Impact of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan)

Impact of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Impact of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan) Daehyuk Kim, Hong-Ryeol Shin, Jae-Hong Moon This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4760644/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The Intrinsic variability in the East Sea (Sea of Japan), excluding the influence of external forcings on interannual variation, is distributed mainly in the southern region, especially in the Ulleung and Yamato Basins. In this study, the effects of interannual variations in surface heat flux on upper layer circulation in the East Sea are analyzed via numerical experiments. The interannual variations in the surface heat flux amplify variability in the Yamato Basin, not in the Ulleung Basin. The variability in the water temperature in the northern region is highly correlated with the variability in the surface heat flux with one month time lag. Winter surface cooling facilitates cold water formation in the northern regions, and it extends toward the Yamato Basin. As the cold water region expands (contracts), the meandering (straight) path of the Tsushima Warm Current flows northeastward due to increasing baroclinic instability. However, the surface heat flux does not significantly impact the separation latitude of the East Korea Warm Current. Instead, the separation latitude of the East Korea Warm Current is influenced primarily by strong winter positive wind stress curl anomalies in the northern region and greatly affects the southward flow of cold water along the Korean coast. surface heat flux cold water extension current meandering variability baroclinic instability East Korea Warm Current wind stress curl Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Many oceanographic researchers are interested in how changes in the global climate system affect ocean circulation in the open oceans and regional marginal seas. The ocean, as a component of the Earth’s climate system, is a crucial factor in predicting future climate changes. Therefore, it is important to understand the physical mechanisms underlying the formation and variability of ocean circulation. Many researchers have analyzed the characteristics of ocean circulation under external forcings through numerical experiments in open oceans and regional seas. There is active research underway regarding the influence of surface heat fluxes among external forcings on the upper layer circulation, which is a crucial physical process in ocean-atmosphere interactions. For example, Yu et al. ( 2006 ) suggested that the surface net heat flux is greatly related to the variability in sea surface temperature (SST) in the tropical Atlantic Ocean. The surface heat flux is important in enhancing the predictability of future SST variability. In the Gulf Stream region, the interannual variability in the mixed layer temperature tendency is primarily dominated by surface heat flux (Dong and Kelly 2004 ). In addition, the mixed layer depth (MLD) is determined by turbulent mixing due to heat exchange at the ocean-atmosphere interface, along with wind stress (Kara et al. 2003 ). Lee et al. ( 2015 ) reported that in the North Pacific, except for the Kuroshio region, the contribution of surface heating is more important than heat transport for the heat budget of the MLD. Surface heat flux is essential for analyzing the heat budget and its variability in the upper ocean. The East Sea (Sea of Japan), which was analyzed in this study, is often referred to as a “miniature ocean” due to similarities with open oceans in terms of circulation systems, the subpolar front, the formation of intermediate water, and deep convection systems (Lee et al. 2011 ; Gamo et al. 2014 ). Moreover, it is more efficient to analyze physical processes using numerical models in the East Sea than in the open seas. The variability in the upper layer circulation appears to be much greater in the southern part, especially in the Ulleung and Yamato Basins, than in the northern region because of the Tsushima Warm Current (TWC) meandering or eddy activity (Choi et al. 2018 ; Kim et al. 2024 ). This variability can occur independently of external forcing variations, and Kim et al. ( 2024 ) suggested that intrinsic variability in the East Sea is represented by fluctuations in the strength of the TWC or changes in its main path caused by baroclinic Rossby waves. Based on the intrinsic variability, the interannual variations in external forcings could function in either strengthening or weakening the variability of the upper layer circulation in the East Sea. In the Ulleung Basin, one of the regions with large variability in the upper layer circulation, the variability is mostly influenced by the changes in the separation latitude of the East Korea Warm Current (EKWC) and eddy activity. In the Yamato Basin, the variability is enhanced by boundary conditions influenced by inflows through the Korea/Tushima Strait (KTS) (Choi et al. 2018 ). However, their results also demonstrated that in experiments with interannual variations in atmospheric forcing, the meandering of the EKWC contributes to the variability in the Yamato Basin. Therefore, we need to consider the physical processes through which atmospheric forcing may affect the variability in the Yamato Basin. Seung and Nam ( 1991 ) suggested that winter cooling affects subsurface waters along the Korean coast based on 24 years of hydrographic data. Ito ( 2014 ) showed that strong cooling at the surface results in a wider distribution of cold water at a depth of 100 m than does weak cooling, potentially influencing the northward extent of the EKWC. Shin et al ( 2013 ) reported that the East Sea Intermediate Water (ESIW) with cold and low salinity water, which is defined range of roughly 26.9 < \(\:{\sigma\:}_{\theta\:}\) < 27.3 (Kim and Kim 1999 ; Yoon and Kawamura 2002 ), distributed in the northern region, extended to the Ulleung and Yamato Basins. Furthermore, the expansion and contraction of cold water areas could influence the position of the subpolar front between the boundary regions of cold and warm water (approximately 40°N) and alter the meandering path of the TWC. Surface heat flux is an important external forcing not only for cold water formation in the northern part of the East Sea but also for the branch of the TWC in the southern part (Kim et al. 2020 ). Our purpose was to analyze how interannual variations of surface heat flux influence not only local variations but also the variability in the upper layer circulation throughout the East Sea, particularly in conjunction with the ventilation activation in the northern region. 