Ocean Sci. J. (2013) 48(1):35-47 http://dx.doi.org/10.1007/s12601-013-0003-4
Available online at http://link.springer.com
Article
Seasonal Variations in the Low-salinity Intermediate Water in the Region South of Sub-polar Front of the East Sea (Sea of Japan) Chang-Woong Shin1*, Sang-Kyung Byun1, Cheolsoo Kim1, Jae Hak Lee1, Bong-Chae Kim2, Sang-Chull Hwang1, Young Ho Seung3, and Hong-Ryeol Shin4 1
Ocean Circulation & Climate Research Division, KIOST, Ansan 426-744, Korea Maritime Security Research Center, KIOST, Ansan 426-744, Korea 3 Department of Oceanography, College of Natural Sciences, Inha University, Incheon 402-751, Korea 4 Department of Atmospheric Science, Health and Environment Institute, Kongju National University, Gongju 314-701, Korea 2
Received 15 August 2012; Revised 6 November 2012; Accepted 11 December 2012 © KSO, KIOST and Springer 2013
Abstract − Seasonal variations in the low-salinity intermediate water (ESIW) in the region south of the sub-polar front of the East Sea were investigated by using historical hydrographic data. The salinity of the representative density (sigma-0=27.2) of the ESIW was minimal in summer and maximal in winter in the region south of the sub-polar front. The selected four subregions showed different salinity variations. In the west of Oki Spur and the Yamato Basin, salinity fluctuated similarly, with a minimum during summer. In the Ulleung Basin and northwest of Sado Island, however, variations in salinity showed two minima, one is in winter and the other is in summer. These results imply differences in the flow path of the ESIW into the region south of the sub-polar front over time. Key words – East Sea Intermediate Water, seasonal variation, sub-polar front, isopycnal surface, Ulleung Basin
1. Introduction The East Sea (Sea of Japan) is a marginal sea of the North Pacific. The bottom of the East Sea is approximately divided into two areas by the 40°N parallel (Fig. 1). The northern half is comparatively flat and deep. In contrast, the southern half is characterized by islands and banks. The latitude 40°N is nearly consistent with the sub-polar front. The southern half is known as a warm region due to the influence of the Tsushima Warm Current, whereas the northern half or cold region is dominated by cyclonic gyre. Kim et al. (2004) divided the water masses vertically into *Corresponding author. E-mail:
[email protected]
five types, excluding the seasonally variable surface water: Tsushima Warm Water (TWW), Intermediate Water, East Sea Central Water, East Sea Deep Water, and East Sea Bottom Water. Two intermediate waters are found in the East Sea: the low-salinity intermediate water or East Sea intermediate water (ESIW), which is characterized by salinityminimum and oxygen-maximum layers and is found in the southwestern area of the East Sea (Kim and Chung 1984), and the high-salinity intermediate water (HSIW) in the eastern Japan Basin (Kim and Kim 1999). The potential temperatures and salinity of the ESIW are 1.0~5.0 °C and less than 34.065 psu, respectively. In contrast, the HSIW is defined by potential temperatures of 0.6~5.0 °C and salinity higher than 34.07 psu. Thus, the potential density range of the HSIW (27.0~27.32 kg/m3) is higher than that of the ESIW (26.9~27.3 kg/m3). Both intermediate waters have high dissolved oxygen levels greater than 250 µmol/l (Kim and Kim 1999). In this study, we exclude descriptions of the HSIW variations because it is located in the region north of the sub-polar front and due to the lack of data. The ESIW occupies a layer between the TWW and East Sea Central Water (Kim et al. 2004) or Japan Sea proper water (Senjyu 1999) in the region south of the sub-polar front. Based on numerical model results, the ESIW is formed off Vladivostok by vertical convection in winter (Kim and Seung 1999; Yoshikawa et al.1999). Another view of the formation mechanism is that low-salinity coastal water along the Russian coast is advected by the current
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system in the northern half and penetrates into the subsurface below the TWW (Yoon and Kawamura 2002). In winter, atmospheric forcing by down-front winds and surface cooling play significant roles in driving subduction and mode water formation along the sub-polar front (Lee et al. 2006; Yoshikawa et al. 2001). These formation processes are repeated yearly. The high levels of dissolved oxygen in the ESIW result from rapid circulation compared with the low-oxygen waters of the East Sea Central Water and East Sea deep and bottom water. Indeed, Kawamura et al. (2007) calculated the turnover time of the ESIW as being 2.2 years using a numerical model. Shin et al. (2007) described the general horizontal distribution patterns of the ESIW on the representative isopycnal surface (sigm-0=27.2 kg/m3) by analyzing historical data. Salinity and potential temperature are low, whereas dissolved oxygen is high in the northwestern East Sea. This implies that the origin of the ESIW is very close to that region. Fresh ESIW is extended from the northwest to the east along the sub-polar front and to the south, such as the Ulleung and Yamato Basins. The ESIW is characterized by the salinity minimum. Shin et al. (2007) showed zonal and meridional salinity sections in a potential density-distance coordinate (Fig. 2). The salinity minimum layer had a density range between 26.9 and 27.3 sigma-0 in the region south of sub-polar front. The salinity minimum density increased to the south and to the east in the region south of the sub-polar front. This means that the salinity minimum layer was mixed mainly with the upper high salinity water. Min et al. (2001) showed by using a 1-D diffusion model that the upper part of the salinity minimum layer was more rapidly mixed with the upper warm and saline water. Thus, the salinity level of the salinity minimum layer might be higher than 34.06 near the Korea Strait and the Japanese coast. In this study, we chose 27.2 sigma-0 as the representative potential density of the ESIW for convenience of description. Although the ESIW is formed annually, the seasonal variation in the ESIW in the region south of the sub-polar front has not been elucidated. In this study, seasonal mean distributions of physical properties based on the representative density of the ESIW were calculated using historical hydrographical data, and then the seasonal variation on the isopycnal surface was examined. The path and timing of the penetration of the ESIW into the south of the sub-polar front are also discussed.
