Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/ East Sea Upper Layer Circulation

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1 Journal of Oceanography, Vol. 61, pp. 155 to 165, 2005 Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/ East Sea Upper Layer Circulation SUNGHYEA PARK 1 *, MOON-SIK SUK 1 and HUI SOO AN 2 1 Marine Environment and Climate Change Laboratory, Korea Ocean Research & Development Institute, Ansan , Korea 2 Department of Earth Science Education, Seoul National University, Seoul , Korea (Received 24 February 2004; in revised form 28 May 2004; accepted 29 May 2004) Numerical experiments were performed in order to investigate the effects of variations of the transport through the Korea/Tsushima Strait, an inlet of the Japan/East Sea, on the upper layer circulation in the JES based on a 10-month transport observation from May 1999 to March 2000 (Perkins et al., 2000). All external forcings to the model were annual mean fields, except the transport variation through the Korea Strait. In the experiments where the periodic variation of the transport repeated continuously sinusoidally by several periods, strong variability of sea surface height (SSH) was detected in the region extending from the Korea Strait to the Japanese coast due to the geostrophy of the buoyancy forcing at the Korea Strait. The region along the Korean coast is more sensitive to the long-term variations than the short-term ( 60- day period) ones. In two experiments forced by realistic and monthly mean transport, the difference of rms of sea surface height was largest at the Japanese coast and relatively large at the East Korean Warm Current separation region (128~130 E, 39~41 N) and to the east of Yamato Rise. The distribution of difference of eddy kinetic energy at 100 m depth between the two experiments was similar to that of the rms of SSH. In the distributions of mean SSH and mean kinetic energy at 100 m depth the realistic transport invokes eddy variability to interact with mean current resulting in the changes of the mean SSH and the mean kinetic energy at the East Korean Warm Current separation region, but it does not produce conspicuous changes in the mean fields of entire JES compared with the mean fields forced by the seasonal transport. Keywords: The Japan Sea, the Korea Strait, inflow transport variation, upper layer circulation. 1. Introduction The Korea Strait (KS) connects the Japan/East Sea (JES), the Yellow Sea, and the East China Sea, and it is divided into western and eastern channels due to the presence of the Tsushima islands in the middle of the strait. The KS relatively shallow with a trough of 220 m in the western channel. The JES exchanges surface water, which is about 10% of its total volume, with the neighboring seas through the KS and the other straits in the north. Therefore, the surface water exchanges of the JES through the straits have very significant effects on upper layer * Corresponding author. spark@nps.edu The Editor-in-Chief does not recommend the usage of the term East Sea in place of Japan Sea. Copyright The Oceanographic Society of Japan. circulation. The Tsushima Warm Current (TWC), bringing warm and saline water into the JES, flows through the KS and is divided into western and eastern branches, the East Korean Warm Current (EKWC) and the Japanese Coast Branch (JCB, referred as the Nearshore Branch in the Japanese literature), respectively; however, there is an additional time-dependent eastern branch. The EKWC separates from the Korean coast at 38~40 N throughout a year, changes its direction to the east, and then a large portion of it eventually flows out through the Tsugaru Strait. It is, therefore, clear that the heat, salt, and momentum flux through the KS affects the variability of the upper layer circulation in the JES. Several estimates of the transport through the KS have been attempted in order to understand its influence on the variability in the JES. Previous estimates of the transport through the KS are classified into two catego- 155