2. Method Numerical Model and Experimental Designs To analyze the variability in the upper layer circulation in response to interannual variations in surface heat flux, a three-dimensional ocean numerical model, known as the Research Institute for Applied Mechanics Ocean Model (RIAMOM; Lee 1996 ; Kim and Yoon 2010 ; Kim et al. 2020 , 2024 ), was employed. The RIAMOM assuming hydrostatic balance with the Bossinesq approximation, solved the nonlinear, primitive equations on the Arakawa-B grid system and z-coordinate. The model domain covered the East Sea (126.5–142.5°E, 33–52°N; Fig. 1 ), with a horizontal grid resolution of 1/12° in both longitude and latitude, and it included 36 vertical layers. The bottom topography encompassed high-resolution data (1/12°) from a combination of ETOPO5 and Sungkyunkwan University (SKKU) (Choi et al. 2002 ). For the boundary data, water temperature and salinity data were obtained from the Hybrid Coordinate Ocean Model (HYCOM; https://tds.hycom.org/thredds/catalog.html ) reanalysis data, and the volume transport through the KTS was employed for climatological monthly mean data estimated from long-term observation data (Takikawa et al. 2005 ). The outflow volume transport for the Tsugaru and Soya straits was split by 65% and 35%, respectively. The atmospheric forcings, including wind stress and surface heat flux were obtained from the Japanese 55-year Reanalysis dataset (JRA-55; Japan Meteorological Agency 2013 ). Detailed descriptions of the parameters used for experiments can be found in Kim et al. ( 2020 , 2024 ). In this study, two numerical experiments were conducted. The first experiment is intrinsic variability (hereafter Intrinsic-Exp), which included seasonal variations in external forcings without interannual variations. The second experiment (hereafter IV_Qnet) applied interannual variations only to surface heat flux while excluding variations in other forcings such as wind stress and boundary forcing (Table 1 ). Two experiments were conducted for 48 years. For the Control Run, the analysis was conducted using data from the last 15 years after a 33-year spin-up period. In IV_Qnet, the spin-up period was 25 years under the same conditions as those in the Control Run, and then from the 26th year, surface heat flux with interannual variation was applied from 1993 to 2015. Due to a discontinuity between seasonal and interannual forcings, the results from 1993 to 2000 were not used in this study. Consequently, IV_Qnet was analyzed using data from 2001 to 2015. Table 1 Design of numerical experiments depending on the applied external forcings. “Seasonal forcing” means that climatological mean data were applied. Experiment Volume transport Wind stress Surface heat flux Intrinsic-Exp Seasonal forcing Seasonal forcing Seasonal forcing IV_Qnet Seasonal forcing Seasonal forcing Interannual variation IV_Wind Seasonal forcing Interannual variation Seasonal forcing Variability in the surface heat flux When conducting numerical experiments in a marginal sea such as the East Sea, the heat in the East Sea exchanges through the three straits (the KTS, Tsugaru, and Soya Straits) and the heat released into the atmosphere must be balanced. To achieve heat balance in the East Sea, the surface heat flux applied in this study was based on the Barnier type (Barnier et al. 1995 ; Noh et al. 2002 ), which is expressed as follows: $$\:\text{Q}\text{n}\text{e}\text{t}=\:{Q}^{*}+\:\rho\:{C}_{p}\varDelta\:{Z}_{1}({T}^{*}-{T}_{1})/\tau\:$$ 1 where \(\:{Q}^{\text{*}}\) is the surface net heat flux obtained from JRA-55 data, \(\:{\rho\:}\) is the seawater density ( \(\:\text{k}\text{g}\:{m}^{-3}\) ), and \(\:{C}_{p}\) is the specific heat of seawater at constant pressure. \(\:\varDelta\:{Z}_{1}\) is the thickness of the first layer in the RIMAMOM, and the restoring time scale ( \(\:{\tau\:}\) ) is 10 days. The annual mean surface heat flux ( \(\:\text{Q}\text{n}\text{e}\text{t}\) in Eq. ( 1 )) was approximately − 65.39 W m -2 , indicating that heat was released from the ocean to the atmosphere. This was characterized by heat release predominantly in winter and the opposite occurred in summer. The monthly variability in the surface heat flux is over 60 W m -2 in winter, which was greater than that in summer (Fig. 2 a). The temporal variability of the surface heat flux in the northern region can represent the temporal variability of the surface heat flux over the entire East Sea (correlation coefficient: 0.90). In this study, we focused on the interannual variability in winter surface heat flux used in December to February and categorized years into strong cooling ( 280 W m -2 ) for analysis based on approximately 1 standard deviation (Fig. 2 b). For example, in this study, strong cooling (weak cooling) periods were defined in winter 2001, 2006, 2012 and 2013 (2002, 2004, 2007, 2009, and 2015). 