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The Tsushima Warm Current Through Tsushima Straits Estimated from Ferryboat ADCP Data

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The Tsushima Warm Current Through Tsushima Straits Estimated from Ferryboat ADCP Data 1154 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 The Tsushima Warm Current through Tsushima Straits Estimated from Ferryboat ADCP Data TETSUTARO TAKIKAWA* Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, Fukuoka, Japan JONG-HWAN YOON Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, Japan KYU-DAE CHO Department of Oceanography, Pukyong National University, Nagu, Pusan, Korea (Manuscript received 20 October 2003, in final form 19 November 2004) ABSTRACT Current structures across the Tsushima Straits are studied using results from long-term acoustic Doppler current profiler (ADCP) observations by a ferryboat between Hakata and Pusan conducted since February 1997. Two maxima of the northeastward current are observed in the central parts of the eastern and western channels, and the maximum velocity in the western channel is stronger than that of the eastern channel. Downstream of the Tsushima Islands, a southwestward countercurrent is observed associated with a pair of cyclonic and anticyclonic eddies. In the western channel, the deep countercurrent is observed pronouncedly on the bottom slope of the Korean side from summer to winter. The volume transport of the Tsushima Warm Current through the straits has strong seasonal variation with a minimum in January and two maxima from spring to autumn (double peaks). The spring peak of the volume transport through the eastern channel is more pronounced than the autumn peak, and the autumn peak of the western channel is more pro- nounced than the spring peak. The inflow volume transport into the Japan Sea through the western channel significantly increases in autumn because of an incrementation of the freshwater transport. The total volume transport averaged over the observation period (5.5 yr) is 2.64 Sv (Sv ϵ 106 m3 sϪ1). The average volume transports through the eastern and western channels are 1.10 and 1.54 Sv, respectively. 1. Introduction Tsushima Islands. According to Hase et al. (1999), the current through the eastern channel of Tsushima Straits The Tsushima Warm Current flows into the Japan feeds the first branch of the Tsushima Warm Current Sea from the East China Sea through the Tsushima roughly following the 200-m isobath of the continental Straits with width, length, and mean water depth of shelf along the Japanese coast. The current through the about 180 km, 330 km, and 100 m, respectively. The western channel of Tsushima Straits feeds the second straits are divided by Tsushima Island into the eastern branch, which flows along the continental shelf break and western channels with widths of about 140 and 40 and slope along the Japanese coast. Another branch km, respectively. Most of the water flows out to the along the Korean coast, which is called the East Korean Pacific Ocean and the Sea of Okhotsk through the Warm Current, flows as a western boundary current Tsugaru and Soya Straits. The Tsushima Warm Current is divided by the (Kawabe 1982; Yoon 1982a,b) and separates from the Korean coast at about 37°–39°N with synoptic mean- ders accompanied by warm and cold mesoscale eddies * Current affiliation: Department of Fishery Science and Tech- (Beardsley et al. 1992; Isoda and Saitoh 1993; Jacobs nology, National Fisheries University, Shimonoseki, Japan. et al. 1999). A part of the East Korean Warm Current flows southward as a countercurrent and the rest of it flows northeastward toward Tsugaru Strait, forming Corresponding author address: Tetsutaro Takikawa, Depart- ment of Fishery Science and Technology, National Fisheries Uni- the polar front at about 40°N with the northern cold versity, 2-7-1 Nagata-Honmachi, Shimonoseki 759-6595, Japan. water (Kim and Yoon 1996). Furthermore, a water E-mail: [email protected] mass with a vertical salinity minimum called the Japan © 2005 American Meteorological Society Unauthenticated | Downloaded 09/29/21 03:19 PM UTC JPO2742 JUNE 2005 NOTES AND CORRESPONDENCE 1155 FIG. 1. (a) The track (thick solid line) of the ferry Camellia along which current measure- ments by ADCP were carried out. The eastern and western channels are defined as the black (south of 34.75°N) and gray (north of 34.75°N) lines, respectively. Stations A and B are the deepest points on the observation line in the eastern and western channels, respectively. CTD observations were carried out at stations 1–5. Contour lines show the water depth in meters. (b) Cross-section view along the ferry track. Solid line indicates the water depth (m). Outflow volume transports associated with countercurrent downstream of the Tsushima Islands and deep countercurrent in the western channel flow through black and gray regions, respectively. Unauthenticated | Downloaded 09/29/21 03:19 PM UTC 1156 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 FIG. 2. Monthly