2 Fig. 1. Configuration of model domain. A rectangle indicates the model domain. An inflow open boundary is located at the Korea Strait, and two outflow open boundaries are at the Tsugaru Strait and the Soya Strait. S-line and N-line are mooring lines where the NRL observed transport through the Korea Strait from May 1999 to March 2000 (Perkins et al., 2000). Contours indicate the bottom topography, and intervals of thin and thick ones are 250 m and 1000 m, respectively. Geographical names are the followings: KP of the Korea Plateau, UB of the Ulleung Basin, OS of the Oki Spur, YB of the Yamato Basin, YR of the Yamato Rise, and JB of the Japan Basin. ries. One uses an indirect dynamic method, such as geostrophic calculation, and the other is based on direct current measurement. Several estimates were obtained by the indirect method, giving annual mean values of 1.35 Sv (Yi, 1966) and 1.5 Sv (Miita and Ogawa, 1984). A number of estimates were also obtained by direct current measurements, giving values of 4.2 Sv (Miita and Ogawa, 1984) and 2.2 Sv (Isobe et al., 1994). The estimates obtained by the direct method are generally higher than those calculated by the indirect method, because the barotropic component of current is not included in the indirect method. The estimates by the indirect method show a minimum value in spring and a maximum value in summer, but those obtained by the actual measurement show a maximum in late autumn, November~December, rather than in summer, June~August (KORDI, 1998b). Since the estimates by the indirect method were underestimated and those by the direct method were obtained from short-term measurements, the transport variation is so far not known exactly. The transport through the KS was measured by the US Naval Research Laboratory (NRL) from May 1999 through March 2000 with an array of bottom-mounted 156 S. Park et al. Fig. 2. (a) Transport across the S-line and the N-line. The Nline is offset by 3 Sv. (b) Mean velocities perpendicular to the N-line (top) and the S-line (bottom). These line plots indicate vertically integrated velocity, which is transport per distance along the section (Jacobs et al., 2001) (c) Power density spectrum of the transport (S-line) through the KS. Acoustic Doppler Current Profiler (ADCP) (Fig. 1). The observed mean transport was 2.8 Sv through the S-line and had a slightly lower value of 2.4 Sv through the Nline (Fig. 2(a)). One reason for this discrepancy is probably that one current meter was not recovered along the N-line (Perkins et al., 2000). This observation shows that the transport varies from 1.0 Sv in January to 4.5 Sv in October. The current core is found at the center of the Sline, while it is split into two cores near the Korean coast and the Japanese coast of the N-line by the Tsushima islands (Fig. 2(b)). The transport variation is evident in the periods of 5~7 day, 10~15 day, and 30~50 day (Fig. 2(c)). This observation reveals that the temporal and quantitative variation of transport through the KS is larger than had so far been supposed, which motivates us to carry out numerical experiments using that realistic inflow transport to investigate influences of the transport variation through the KS on the upper layer circulation of the JES. Numerical modeling studies of the JES have been actively pursued since the 1980s. The transport through the KS has been described only in simple form in various

3 Authors Table 1. Summary of inflow transport at the Korea Strait in numerical experiments. Inflow transport at the KS in numerical experiments Yoon, Sv Kawabe, 1982 A constant of 0.5 Sv at the eastern half of the entrance and changing from 1.0 Sv to 2.0 Sv at the western half Seung and Kim, Sv based on the previous data (Yi, 1966; Miita and Ogawa, 1984) Kim and Yoon, 1999; Hirose and Ostrovskii, 2000 A sine curve of 2.2 Sv ± 0.35 Sv (maximum in the middle of September and minimum in the middle of March) according to Isobe s observation (Isobe et al., 1994) Hogan and Hurlbert, 2000 A sine curve of 2.0 Sv ± 0.66 Sv (with the maximum in July and the minimum in January) Chu et al., 2000 An annual mean value of 1.3 Sv with the maximum value of 2.2 Sv in October and the minimum value of 0.3 Sv in February from Yi s estimation (Yi, 1966) numerical experiments due to the lack of observed data. According to Table 1 the annual mean values of the transport imposed on the inflow boundary range from 1.3 Sv to 2.3 Sv, and amplitudes, i.e. seasonal variation, are less than 35% of the annual mean value. Few numerical experiments have adopted horizontal velocity shear across the KS and the short-term variation of the inflow transport. In their report of numerical experiments to examine the influences of the transport variation through the KS, Kim et al. (1997a, b) investigated the effects of temporal variations of inflow transport through