3. Results and Discussion Variability in upper layer circulation The main features of upper layer circulation in the East Sea have investigated through observations and numerical experiments in previous studies (Kawabe 1982 ; Yoon 1982a , b , c ; Hase et al. 1999 ; Park et al. 2013 ; Kim et al. 2020 ). One of the main currents, the TWC inflowing through the KTS, bifurcates into two branches: the EKWC flowing northward along the Korean coast and the Nearshore Branch flowing northeastward along the Japanese coast. In addition, a cyclonic gyre appears in the northern part (Fig. 1 ). The model results clearly reproduce distinct features of the upper layer circulation. The distributions of the intrinsic variability (Intrinsic-Exp) were large around meandering and eddy activities regions in the southern part of the East Sea (Fig. 3 a), similar to previous studies (Trusenkova, 2014 ; Choi et al. 2018 ; Kim et al. 2024 ). Based on the Control Run, the IV_Qnet experiment revealed that the variability in upper layer circulation increases, especially in the Yamato Basin, but does not significantly increase in the Ulleung Basin (Fig. 3 b). The surface heat flux plays an important role in the formation of cold water in the northern region. The correlation between the surface heat flux and nonseasonal SST, with a 1-month time lag in the northern region was approximately 0.44. This means that the effects of the surface heat flux on the upper ocean have a time lag of approximately one month. Therefore, to analyze the hydrographic conditions of the upper ocean caused by surface heat flux, winter data such as temperature and density data, were averaged from January to March. It is necessary to consider how interannual variations in the winter surface heat flux in the northern region could influence the variability in the upper layer circulation in the southern region through which physical processes occur. The surface heat flux not only contributes to cold water formation in the northern region but also enhances ventilation. Consequently, the planetary (topographic) beta effects strengthen (weaken), leading to the formation of the EKWC, one of the TWC branches (Kim et al. 2020 ). Considering to the role of surface heat flux in the East Sea based on previous researches, two possibilities can be considered for inducing variability in the upper layer circulation due to interannual variation of the surface heat flux: 1) the changes in the meandering path due to the formation and expansion of the cold water in the northern region and 2) how it affects the separation latitude of the EKWC, although the variability in the Ulleung Basin did not increase significantly because of interannual variations in the surface heat flux. As mentioned earlier, the water temperature and density at a depth of 100 m were composited for periods of strong and weak cooling periods, and the spatial distributions during these periods were analyzed (Fig. 4 ). During strong cooling in winter, cold water corresponding to temperatures less than 1°C was distributed over the Japan Basin at a depth of 100 m. Furthermore, the cold water extended to the entrance of the Yamato Basin, leading to meandering of the TWC and development of the subpolar front between the southern and northern region. In contrast, during weak cooling, cold water was limited to the eastern Japan Basin. When the 6°C isotherms (white lines in Fig. 4 a, b), which represent the northern boundary of the main TWC at a depth of 100 m, with strong cooling and weak cooling conditions were compared, significant differences in the meandering phase of the TWC appeared in the Yamato Basin. In the water density distribution at a depth of 100 m, the 27.13 \(\:{\sigma\:}_{\theta\:}\) isopycnal, corresponding to the center of the ESIW, extended distinctly into the Yamato Basin (white dashed line in Fig. 4 c, d). In these cases, the buoyancy frequency ( \(\:N=\:\sqrt{-(g/{\rho\:}_{0})(\:\partial\:\rho\:/\partial\:z)}\) ) at point A´ peaks at approximately 80 m depth under strong cooling conditions, which is approximately 50 m shallower than that under weak cooling conditions (Fig. 5 ). This indicates that, under strong cooling conditions, the cold water widely formed in the northern part extends into the southern part and penetrates beneath the warm TWC at relatively shallower depths compared to weak cooling conditions. At shallow depths, cold water subduction can induce baroclinic instability. The intensity of baroclinic instability can be estimated by the eddy growth rate (Stammer 1998 ; Zhai et al. 2008 ), which is expressed as follows: $$\:EGR\:\left(eddy\:growth\:rate\right)=\:\frac{f}{N}\:\frac{dU}{dz}$$ 2 where \(\:f\) and \(\:N\) indicate the Coriolis parameter and the buoyancy frequency, respectively. \(\:du/dz\) represents vertical shear. In this study, monthly mean EGR was calculated from the sea surface to a depth of 100 m in the Yamato Basin (Fig. 6 ), where the large variability in the upper layer circulation appears to be due to the extension of the cold water in the northern region. At this time, the EGR during strong cooling periods was greater than that during weak cooling periods, especially in February. Qiu and Miao ( 2000 ) suggested that variations in the Kuroshio path were influenced by baroclinic instability leading to the development of meandering system. In the East Sea, increased baroclinic instability led to the development of the meandering TWC path (Fig. 6 ). However, over time, the difference in the EGR between the strong and weak cooling periods decreased gradually. By August, the EGR was similar in both cases, and the main path of the TWC was similar. Another possibility for increasing variability in the southern part of the East Sea is variability in the separation latitude of the EKWC due to interannual variation in surface heat flux. Previous studies have analyzed numerical experiments on wind forcings as the major atmospheric forcings affecting the separation latitude of the EKWC (Kim and Yoon, 1996 ; Yoon et al. 2005 ; Kim et al. 2020 ). However, this study focused on the surface heat flux, highlighted as a significant external forcing influencing the formation of the EKWC, as demonstrated by Kim et al. ( 2020 ). We analyzed how interannual variation in surface heat flux affects variability in upper layer circulation, especially the separation latitude of the EKWC. Seung and Nam ( 1991 ) and Ito ( 2014 ) reported that during strong cooling, cold water in the northern part of the East Sea extended southward along the Korean coast, influencing the separation latitude of the EKWC. However, in the results from IV_Qnet, during strong cooling, the ESIW in the northern region was widely distributed and the cyclonic eddy called the Dok cold eddy is simulated west of the Yamato Rise, but the separation latitude of the EKWC is not significantly different from that under weak cooling conditions (Fig. 4 and Fig. 6 b, c). Because Ito ( 2014 ) analyzed hydrographic conditions