JFM SE, 26(3), pp. 455 473, 2014. www.ksfme.or.kr 수산해양교육연구 제 권 제 호 통권 호, 26 3 , 69 , 2014. http://dx.doi.org/10.13000/JFMSE.2014.26.3.455
. Introduction
Ⅰ
Ulsan Bay has complex seawater behavior, because it receives freshwater from the Taehwa River and it occurs in an area where water circulates due to rapid variations in depth around Bangeojin (Lee and Cho, 1999). In addition, the Ulsan coastal waters (UCW) are located where the Tsushima Current (TC) and the North Korean Cold Water (NKCW) meet and pass through, and receive inflow from a neighboring industrial complex. Thus,
the marine environment is extremely complex and diverse. In particular, cold water (CW) often appears in the summer, which greatly affects the marine habitat in the southeast area of the East Sea (Han et al., 2002; Lee, 2007; Seo et al., 2008).
Many studies have been conducted on the marine environments and ecological problems in the UCW, but only a few studies have considered the physical environments (Lee et al., 1994; Na and Paeng, 1992; Seung and Nam, 1992). An(1974) investigated the scope and passage of CW in the
Characteristics of Cold Water Appeared in the Southwestern East Sea
Moon-Ock LEE ㆍShinya OTAKE
*ㆍ JongKyu KIM ( Chonnam National Universityㆍ
*Fukui Prefectural University)
동해 남서부해역에 출현하는 냉수 특성
이문옥 ㆍ오오다케 신야*ㆍ김종규
( 전남대학교ㆍ
*일본후쿠이현립대학)
Abstract
하계 동해의 남서부해역에 냉수가 왜 그리고 어떠한 해양환경하에 출현하는가를 밝히기 위하여 현장조사 위성자료의 분석 및 수치실험을 실시하였다 이 해역에서 냉수는 비정상년의 경우는 정상, . 년에 비해 보다 더 연안 가까이에서 출현하였고 수온도 낮았다 이것은 냉수가 비정상년에는 정상년, . 에 비해 크게 발달하여 연안역으로 확장함을 의미하였다 흐름장의 계산결과는 지형류적인 관점에서 . 수온의 관측결과를 잘 재현하였다 한편 정상년의 경우는 북한한류. , , (NKCW)가 쓰시마난류(TC)와 균 형을 유지하면서 동해의 북서쪽에 머물고 있었다 이에 반해 비정상년의 경우는 북한한류가 점차 . , , 남쪽으로 남하하여 동해 서부역의 대부분이 북한한류의 세력하에 놓였다 그래서 하계 동해 서부 연. , 안역에서의 냉수 출현은 남쪽으로의 북한한류의 확장에 의한 부산물인 것으로 판단되었다 울산 연. 안역에 대한 유동계산결과는 하계 남풍이 불 경우 표층과 저층 사이에 흐름의 역전현상이 나타났다, . 따라서 하계 동해의 남서부 연안역 특히 수심의 변화가 급한 방어진 부근에서 냉수의 용승이 일어, , 날 수 있음을 시사하였다.
Key words : Cold Water, Ulsan Coastal Waters, NGSST Satellite Image, Tsushima Current, North Korean Cold Water
Corresponding author : 061-654-7151, [email protected]
southern area of the East Sea based on CSK data.
He reported that CW appears mainly in the summer from Yeongil Bay to Pusan and postulated that this CW resulted from uprising of the North Korean Cold Water (NKCW) that descended along the coast. In particular, he insisted that CW can be created when the TC flows with a velocity > 50 cm s ･
-1and when there is a strong south westerly wind. Lee and Kunishi(1975) also studied the reason why CW frequently appears in the summer near the UCW using CSK data as well as ship-stop observation data. They inferred that the cyclonic circulation created near the UCW, turns into cold water after its upsurge to the surface due to topographical effects. However, they did not present any critical evidence. Kim and Kim(1983) elucidated that the cold water appearing in the Uljin-Yeongdeok coastal waters in the summer originated from NKCW, on the basis of a long-term observational data of salinity and dissolved oxygen. Lee and Na(1985) attempted to explain why sea surface temperature (SST) only near the Gampo coastal waters was low due to the upwelling caused by the summer wind. They attributed it to a relatively strong baroclinic tilting of the East Korean Warm Current (EKWC) due to topographic effects around the southeast coast.
Byun(1989) investigated CW appearing in the UCW in the summer using field observational data, as well as satellite image data. He found that the upsurge phenomena appeared in these coastal waters after a few days of persistent southwesterly winds.