the KS on the generation of Ulleung Eddy as well as meanders and eddies in the Subploar Front (SPF) using a nonlinear layer model with a simple inflow-outflow system. More realistic responses were not studied due to the simplicity of model. Chu et al. (2000) reported that lateral transport affects the driving of the Ulleung Eddy, generating the EKWC, and generating the JCB according to an ocean general circulation model. The temporal variation of inflow transport in simple or seasonal form was imposed in those experiments. In this study we investigate the following aspects through numerical experiments in which the inflow transport variation is controlled based on the observed, realistic transport by the NRL: Does the transport variation with short period from the observed transport influence the upper layer circulation of the JES? If so, where is the influence detected? What difference is shown between a numerical experiment forced by seasonal transport and that forced by realistic transport? The following sections describe the configuration of the model and the implementation of experiments. The results and discussion are presented thereafter. 2. Model Design 2.1 Model configuration The Princeton Ocean Model (POM) was used in these experiments (Mellor, 1996). The model domain covers the interior of (130 E, 31 N), (147 E, 43 N), (142 E, 51 N), and (125 E, 39 N) with three ports (Fig. 1): one for inflow boundary (the KS) and two for outflow boundary (the Tsugaru Strait and the Soya Strait). A grid system is referred to the Operational Korea Ocean Prediction System, which makes an angle of 35 with the parallel of latitude (Suk et al., 1996, 2001). This inclined grid system has several advantages. For instance, it needs shorter computational time than the parallel grid system with the same resolution and has the convenience that velocities can be imposed on the inflow boundary because the axis of the TWC at the KS is parallel to the i-direction of the inclined grid (KORDI, 1998a). The grid resolution of the model varies from 16 km in the northern part to 18 km in the southern one. The model has cells horizontally and 18 layers vertically. The bottom topography is smoothed from Choi s data (Digital Atlas for Neighboring Seas of Korean Peninsula, Choi et al., 2002), 1 1 resolution (Fig. 1). The smoothed topography is set up under the condition that the ratio of the depth of a grid cell to that of an adjacent one stays less than 1.5, but the coastal region is excluded from this smoothing. 2.2 Initial and forcing data The present model is initialized by the mean temperature and salinity in January obtained from the Japan Oceanographic Data Center (JODC). Temperature and salinity at open lateral boundaries and at the sea surface are forced with the Generalized Digital Environmental Model (GDEM, version 2.5) data (Figs. 3(a) and (b), respectively). The wind field, shown in Fig. 3(c), is forced by Na and Seo data (1998 version). All external forcings are annual mean fields except the inflow transport. The momentum flux through the sea surface is found from the distribution of annual mean wind stress curl (Fig. 3(c)). Generally, the positive (negative) wind stress curl in the north (south) of 40 N produces a cyclonic (anticyclonic) torque. The line of zero dyne/cm 3, the boundary between the northern cyclonic gyre and the southern anticyclonic gyre, stretches zonally in 40~42 N. The location of the Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/East Sea Circulation 157

4 Fig. 4. Transport distributions across the Korea Strait in the model. Bars exist at grid points. The value is the ratio of the transport of each grid to the total transport through the Korea Strait. Fig. 3. The surface forcing fields: (a) temperature ( C), (b) salinity (psu), and (c) wind stress curl (10 9 dyne/cm 3 ). Subpolar Front is associated with this. Hogan and Hurlbert (2000) demonstrated that the northern cyclonic gyre is associated with positive wind stress curl and that realistic separation of the EKWC in a 1/8 resolution ocean model may be substantially influenced by wind-driven circulation according to comparative experiments using European Centre for Medium-Range Weather Forecasts (ECMWF) wind and Hellerman Rosenstein wind. Large positive values are found in the northern Japan Basin ( dyne/cm 3 ) and the Vladivostok coast ( dyne/cm 3 ). Large negative values are spread over the south of the JES from the Ulleung Basin (UB) to the Japanese coast. 