current measurements of the japan sea proper water … · current measurements of the japan sea...

359

Journal of Oceanography, Vol. 59, pp. 359 to 368, 2003

Keywords:⋅ ALACE float,⋅ Japan Sea,⋅ current measure-ment,

⋅ intermediatecirculation.

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

Copyright © The Oceanographic Society of Japan.

Current Measurements of the Japan Sea Proper Waterand the Intermediate Water by ALACE Floats

DAIGO YANAGIMOTO1* and KEISUKE TAIRA2

1Ocean Research Institute, University of Tokyo, Minamidai, Nakano-ku, Tokyo 164-8639, Japan2Japan Society for the Promotion of Science, Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan

(Received 6 March 2002; in revised form 2 December 2002; accepted 3 December 2002)

The subsurface current of the Japan Sea was observed by two Autonomous LagrangianCirculation Explorer (ALACE) floats. One float, having a 20-day cycle, was deployedon 29 July 1995 in the eastern Japan Basin and drifted in the northeastern part of thebasin until 15 September 2000. The other float, with a 10-day cycle, was deployed on4 August 1995 in the western Japan Basin and drifted in the western Japan Basin, inthe Yamato Basin and around the Yamato Rise until it reached its life limit in mid-May 2000. An anticlockwise circulation in the eastern Japan Basin was observed andit was assumed to be in the upper portion of the Japan Sea Proper Water (UJSPW) orin the intermediate water. The spatial scale of the circulation increased as the depthdecreased. A clockwise circulation was observed around the Yamato Rise in the UJSPW.Smaller clockwise and anticlockwise rotations were observed in the western JapanSea, where a seasonal variation was seen in drift speed with different phase by depth.The correlation coefficient between drift speeds of two floats indicated little coher-ence among the subsurface circulation between the east and the west of the JapanBasin, or between the north and the south of the subpolar front.

The deep layer below the thermocline is filled withan almost homogeneous water mass colder than 1°C,which is called the Japan Sea Proper Water. Sudo (1986)and Senjyu and Sudo (1993) identified the upper portionof the Japan Sea Proper Water (UJSPW) at 300–1000 mdepth as a mode water formed by winter convection offthe Siberian coast west of 136°E between 40°N and 43°N.Senjyu and Sudo (1993, 1994) showed that the UJSPWhas potential density σ1 of 32.00–32.05 kg/m3 or σθ of27.31–27.34 kg/m3 and salinity of 34.04–34.07 PSU, andthat the depth of the top of the UJSPW (σ1 = 32.00) isless than 200 m in the eastern Japan Basin and is between200 m and 400 m around the Yamato Rise. Above theProper Water, and below the surface water, there existintermediate waters with a temperature of 1–5°C, lowsalinity and high oxygen concentration (Kim et al., 1999;Kim and Kim, 1999; Senjyu, 1999). Senjyu (1999) iden-tified the Japan Sea Intermediate Water (JIW) at the sa-linity minimum layer lying on the isopycnal surface27.28σθ in the southern Japan Sea, and Kim and Kim(1999) identified the High Salinity Intermediate Water(HSIW) as the high salinity and high oxygen water lyingon the isopycnal surface of 27.2σθ in the eastern JapanBasin. The depth of the JIW ranges from more than 300m in the southern Japan Sea to less than 200 m in the

1. IntroductionThe Japan Sea, which has a maximum depth greater

than 3500 m, is connected to open oceans by four nar-row, shallow straits: Tsushima Strait at the southern end,Tatar Strait at the northern end, and Tsugaru Strait andSoya Strait to the south and the north of Hokkaido Is-land, respectively. The deepest parts of Tsushima andTsugaru Straits are about 150 m deep, that of Soya Straitis about 60 m, and that of Tatar Strait less than 10 m. Atthe surface layer, a warm, saline water is carried by theTsushima Current through Tsushima Strait and forms a“warm water region” in the southern area of the JapanSea. The Tsushima Current separates into two or threebranches including the East Korean Warm Current flow-ing northward along the Korean coast. On the other hand,in the northern area of the Japan Sea, a cold, fresh waterforms a “cold water region” and cold coastal currentsnamed the Liman Current and the North Korean ColdCurrent are dominant along the northern coast. The warmwater and the cold water form the subpolar front around40°N.

360 D. Yanagimoto and K. Taira

western Japan Sea (Senjyu, 1999). The depth of the HSIWis about 50 db to 400 db (Kim and Kim, 1999). The move-ment of the UJSPW and the intermediate waters has notyet been clarified.