under strong and weak winter cooling conditions using observational data, it was difficult to separate the signals related to wind and surface heat flux independently. Combining these numerical experiments with previous research, it is inferred that only surface heat flux does not influence the separation latitude of the EKWC. However, if a positive wind stress curl acts along with strong winter cooling conditions, cold water in the northern part of the East Sea could extend to the Korean coast, thereby affecting the separation latitude of the EKWC. Distribution of the temperature from the observation data The characteristics of the hydrographic conditions in the southern part of the East Sea, depending on the strong cooling and weak cooling periods selected from JRA-55, were analyzed via ship observations from the National Institute of Fisheries Science (NIFS) and Japan Meteorological Agency (JMA) (Fig. 7 ). Owing to the rarity of both institutions observing data concurrently, we defined winter as the months of January, February, and March. Since the characteristics of the hydrographic conditions derived from observation data reflect the combined influence of both surface heat flux and wind stress, analyzing the effects of only surface heat flux is difficult. Nevertheless, even in observational data, during strong cooling periods, the 2°C isotherm extends as far south as 40°N near the Yamato Basin. In contrast, during weak cooling periods, cold water does not extend southward beyond 40°N. According to observational data, the cold water extending southward along the Korean coast also expanded much further south during strong cooling periods than during weak cooling periods. These results are consistent with those of previous studies (Seung and Nam 1991 ; Ito 2014 ). As mentioned earlier, cold water extending southward along the Korean coast was considered to be influenced not only by surface heat flux but also significantly by wind stress. Therefore, in the next section, we discuss how wind stress affects the separation latitude of the EKWC. Wind stress response to the latitude of the EKWC According to the results of IV_Qnet in this study, the variability in the separation latitude of the EKWC seems to be unrelated to the interannual variation in the surface heat flux. Although cold water forms in the northern part of the East Sea during strong winter cooling, if the cyclonic gyre does not develop significantly, cold water cannot expand southward along the Korean coast. Therefore, to analyze the relationship between the interannual variation in wind stress and the separation latitude of the EKWC, an additional experiment, which applied the interannual variation in wind stress (IV_Wind), was conducted. Similar to the IV_Qnet experiment, during the analysis period, winter wind stress curls exceeding 1 standard deviation were classified as strong and weak positive wind stress curl conditions. The time series of the winter wind stress curl in the northern part of the East Sea is similar to that in the entire East Sea (not shown here). From winter to summer, under strong positive wind stress conditions, cold water below 2°C was widely distributed at a depth of 100 m, and the separation latitude of the EKWC was farther south than that under weak positive wind stress conditions (Fig. 8 ). The difference in the separation latitude of the EKWC between the two conditions distinctly appears in summer. In the vertical structures along the 130°E line in August, during strong positive wind stress conditions, the thickness between 1°C and 2°C isotherms is prominently thickened and the subpolar front is located further south, because cold water in the northern part flows southward in the subsurface (Fig. 9 ). In other words, when the upper layer circulation in the northern part of the East Sea strengthens due to a positive wind stress curl rather than strong cooling conditions, the 2°C isotherm in the northern part prominently extends further southwestward. At a result, the northward flow of the EKWC was impeded, but the isotherm was not clearly different in the Yamato Basin. 4. Conclusions The total variability of the upper layer circulation in the East Sea consists of intrinsic variability and forced variability. In the East Sea, the variability of the upper layer was largely distributed in the southern region, especially in the Ulleung and Yamato Basins (Trusenkova, 2014 ; Choi et al. 2018 ; Kim et al. 2024 ). This study analyzed how interannual variations in surface heat flux, which play an important role in the TWC branches, affect the variability in upper layer circulation in the East Sea through physical processes. In the experiment (IV_Qnet) with only interannual variations in the surface heat flux, the variability was greater in the Yamato Basin and along the coast of Russia and off Vladivostok than the intrinsic variability (Intrinsic-Exp). The annual mean surface heat flux in the East Sea releases heat from the ocean to the atmosphere, with dominant cooling especially noticeable during winter. Under strong winter cooling, cold water formation in the northern region was facilitated, and the isotherms exhibited a trough-like shape in the Yamato Basin due to the southward extension of cold water. In contrast, weakening of southward cold water in the Yamato Basin resulted in the TWC flowing northeastward with a mainly straight path rather than a meandering path. In other words, it is inferred that the variability due to interannual variation in the surface heat flux was greater than the intrinsic variability in the Yamato Basin because the phase of the TWC main path was altered depending on the winter cooling conditions. In observational data, during strong winter cooling conditions, cold water also extends toward the Yamato Basin, and the subpolar front moves southward and becomes more pronounced. In other words, interannual variations in the surface heat flux affect local changes in the SST and the TWC meandering path due to increasing baroclinic instability. However, the variability in the separation latitude of the EKWC was not directly related to only the winter cooling conditions. Instead, the interannual variation