2.3 Boundary conditions Sea surface temperature and salinity are restored to boundary values in a time scale of 10 days, i.e. surface forcings. The barotropic velocities normal to the lateral boundaries are assigned as the transport according to the cross-sectional area of the lateral boundaries. The normal baroclinic velocities are calculated using the Olranski radiation condition (Olranski, 1976). The barotropic and baroclinic velocities parallel to the lateral boundaries are null. Temperature and salinity at the lateral boundaries are calculated using the upstream method. The horizontal distributions of the transport at the inflow boundary are described according to those of the N-line (Fig. 4), considering that the inflow boundary is divided into two by Tsushima islands. Bars in Fig. 4 indicate the ratio of the transport of each grid cell to the total transport through the KS. At each given time the inflow transport is segmented, so that the same volume of transport enters through the western and eastern channels based at the top of Fig. 2(b). For the sake of total volume conservation within the model domain the value of 75% of inflow transport is forced to flow out through the Tsugaru Strait and the rest through the Soya Strait. These conditions are applied identically in all experiments that we conducted. The transport at the inflow boundary is assigned differently in each experiment and is described next. 2.4 Design and implementation of experiments A series of experiments is designed to examine whether or not the inflow transport variation with short period from the observation influences the upper layer circulation of the JES. The inflow transport of the experiments varies at the prominent periods (5, 10, 30, 40, and 50-day) which were detected in the realistic transport and at longer periods (60, 90, 120, 180, and 360- day) under identical conditions except for the inflow transport. The periodic variation of the inflow transport repeats continuously sinusoidally with a mean of 2.8 Sv and an amplitude of 0.7 Sv (rms of the observed transport), by the above periods for one year. The experiments are referred to by the period, such as Exp5-day, Exp10- day, Exp30-day, and so forth. Two experiments are designed to examine difference between a numerical experiment forced by seasonal transport (ExpPS) and that forced 158 S. Park et al.

5 Table 2. List of experiments. Name of experiments ExpRF (reference) Exp5-day, Exp10-day, and etc. ExpPS (Period: Seasonal) ExpPR (Period: Realistic) Imposed inflow transport Annual mean value of 2.8 Sv Periodic variation by periods of 5-day, 10-day and etc. Monthly mean transport (averaged the observed transport over one month) Realistic (observed) transport Conditions applied identically to all experiments. Same volume of the transport passes through western channel and eastern channel of the KS. All external forcings are annual mean fields. The horizontal distribution of transport at the inflow boundary is described as parabolic shape based on the observation. by realistic transport (ExpPR). ExpPR is forced by the realistic transport with observational noise filtered out, and ExpPS is forced by the transport averaged from of ExpPR over one month. The monthly mean transport reveals only seasonal variation. The realistic transport, however, shows a short-period variation (shorter than seasonal) as well as seasonal variation. A reference experiment (ExpRF) is constantly forced by 2.8 Sv. A summary of the experiment design is represented in Table 2. Model parameters are described as follows (Mellor, 1996): coefficient of the Smagorinsky (1963) diffusivity (horizontal) 0.12, non-dimensional; background vertical diffusivity 50.0 m 2 /s; external time step 30 sec and internal time step 900 sec. Integration is prognostic in all experiments. Total kinetic energy (TKE) is monitored every day (not shown) for stability of integration. The TKE of ExpRF appears to be steady after four years of integration, and the integration is then carried out for 15 years in total. The series of experiments, such as Exp5-day and Exp10-day, is integrated for one year starting from the 14th-year result of ExpRF. ExpPS and ExpPR are integrated for three years starting from the 12th-year result of ExpRF. The results of the last year obtained at intervals of two days are discussed in next section. 