CREAMS (Circulation Research of the East AsianMarginal Seas) is a research program conducted by inter-national cooperation in 1993–1997 (Takematsu, 1999); itintensified the oceanographic studies of the Japan Sea.Direct current measurements by moored current meters(Takematsu et al., 1999), and by a towed ADCP (Acous-tic Doppler Current Profiler) (Isobe and Isoda, 1997) weredone in CREAMS. Also as a part of CREAMS, twoALACE (Autonomous Lagrangian Circulation Explorer)floats were tracked from August 1995 to September 2000.

This paper describes the result of the Lagrangiancurrent measurements at the intermediate depth in thenortheastern Japan Basin and in the area around theYamato Rise. The deployment of the floats and methodof data analysis are described in Section 2. The pressureand temperature which ALACE floats observed at the driftdepth are shown in Section 3. The trajectories of theALACE floats and current variation are discussed in Sec-tion 4, and a conclusion and discussion, with reference toprevious studies, are summarized in Section 5.

2. MethodTwo ALACE floats, manufactured by Webb Research

Corporation, were deployed from R/V Professor Khromovduring the CREAMS-1995 summer cruise. An ALACEfloat cycles vertically between a balanced depth and thesea surface by powering an oil pump and changing itsown buoyancy (Davis et al., 1991). The two floats wereoriginally ballasted for an in-situ density of 28.951kg/m3, potential density (σθ) of 27.299 kg/m3, calculatedfrom typical temperature of 1.0°C and salinity of 34.07PSU at a depth of 350 db in the central part of the JapanSea. One of them, “A77”, was set to have a cycle of 20days, i.e., it repeated the cycle of descending to the bal-anced depth, where it drifted for 19 days, and ascendingto the surface, where it drifted for one day. The other,“A78”, was set to have a cycle of 10 days, i.e., 9 days atthe balanced depth and one day at the surface in eachcycle. Float A77 with a 20-day cycle was deployed on 29July 1995 at 41°42.6′ N, 136°17.5′ E, where the waterdepth was 3450 m, in the eastern Japan Basin. Float A78with a 10-day cycle was deployed on 4 August 1995 at40°54.0′ N, 132°24.0′ E, where water depth was 3100 m,in the western Japan Basin.

A float drifting at the surface is located by SystemArgos satellites. The float also sends pressure and tem-perature averaged for the first half and the last half of thedrifting time in the water, pressure averaged while at thesurface before the previous dive, and voltage of the bat-teries powering the computer and the pump. These te-

lemetry data and location data were collected by SystemArgos, and were sent by a floppy disk every month. FloatA78 completed 169 cycles until it stopped sending sig-nals after it had sent its last data off Akita Prefecture on 7May 2000. Float A77 completed 92 cycles on 15 Septem-ber 2000.

The subsurface currents were estimated from thedisplacements from the last position of the floats beforediving to the first position after surfacing. There wouldbe some errors in estimating the subsurface currents dueto the surface currents during the following periods. First,in the Japan Sea, location was done approximately every1 1/2 h by two System Argos satellites. The accurate div-ing time of floats is different from the last locating timebefore their dives by ≤1 1/2 h. The accurate surfacingtime is also different from the first locating time aftertheir dives in the same way. Second, it takes about 165minutes for a float to descend to 350 m depth and about35 minutes to ascend to the sea surface. As a result, floatsmay be subject to drift by the surface current for about 61/2 h. Though drift speeds at the surface estimated fromArgos tracking are dependent on time and area, averagespeeds of both floats were about 30 cm/s at the surface.The distance between diving and surfacing points had anerror of 7 km due to drift by the surface current with anaverage speed of 30 cm/s during the period of 6 1/2 h.Therefore, errors of the average subsurface speeds were0.42 cm/s for float A77 which stayed underwater for 19days, and 0.90 cm/s for float A78 which stayed underwa-ter for 9 days.

The accuracy (ε) of Argos location is classified intothree bands. Frequencies for two floats were 17% for Class3 (ε < 150 m), 44% for Class 2 (150 m ≤ ε < 350 m) and39% for Class 1 (350 m ≤ ε < 1000 m). The errors oflocation by satellites were less than the drift by surfacecurrents.

3. Pressure and TemperatureFigures 1 and 2 show the time series of pressure and

temperature observed by floats A77 and A78, respectively.The day of the abscissa starts on 28 July 1995 as the zeroday for both floats. Winter, spring, summer and autumnmean January to March, April to June, July to Septemberand October to December, respectively. Pressure observedin the water was corrected with pressure values observedat the surface by the floats, which were less than 10 db.