2. Data and Methods Historical hydrographical data were used to analyze the seasonal variation in the ESIW. To identify the salinity minimum below the thermocline and to analyze the ESIW, data of high accuracy and resolution were required. Thus, we selected data gathered by the Maizuru Marine Observatory (MMO) of the Japan Meteorological Agency from 1963 to 1992 (Fig. 1). These data cover the whole East Sea except for the northwestern East Sea. As for the south of the subpolar front, however, there are no data for the area near the east coast of South Korea, and these data are essential to investigate seasonal variation in the region south of the subpolar front. Therefore, data gathered by the Korea Institute Ocean Science and Technology (KIOST) from 1990 to 2005 were included with the former data. Dissolved oxygen (DO) is a useful tracer to examine the age of the water mass. DO distributions were analyzed with the MMO data only, because the KIOST data did not include DO. Basically, this data set was the same as that in Shin et al. (2007), except for recent cruise data surveyed around the east coast of Korea and the Ulleung Basin from 1999 to 2005. This data set was classified into four seasons: winter (Jan.-Mar.), spring (Apr.-June), summer (July-Sep.) and autumn (Oct.-Dec.). Fig. 3 shows seasonal distributions of the number of data averaged in each segment. In the region south of the sub-polar front, the number of data enough to average, particularly in the southwestern part. The most data were collected in the summer, while in the spring and autumn data were sparse. All the observations were interpolated for standard depth after filtering through a stability test. To obtain the physical properties of the ESIW on the isopycnal surface, measured data and calculated values, such as potential density and potential vorticity, were interpolated for every 0.01 interval of potential density. Potential vorticity was calculated by the following equation f dρθ - -------Q = ---ρθ dp = fE where f is Coriolis parameter, ρθ potential density, p pressure, and E stability (UNESCO 1991; Luyten et al. 1983). The interpolated physical properties were averaged at each potential density into 1°N × 1°E segments by moving 0.5° in latitude and longitude. With regard to the conservation of water-mass properties, the density level average method is
Seasonal variations of the East Sea Intermediate Water
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Fig. 1. Maps show bottom topography with locations of the East Sea (left) and observed stations (right). Depths are in meters. The four red boxes were drawn to analyze seasonal variation of the ESIW salinity on the isopycnal surface (sigma-0=27.2). Blue and red dots mean stations observed by Maizuru Marine Observatory (M) and Korea Institute of Ocean Science and Technology (K), respectively. Numbers of stations are shown upper left corner at each season map
considered superior to the depth level average method (Lozier et al. 1994; Shin et al. 2007).