in the wind stress curl rather than the surface heat flux in winter distinctly influenced the northward latitudinal variations in the EKWC. In IV_Wind, because the cold water that formed in the northern region extended southwestward, the separation latitude of the EKWC and thickness of the ESIW (between 1°C and 2°C isotherms) were influenced. In the real ocean, the combined effects of two external forcings (the wind stress curl and surface heat flux) influence upper layer circulation through more complex physical processes. However, during the analysis period of this study, there are insufficient examples to analyze the variability in upper layer circulation by categorizing cases where the expansion of cold water is in-phase and/or out-of-phase due to these external forcings. Therefore, to analyze the effects of these external forcings on the variability of upper layer circulation in the East Sea, planning and analyzing new experimental methods are needed. Declarations Acknowledgments . This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (NRF-2022M3I6A1086449) and the Korea Institute of Marine Science and Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (KIMST-20220033). The main calculations were performed by using the supercomputing resource of the Korea Meteorological Administration (National Center for Meteorological Supercomputer). Availability of data and materials The data analyzed in this study are available from public websites. The observation data (temperature) are freely available online at National Institute of Fisheries Science (NIFS; https://www.nifs.go.kr/kodc/index.kodc) and Japan Meteorological Agency (JMA; https://www.data.jma.go.jp/gmd/kaiyou/db/vessel_obs/data-report/html/ship/ship_e.php). For the atmospheric external forcings, JRA-55 reanalysis data were provided by the NCAR/UCAR Research Data Archive (accessible via http://rda.ucar. edu/ under dataset number ds628.0). Author contributions D Kim conceived the idea, performed the analysis, and wrote the manuscript. HR Shin supported the idealized experiments and HR Shin and JH Moon continuously discussed the results. All authors participated in discussions during this study and contributed to writing and editing of the article. Competing interests The authors declare that they have no competing interests. References Barnier B, Siefridt L, Marchesiello P (1995) Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analysis. J Mar Syst 6:363-380. https://doi.org/10.1016/0924-7963(94)00034-9 Choi BH, Kim KO, Eum HM (2002) Digital bathymetric and topographic data for neighboring seas of Korea. J Korean Soc Coastal Ocean Eng 14:41–50. Choi BJ, Cho SH, Jung HS, Lee SH, Byun DS, Kwon K (2018) Interannual variation of surface circulation in the Japan/East Sea due to external forcings and intrinsic variability. Ocean Sci J 53 (1):1-16. https://doi.org/10.1007/s12601-017-0058-8 Dong S, Kelly KA (2004) Heat budget in the Gulf Stream region: the importance of heat storage and advection. J Phys Oceanogr 34(5):1214-1231. https://doi.org/10.1175/1520-0485(2004)0342.0.CO;2 Gamo T, Nakayama N, Takahata N, Sano Y, Zhang J, Yamazaki E, Taniyasu S, Yamashita N, (2014) The Sea of Japan and its unique chemistry revealed by time-series observations over the last 30 years. Monogr Environ Earth Planets 2(1):1-22. https://doi.org/10.5047/meep.2014.00201.0001 Hase H, Yoon JH, Koterayama W (1999) The current structure of the Tsushima Warm Current along the Japanese coast. J Oceanogra 55:217-235. https://doi.org/10.1023/A:1007894030095 Ito M (2014) Seasonal and interannual variations of the Tsushima Warm Current paths in the southwestern part of the Japan Sea. Dissertation, Nagoya University. Japan Meteorological Agency (2013) Japanese 55-year Reanalysis, daily 3-hourly and 6-hourly data. National Center for Atmospheric Research Computational and Information Systems Laboratory Research Data Archive, https://doi.org/10.5065/D6HH6H41 Kara AB, Rochford PA, Hurlburt HE (2003) Mixed layer depth variability over the global ocean. J Geophys Res:Oceans 108:3079. https://doi.org/10.1029/ 2000JC000736 Kawabe M (1982) Branching of the Tsushima Current in the Japan Sea, Part I. Data analysis. J Oceanogr Soc Japan 38:95–107. https://doi.org/10.1007/BF02110295 Kim CH, Yoon JH (1996) Modeling of the wind-driven circulation in the Japan Sea using a reduced gravity model. J Oceanogr 52:359-373. https://doi.org/10.1007/BF02235930 Kim D, Shin HR, Kim CH, Hirose N (2020) Characteristics of the East Sea (Japan Sea) circulation depending on surface heat flux and its effect on branching of the Tsushima Warm Current. Con Shelf Res 192:104025. https://doi.org/10.1016/j.csr.2019.104025 Kim D, Shin HR, Kim CH, Kim J, Hirose N (2024) Intrinsic low-frequency variability in the upper-layer circulation of the East Sea (Sea of Japan). J Phys Oceanogr 54(3):697-716. https://doi.org/10.1175/JPO-D-23-0030.1 Kim YG, Kim K (1999) Intermediate waters in the East/Japan Sea, J Oceanogr 55:123-132. https://doi.org/10.1023/A:1007877610531 Kim T, Yoon JH (2010) Seasonal variation of upper layer circulation in the northern part of the East/Japan Sea. Con Shelf Res 30:1283-1301. https://doi.org/10.1016/j.csr.2010.04.006 Lee E, Noh Y, Qiu B, Yeh SW (2015) Seasonal variation of the upper ocean responding to surface heating in the North Pacific. J Geophy Res:Ocean 120:5631-5647. https://doi.org/10.1002/2015JC010800 Lee HC (1996) A numerical simulation for the water masses and circulations of the Yellow Sea and East China Sea. Dissertation, Kyushu University. Lee HJ, Park JH, Wimbush M (2011) Tidal effects on intermediate waters: A case study in the East/Japan Sea. J Phys Oceanogr 41:234-240. https://doi.org/10.1175/2010JPO4510.1 Noh Y, Jang CJ, Yamagata T. Chu PC, Kim CH (2002) Simulation of more realistic upper-ocean processes from an OGCM with a new ocean mixed layer model. J Phys Oceanogr 32:1284-1307. https://doi.org/10.1175/1520-0485(2002)0322.0.CO;2 Park KA, Park JE, Choi BJ, Byun DS, Lee EI (2013) An oceanic current map of the East Sea for science textbooks based on scientific knowledge acquired from oceanic measurements (in Korean with English abstract). J Korean Soc Oceanogr 18(4):234-265, https://doi.org/10.7850/jkso.2013.18.4.234 Qiu B, Miao W (2000) Kuroshio path variations south of Japan: Bimodality as a self-sustained internal oscillation. J Phys Oceanogr 30(8):2124-2137. https://doi.org/10.1175/1520-0485(2000)0302.0.CO;2 Seung YH, Nam SY (1991) Effects of winter cooling on subsurface hydrographic conditions off Korean coast In the East (Japan) Sea. In: Takano K.