3. Results This section briefly presents, first, the circulation of the reference field, ExpRF. We analyze sea surface height (SSH) variability to examine whether or not the short periodic variation of inflow transport influences the upper layers circulation of the JES. We compare ExpRF, ExpPS, and ExpPR. Finally, we present the temporal variability of sea surface height, temperature, and velocity at one location near the Japanese coast from ExpRF, ExpPS and ExpPR. 3.1 Reference field The reference field for the following discussions is obtained by averaging over the last 30 days. Since the Fig. 5. Current distributions represented by streaks of floating points at the surface, 100 m, and 500 m depth in ExpRF. inflow transport variation generally has more effects on the upper layer than the intermediate and deep layers, the following discussion is confined to the results for the upper 500 m of water. At the surface the EKWC flows northward along the Korean coast at a velocity of 40~50 cm/s (Fig. 5). Part of the EKWC leaves the Korean coast Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/East Sea Circulation 159

6 Fig. 7. Vertical distributions of the temperature and salinity at 37 N transect: (a) temperature ( C) from ExpRF; (b) temperature ( C) from GDEM; (c) salinity (psu) from ExpRF; (d) salinity (psu) from GDEM. Fig. 6. (a) Distributions of sea surface height of ExpRF. (b) Combined mean dynamic topography (RIO-03) from the Collecte Localisation Satellites (CLS). Contour interval is 2 cm. at 37 N to form a warm anticyclonic eddy. The rest retains a northward motion, turns eastward at the Korea Plateau (KP, near 38 N) and near 40~41 N, meets the North Korean Cold Current (NKCC), and eventually forms a strong current belt, the SPF. That belt is located close to the zero line of the wind stress curl. The water entering through the eastern channel forms the JCB flowing along the Japanese coast. Strong warm anticyclonic eddies are found in the Ulleung Basin, in the Yamato Basin (YB), and to the north of the YB (137.5 E, 39 N). Small cold cyclonic eddies exist within the large cyclonic gyre in the northern JES. The current distribution at 100 m depth resembles that at the surface. Several small eddies are found in the current distribution at 500 m depth, such as four eddies near the Korean coast, two in the YB, and two to the northeast of the YB. These eddies seem to be related to the bottom topography. The sea surface height of ExpRF varies around 40 cm, from 24 cm in the south to 16 cm in the north (Fig. 6(a)). This variation range is reasonable compared to the Combined Mean Dynamic Topography (CMDT) of Collecte Localisation Satellites (CLS), which varies around 40~50 cm (Fig. 6(b)): this dataset, named RIO-03 and computed at a resolution of 1 1 over the period with a multi-variate analysis using hydrographic data, surface drifter velocities, and altimetry, was distributed by Rio and Hernandez in 2003 (ftp://ftp.cls.fr/pub/oceano/enact/mdt). It is difficult to compare detailed SSH features between ExpRF and CMDT due to differences of spatial resolution; ExpRF presents five times the spatial resolution of CMDT. Nevertheless, the SPF of central JES and northward convex features along 131 E and 134 E in southern JES appear in CMDT. These features are consistent with the SSH distributions of ExpRF. In vertical distribution of temperature at 37 N transect, concave structures of isotherms are detected around 131 E and 133 E in ExpRF (Fig. 7(a)) and observation (GDEM) (Fig. 7(b)). These seem to be warm eddies in UB and in YB. The isotherms around 136 E are inclined toward the east at a depth of 100 m or deeper in both distributions. These are associated with a southwestward undercurrent off the Japanese coast. In vertical distribution of salinity at 37 N transect, low salinity water is found near the sea surface, while high salinity water is distributed at around 50~200 m depth (Figs. 7(c) and (d)). Below that, low salinity water is present. However, the values of salinity and temperature from ExpRF are in more or less disagreement with those from observation. In particular, the temperature from ExpRF is 3 C warmer than the observed values at a depth of 200 m or deeper. The salinity from ExpRF is shifted toward low values compared to the observed values. As a result, the present model shows some difficulties in reproducing real hydrographic fields in deeper layers of JES. This can be considered as being due to computational diffusion, 160 S. Park et al.