Float A77 did not send any data on 1 and 20 March2000 although the float was scheduled to surface. FloatA78 did not send any data on scheduled dates of 25 Janu-ary, 13 February, 2 June and 8 November in 1997, and 17April in 2000. Weather reports at Ulleungdo andVladivostok showed that the wind speeds were not al-ways stronger than on other dates of successful transmis-sion. It is thought that the Argos antenna was kept in the

Current Measurements of the Japan Sea Proper Water and the Intermediate Water by ALACE Floats 361

water even at the surface for some unknown reason.Figure 1 shows that float A77 observed temperatures

below 1.0°C at 331–256 db depth during the first 420 days.Pressure gradually decreased from 331 db to 51 db dur-ing the whole observation period and the speed of pres-sure decrease was faster in spring to summer than in otherseasons. In summer of 1999, the pressure measurementsshowed very low values of 43 db and 16 db. This is be-cause the float grounded on the shallow bottom nearHokkaido Island. After the grounding, the float was drift-ing in the water of 10.6–1.4°C temperature at 110–51 dbdepth.

Figure 2 shows that float A78 observed temperaturesaround 1.0°C at 325–238 db depth during the first 210days. Pressure from float A78 also decreased from 325db to 162 db during the first 450 days, and then, a re-peated increase and decrease was observed between about130 db and 240 db until April 2000. It reached the mini-

mum pressure in autumn of 1996 (159 db), in summer toautumn of 1998 (136 db), and in summer of 1999 (126db), and reached the maximum pressure in autumn of 1997(238 db), in winter to spring of 1999 (246 db), and inwinter of 2000 (221 db). The float grounded off Akitacoast in three dives before it stopped transmitting sig-nals.

Pressure did not return to the level at the start of theobservation and temperature apparently tended to increasethrough the observation period. The trend of pressuredecrease was 51 db/year for A77 and 20 db/year for A78.The trend of temperature increase was 0.65°C/year forA77 and 1.25°C/year for A78. These trends imply thatfloats’ density decreased. The two floats were guaranteedfor about 50 cycles. The voltage of the batteries that pow-ered the pump and the computer of float A77 remainednearly constant during the observation period. The volt-age of the battery powering the pump of float A78 de-creased rapidly from 14.5 to 11.3 V around 1480 days,although the float dived to almost the same depth as be-fore until the voltage fell below 10 V in mid-May, 2000.The voltage of the computer batteries stayed constant for

Fig. 1. Time series of pressure (a) and temperature (b) observedby float A77. Day of the abscissa starts on 28 July 1995(0th day). Vertical broken lines show partitions of seasons.Winter, spring, summer and autumn mean January to March,April to June, July to September and October to December,respectively.

Fig. 2. Time series of pressure (a) and temperature (b) observedby the float A78, otherwise as Fig. 1.


the entire period. The decrease of the depth at which thesubsurface floats drifted would not be caused by batterydeterioration, but by leakage of the hydraulic oil or ero-sion of the bodies.

Besides the long term trend described above, therewas an apparent tendency for pressure from A78 to de-crease in summer, and the difference of pressure betweenthe minimum and the maximum was about 100 db. Pres-sure from A77 also showed a more rapid decrease in springto summer than in other seasons. The variance betweenthe minimum and the maximum after trend removal wasabout 40 db, which is smaller than the pressure variationfrom A78. This quite large variation of pressure will bediscussed in Section 5.

4. Trajectories of Subsurface Floats and Variationof Current VelocityFigure 3 shows subsurface trajectories of two

ALACE floats during the whole observation period. Thegap between arrows indicating the drift at the sea surfaceis smaller than the length of the arrows indicating thesubsurface flow. These small gaps imply that the estima-tion of the subsurface flow does not include much errordue to the surface current. Float A77 revealed an anti-clockwise circulation in the eastern Japan Basin. FloatA78 showed complex flows in the western Japan Basinand around Yamato Rise, and an outflow to Yamato Ba-sin. Since the drift depth varied greatly, the trajectoriesare examined in 3-monthly periods.

Each subpanel of Fig. 4 shows the trajectory of floatA77 every 3 months. The average values of temperatureand pressure in each period are shown at the upper leftcorner of each subpanel. Float A77 showed the first anti-clockwise turn from summer of 1995 to spring of 1996 atdepths of 323–304 db. The average speed of the circula-tion was 2.7 cm/s. The next turn was observed from sum-mer of 1996 to spring of 1998 at depths of 268–184 db.In the second turn, the float was trapped above the localtopography of the northern periphery of the 3000 misobath for one year.