Thus, he confirmed that warm seawater with a temperature of approximately 25 °C moves off the coastal area and a cold intermediate layer of seawater is exposed to the sea surface. Min(1994) analyzed behaviors of the thermal fronts that form between the Korean Coastal Waters (cold water)
and Tsushima Current Intermediate Waters(warm water),based on the past ten-year water temperature data observed on a cross-section of the Korean Strait. He elucidated from this that the drop of water temperature in the Korean Coastal Water was prominent when the thermal fronts formed in the outer sea. Teague et al.(2002) conducted systematic and comprehensive current measurements in several cross-sections of the Korea Strait for almost one year. They conducted empirical orthogonal function (EOF) analyses and then concluded that total transport variations in the summer are mainly due to transport variations near the Korean coast. Lee et al.(2003) investigated the coastal upwelling process in the southeast coast of Korea, using the data of wind, water temperature, sea level and current. They found that the magnitude of water temperature change is not quantitatively proportional to the intensity or duration of the wind, since it depends on a baroclinic tilting of isotherms by the strong TC. In addition, they stated that the current was particularly strong near the coast and has a large vertical shear when the upwelling events occur. Kim et al.(2006) analyzed the long-term hydrographic and current data acquired in the Korea Strait to understand temporal variations of the Korea Strait Bottom Cold Water (KSBCW).
They noticed that KSBCW has two temperature minimums with the seasons, and these drops of water temperature appear when bottom currents were intensified. In addition, they insisted that these seasonal variations of KSBCW were associated with the maximum southwestward baroclinic component.
Lee(2007) concluded that the UCW is under the
influence of the TC based on analytical
foraminifera and dinoflagellate results. Lee and
Cho(1999) conducted current measurements in the
summer and found that three distinctive water
masses exist in the southwestern area of the East Sea. Kim et al.(2001) reported using the CTD observation data that CW with a temperature < 11
°C develops in August near the UCW. Kim et al.(2003) performed tide and current measurements in the summer in the UCW. They found that northeastward currents prevail at ebb tide, whereas the currents weaken at flood tide, because of the northward TC. Lee et al.(2004) inferred that the appearance of CW near the UCW contributes to enhancing zooplankton biomass, as it supplies nutrients during upsurge. Takikawa et al.(2005) investigated the volume transports of the TC through the Tsushima Straits using over five years of ADCP data. They found that the volume transport of the TC is at a minimum in January, but that two maxima occur from the spring to the autumn. In particular, they insisted that the inflow volume transport into the East Sea through the western channel significantly increases in autumn due to the increase in freshwater transport. Choi et al.(2012) attempted to produce simultaneous surface current fields from satellite altimeter data for the East Sea. In particular, they analyzed 16-year surface current data using EOF, and found that surface circulation patterns in the Korea Sea could be classified as inertial boundary current, TC, meandering, and offshore branch by the time coefficient of the first two EOF modes.
The present study was performed to find why CW appears in the summer in the southwest part of the East Sea, and to understand the mechanism of CW, based on data analyses of water temperature, salinity, satellite images, and numerical simulation results.
. Materials and Methods
Ⅱ
1. Observational data
First, we defined CW as water masses that have lower temperatures in the summer than those in the winter at the same depth. That is why local people call it CW when colder water appears at the same location in the summer rather than in the winter.
Additionally, it is difficult to define CW by a particular water temperature. We also define UCW as shallow areas < 100 m near Ulsan, as indicated in the lower panel of [Fig. 1]. Thus, we performed field measurements of the tides and currents in the summer from August 14 to September 12, 2009 and in the winter from January 15 to February 19, 2010 at stations T1 and C1 using a TGR-2050 tide recorder (RBR, Ottawa, ON, Canada) and a 600 Hz ADCP (RD Instruments, Poway, CA, USA) to understand the physical environments of the UCW (see [Fig. 1]). These instruments were mounted at the same location of the seabed to prevent from being lost using TRBM (Trawl Resistant Bottom Mount). We also collected CTD data of water temperature and salinity the National Fisheries Research and Development Institute (NFRDI;
http://portal.nfrdi.re.kr) observed at fixed stations A and B, and six NFRDI serial oceanographic observation lines from 208 to 105, as indicated in [Fig. 1]. The New Generation Sea Surface Temperature (NGSST) satellite image data were used to find the distributions of the water temperature when CW appeared (Korea Meteorological Agency (KMA; http://www.kma.go.
kr); NFRDI). The data collected from 2005 to 2011
was used here for NGSST and KMA while the
data from 1997 to 2011 for NFRDI. In contrast,
we referred to the statistics of aquatic products
collected from 1998 to 2011, in order to examine
the influence of CW (http://www.suhyup.co.kr).