3. Results Depth, potential temperature, salinity, potential vorticity and DO on the isopycnal surface (sigma-0=27.2 kg/m3) for each season are presented in Fig. 4. Their standard deviations are shown in Fig. 5. Outcrops of high-density areas greater than 27.2 at the surface during winter are displayed in white. The depth of the sub-polar front is greater in the south than in the north due to the subduction of the ESIW below the TWW. Deeper isopycnal in the south than the
north around the sub-polar front results from the eastward flow. In the northern part of the sub-polar front (higher latitude than 40°N), the depth was 50-100 m depth in summer. Depths in the Ulleung and Yamato Basins are especially deep. The extreme depth found in the Ulleung and Yamato Basins is associated with the steady occurrence of mesoscale warm eddies (Lee and Niiler 2005; Shin et al. 2005; Shin 2009). Potential temperature and salinity were low in the northwestern East Sea and increased southeastward. The ESIW had low salinity and temperatures in the northwest. As the fresh ESIW penetrated below the TWW to the region south of the sub-polar front on the isopycnal layer, the
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Fig. 2. Mean salinity sections in potential temperature-distance coordinate along 38°N and 134°E
Fig. 3. Distributions of number of data used to average in each segment for potential temperature and salinity (a) and oxygen (b)
salinity and potential temperature increased due to mixing with surrounding waters. In the Yamato Rise, the potential temperature was warmer than 2.2 °C in winter except in the western part. In spring and summer, the potential temperature became colder compared with winter over the whole Yamato Rise. The potential temperature became warmer in autumn except in the western part of the Yamato Basin. Similarly, the salinity in the Yamato Rise changed as the
potential temperature. In winter, the salinity was higher than 34.06 except in the western part. In spring and summer the salinity dropped to between 34.02 and 34.06. In autumn, the salinity was restored to the same level as in winter. Potential vorticity just south of the sub-polar front was consistently low throughout the year, except during winter, when an area of high vorticity in the west of Yamato Rise separated the regions of low vorticity, isolating the east
Seasonal variations of the East Sea Intermediate Water
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Fig. 4. Seasonal variations of mean values in depth (m) (a), potential temperature (°C) (b), salinity (c), potential vorticity (10-11m-1s-1) (d) and dissolved oxygen (ml/l) (e) on the isopycnal surface (sigma-0=27.2 kgm-3). Areas with potential density higher than sigma0=27.2 (thick solid line in winter) are displayed in white. The thick dashed line indicates a 2,000-m depth contour
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Fig. 5. The same as Fig. 4 except standard deviations
from the west. The potential vorticity was higher in the center of the Japan Basin than in the peripheral waters in summer and autumn. This high vorticity region is almost consistent with the outcropped area in winter.
DO was high all the year round in the region north of the sub-polar front. There were two areas where the DO was more than 7.2 ml/l. One was the northwestern Japan Basin and the other was the southeastern Japan Basin. High
Seasonal variations of the East Sea Intermediate Water
Fig. 6. Relations of salinity-DO, potential temperature-DO, salinityAOU, and salinity-potential vorticity in the region south of sub-polar front (south of 40°N) on the isopycnal surface of 27.2 sigma-0
density (>27.2 sigma-0) isopynal surfaces were outcropped during winter in both areas. The former area was coincident with the low potential temperature and the low salinity area. However, the latter area has high potential temperature and high salinity compared with the former area. This suggests that the ESIW is formed in the northwestern Japan Basin during winter. Fig. 6 shows the relations of salinity-DO, potential temperature-DO, salinity-Apparent Oxygen Utilization (AOU), and salinity-potential vorticity in the region south of the sub-polar front (south of 40°N) on the isopycnal surface of 27.2 sigma-0. The DO decreased linearly with the salinity and potential temperature. The AOU increased with the salinity. These mean that newly formed fresh ESIW has high DO, low potential temperature and low salinity. The potential vorticity showed an exponential increasing pattern with salinity rather than a linear relationship. This pattern is somewhat different from the linear relationship described by Shin et al. (1998). Their study area was confined to the southwestern East Sea. Fig. 7 shows seasonal variations of salinity in a potential density-distance coordinate. In a meridional section along 131°E, distributions of an isohaline of 34.04 changed seasonally. Low salinity water was intruded into southward
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from the sub-polar front around 40°N along the isopycnal layer of 27.2 sigma-0. In winter, the southern limit of the isohaline of 34.04 was 38.0°N. The limit moved southward in spring and reached 37.0°N in summer. In autumn, there was no 34.04 water in the region south of 38.5°N. In section 135°E, there was no lower salinity water than 34.04 in winter. This low salinity water existed around 38.5°N and north of the 40°N sub-polar front in spring and summer. In autumn, the low salinity water decreased in the southern region. Similar to the meridional section, salinity of the salinity minimum layer also changed seasonally in a zonal section along 37°N and 38.0°N. The isohaline moved eastward from winter to summer and retreated in autumn except around 37°N, 131°E (Ulleung Basin). This seasonal change means that the low salinity intermediate water spreads form the northwestern part of the East Sea to the south and the east. The retreat of low salinity isohalines in autumn indicates a reduction or interruption in supply of the low salinity intermediate water. Seasonal salinity variations with standard deviations and the 95% confidence intervals in each region (Fig. 1) are presented in Fig. 8 and statistics of their differences with neighboring seasons are presented in Table 1. The salinity in the region south of the sub-polar front (south of 40°N) decreased from winter to summer and increased in autumn. The salinity differences between neighboring seasons were not statistically significant with the 5% significance level except for the difference between summer and autumn. The difference (-0.008) between winter and summer was statistically significant with the 5% significance level (-0.0114