Two anticlockwise turns were confined to a deepbasin surrounded by Hokkaido Island, the northern pe-riphery of the Japan Basin, and a north-south seamountwith a summit depth of 2200 m between 136°E and 137°Ein the Japan Basin. These were composed of the follow-ing flows; a northward flow of 2 to 7 cm/s at 250–310 dbalong a steep slope off Hokkaido Island (seen in autumnof 1995 and summer to autumn in 1996), an eastward flowdominant between 41°N and 42°N at 270–310 db (in sum-mer to autumn of 1995 and summer of 1996), a westwardflow that dominated between 42°N and 44°N at 200–310db (in winter of 1996 and summer of 1997), and a south-ward flow dominating in the east of the seamount, wherecyclonic eddies with horizontal scale of 50 km were also

observed (e.g., spring of 1996, autumn of 1997, and win-ter of 1998).

After the float entered into a shallow sea 16–43 mdeep off Hokkaido Island in summer of 1999, it floatednorthward at depths of 50–110 db to the west of SakhalinIsland at a speed of 13 to 14 cm/s with warm water of10.6–5.4°C, and then drifted southwestward at a speed of8 to 17 cm/s along the Russian coast with cold water of1.6–2.2°C. The strong northward flow along HokkaidoIsland would be a part of the warm Tsushima Current to-ward Soya Strait, and the strong southwestward flowalong the Russian coast would be the cold Liman Cur-rent.

Figure 5 shows the trajectory of float A78 every 3months. From summer to autumn in 1995, float A78 madea clockwise turn with a horizontal scale of about 200 kmat depths of 305–257 db around the Yamato Rise. Thespeed of the turn was from 2 to 6 cm/s and the averagespeed was 5.3 cm/s.

Fig. 3. Subsurface trajectories of floats A77 and A78. Whitecircles show points where floats were deployed. Float A77was deployed at 41°42.6′ N, 136°17.5′ E (white circlemarked with “1”) on 29 July 1995, and float A78 was de-ployed at 40°54.0′ N, 132°24.0′ E (white circle marked with“2”) on 4 August 1995. Each solid arrow starts at divingpoint and ends at surfacing point. Broken arrows mean thatthe floats did not send signals on scheduled dates. Isobathsare of 1000 m, 2000 m and 3000 m. “YR” stands for YamatoRise, “UI” for Ulleungdo Island, and “OS” for Oki Spur.


After the turn around the Yamato Rise, the floatstayed to the west of the rise until winter of 1998 at depthsof 243–166 db, and showed many small clockwise andanticlockwise turns. Anticlockwise turns of about 50 kmscale were seen twice at 188–205 db depth in almost thesame area around 39°N, 132°30′ E in summer of 1996and in spring and summer of 1997. These turns are re-lated to a large decrease of pressure from float A78 insummer. It was supposed that the float was trapped in acold eddy in the small basin west of the Yamato Rise.

In winter of 1997, it showed a large clockwise turnat 180 db depth around 41°N, 132°E. The southward flowin this turn accompanied a very cold water of tempera-ture lower than 1°C. From its extremely low temperaturebut shallow depth, this flow might have carried low sa-linity water. This implies the advection of the low salin-ity intermediate water just formed by winter convection.

In spring (June) of 1998, the float escaped from thewest of the Yamato Rise and went southwestward to 37°Nat a speed of 13.4 cm/s at 151–160 db depth. The ob-served temperature was about 4°C. The float might havebeen carried by the southward penetration of the coldwater north of the subpolar front.

After the escape, in summer of 1998, the float turnedto the Yamato Rise along Oki Spur at a high speed of10.9–15.5 cm/s at 150–170 db depth. This flow accom-panied warm water of 6–7°C and this implies that the floattransferred to the Tsushima Current. The float then stayedabove the ridge system from Oki Spur to the Yamato Riseuntil winter of 2000. Above Oki Spur in winter and springof 1999, it drifted in the zonal and meridional directionsat depths of 160–250 db where it observed the highesttemperature of 6.4–8.6°C in the observation period, ex-cept for the eastward and the westward drifts with colderwater of 4.6–5.4°C in winter of 1999. Above the Yamato

Rise in summer to autumn of 1998 and 1999, it showedalmost only clockwise turns with a horizontal scale of 50km at depths of 142–155 db and observed a cold tem-perature of 4°C. This depth was one of the minimumdepths through the observation period.

In winter of 2000, the float traveled eastward in wa-ter of temperature 4.0–4.6°C at 160 db depth, while itflowed southwestward once with temperature of 6.4–7.0°C at 200 db depth (Fig. 3). It is supposed the floatapproached the Tsushima Current region near the coastof Japan and caught a warm and saline eddy. It then de-creased drift depth due to grounding and finished its lifeoff Akita, Japan in mid-May 2000.