7 Fig. 8. Distributions of the difference of SSH: (a) between Exp10-day and ExpRF; (b) between Exp30-day and ExpRF; (c) between Exp90-day and ExpRF. Contour interval is 0.5 cm. Dotted and solid lines indicate negative and positive differences, respectively. regardlessness of heat flux through sea surface in surface boundary forcing, coarse model resolution, imperfection of forcing data, etc. Nevertheless, in general, ExpRF is deemed to give a good reproduction of the mean field of JES in terms of depicting upper layer circulation of JES, such as SPF, EKWC, JCB, NKCC, and eddies in southern JES. 3.2 Response to periodic variation of inflow transport based on observation To analyze the response to periodic variation of inflow transport we calculate the difference of the sea surface height (DSSH) from ExpRF at each experiment. In Exp10-day the DSSH is negligible over the entire JES, except in the direct response region covering the KS to the Japanese coast in Exp10-day (Fig. 8(a)). In Exp30day the DSSH is found not only along the Japanese coast but also along the Korean coast (Fig. 8(b)). In Exp90-day the DSSH distribution is different from that in Exp 30day (Fig. 8(c)); that is, the regions showing the DSSH expand and stronger DSSH signals are detected near the UB and south of the YR (in particular, between the YR and the YB). When the inflow transport is increased, the SSH near the Japanese coast becomes high and the DSSH spreads offshore. In the Oki Spur a positive tongue-shaped feature of the DSSH is generated (24~40 days of Fig. 8(c)). This then separates from the Japanese coast due to decreased supply of the inflow transport and spreads out, interacting with the surrounding water (48~96 days of Fig. 8(c)). This suggests that eddy mean current interaction results in expansion of spatial variability of SSH and its intensification. Moreover, this feature is found in longer than 90-day variation of the transport (not shown). In order to estimate the intensity of the response to the temporal variation of the transport, the rms of SSH is calculated in each experiment. The rms is defined as N ηrms (i ) = (η(i, t ) ηref (i, t )) t =1 N 2, where i is the index of space; N is number of data in time series of sea surface height; η is sea surface height of each experiment; η ref is sea surface height of ExpRF. In each experiment the rms with respect to each period is calculated from time series of the SSH over one year at two-day intervals. Since the influences of the transport variation seemed to be dominant in the southern JES, the rms is calculated for the south of 40 N. The rms of the Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/East Sea Circulation 161

8 Fig. 10. Difference distributions of several physical quantities between ExpPR and ExpPS: (a) mean SSH (cm); (b) rms of SSH (cm); (c) MKE (cm 2/s 2 ) at 100 m depth; (d) EKE (cm2/s 2) at 100 m depth. Fig. 9. The rms of SSH at the south of 40 N, the Korean coast (X), the Japanese coast (Y), and the south of the Yamato Rise (Z). Horizontal axis presents the period of the inflow transport applied to each experiment. three regions are shown in Fig. 9: near the Korean coast (region X in Fig. 9), near the Japanese coast (region Y in Fig. 9), and south of the YR (region Z in Fig. 9). The rms in the south of 40 N shows that the SSH variability is relatively higher on the long-term variation ( 90-day period) of the transport. The region along the Japanese coast is the most sensitive to all temporal variations adopted in the experiments. The region along the Korean coast is more sensitive to the long-term variations than the shortterm ( 60-day period) variations. The region of south of the YR is also sensitive to the long-term variations. Spall (2002) showed that inflow transport branches into eastern and western boundary currents due to anticyclonic wind stress curl and atmospheric cooling in marginal seas, especially the southern JES. The temperature in the interior of the JES is colder than that in the open sea, so warm water enters the colder JES. This warm 162 S. Park et al. water is deflected to the eastern boundary, the Japanese coast, by geostrophy and forms an eastern boundary current (Spall, 2002). The generation and variability of the eastern boundary current in the JES is related to buoyancy forcing at the KS (Hogan and Hurlburt, 2000; Spall, 2002). This demonstrates that the temporal variation of the inflow is most influential along the Japanese coast. Using hydrographic and direct current measurements Hase et al. (1999) suggested that First Branch of the TWC (FBTWC), the JCB, starts from the eastern