(ed.) Oceanography of Asian Marginal Seas. Ed.Takano K. Tokyo, p.163-178. Shin CW, Byun SK, Kim C, Lee JH, Kim BC, Hwang SC, Seung YH, Shin HR (2013) Seasonal variations in the low-salinity intermediate water in the region south of subpolar front of the East Sea (Sea of Japan). Ocean Sci J 48(1):35-47. https://doi.org/10.1007/s12601-013-0003-4 Stammer D (1998) On eddy characteristics, eddy transports, and mean flow properties. J Phy Oceanogr 28(4):727-739. https://doi.org/10.1175/1520-0485(1998)0282.0.CO;2 Takikawa T, Yoon JH, Cho KD (2005) The Tsushima Warm Current through Tsushima Straits estimated from ferryboat. J Phys Oceanogr 35:1154–1168. https://doi.org/10.1175/JPO2742.1 Trusenkova OO (2014) Variability of eddy kinetic energy in the Sea of Japan from satellite altimetry data. Oceanology 54(1):8-16. https://doi.org/10.1134/S0001437014010111 Yoon JH (1982a) Numerical experiment on the circulation in the Japan Sea, Part I. Formation of the East Korean Warm Current. J Oceanogr Soc Japan 38:43–51. https://doi.org/10.1007/BF02110289 Yoon JH (1982b) Numerical experiment on the circulation in the Japan Sea, Part II. Influence of seasonal variation in atmospheric conditions on the Tsushima current. J Oceanogr Soc Japan 38:81–94. https://doi.org/10.1007/BF02110294 Yoon JH (1982c) Numerical experiment on the circulation in the Japan Sea. Part III: mechanism of the nearshore branch of the Tsushima Current. J Oceanogr Soc Japan 38:125–130. https://doi.org/10.1007/BF02110283 Yoon JH, Abe K, Ogata T, Wakamatsu Y. (2005) The effects of wind-stress curl on the Japan/East Sea circulation. Deep Sea Res II 52:1827-1844. https://doi.org/10.1016/j.dsr2.2004.03.004 Yoon JH, Kawamura H (2002) The formation and circulation of the Intermediate Water in the Japan Sea. J Oceanogr 58:197-211. https://doi.org/10.1023/A:1015893104998 Yu L, Jin Z, Weller RA (2006) Role of net surface heat flux in seasonal variations sea surface temperature in the tropical Atlantic Ocean. J Clm 19(23):6153-6169. https://doi.org/10.1175/JCLI3970.1 Zhai X, Greatbatch R.J., Kohlmann JD (2008) On the seasonal variability of eddy kinetic energy in the Gulf Stream region. Geophy Res Lett 35:L24609. https://doi.org/10.1029/2008GL036412 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4760644","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":333983002,"identity":"a179a4bd-15fc-4462-b73c-f0245a5e5206","order_by":0,"name":"Daehyuk Kim","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA9UlEQVRIiWNgGAWjYDCCAwxsQDJBTl7++MEHQBYPH7FajA1n8CQbgLSwEaslseEGg5kESICgFr7jh4895m1LY2yc3ZBW+TXHToaNgfnhoxt4tEieSUs35m3LYWaXOXjstuy2ZKDD2IyNc/BoMTiQYybN21bBxtiQkHZbchszUAsPmzReLefffwNp4WE4kGBWLLmtnggtN3LYgFpyJBhuJJgxftx2mLAWyRvPzCTnnEszMOw5kyzNuO04DxszAb/wnU9+JvGmLLl+Pnv7wY8/t1Xb87M3P3yMTwsIMPFAGcxgBjMB5SDA+AOdMQpGwSgYBaMAGQAAB0pJTk/oIWQAAAAASUVORK5CYII=","orcid":"","institution":"Center for Sea Level Changes, Jeju National University","correspondingAuthor":true,"prefix":"","firstName":"Daehyuk","middleName":"","lastName":"Kim","suffix":""},{"id":333983003,"identity":"ee15e9c7-2c5c-4418-976b-b22ea78e707e","order_by":1,"name":"Hong-Ryeol Shin","email":"","orcid":"","institution":"Kongju National University","correspondingAuthor":false,"prefix":"","firstName":"Hong-Ryeol","middleName":"","lastName":"Shin","suffix":""},{"id":333983004,"identity":"80ed23e8-c6d4-4574-94e1-2016c8e8f2c2","order_by":2,"name":"Jae-Hong Moon","email":"","orcid":"","institution":"Jeju National University","correspondingAuthor":false,"prefix":"","firstName":"Jae-Hong","middleName":"","lastName":"Moon","suffix":""}],"badges":[],"createdAt":"2024-07-18 07:18:53","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4760644/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4760644/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":62096901,"identity":"7268491c-ef76-4bbe-81bc-104199243565","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":169907,"visible":true,"origin":"","legend":"\u003cp\u003eBathymetric map and schematic surface current in the East Sea (Sea of Japan). KTS: Korea/Tsushima Strait, TS: Tsugaru Strait, SS: Soya Strait, UB: Ulleung Basin, YB: Yamato Basin, JB: Japan Basin. NB: Nearshore Branch, EKWC: East Korea Warm Current, OB: Offshore Branch, LCC: Liman Cold Current, NKCC: North Korea Cold Current.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/aca221427f5e75e911b53bcc.png"},{"id":62097378,"identity":"6abb16e2-5723-4d09-bbf5-8430041a87a5","added_by":"auto","created_at":"2024-08-09 09:01:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":77139,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Monthly variability in the surface heat flux and (b) time series of the surface heat flux in winter. The error bars indicate one standard deviation (σ). The dashed line represents the average winter surface heat flux. Strong and weak cooling periods were classified as periods exceeding approximately ±1σ from the mean. The blue and red triangles indicate strong and weak cooling periods, respectively, in this study.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/a25b801213bd2bf262ad4ace.png"},{"id":62096896,"identity":"16824d9a-395b-45fb-a55a-611197a4d2b0","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":266334,"visible":true,"origin":"","legend":"\u003cp\u003eVariability of the sea surface height (SSH; units: cm) for 15 years in (a) Intrinsic-Exp and (b) IV_Qnet. The white line indicates the depth of the bottom topography in the East Sea. Black rectangular box indicates regions with especially increasing SSH variability in IV_Qnet.