Parameter Source/Value Sea Surface Temperature(SST) NGSST(A-Higher)
Eddy viscosity(m2∙sec-1) Horizontal : 20 Vertical : 2x10-2 Eddy diffusivity(m2∙sec-1) Horizontal : 20
Vertical : 1x10-4
Sea Surface Height(SSH) TOPEX/POSEIDON(Jason-1, GFO, ERS, Envisat)
Rainfall JMA/GSM
Wind JAM/MSM(hourly surface wind, sea level pressure, air temperature, humidity) Surface Flux TOGA-COARE(Transport fluxes of bulk algorithm)
Tide NA099(16 partial tides)
Freshwater Discharge Yearly mean inflows from 5 major rivers
Water temperature/Salinity KODC/JODC
<Table 1> Parameters used in the numerical simulation by RIAMOM
[Fig. 1] Study area and oceanographic stations
2. Numerical simulation
We also performed numerical simulations to investigate the behavior of the CW in the southern area of the East Sea, using the Environmental Fluid Dynamics Code (EFDC) developed by Hamrick (1992) and the Research Institute for Applied Mechanics Ocean Model (RIAMOM) developed by
Lee and Yoon(1994). The computational domain
comprised 16.1 km east-westward and 15.4 km
south-northward when we simulated the UCW using
the EFDC model (Mellor, 1991). The grid nets
were composed of rectangular systems of 50 200 m –
in the x and y directions with variable grid sizes
from the coast. The layers were vertically divided
three times, and four major components of the tide,
i.e. M
2, S
2, K
1and O
1, are given as the open
boundary condition. However, the computational
domain consisted of a wide range from 126.5°E to
142.5°E in the longitudinal direction and from
33°N to 52°N in the latitudinal direction, when
simulating the entire coastal area of the East Sea
using RIAMOM. In this simulation model, the
number of inlets was set to two, i.e. one from the
Jeju Strait and the other from a cross-section that
connects the south of Jeju Island and the west
coast of Kyushu. The number of outlets was also
set to two, i.e., one from the Tsugaru Strait and
the other one from the Soya Strait, as shown in
[Fig. 2]. The horizontal grid sizes in the x and y
directions are 0.083°, i.e., 8 10 km, and the layers –
are divided into 64 vertically. Thus, the
computational domain comprises 187 × 224 × 64
grid nets with a 5 m thickness of the surface layer.
The time interval, ∆ t, was 10 sec for fast external mode and 360 sec for slow internal mode. We adopted the monthly mean inflows and outflows of the three straits in the coastal area of the East Sea for the open boundaries (Takikawa et al., 2005).
Satellite image data acquired by some satellites (Jason-1, GFO, Envisat) were also used as the input data of the sea surface height in the model after assimilating the data using a Karman filter.
The other parameters used in the numerical simulation are indicated in <Table 1>. All procedures associated with this numerical simulation were obtained from the results of Lee(1996).
[Fig. 2] Numerical model domains and depth distributions
. Results
Ⅲ
1. General features of the tides and currents around Ulsan Bay and UCW
First, we conducted a harmonic analysis of the
tides using data obtained at station T1, as shown in [Fig. 1]. We found that four major partial tides of amplitudes, M
2, S
2, K
1and O
1were more prevalent than any other partial tides. Additionally, the currents observed at station C1 mainly moved in a southwestwardly direction at flood tide, but a northeastwardly direction at ebb tide. However, the ebb currents were stronger than the flood currents in the summer, whereas the opposite was true in winter. After validating the numerically simulated results by comparing the observed results of the tide and current at stations T1 and C1, we computed the flow structure for UCW. [Fig. 3]
represents the simulated flow patterns for UCW in the horizontal and vertical directions along line 208, as shown in [Fig. 1]. The flood currents move in a SSW direction, whereas the ebb currents move in a NNE direction, with the latter being slightly stronger than the former([Fig. 3(a)]). In addition, the flood and ebb currents tended to slightly accelerate around Bangeojin, probably due to topographical effects([Fig. 3(b)]).
(a)
(b)
[Fig. 3] Horizontal surface currents and cross-
sectional currents along line 208 at flood
and ebb tides
We examined the possibility of the appearance of CW in UCW when considering the southerly wind in the summer. [Fig. 4] represents the flow structure when the summer maximum wind in 2011, i.e., 8.7 m s ･
-1of southerly wind, is considered. Here positive signs in degree denote eastward flows whereas negative signs denote westward flows, respectively. Compared to the previous result of [Fig. 3(a)], the flood current weakened, whereas the ebb current strengthened, due to the southerly wind.