averaged current vectors at 18-m depth on the Camellia line from Feb 1997 to Aug 2002 (1 kt ϵ 51.4 cm sϪ1). Unauthenticated | Downloaded 09/29/21 03:19 PM UTC JUNE 2005 NOTES AND CORRESPONDENCE 1157 short-term direct measurements using current meters (Miita 1976). Miyazaki (1952) concluded that the vol- ume transport of the Tsushima Warm Current had a large seasonal variation with maximum of 2.5 Sv (Sv ϵ 106 m3 sϪ1) in summer and minimum of 0.2 Sv in winter, and Yi (1966) obtained almost the same result. Isoda and Yamaoka (1991) estimated the volume transport in summer to be about 3 Sv, where the ratio of the trans- port through the eastern channel to that of the western channel was about 1 to 3. Miita (1976) also showed that the volume transport attained a maximum in summer (3.3–3.7 Sv). Over the past decade, there have been many direct current observations using a ship-mounted or towed acoustic Doppler current profiler (ADCP) to elucidate the volume transport and spatial current structure of the Tsushima Warm Current in Tsushima Straits (Kaneko et al. 1991; Kawano 1993; Egawa et al. 1993; Katoh 1993; Isobe et al. 1994). These results were av- eraged by Isobe (1994), who obtained an annually av- eraged volume transport of 2.2 and 0.7 Sv through Tsushima Straits and the eastern channel, respectively. Teague et al. (2002) carried out current measure- ments at 12 stations along two lines (northeast and southwest of Tsushima Islands) across Tsushima Straits FIG. 3. Current vectors (kt) at 26-m depth from 16 to 18 Jun for about 11 months using ADCPs housed in trawl- 2000, using Camellia ADCP observations (west) and ADCP ob- servations from the T/V Kakuyo-Maru of Nagasaki University resistant bottom mounts (TRBM). Annually averaged (east). volume transports of the Tsushima Warm Current were estimated to be 2.3 Sv at the northeastern section and 2.7 Sv at the southwestern section. The disagree- Sea Intermediate Water is found below the Tsushima ment of the two transports between the northeastern Warm Current (Senjyu 1999; Yoon and Kawamura 2002). and southwestern sections is considered to be due to So far, many studies have been made to estimate the the large interpolation errors of the ADCP data near volume transport of the Tsushima Warm Current. How- the surface layer, especially near the Korean coast, ever, long-term current measurements in Tsushima where strong northeastward currents are observed (Ja- Straits are difficult because of heavy fishing and trawl- cobs et al. 2001). ing activities (Kawatate et al. 1988). Hence, most of the The Research Institute for Applied Mechanics estimates were given by geostrophic calculations (RIAM) of Kyushu University has been conducting (Miyazaki 1952; Yi 1966; Isoda and Yamaoka 1991) or long-term ADCP observations since 21 February 1997 Ϫ FIG. 4. Numerical simulation of the Tsushima Straits velocities (cm s 1) using the 1.5-layer reduced-gravity model of Maruyama et al. (2003). The horizontal grid spaceis2km. Unauthenticated | Downloaded 09/29/21 03:19 PM UTC 1158 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 Ϫ FIG. 5. Monthly averaged velocities (cm s 1) on the vertical section along the ferry track from Feb 1997 to Aug 2002. The velocities are normal to the section, and positive velocities are toward the Japan Sea. 6 times per week using the ferryboat Camellia between multilevel ADCP (VMBBADCP, 300 kHz, RD Instru- Hakata and Pusan (Fig. 1). We use these data to esti- ments) mounted on the ferryboat Camellia. The Camel- mate the averaged volume transport and seasonal lia makes round trips between Hakata and Pusan 3 variations through Tsushima Straits. The characteristics times per week at a cruising speed of about 17 kt. The of the Tsushima Warm Current structure are discussed data sampling intervals are about 24 s and8mindepth first, and the volume transport is discussed second. from 18 to 258 m. However, the data within 15% of the total depth from the bottom are not reliable (missing 2. Data and method data). Surface and bottom velocities are obtained by The data used in this study were obtained for 5.5 yr extrapolating the values at 18-m depth and at the deep- from 21 February 1997 to 25 August 2002 from the est depth of reliable ADCP measurements to calculate Unauthenticated | Downloaded 09/29/21 03:19 PM UTC Fig 5 live 4/C JUNE 2005 NOTES AND CORRESPONDENCE 1159 Ϫ FIG. 6. Standard deviations (cm s 1) of the velocities in Fig. 5. the volume transport through the section. Since the current sections is roughly one day, the current data maximum water depth is about 240 m along the ferry may contain significant aliasing errors associated with track, the ship velocity relative to the bottom can be some of the tidal constituents. However, the sampling measured everywhere through the bottom tracking by intervals (the time between two successive cruises) are the ADCP, implying that the current velocity can be not constant and vary from point to point along the measured relative to the bottom.
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