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/98535896fc55fe5bada73b7d.png"},{"id":62096897,"identity":"3feba14a-3ce4-4bc7-9645-f1120cff31a0","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":525201,"visible":true,"origin":"","legend":"\u003cp\u003eDistributions of the composite (top) temperature and (bottom) density in winter at a depth of 100 m in (a, c) strong (\u0026lt; 330 W m\u003csup\u003e-2\u003c/sup\u003e) and (b, d) weak (\u0026gt; 280 W m\u003csup\u003e-2\u003c/sup\u003e) cooling periods. The white line in the composite water temperature plot indicates the 6°C isotherm (top panel), and the white dashed and solid lines represent 23.13 \u0026nbsp;and 27.3 isopycnals, respectively. Buoyancy frequency is calculated at point A´.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/35c824249fe1c53e5ba4d501.png"},{"id":62096900,"identity":"88ed719d-2c4b-4403-8683-0a4c487ce299","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":57488,"visible":true,"origin":"","legend":"\u003cp\u003eVertical structures of the winter buoyancy frequency (×10\u003csup\u003e-3 \u003c/sup\u003es\u003csup\u003e-1\u003c/sup\u003e) at point A´ (the black dot in Fig. 4, 135.5°E, 38.92°N) in IV_Qnet. The blue and red lines indicate the buoyancy frequency during the strong and weak cooling periods, respectively.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/3c88190b6656d725b22eb351.png"},{"id":62096898,"identity":"95a5bd9e-3c92-4423-a1f1-0930c9edb809","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":127328,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Monthly eddy growth rate (day\u003csup\u003e-1\u003c/sup\u003e) in the Yamato Basin (134­­­­­­­°E –137°E, 37°N –40°N) and composite 6°C isotherm at a depth of 100 m in strong and weak cooling years in (b) February and (c) August in IV_Qnet. The black rectangular box indicates regions with increasing variability in upper layer circulation when interannual variations in the surface heat flux are applied.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/5316992e7541958bef8edf13.png"},{"id":62097379,"identity":"dc784bb9-7ca5-4871-91a0-f46016d49147","added_by":"auto","created_at":"2024-08-09 09:01:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":204505,"visible":true,"origin":"","legend":"\u003cp\u003eDistributions of temperature observed from the NIFS (National Institute of Fisheries Sciences) and JMA (Japan Meteorological Agency) during (a) strong and weak (b) cooling in winter. The thick black line indicates 2°C isotherm line.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/fc25493eeb89a9dd4e832a73.png"},{"id":62096904,"identity":"f4890880-80d7-4444-acf4-1789d2d54e5a","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":528835,"visible":true,"origin":"","legend":"\u003cp\u003eDistributions of the composite water temperature at a depth of 100 m during positive (red) and negative (blue) wind stress curl anomalies periods in (a) February, (b) April, (c) June, and (d) August in IV_Wind. The lines and shaded regions indicate the 10°C isotherm and cold water region (\u0026lt; 2°C), respectively.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/49d7044c1839048c34ed60d2.png"},{"id":62096902,"identity":"4a3784d0-4739-4c00-8590-58b70b7203ee","added_by":"auto","created_at":"2024-08-09 08:53:15","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":213338,"visible":true,"origin":"","legend":"\u003cp\u003eVertical structures of the composite water temperature along 130°E during (a) positive and (b) negative wind stress curl anomalies in August in IV_Wind. The white dashed and solid lines indicate 2°C and 1°C isotherm, respectively.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/88ab6058dff2ee032f30e346.png"},{"id":74990383,"identity":"af3d1b5a-99af-48fa-8c3b-0e602b04c2a5","added_by":"auto","created_at":"2025-01-29 07:16:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2597136,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4760644/v1/9ad8501d-4080-4052-959a-642c1356b1b3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Impact of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan)","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMany oceanographic researchers are interested in how changes in the global climate system affect ocean circulation in the open oceans and regional marginal seas. The ocean, as a component of the Earth\u0026rsquo;s climate system, is a crucial factor in predicting future climate changes. Therefore, it is important to understand the physical mechanisms underlying the formation and variability of ocean circulation.\u003c/p\u003e \u003cp\u003eMany researchers have analyzed the characteristics of ocean circulation under external forcings through numerical experiments in open oceans and regional seas. There is active research underway regarding the influence of surface heat fluxes among external forcings on the upper layer circulation, which is a crucial physical process in ocean-atmosphere interactions. For example, Yu et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) suggested that the surface net heat flux is greatly related to the variability in sea surface temperature (SST) in the tropical Atlantic Ocean. The surface heat flux is important in enhancing the predictability of future SST variability. In the Gulf Stream region, the interannual variability in the mixed layer temperature tendency is primarily dominated by surface heat flux (Dong and Kelly \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). In addition, the mixed layer depth (MLD) is determined by turbulent mixing due to heat exchange at the ocean-atmosphere interface, along with wind stress (Kara et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Lee et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) reported that in the North Pacific, except for the Kuroshio region, the contribution of surface heating is more important than heat transport for the heat budget of the MLD. Surface heat flux is essential for analyzing the heat budget and its variability in the upper ocean.