In particular, the flood current on line 208 that flowed toward the coast with no wind, became fairly weak and flowed toward the open sea, just like the ebb current. Furthermore, this current changed direction near the edge of Bangeojin and then flowed in reverse. This suggests that an upsurge of CW occurs from the bottom layer. In contrast, the ebb current intensified due to the southerly wind. [Fig. 5] represents the flow structure when the winter maximum wind in 2011, i.e. 7.9 m s ･
-1of northerly wind, is considered. We see that the flood current strengthened, whereas the ebb current weakened, compared to the previous result of [Fig. 3(a)]. In addition, the flood current on line 208 became fairly stronger whereas the ebb current became a little weaker, compared to the result of [Fig. 3(b)], although the entire pattern of the currents looked similar.
(a)
(b)
[Fig. 4] Horizontal surface currents (a) and cross-sectional currents along line 208 (b) during the flood and ebb tides in consideration of the summer maximum wind
Item Water Temperature(°C) Salinity(psu)
Season Summer Winter Summer Winter
Station Surface Bottom Surface Bottom Surface Bottom Surface Bottom
A 14.7-26.4 (22.0)
9.4-20.2 (13.5)
11.3-13.5 (12.2)
11.2-12.9 (12.0)
31.26-33.59 (32.57)
29.97-34.32 (33.53)
33.59-34.68 (34.12)
33.65-34.45 (34.18)
B 15.1-26.5 (21.6)
8.3-21.4 (13.8)
11.4-13.1 (12.2)
11.3-12.9 (12.0)
22.88-33.42 (31.05)
32.27-34.18 (33.79)
33.04-34.61 (33.85)
33.62-34.23 (33.89)
<Table 2> Water temperature and salinity observed at stations A and B from 2001 to 2010
(a)
(b)
[Fig. 5] Horizontal surface currents (a) and cross-sectional currents along line 208 (b) during the flood and ebb tides in consideration of the winter maximum wind
2. Distributions of water temperature and salinity in the UCW
<Table 2> shows the CTD measurement results NFRDI observed at stations A and B, as indicated in [Fig. 1], for 2001 2010. According to the results, – the surface water temperature and salinity in the UCW were roughly 22 °C and 32.0 psu in the summer, whereas they were roughly 12.0 °C and
34.0 psu in the winter, respectively. In particular, the surface water temperature in the summer was much higher than that at the bottom layer, suggesting that stratification occurs. In addition, the salinities at station B were slightly lower than those at station A. This may be attributed to the influence of the Taehwa River since it has much more flow rates in July of 2009 and 2010 than in any other month([Fig. 6]).
[Fig. 6] Flow rates of the Taehwa River from January 2009 to December 2010 [Fig. 7] shows the variations of the water temperature NFRDI observed at station 208-01 (near Bangeojin) of line 208 from 1997 to 2011. A significant fluctuation in the water temperature was observed in the summer, whereas the water temperature in the winter appeared relatively stable.
[Fig. 7] Variations in surface water temperature
at station 208-01 of line 208
In particular, 2005 recorded the lowest water temperature in the summer during the observational period.
In addition, [Fig. 8] shows the aquatic product statistics from the Ulsan coastal areas in August for the past 14 years. The 2005 statistics exceeded 90 tons, extremely high compared to that produced in other years. At present, sea mustard, kelp, and abalone are cultured in the shallow waters of UCW, but sea mustard predominates among these species.
[Fig. 8] Statistics of Aquatic products cultured in Ulsan coastal areas in August
3. Distributions of water temperature and salinity based on NFRDI serial oceanographic observation data
<Table 3> shows the typical marine environmental conditions on the days when NFRDI serial oceanographic observations were performed at station 208-01 of line 208, as the NGSST image data first began to be produced in 2005. The water temperature, salinity, and dissolved oxygen (DO) here denote the sea surface values. The surface water temperatures of the UCW in 2005 became extremely low, whereas the other environmental factors, such as salinity and DO, were similar overall. Thus, we inferred that the drop in water temperature resulted from the upsurge of CW, and decided to call 2005 an abnormal year, but 2011 a normal year, for convenience. This is reasonable because 2005 recorded the lowest water temperature since 1997, as described in [Fig. 7]. Accordingly, we compared and examined the distributions of water temperature, salinity, and current structure to identify the characteristics of the marine environments in 2005 and 2011. We representatively investigated three of the six observation lines, i.e., 208, 102, and 105, for convenience.
