack of resolution of previous numerical models have been obscuring the mean path, especially the Nearshore Branch of the TWC along the Japanese coast. As seen in Fig. 2, the model results show that the Nearshore Branch consists of two branches, the first and second branches in the warm period (roughly from April to November). In the cold period (roughly from December to March), the second branch becomes indiscernible whereas the first branch continues to exist along the Japanese coast. The first branch of the TWC, which flows into the JES through the eastern channel of the TKS is trapped within an area shallower than 200 m along the Japanese coast during a year with maximum transport in summer and minimum in winter. This current is very difficult to detect by the traditional geostrophic calculation due to its relatively strong barotropy and shallowness of the bottom. In recent years, many direct current measurements using such as a moored current meter (Hase et al., 1999), an ADCP, or an ARGOS buoy have shown the strong coastal trapped current within the area shallower than 200 m along the Japanese coast. The track of an ARGOS buoy, which was deployed at the TKS in summer (Ishii and Michida, 1996) and reached the Tsugaru Strait in a month, shows the existence of a very strong current of mean speed of 50 cm/s along the Japanese coast in the summer season, corresponding well with the model results. The second branch, which flows into the JES through the eastern channel of the TKS, flows along the offshore side of the first branch, following the isobath of 500 m–1000 m in the warm period. In the cold period, the second branch, originating from the eastern channel of the TKS, joins the first branch west of the Oki Islands. The current (dotted line) branching from the third branch might feed the second branch. However, the mesoscale variability in the model is so strong that the mean path of the second branch is not identifiable. 2.4. Submesoscale variabilities in the Tsushima/Korea Straits (TKS) In the TKS, the TWC structure has been monitored since February 1997 using an ADCP mounted on a ferry boat that made three (six times from July 2004) round trips between Pusan and Hakata (Takikawa et al., 2003, 2005a,b). Takikawa et al. (2005a) showed that the monthly volume transport of the TWC averaged from February 1997 to August 2002 has a remarkable seasonal change with a minimum (1.75Sv) in winter and two maxima (2.8 Sv in June and 3.2 Sv in October) with an average volume transport of 2.64 Sv. The ADCP measurements showed another salient feature of the TWC, submesoscale variaibilities that show strong seasonal dependence (Seung et al., 2007). The image of chlorophylla/MODIS on 12 January 2006 (NPEC) in Fig. 8 is an example of submesoscale variability with
Fig. 8. Chlorophyl-a/MODIS on January 2, 2006 (NPEC, Nothwest Pacific Region Environmental Cooperation Center). Color bar unit is mg/m3.
a length scale of about 60 km or less along the southern coast of Korea in the western channel of the TKS. Fig. 9 shows a time series of eddy kinetic energy with a period shorter than 10 days calculated from the ADCP measurements between Hakata and Pusan showing the distinct seasonal dependence of eddy activities at the western channel and the lee side of the Tsushima Islands (Kim, 2007). The eddy kinetic energy at station A in Fig. 9 in the western channel shows a strong eddy activity from January to April and a weak one from May to November, whereas that at station C on the lee of the Tsushima Islands, becomes strong from May to November and weak in the other month. Fig. 10 shows snapshots of surface velocity at the TKS on 15 February and 15 June (Kim, 2007), where submesoscale eddy activities are clearly seen in the western channel in February, corresponding to the chlorophylla/ MODIS image in Fig. 8 and on the lee side of the Tsushima Islands in June. Kim (2007) successfully reproduced some features of submesoscale variabilities, which are captured by observations mentioned above. From January to April, submesoscale variabilities with a length scale of about 80 km and time scale of 5–6 days develop in the western channel of the TKS and propagate toward the JES with a phase speed of 16 cm/s, whereas from May to December, the submesoscale variabiility does not appear. The development of the submesoscale variability in the western channel seems to be closely related to the seasonal change in the horizontal shear of the background mean current flowing through the western channel. As shown in Fig. 11, the horizontal shear of the background mean current becomes strong in winter with a maximum at the deepest part of the western channel as the vertical shear of the mean current decreases due to cooling. Then, the horizontal shear gradually decreases toward summer as the vertical shear of the mean current increases. Seung et al. (2007) and Kim (2007) showed that the cross strait ðxÞ = dx profile of the model background potential vorticity (q = f + dV ) H ðxÞ embedded by the submesoscale eddies satisfies the necessary condition for barotropic instability using the depth-averaged model velocity V(x) and the model topography H(x) across the ferry line with an ADCP (the solid line in Fig. 10).