channel of the KS, flows along the Japanese coast trapped near the 200 m isobath, and exists throughout the year. The Second Branch of the TWC (SBTWC) is fed from the western channel of the KS, found well offshore of the shelf break in the Japanese coast, and is much more time dependent. Total transport variations through the KS in summer are mainly due to transport variations in the western channel, while in winter, contributions to total transport variations are more uniformly distributed across the KS, as direct current measurements have revealed (Jacobs et al., 2001; Teague et al., 2005). It is shown that a relatively large amount of the inflow from the western channel is deflected to the Japanese coast (all of Fig. 8), corresponding with the development of the SBTWC due to temporal variation of inflow transport in the western channel. 3.3 Difference of experiments according to seasonal transport and realistic transport The distribution of mean SSH is similar in two ex-

9 periments (ExpPS and ExpPR, not shown). The short-term variation of the transport appearing in realistic transport has little effect on the mean SSH field; in Fig. 10(a) there is small a difference of mean SSH between the two experiments at the EKWC separation region (128~130 E, 39~41 N) and to the east of YR (136~138 E, 39~41 N) and almost no difference along the Japanese coast. The rms of SSH is high at the EKWC separation region, the UB (130~131 E, 37~38 N), and the Japanese coast in both experiments (not shown). The difference of the rms of SSH is found to the east of the YR and at the EKWC separation region (Fig. 10(b)). Moreover, the biggest difference is found along the Japanese coast, which indicates that the region along the Japanese coast is the most sensitive to the realistic transport, as deduced from the previous experiments forced by the periodic variation of inflow transport. A mean kinetic energy (MKE) and an eddy kinetic energy (EKE) are also calculated at 100 m depth. The MKE and the EKE are defined as N 1 U()= i Uit (, ), N t = 1 U ( i, t)= U( i, t) U() i, 1 MKE = U () i 2 2, EKE = U ( i, t), 2 N N t = 1 where U is velocity and N is the number of data in time series of velocities. The spatial mean value of MKE is nearly equal in both experiments, but that of EKE is higher in ExpPR than ExpPS. In both experiments high MKE (>600 cm 2 /s 2 ) is detected at the Korean coast, the YR, and the YB, that is, a path of the EKWC, whereas relatively low MKE (50~200 cm 2 /s 2 ) is detected along the Japanese coast (not shown). The difference of MKE between the two experiments (Fig. 10(c)) seems to be similar to the difference of the mean SSH between the two experiments (Fig. 10(a)). The EKE is much lower than the MKE and is distributed locally due to the annual mean forcings, except for the inflow transport in both experiments (not shown). The ratio of the EKE to the MKE is the highest at the Japanese coast and is relatively high in the EKWC separation region, suggesting that there is considerable interaction between the eddy and the mean current. This result corresponds to the estimates published by Lee et al. (2000); they estimated conversion rates of EKE due to eddy and mean current interaction from Fig. 11. Time series of transport, SSH, temperature, and salinity at a location, denoted as a star in Fig. 1, at 100 m depth. Thin solid, thick two-dot, and dotted lines indicate ExpPR, ExpPS, and ExpRF, respectively. Argos-tracked drifters, which showed that fast eddy growing areas are found along the path of the EKWC and along the Japanese coast. Furthermore, Jacobs et al. (1999) indicated a band of high EKE in those regions through calculation of the cross-track EKE from altimeter data of the Geosat-Exact Repeat Mission and TOPEX/Poseidon. In addition, it was indicated that eddy variability impacts on the EKWC separation from the Korean coast based on the Naval Research Laboratory Layered Ocean Model (NLOM) results (Jacobs et al., 1999). We also find that the difference of MKE, even though it is small, and that of EKE are detected in the EKWC separation region, suggesting increased eddy variability due to the transport variation influences on mean current. The most conspicuous difference is found along the Japanese coast when the difference of EKE between the two experiments (Fig. 10(d)) is compared to the difference of MKE between the two experiments (Fig. 10(c)). This is the same result that was deduced from the difference of rms of SSH between the two experiments. 3.4 Responses from Japanese coast Since the Japanese coast was revealed to be the most sensitive region to the temporal variation of the inflow transport, the time series of SSH, temperature, and salinity at 100 m depth are acquired at a location, denoted by a star in Fig. 1, from ExpRF, ExpPR, and ExpPS in order Numerical Experiments of the Influences of the Transport Variation through the Korea Strait on the Japan/East Sea Circulation 163