Date Water Temp.( )℃ Salinity (psu)
DO (mg l-1)‧
Wind Air Pressure (hPa) Direction Speed(m s-1)‧
2005. 8. 9. 15.06 33.36 6.94 SSW 1.6 1011.5
2006. 9. 7. 24.96 30.71 3.96 NNE 2.0 1015.0
2007. 8. 9. 22.70 33.03 6.07 SW 1.6 1013.5
2008. 8. 19. 22.86 32.24 4.17 NW 2.4 1005.0
2009. 8. 15. 23.90 31.89 5.46 S 3.5 1014.0
2010. 9. 10. 23.97 29.91 4.95 ─ ─ ─
2011. 9. 1. 23.85 32.16 4.91 S 2.0 1010.0
<Table 3> Marine environmental conditions during the observation at station 208-01 of line 208
3.1 Normal year
For Line 208, the surface water temperature on
September 1 in a normal year, 2011, was 23.9 26.5 –
°C, increasing towards the outer sea, whereas it
was 11.4 13.5 °C on February 23, increasing – towards the outer sea([Fig. 9(a)]). In particular, the bottom water temperature, reaching a depth of 50m in coastal areas to 100m in the outer sea, was
(a)
(b)
[Fig. 9] Distributions of water temperature and salinity in line 208 during a normal year, 2011(a), and NGSST image on September 1(b)
1.14 6.01 °C (mean: 3.23 °C) lower than that at – the same depth in February. In contrast, the surface salinity on September 1 was 32.16 33.41 psu, – slightly increasing towards the outer sea. NGSST image data showed surface water temperature in the UCW was 25 26 °C([Fig. 9(b)]). Thus, the water – temperature in these regions was slightly lower compared to that of the surrounding areas.
However, most of the eastern coastal regions of Korea have water temperatures > 26 °C.
For Line 102, the surface water temperature on August 30 in a normal year, 2011, was 24.2 26.6 –
°C, increasing towards the outer sea([Fig. 10(a)]).
In addition, the bottom water temperature in August, reaching a depth of 30 m in coastal areas to 300 m in the outer sea, was 0.04 7.56 °C – (mean: 2.7 °C) lower than that at the same depth in February. Surface salinity on August 30 was 32.10 32.99 psu. NGSST image data showed the – surface water temperature in UCW in August was 25 26 °C([Fig. 10(b)]). –
For Line 105, the surface water temperature on August 27 in a normal year, 2011, was 24.0 25.0 –
°C([Fig. 11(a)]). However, the bottom water temperature in August, reaching a depth of 30 m in the coastal areas to 500 m in the outer sea, was 0.02 7.8 °C (mean: 1.76 °C) lower than that at the – same depth in February. In particular, the surface water temperature at a depth < 10 m appeared constant along line 105 in the summer. Surface salinity on August 27 was 31.58 33.46 psu. In – contrast, the NGSST image data showed that the surface water temperature was 25 26 °C in most of – the eastern coastal areas, including Ulsan, except for parts of some areas in August([Fig. 11(b)]).
3.2 Abnormal year
For Line 208, the surface water temperature on
(a)
(b)
[Fig. 10] Distributions of water temperature and salinity on line 102 in a normal year, 2011(a), and NGSST image on August 30(b)
August 10 in an abnormal year, 2005, was 15.1–
26.4 °C, increasing towards the outer sea([Fig.
12(a)]). In particular, the bottom water temperature in August, reaching a depth of 30 m in coastal
(a)
(b)
[Fig. 11] Distributions of water temperature and salinity on line 105 in a normal year, 2011(a), and NGSST image on August 27(b)
areas to 140 m in the outer sea, was 0.03 8.33 –
°C (mean: 2.43 °C) lower than that at the same
(a)
(b)
[Fig. 12] Distributions of water temperature and salinity on line 102 in an abnormal year, 2005(a), and NGSST image on August 12(b)
depth in February. In contrast, surface salinity on August 10 was 32.42 33.36 psu, decreasing towards – the outer sea. NGSST image data showed that the surface water temperature in the UCW in August is
was 25 26 °C. Thus, the UCW was slightly colder – than that of other regions([Fig. 12(b)]).