Drift speeds and velocity components have been ex-amined to detect any seasonal variation. Figure 6 showsthe drift speed and the eastward and northward velocitycomponents (U, V) of floats A77 and A78. The drift speedof A77 was slower than that of A78. The correlation co-efficient between drift speeds of A77 and A78 was 0.117.Drift speeds were classified simply by pressure and sea-son and averaged in each class (Table 1). Speeds above100 db depth are not included in the table because thefloat drifted in apparent strong surface currents such asthe Tsushima Current or the Liman Current. Although thevariation in each class was very large and the seasonalchange was very weak in the speed time series, averagedrift speeds of both the floats, particularly float A78, wereslightly faster in winter to spring at the depth of 100–200db, and in summer to autumn at the depth of 200–300 db.

5. Discussion and SummaryWe have tracked two ALACE floats in the Japan Sea.

One of them, float A77, having a 20-day cycle, was de-ployed on 29 July 1995 in the east Japan Basin, andtracked in the northeastern part of the basin until 15 Sep-

Table 1. Depth and seasonal dependency of drift speed of the floats. Values are average speeds (cm/s) and standard deviations(cm/s, shown after ±) at each pressure of every 100 db below 100 db depth in each season (“winter” to “autumn”) and throughthe observation period (“total”). Numbers of data points are shown in parentheses.

100 dbar–200 dbar 200 dbar–300 dbar 300 dbar–400 dbar 100 dbar–400 dbar

A77winter 2.3 ± 1.9 (9) 2.7 ± 1.7 (5) 2.5 ± 0.9 (4) 2.5 ± 1.4 (18)spring 3.7 ± 2.0 (9) 2.8 ± 2.1 (5) 1.9 ± 0.9 (4) 3.0 ± 2.0 (18)summer 2.0 ± 1.1 (9) 3.2 ± 1.5 (8) 1.6 ± 0.5 (3) 2.4 ± 1.4 (20)autumn 2.5 ± 1.8 (9) 3.4 ± 1.0 (4) 2.9 ± 0.8 (5) 2.8 ± 1.5 (18)total 2.6 ± 1.7 (36) 3.0 ± 1.6 (22) 2.3 ± 1.0 (16) 2.7 ± 1.6 (74)

A78winter 7.0 ± 4.2 (24) 3.2 ± 2.6 (20) –(0) 5.3 ± 4.0 (44)spring 8.1 ± 5.0 (15) 4.2 ± 2.5 (21) –(0) 5.8 ± 4.2 (36)summer 5.2 ± 3.4 (32) 5.5 ± 1.8 (8) 4.4 ± 2.2 (3) 5.2 ± 3.1 (43)autumn 3.4 ± 1.3 (27) 4.1 ± 2.3 (17) –(0) 3.7 ± 1.8 (44)total 5.6 ± 3.9 (98) 4.0 ± 2.5 (66) 4.4 ± 2.2 (3) 5.0 ± 3.5 (167)


Fig. 4. (a) Subsurface trajectory of float A77 in each season of 1995 and 1996. Drift speed scale is shown by an arrow for 5cm/s in the upper left subpanel. P and T in each subpanel are average pressure and temperature at drift depth. Isobaths of2000 m and 500 m are shown by solid and broken lines, respectively. (b) Subsurface trajectory of float A77 in each season of1997 and 1998. (c) Subsurface trajectory of float A77 in each season of 1999 and 2000.

tember 2000. The other float, A78 with a 10-day cycle,was deployed on 4 August 1995 in the western JapanBasin, and drifted in the western Japan Basin, in theYamato Basin and around Yamato Rise until it expired inmid-May 2000.

The main conclusions may be summarized as fol-lowing four points:

(1) Anticlockwise circulation in the eastern JapanBasin.

Float A77 showed anticlockwise turns, one from

summer of 1995 to spring of 1996 at a depth of 323–304db, and the other from summer of 1996 to spring of 1998at depth of 268–184 db. The third anticlockwise turn fromspring of 1998 to summer 2000 was large in horizontalscale, entering into a shallow sea off Hokkaido Island,flowing northward to the west of Sakhalin Island andflowing southwestward along the Russian coast.