10 nificant on 30~50-day period in both Figs. 12(e) and (f), though it is a little difficult to clearly distinguish dominant periods from neighboring periods in 20-day or shorter. The time lag with the transport is estimated at 20~30 days in all three time series (Figs. 12(g), (h) and (i) as roughly shown in Fig. 11). Therefore, all three time series of ExpRF well reflect the temporal variation of the transport. Fig. 12. Power density spectra of SSH (a), temperature (b), and salinity (c) in ExpPR at a location, denoted as a star in Fig. 1, at 100 m depth. Coherencies between the transport and the SSH (d), between the transport and the temperature (e), and between the transport and the salinity (f). Lag-correlations between the transport and the SSH (g), between the transport and the temperature (h), and between the transport and the salinity (i). to examine the variability that appears in those parameters. The time series of those parameters in ExpPR and ExpPS show similar behavior to that of the transport with some time lags (Fig. 11). The temperature ranges from 13.8 C to 15.6 C, and SSH ranges from 10 cm to 16 cm in ExpPR. The salinity also ranges from 34.0 psu to 34.1 psu. The power density and the coherency are calculated to estimate the correlations between the transport variation and variations of those parameters from ExpPR. The power density of the SSH is concentrated in a 30~50-day period and is relatively high in a 10~15-day period (Fig. 12(a)), as shown in the observed transport (Fig. 2(c)). Furthermore, the coherency between the SSH and the transport is high on 5~15-day and on 30~50-day period (Fig. 12(d)). The power density distributions of the temperature and salinity seem to be similar to that of SSH (Figs. 12(b) and (c), respectively). The coherency is still sig- 4. Conclusions The effects of the transport variations through the KS on the upper layer circulation in the JES have been investigated using numerical experiments based on the observed transport. In experiments where the periodic variation of the inflow transport repeats continuously sinusoidally for several periods, strong SSH variability is detected in the region covering the KS to the Japanese coast, resulting from the geostrophy of the buoyancy forcing at the KS. The SSH variability along the Korean coast is relatively high in the period of 90-day and longer of transport variation. Between the experiment forced by the realistic transport (ExpPR) and the experiment forced by monthly mean transport (ExpPS), the differences of rms of SSH and the differences of EKE at 100 m depth are large along the Japanese coast. On the other hand, the distribution of mean SSH is similar in both experiments. The short-term variation (shorter than seasonal) of the transport stimulates SSH variability of the JES and increases EKE. However, it is not thought to change entire mean fields of SSH and kinetic energy, even though it drives eddy variability to interact with mean current, resulting in a change of EKWC separation latitude. A numerical ocean model, especially a prediction model, should therefore include the realistic transport at the Korea Strait to simulate variabilities of JCB, SBTWC, and EKWC. It is difficult to determine whether or not the large values of rms of SSH and EKE at the east of the YR shown in Figs. 10(b) and (d) are directly related to the transport variation. It was reported that temporal variability of SSH from TOPEX/Poseidon, such as the biennial variability, was high in that region (Hirose and Ostrovskii, 2000; Morimoto and Yanagi, 2001). It is difficult to examine variability of two eastern branches, JCB and SBTWC shown by Hase et al. (1999) using the present model due to its coarse resolution and the annual mean forcings. Since the Japanese coast is the most sensitive to the transport variation, its influence on the JCB and the SBTWC would be detected. The responses of these two branches are expected to be different because the JCB (the SBTWC) has characteristics of barotropic (baroclinic) flow. Hence, a study of their variability needs to be continued by adopting a higher grid resolution and temporal variations of other forcings, including the transport variation. 164 S. Park et al.

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