For Line 102, the surface water temperature on August 12 in an abnormal year, 2005, was 19.3–
25.7 °C ([Fig. 13(a)]). In particular, the bottom water temperature in August, reaching a depth of 20 m in the coastal areas to 300 m in the outer sea, was 0.09 8.92 °C (mean: 3.6 °C) lower than – that at the same depth in February. Surface salinity on August 12 was 32.27 33.66 psu. However, the – salinity distribution in August is associated with vertical mixing near the coastal area, whereas it was stratified in the outer sea at other times.
NGSST image data surface water temperature in the UCW in August was < 23 °C. Additionally, the entire area of the eastern coast of Korea showed relatively low temperatures compared to those of the surrounding area([Fig. 13(b)]).
For Line 105, surface water temperature on August 27 in an abnormal year, 2005, was 20.1–
24.6 °C ([Fig. 14(a)]). In addition, the bottom water temperature in August, reaching a depth of 30 m in the coastal areas to 500 m in the outer sea, was 0.01 6.6 °C (mean: 2.23 °C) lower in – August than that at the same depth in March. In particular, the surface water temperature at a depth
< 10 m appeared to alternately increase or decrease towards the outer sea in the summer. Surface salinity on August 27 was 32.03 33.74 psu. In – particular, the salinity distribution in August was associated with vertical mixing near the coastal area, whereas it was stratified in the outer sea.
NGSST image data showed the surface water
temperature in UCW in August was 26 27 °C. –
However, the eastern coastal areas at the north of
UCW were covered with relatively low temperature
waters < 23 °C. ([Fig. 14(b)]).
(a)
(b)
[Fig. 13] Distributions of water temperature and salinity on line 102 in an abnormal year, 2005(a), and NGSST image on August 12(b)
(a)
(b)
[Fig. 14] Distributions of water temperature and
salinity on line 105 in an abnormal
year, 2005(a), and NGSST image on
August 27(b)
4. Flow structures in the western boundary of the East Sea by numerical simulations
4.1 Normal year
[Fig. 15] shows the surface current structures at the southwestern boundary of the East Sea created by the RIAMOM numerical model. Some of the simulated results that Yoon(1982a, b) generated for the entire East Sea were employed as open boundary conditions for the numerical simulation of the currents in this computational domain. First, on August 27 and September 1 in a normal year, 2011, the TC was somewhat intensified, and northward currents prevailed throughout the eastern coastal area of the Korean Peninsula, so they reached the north of line 105. Furthermore, two anti-cyclonic circulations were created south and north of the main axis of the TC. The TC remarkably weakened on February 18 and 23, compared to the summer, and then cyclonic circulation was created south of the open sea near line 103. In particular, all currents travelled north
[Fig. 15] Surface currents in the western coastal area of the East Sea in the summer (August 27 and September 1) and winter (February 18 and February 23), 2011
along the western coastal area of the East Sea, whereas the CW weakened and was restricted, so that the northward current became dominant in nearly all of the eastern coasts of Korea when the TC intensified.
4.2 Abnormal year
[Fig. 16] shows the surface current structures in an abnormal year, created by the RIAMOM numerical model. On August 13, the current (probably NKCW) came down to line 103 along the eastern coast of Korea, whereas the TC went up to the north along the southeast coast of Korea.
Then, it began retreating away from the coast near line 102, probably due to a beta-effect, and then its direction turned southward near line 104. This flow eventually reached line 209, but its direction turned once again northward. In addition, the southward current travelling down along the eastern coast of Korea on August 27 intensified compared to the current on August 13, whereas the TC was slightly weakened compared to that on August 13. In contrast, the TC significantly weakened on February 28, compared to the summer and then met the southward current, i.e. CW, in the vicinity of line 102. Then, it proceeded northeast toward the open sea. In addition, the direction of the CW heading south of the open sea turned toward the north along the eastern coast of Korea at the northern part of line 103. In particular, two anti-cyclonic circulations were created between the northward and southward currents at the boundary of line 105.
These flow patterns seemed to be almost equivalent
on February 28 and March 15. However, the
current heading south along the eastern coast of
Korea on August 27, expanded to near line 102,
whereas the TC headed north, leading to a relative
weakening of its power.
[Fig. 16] Surface currents in the western coastal area of the East Sea in the summer (August 13 and 27) and winter (February 28 and March 15), 2005
. Discussion and conclusions
Ⅳ
1. General current patterns in the UCW
The field measurement results of the tides and currents in the UCW showed that the ebb currents were stronger than the flood currents in the summer, whereas the flood currents were stronger than the ebb currents in the winter. This is probably due to the TC in the summer and the northerly wind in the winter (Kim et al., 2003).