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Fig. 9. Temporal variation of eddy kinetic energy (EKE in cm2/s2) with period shorter than 10 days at the points A (129.3°E, 34.9°N), B (129.9°E, 34.1°N), and C (129.5°E, 34.7°N) shown in Fig. 10 from January 2005 to May 2006, estimated from the Camellia ADCP observations.
On the lee side of the TKS, a Karman-like vortex pair develops in the warm period from May to November as shown in Fig. 10. An anticyclonic vortex generated at the northern tip of the Tsushima Islands has a time scale of 5–8 days, length scale of 35 to 60 km and propagates toward the JES with a phase speed of 8 cm/s. A cyclonic vortex south of the anticyclonic counter part of the vortex pair is rather stationary with intermittent occasional propagation toward the east.
The development of the Karman-like vortex pair in the warm period seems to be closely related to the development of stratification in the warm period. Maruyama et al. (2003) showed that a Karmanlike vortex pair could not be generated in a barotropic model with an idealized model of the Tsushima Islands and a varying bottom topography, whereas a reduced gravity model could produce a Karman-like vortex pair, suggesting that the release from the bottom constraint as the stratification increases in the warm period is
Fig. 10. Snapshots of surface velocity at the Tsushima Straits on 15 February and 15 June (Kim, 2007).
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Fig. 11. Seasonal variations of surface velocity fields at the Tsushima Straits. Left: Camellia ADCP observation. Right: model results of Kim (2007).
necessary for the development of the submesoscale variability on the lee side of the Tsushima Islands. It should be noted that submesoscale eddies are also generated east of the eastern channel and propagate toward Yamaguchi prefecture as seen in the region (34°N–35°N, 130°E–131°E)) in Fig. 10. Although these submesoscale eddies are not captured in the kinetic energy analysis at station C in Fig. 9, the characteristics of the eddy activities listed in Kim (2007) compare well with those of Senjyu and Sugihara (2001). 3. Summary and remarks The seasonal variation in the surface circulation of the JES and the TKS were reviewed with special focus on mesoscale and submesoscale variabilities. The seasonal variability of the JES is summarized as follows. 1. The winter monsoon modified by the coastal geographical feature near Vladivostok generates a dipole of vortex off Vladivostok which induces dramatic seasonal changes in the circulation in the northwest JES, generating the NWTF, the current reversal along the North Korean coast. The monsoon also induces a current reversal along the Sakhalin coast. 2. The sea ice and snow melting in the northern JES as well as the winter monsoon and associated cooling play important roles in the seasonal change in the Subpolar Gyre. A shallow and narrow surface coastal jet as a density current is driven by buoyancy forcing resulting from the sea ice and snow melting along the Russian and North Korean coasts. The volume transport of the surface Subpolar Gyre of the JES has two maxima in January and August. The intensification of the LCC–NKCC due to the winter monsoon with positive wind stress curl over the JES and related cooling is
responsible for the increase in winter. The development of the surface coastal jet along the Russian coast is partly responsible for the increase in Transport A from April toward August. The remaining part of the increase is contributed by the intensification of the LCC–NKCC in summer. 3. A salient feature in the TKS is the submesoscale eddy variability. In the western channel, submesoscale eddies with length scale of about 80 km and time scale of 5–6 days develop in the cold period. On the lee side of the Tsushima Islands, a Karman-like vortex pair is generated in the warm period. The anticyclonic vortex of this vortex pair generated at the northern tip of the Tsushima Islands has a time scale of 5–8 days, length scale of 35 to 60 km, and propagates toward the east with a phase speed of 8 cm/s. The cyclonic vortex south of the anticyclonic counter part of the vortex pair is rather stationary with intermittent occasional propagation toward the east. The development of stratification in the warm period seems to be crucial for the development of the Karman-like vortex pair. A schematic surface circulation with more detailed structures than ever reported previously is shown in Fig. 2, summarizing results above. The modeling of the seasonal variation described here with special attention ton the mesoscale and submesoscale variability is considered to be an initial model. There are many issues to be studied. One of those issues is the role of the interaction between the surface and deep circulation. As suggested by Hogan and Hurlburt (2000), the mean surface circulation can be strongly influenced by abyssal circulation via the eddy–mean flow interaction (eddy–topography interaction). Therefore, it is necessary to reproduce mesoscale eddies as accurately as possible, so that the energy flow from mesoscale eddies to the mean deep circulation through the eddy–topography