The first two turns were confined in a deep basinsurrounded by Hokkaido Island, the northern peripheryof the Japan Basin, and a seamount in the west. A broad


Fig. 4. (continued).

anticlockwise circulation in the Japan Basin has been re-vealed in other studies. For example, using hydrographicdata, Nishiyama et al. (1993), Senjyu and Sudo (1994),and Kim and Kim (1999) suggest an anticlockwise circu-lation from dynamic height anomaly at 500 db depth re-ferred to 1000 db, depth of the isopycnal surface of σ1 =32.03, and acceleration potential at σθ = 27.2 surface re-ferred to 700 db, respectively. From ADCP observationin the CREAMS-1993 cruise, Isobe and Isoda (1997) de-tected an eastward flow south of 42°N and a southwest-ward flow north of 42°N along 136°E in the upper 300m. Because the western edge of the anticlockwise circu-lation observed by float A77 was located around 136°E(Fig. 3), these studies suggest the existence of anothercell to the west of our observation area, or a larger out-side cell. The anticlockwise intermediate circulation con-fined to the eastern Japan Basin is also suggested fromgeostrophic volume transports measured by Watanabe etal. (2001).

Water masses involved in these circulations are in-teresting. Float A77 reported a low temperature (below1°C) for the first 420 days, or 21 cycles, while pressuredecreased from 331 to 256 db. This depth and tempera-ture imply that the float drifted in the UJSPW or the HSIW.The combination of salinity of 34.07 PSU and density ofσθ = 27.3 is possible for temperature and pressure ob-served by float A77 for the first 420 days. The anticlock-wise circulation might supply high salinity water fromthe Tsushima Current area to the eastern part of the JapanBasin, as suggested by Watanabe et al. (2001).

Since our float showed that the spatial scale of thecirculation increased as the depth decreased, the anticlock-wise circulation was also expected at the surface. Lee et

al. (2000) showed that two surface drifters revealed south-ward flows between 136°E and 138°E from 43°N to 42°Nin the eastern Japan Basin. One of the southward flows isa part of a clockwise turn ranging from 136°E to 137°Eand the other is a part of an anticlockwise turn separatingfrom the northward flow along Hokkaido Island.Morimoto and Yanagi (2001) calculated monthly meansea surface dynamic height by composing the altimetricdata of satellite ERS-2 and showed a weak low core ofsea surface dynamic height around 43°N, 138°E in Octo-ber 1996 (figure 7 in Morimoto and Yanagi, 2001). It issuggested that the anticlockwise circulation would be seenfrom the surface to the intermediate depth of 320–180db, but its existence was clearer in the intermediate depththan at the surface.

The mechanism by which the anticlockwise circula-tion exists is also an interesting problem. According tothe location between the unknown seamount andHokkaido Island, topographic control is suggested as areason for the flow taking the shallow to the right handside (e.g., Fujio et al., 2000). Jacobs et al. (1999) did notmention this circulation but showed an anticlockwise cir-culation southwest of Hokkaido Island in the mean cur-rents from a numerical model using a low viscosity pa-rameter and realistic bottom topography (figure 3 inJacobs et al., 1999). It is suggested that eddy kinetic en-ergy interacting with the seamount would have producedthe mean current, as proposed by Holloway (1992). Eddykinetic energy would be supplied by the variability of thesubpolar front and the Tsushima Current near the circula-tion. The numerical model study using realistic bottomtopography published by Yoon and Kawamura (2002) alsoshowed the distribution of stream function where a weak


Fig. 5. (a) Subsurface trajectory of float A78 in each season of 1995 and 1996. Drift speed scale is shown by an arrow for 10cm/s in the upper left subpanel. Otherwise as Fig. 4. (b) The subsurface trajectory of float A78 in each season of 1997 and1998. (c) The subsurface trajectory of float A78 in each season of 1999 and 2000.

negative core exists in the eastern part of the Japan Ba-sin.

(2) Clockwise circulation around the Yamato Rise.From summer to autumn of 1995, float A78 traced a

clockwise circulation around the Yamato Rise at depthsof 299–259 db with a horizontal scale of about 200 km.Float A78 reported a temperature around 1°C for the first210 days, or 21 cycles, while pressure decreased from325 to 238 db. This depth and temperature imply that thefloat drifted in the UJSPW. Assuming that the float driftedon the density layer of 27.3 σθ, the resulting salinity wouldbe 34.07 PSU for the temperature and pressure observedby float A78 for the first 210 days. The clockwise turnaround the Yamato Rise might be the flow of the UJSPW.Senjyu and Sudo (1993) suggest the UJSPW turns clock-wise around the Yamato Rise into the Yamato Basin onthe basis of the distribution of water properties.

(3) Clockwise and anticlockwise eddies to the westof the Yamato Rise, above the Yamato Rise and aboveOki Spur.

Float A78 observed many clockwise and anticlock-

wise turns of about 50 km scale and remained in the west-ern area, especially to the west of the Yamato Rise andabove Oki Spur and the Yamato Rise. Above the YamatoRise, only clockwise eddies of about 50 km were revealedin summer and autumn of 1998 and 1999 at depths of142–155 db. The eddies observed above Oki Spur did nothave clear shapes but were accompanied by the warmestwater (7.6–8.8°C) through the whole observation periodat 212–246 db depth. A cold water path into or out of theYamato Basin was also seen in the northern part of OkiSpur.