The numerical simulation results also revealed that the ebb currents were slightly stronger than flood currents([Fig. 3(a)]), suggesting that the TC affects the flows in this area every season. Lee(1983) reported that the TC had a water temperature of >
26 °C and salinity < 31.5 psu in summer, whereas it had a water temperature of < 15 °C and salinity
> 34.4 psu in winter. According to <Table 2>, the surface water temperature and salinity in the UCW were roughly 22 °C and 32.0 psu in the summer, but 12.0 °C and 34.0 psu in the winter,
respectively. Consequently, the UCW seems to be located with a slight deviation from the main path of the TC in the summer, whereas it was almost located on the main path of the TC in the winter.
In contrast, the previous result of [Fig. 4] revealed that an upsurge of CW could occur in UCW under the summer southerly wind, and qualitatively coincided with the results of An(1974), Lee and Kunishi(1975), and Byun(1989).
2. Characteristics of marine environments in the UCW
UCW receives freshwater from Taehwa River and runoff from the land, so it appeared to be stratified in the summer(<Table 2>). In addition, the surface water temperature exhibited considerable ups and downs, particularly in the summer([Fig.
7]). Thus, the unstable statistics of the aquatic products for the month in August may have been due to fluctuations in surface water temperature during the summer([Fig. 8]). Furthermore, we infer that the CW may have caused this, as Lee et al.(2004) pointed out that CW contributes to enhance biomass in terms of nutrient supply. For example, we found that the concentrations of NO4-N and PO4-P at station 208-01 were 1.57 m g L ･
-1and 0.05 mg L ･
-1on August 9, 2005, whereas they were 0.93 mg L ･
-1and 0.03 mg L ･
-1on September 1, 2011, respectively (NFRDI). In contrast, CW < 11 °C appeared in the UCW in the summer([Figs. 9(a) 11(a)]), as Kim et al.(2002) – reported, suggesting that CW can appear anytime in the summer in the UCW, regardless of it being a normal or abnormal year. In addition, on August 12 in the abnormal year of 2005, as shown in Fig.
13(a), the surface water temperature around the
UCW was < 23 °C, approximately 3 °C lower than
that in the normal year of 2011. Thus, we inferred that CW might have originated from NKCW. This is also reasonable in terms of the simulated result of the current after-mentioned.
3. Characteristics of marine environments in the western boundary of the East Sea
3.1 Distributions of water temperature and salinity based on ship-stop observation data
<Table 4> summarizes previous results of [Figs.
9 14], and we can recognize the following: First, – the SST for each observation line in summer during an abnormal year tended to be lower overall than that in a normal year, whereas surface salinity was almost the same in normal and abnormal years. Second, a remarkable difference existed in
the summer between the water temperature at the sea surface and that at a depth of 20 m, suggesting that stratification occurs, while the salinities of the two layers are similar. Third, the SST and winter water temperature at a depth of 20 m in an abnormal year were slightly higher compared to those in a normal year, whereas the salinities were similar. In addition, no difference was observed between the water temperature at the sea surface and at a depth of 20 m, suggesting that a mixed layer develops. Fourth, the water temperature tended to decrease toward the north, regardless of the season. This suggests that the LC is intensified towards the north, whereas the TC relatively weakens towards the south.
Year Normal(2011) Abnormal(2005)
Season Summer Winter Summer Winter
Line SST ( )℃
Sal.
(psu)
SST ( )℃
Sal.
(psu)
SST ( )℃
Sal.
(psu)
SST ( )℃
Sal.
(psu)
208 23.9-26.5 (18.8-23.0)
32.16-33.41 (32.36-32.90)
11.4-13.5 (11.5-13.5)
34.34-34.61 (34.45-34.62)
15.1-26.4 (12.6-22.5)
32.42-33.36 (32.48-33.90)
12.2-14.5 (12.4-14.7)
34.22-34.61 (34.44-34.68)
102 24.2-26.6 (13.4-24.4)
32.10-32.99 (32.73-34.17)
10.5-11.9 (10.1-11.8)
34.30-34.62 (34.21-34.46)
19.3-25.7 (12.1-23.7)
32.27-33.66 (32.78-34.23)
9.8-13.6 (9.8-13.8)
34.20-34.55 (34.30-34.61)
105 24.0-25.0 (9.9-15.6)
31.58-33.46 (33.54-34.19)
7.2-10.4 (7.1-10.4)
33.40-34.17 (34.09-34.27)
20.1-24.6 (9.8-21.2)
32.03-33.74 (32.77-34.90)
7.6-10.4 (7.6-10.3)
34.22-34.41 (34.21-34.47) The numbers in the parenthesis denote values at the depth of 20m
<Table 4> Distributions of water temperature and salinity based on data of NFRDI serial oceanographic observations and satellite imaging
<Table 5> summarizes the appearance of the CW in the summer along lines 208 105, where – depth and TD denote the apparent depth of the CW and temperature difference between the two water masses that appeared in the summer and winter at the same depth, respectively. According to the results, CW in an abnormal year approached
shallower areas and its temperature tended to
become relatively lower compared to that in a
normal year. This suggests that CW develops and
extends to the coastal waters in abnormal years to
a greater extent than in normal years. Furthermore,
on August 27 when the power of the southward
current (probably NKCW) intensified, an upsurge or
considerable undulation of CW in the coastal area of line 105, as well as the outer sea, occurred, as shown in [Fig. 13(a)]. These phenomena support the postulation reported by An(1974) that CW is caused by an upsurge of NKCW (a tributary of the Liman Current) in the summer.