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interaction. The model by Kim (2007) was successful in reproducing the flow pattern of the deep mean flow, but the amplitude was less than one-fifth of the real amplitude. However, the requirement to have a finer resolution than 1/36° is unrealistic since it requires tremendously large computing power. One of the realistic ways to study the effect of deep circulation on the surface circulation is to incorporate the parameterization of eddy–topography interaction in the model which does not require huge computing power. References Chang, K.I., Teague, W.J., Lyu, S.J., Perkins, H.T., Lee, D.K., Watts, D.R., Kim, Y.B., Mitchell, D.A., Lee, C.M., Kim, K., 2004. Circulation and currents in the southwestern East/ Japan Sea: overview and review. Progress in Oceanography 61 (2–4), 105–156. Danchenkov, M.A., Riser, S.C., 2000. Observations of currents, temperature and salinity in the Japan Sea in 1999–2000 by PALACE floats. Proc. CREAMS'2000, pp. 33–40. Danchenkov, M.A., Aubrey, D.G., Feldman, K.L., 2003a. Oceanography of area close to the Tumannaya River mouth (The Sea of Japan). Pacific Oceanography 1 (1), 61–69. Danchenkov, M.A., Riser, S.C., Yoon, J.H., 2003b. Deep current of the central sea of Japan. Pacific Oceanography 1 (1), 6–11. Danchenkov, M.A., Lobanov, V.B., Riser, S.C., Kim, K., Takematsu, M., Yoon, J.H., 2006. A history of physical oceanographic research in the Japan/East Sea. Oceanography 19 (3), 18–31. Greatbatch, R.J., Li, G., 2000. Alongslope mean flow and an associated upslope bolus flux of tracer in a parameterization of mesoscale turbulence. Deep-sea Research 2000, 709–735. Hase, H., Yoon, J.H., Koterayama, W., 1999. The current structure of the Tsushima Current along the Japanese coast. Journal of Oceanography 55 (2), 217–236. Hogan, P.J., Hurlburt, H.E., 2000. Impact of upper ocean-topographical coupling and isopycnal outcropping in Japan/East Sea models with 1/8° to 1/64° resolution. Journal of Physical Oceanography 30 (10), 2535–2561. Holloway, G., Sou, T., Eby, M., 1995. Dynamics of circulation in the Japan Sea. Journal of Marine Research 53, 539–569. Ishii, H., Michida, Y., 1996. The tracking of the first branch of the Tsushima Warm Current with surface drifter. Report of Hydrographic Research 32, 37–47 (in Japanese with English abstract). Isobe, A., Isoda, Y., 1997. Circulation in the Japan Basin, the northern part of the Japan Sea. Journal of Oceanography 53, 373–381. Kawai, H., (1974): Transition of current images in the Japan Sea. The Tsushima Warm Current-Oceanic-structure and fishery, 7-26, ed. by Fishery Soc. Japan, Koseisha Koseikaku, Tokyo (in Japanese). Kawabe, M., 1982. Branching of the Tsushima Current in the Japan Sea. Part II: Numerical experiment. Journal of the Oceanographic Society of Japan 38, 183–192. Kim, C.H., Yoon, J.H., 1996. Modeling of the wind-driven circulation in the Japan Sea using a reduced gravity model. Journal of Oceanography 52, 359–373. Kim, Y.J., (2007): A study on the Japan/East Sea oceanic circulation using an extra-fine resolution model. Ph.D. thesis, Interdisciplinary graduate school of engineering sciences, Kyushu University. Kim, K.R., Kim, K., Kang, D.J., Volkov, Y., Yoon, J.H., Takematsu, M., 2002a. The changes in the East/Japan Sea found by CREAMS. Oceanography in Japan 11 (3), 419–429. Kim, K., Kim, K.R., Kim, Y.G., Cho, Y.K., Kang, D.J., Takematsu, M., Volkov, Y., 2004. Water masses and decadal variability in the East Sea (Sea of Japan). Progress in Oceanography 61, 157–174. Kim, K.R., Kim, G., Kim, K., Lobanov, V., Ponomarev, V., Salyuk, A., 2002b. A sudden bottom-water formation during the severe winter 2000–2001: the case of the East/ Japan Sea. Geophysical Research letters 29 (8), 75-1. Kim, K.J., Seung, Y.H., 1999. Formation and movement of the ESIW as modeled by MICOM. Journal of Oceanography 55, 369–382. Kawamura, H., Yoon, J.H., 2007. Formation rate of water masses in the Japan sea. J. Oceanography 63, 243–253. Lie, H.J., Byun, S.K., 1985. Summertime southward current along the east coast of Korea. J. Oceanological Society, Korea 20, 22–27. Maruyama, N., Hirose, N., Yoon, J.H., 2003. Numerical experiments on the mechanism of the southwestward countercurrent east of the Tsushima Islands. Engineering science report, vol. 25. Kyushu Univ., pp. 279–283.
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