Isoda and Nishihara (1992) show that warm eddiesfrequently stay to the east of the Korean Peninsula, in thewestern part of the Yamato Rise, in the southern part ofOki Spur and in the central part of Yamato Basin from ananalysis of historical temperature data at 200 m depth.The result of the float observation, which revealed thefrequent existence of warm eddies, is consistent with thishistorical data analysis. Isoda and Nishihara (1992) alsoshowed that the warm eddies above the Yamato Rise havea low temperature of 5°C at 200 m depth. This is also


consistent with our float observation, which showedclockwise eddies with a temperature of 4°C at 142–155db depth. While Isoda and Nishihara (1992) unfortunatelydid not have any data west of the Yamato Rise, the driftof the float implies that cold eddies would be stable inthis area.

(4) Seasonal variation of the flow.Although our observation was made over five win-

ter seasons, the seasonal variation in the flow patterns orin current speeds was not apparent from statistical analy-sis, including spectrum analysis (not shown). The corre-lation coefficient between drift speeds of floats A77 andA78 was 0.117, and this indicates little coherence amongthe strength of intermediate circulation between the eastand the west of the Japan Basin, or between the north andthe south of the subpolar front. It is suggested that thereare many causes of the seasonal variation, all having dif-ferent phases and different spatial scales, even at the in-termediate depth. A numerical model study by Yoon andKawamura (2002) and a satellite altimetric data analysisby Morimoto and Yanagi (2001) demonstrated the devel-opment of surface currents in summer due to the increaseof the inflow of the Tsushima Current. This was supposed

to strengthen the average drift speed of float A78 at 200–300 db depth in summer. On the other hand, its drift speedat 100–200 db depth was also strong in winter, perhapsdue to development of wind-driven circulation as shownby numerical model studies (e.g., Yoon, 1994; Seung andYoon, 1995; Kim and Yoon, 1996).

The drift depth of the floats seemed to have a sea-sonal variation. The differences of the minimum and maxi-mum pressure, besides decreasing trends due to mechani-cal problems with the floats, were 40 db for float A77and 100 db for float A78, and the minima tended to occurin spring to autumn. There may be several possible causesof the pressure variation. One cause would be verticalmovements of isopycnal surfaces corresponding to cur-rent variation. The drift area of float A78 in particular isinvolved with the large variability around the subpolarfront. Morimoto and Yanagi (2001) show that thegeostrophic surface current pattern in the Japan Sea hastwo branches, i.e., the East Korean Warm Current and

Fig. 6. Time series of subsurface drift velocity and speed offloat A77 (a) and of float A78 (b). Upper solid line and dot-ted line in each panel mean zonal (U) and meridional (V)velocity components (see the left ordinate). Lower solid linein each panel means absolute speed (see right ordinate).

Fig. 5. (continued).


the second branch of the Tsushima Current, to the westof the Yamato Rise in summer, while the second branchof the Tsushima Current is not seen and the East KoreanWarm Current is stronger in winter. Such a seasonal vari-ation in the current pattern would cause the seasonal vari-ation of the isopycnal surface depth at a geographic point.Another cause would be vertical movements of isopycnalsurfaces due to temporal change of the water mass distri-bution. For example, the UJSPW west of the Yamato Rise,as shown by Senjyu and Sudo (1994), has a core at thedepth of about 400 m and a thickness of about 200 m inwinter, and has a core at the depth of about 500 m and athickness of about 300 m in summer. The UJSPW in theeastern part of the Japan Basin has a core depth of about400 m and a thickness of 200–300 m in winter, and a coredepth of about 400–500 m and a thickness of 300–400 min summer. Although the core depth of the UJSPW hasthe opposite seasonal tendency to the drift depth varia-tions of the floats, it is interesting that the UJSPW layerbecomes thick in summer. One might speculate that theintermediate waters and the UJSPW become thicker insummer than just after their formation in winter and pushup the pycnocline against the surface warm water, thevolume of which also increases in summer. The last causewould be the spatial distribution of water density, whichis thought to be significant in the case of ALACE floatobservation. As seen in Section 4, the drift depth varia-tion of float A78 is particularly supposed to be largelyrelated to the local current condition in the area wherethe float drifted. For example, in summer to autumn of1998 and 1999, float A78 drifted above the Yamato Rise,where the pycnocline would be shallow. Unfortunatelythe data from the ALACE floats is insufficient for esti-mating how these causes actually effect the variation ofthe drift depth. The spatial and temporal structure of thewater density field should be surveyed in detail from pro-filing float data, historical hydrographic data and so on.