3.2 Relationship between calculated current structures and water temperature distributions
The simulated current patterns, as indicated in [Fig. 14 and 15], seem fairly well reproduced compared to the previous results of Yoon(1982a, b), and Kim and Yoon(1999), particularly for separating the currents from the eastern coast of the Korean Peninsula, and generating cyclonic and anti-cyclonic gyres. Also, the computational results of the currents seem to qualitatively agree with the water temperature observational results. In particular, the current structures in the summer revealed that the TC (more precisely EKWC) starts separating from the eastern coast of Korea over line 103. This was inferred from the gradual weakening of the power of the TC northward, due to the temperature difference between the coast and outer seas, as well as the β -effect (Yoon, 1982a). Therefore, CW can be helpful to the marine environments, as An(1974) reported.
3.3 Characteristics of marine environments between normal and abnormal years based on NGSST satellite image data
First, we considered the 25 °C isothermal lines as boundaries of the TC in the summer, as Lee(1983) described. In the summer, i.e. between August 17 and September 1, 2011(see [Figs. 11(a), 9(a), and 8(a)]), NGSST image data indicate NKCW extends or retreats at the northern tip of the eastern coast of Korea. However, NKCW remained at the northwestern tip of the East Sea
overall, so that it did not change the balance of power with the TC. In contrast, in the summer, i.e.
between August 10 and 27, 2005(see [Figs. 12(a), 13(a), and 14(a)]), NGSST data indicate that NKCW gradually extended south. Thus, most of the Japan Sea, including the eastern coastal areas of Korea, is held under the power of NKCW. As a result, the CW appearing in the summer in the western coastal areas of the East Sea was a by-product of the extension of NKCW southward.
This observation suggests there is a general retreat of the TC, even if it is not clear why this occurs.
As a result, in 2005, i.e., an abnormal year, water temperatures in the eastern coast of China and on the southern coast of Korea, decreased compared to those in 2011, i.e. a normal year.
3.4 T-S diagrams based on NFRDI serial oceanographic observation data
[Fig. 17] represents the T-S diagrams plotted
using the water temperature and salinity data
observed at each station of lines 208 to 105, as
described in [Fig. 1]. Here the Arabic numbers
added behind lines 208 to 105 in the legend,
denote stations located in order from the coast to
the outer seas. Furthermore, all stations in the
offshore zones were selected to be in line with the
main axis of the TC, described in [Figs. 15 16]. –
We can make several observations based on these
results: First, colder water appeared at the bottom
layer of the offshore zones than in the coastal
zones during both normal and abnormal years. In
addition, colder water, with a temperature close to
0 °C, appeared at the bottom layer in the summer
compared to winter. Second, a relatively low
temperature and high salinity of water mass
appeared in the summer during abnormal years
compared to in normal years. Third, the vertical
Year Normal Year(2011) Abnormal Year(2005) Season
Observation Line
Summer
CW Appearance Winter Summer
CW Appearance Winter 208 Depth(m) : 50-100
TD(°C) : 1.14-6.01(3.23) ─ Depth(m) : 30-140
TD(°C) : 0.03-8.33(2.43) ─ 102 Depth(m) : 30-300
TD(°C) : 0.04-7.56(2.70) ─ Depth(m) : 20-300
TD(°C) : 0.09-8.92(3.60) ─ 105 Depth(m) : 30-500
TD(°C) : 0.02-7.85(1.76) ─ Depth(m) : 30-500
TD(°C) : 0.01-6.6(2.20) ─ The numbers in the parenthesis denote mean values