AcknowledgementsWe express our sincere thanks to the CREAMS

Group, especially Professors M. Takematsu, W.Koterayama, J-.H. Yoon, of Kyushu University, and KuhKim of Seoul National University for their help. Thanksare extended to the Captain and crew of R/V ProfessorKhromov for their excellent operation during deploymentof the floats. Mr. S. Kitagawa, Ocean Research Institute,the University of Tokyo, helped with the data analysis.

This study was supported by the Scientific Researchof Priority Area (B), No. 11205201, sponsored by theMinistry of Education, Culture, Sports, Science and Tech-nology, Japan.

ReferencesDavis, R. E., D. C. Webb, L. A. Regier and J. Dufour (1991):

The autonomous Lagrangian circulation explorer (ALACE).J. Atmos. Oceanic Technol., 9, 264–285.

Fujio, S., D. Yanagimoto and K. Taira (2000): Deep currentstructure above the Izu-Ogasawara Trench. J. Geophys. Res.,105, 6377–6386.

Holloway, G. (1992): Representing topographic stress for largescale ocean models. J. Phys. Oceanogr., 22, 1033–1046.

Isobe, A. and Y. Isoda (1997): Circulation in the Japan Basin,the northern part of the Japan Sea. J. Oceanogr., 53, 373–381.

Isoda, Y. and M. Nishihara (1992): Behavior of warm eddies inthe Japan Sea. Umi to Sora, 67, 53–65 (in Japanese withEnglish abstract and legend).

Jacobs, G. A., P. J. Hogan and K. R. Whitmer (1999): Effectsof eddy variability on the circulation of the Japan/East Sea.J. Oceanogr., 55, 247–256.

Kim, C.-H. and J.-H. Yoon (1996): Modeling of the wind-drivencirculation in the Japan Sea using a reduced gravity model.J. Oceanogr., 52, 359–373.

Kim, K., Y.-G. Kim, Y.-K. Cho, M. Takematsu and Y. Volkov(1999): Basin-to-basin and year-to-year variation of tem-perature and salinity characteristics in the East Sea (Sea ofJapan). J. Oceanogr., 55, 103–109.

Kim, Y.-G. and K. Kim (1999): Intermediate waters in the East/Japan Sea. J. Oceanogr., 55, 123–132.

Lee, D.-K., P. P. Niiler, S.-R. Lee, K. Kim and H.-J. Lie (2000):Energetics of the surface circulation of the Japan/East Sea.J. Geophys. Res., 105, 19,561–19,573.

Morimoto, A. and T. Yanagi (2001): Variability of sea surfacecirculation in the Japan Sea. J. Oceanogr., 57, 1–13.

Nishiyama, K., M. Inagawa and T. Mizuno (1993): The JapanSea Proper Water and water circulation in the Japan Sea.Pap. Meteorol. Geophys., 44, 1–10.

Senjyu, T. (1999): The Japan Sea Intermediate Water; its char-acteristics and circulation. J. Oceanogr., 55, 111–122.

Senjyu, T. and H. Sudo (1993): Water characteristics and cir-culation of the upper portion of the Japan Sea Proper Wa-ter. J. Mar. Sys., 4, 349–362.

Senjyu, T. and H. Sudo (1994): The upper portion of the JapanSea Proper Water; its source and circulation as deduced fromisopycnal analysis. J. Oceanogr., 50, 663–690.

Seung, Y. H. and J.-H. Yoon (1995): Robust diagnostic modelingof the Japan Sea circulation. J. Oceanogr., 51, 421–440.

Sudo, H. (1986): A note on the Japan Sea Proper Water. Prog.Oceanogr., 17, 313–336.

Takematsu, M. (1999): Foreword to CREAMS Volume. J.Oceanogr., 55, 101.

Takematsu, M., Z. Nagano, A. G. Ostrovskii, K. Kim and Y.Volkov (1999): Direct measurements of deep currents in thenorthern Japan Sea. J. Oceanogr., 55, 207–216.

Watanabe, T., M. Hirai and H. Yamada (2001): High-salinityintermediate water of the Japan Sea in the eastern JapanBasin. J. Geophys. Res., 106, 11,437–11,450.

Yoon, J.-H. (1994): The recent trend of the study on the circu-lation in the Japan Sea. Kaiyo Monthly, 6, Suppl., 130–134(in Japanese).

Yoon, J.-H. and H. Kawamura (2002): The formation and cir-culation of the intermediate water in the Japan Sea. J.Oceanogr., 58, 197–211.

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