\u003c/p\u003e \u003cp\u003eThe East Sea (Sea of Japan), which was analyzed in this study, is often referred to as a \u0026ldquo;miniature ocean\u0026rdquo; due to similarities with open oceans in terms of circulation systems, the subpolar front, the formation of intermediate water, and deep convection systems (Lee et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Gamo et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Moreover, it is more efficient to analyze physical processes using numerical models in the East Sea than in the open seas. The variability in the upper layer circulation appears to be much greater in the southern part, especially in the Ulleung and Yamato Basins, than in the northern region because of the Tsushima Warm Current (TWC) meandering or eddy activity (Choi et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). This variability can occur independently of external forcing variations, and Kim et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) suggested that intrinsic variability in the East Sea is represented by fluctuations in the strength of the TWC or changes in its main path caused by baroclinic Rossby waves. Based on the intrinsic variability, the interannual variations in external forcings could function in either strengthening or weakening the variability of the upper layer circulation in the East Sea.\u003c/p\u003e \u003cp\u003eIn the Ulleung Basin, one of the regions with large variability in the upper layer circulation, the variability is mostly influenced by the changes in the separation latitude of the East Korea Warm Current (EKWC) and eddy activity. In the Yamato Basin, the variability is enhanced by boundary conditions influenced by inflows through the Korea/Tushima Strait (KTS) (Choi et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). However, their results also demonstrated that in experiments with interannual variations in atmospheric forcing, the meandering of the EKWC contributes to the variability in the Yamato Basin. Therefore, we need to consider the physical processes through which atmospheric forcing may affect the variability in the Yamato Basin. Seung and Nam (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1991\u003c/span\u003e) suggested that winter cooling affects subsurface waters along the Korean coast based on 24 years of hydrographic data. Ito (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) showed that strong cooling at the surface results in a wider distribution of cold water at a depth of 100 m than does weak cooling, potentially influencing the northward extent of the EKWC. Shin et al (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) reported that the East Sea Intermediate Water (ESIW) with cold and low salinity water, which is defined range of roughly 26.9 \u0026lt; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{\\theta\\:}\\)\u003c/span\u003e\u003c/span\u003e \u0026lt; 27.3 (Kim and Kim \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Yoon and Kawamura \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), distributed in the northern region, extended to the Ulleung and Yamato Basins. Furthermore, the expansion and contraction of cold water areas could influence the position of the subpolar front between the boundary regions of cold and warm water (approximately 40\u0026deg;N) and alter the meandering path of the TWC. Surface heat flux is an important external forcing not only for cold water formation in the northern part of the East Sea but also for the branch of the TWC in the southern part (Kim et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Our purpose was to analyze how interannual variations of surface heat flux influence not only local variations but also the variability in the upper layer circulation throughout the East Sea, particularly in conjunction with the ventilation activation in the northern region.\u003c/p\u003e"},{"header":"2. Method","content":"\u003cp\u003e \u003cb\u003eNumerical Model and Experimental Designs\u003c/b\u003e \u003c/p\u003e \u003cp\u003eTo analyze the variability in the upper layer circulation in response to interannual variations in surface heat flux, a three-dimensional ocean numerical model, known as the Research Institute for Applied Mechanics Ocean Model (RIAMOM; Lee \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Kim and Yoon \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), was employed. The RIAMOM assuming hydrostatic balance with the Bossinesq approximation, solved the nonlinear, primitive equations on the Arakawa-B grid system and z-coordinate. The model domain covered the East Sea (126.5\u0026ndash;142.5\u0026deg;E, 33\u0026ndash;52\u0026deg;N; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), with a horizontal grid resolution of 1/12\u0026deg; in both longitude and latitude, and it included 36 vertical layers. The bottom topography encompassed high-resolution data (1/12\u0026deg;) from a combination of ETOPO5 and Sungkyunkwan University (SKKU) (Choi et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). For the boundary data, water temperature and salinity data were obtained from the Hybrid Coordinate Ocean Model (HYCOM; \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://tds.hycom.org/thredds/catalog.html\u003c/span\u003e\u003cspan address=\"https://tds.hycom.org/thredds/catalog.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) reanalysis data, and the volume transport through the KTS was employed for climatological monthly mean data estimated from long-term observation data (Takikawa et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The outflow volume transport for the Tsugaru and Soya straits was split by 65% and 35%, respectively. The atmospheric forcings, including wind stress and surface heat flux were obtained from the Japanese 55-year Reanalysis dataset (JRA-55; Japan Meteorological Agency \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Detailed descriptions of the parameters used for experiments can be found in Kim et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this study, two numerical experiments were conducted. The first experiment is intrinsic variability (hereafter Intrinsic-Exp), which included seasonal variations in external forcings without interannual variations. The second experiment (hereafter IV_Qnet) applied interannual variations only to surface heat flux while excluding variations in other forcings such as wind stress and boundary forcing (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Two experiments were conducted for 48 years. For the Control Run, the analysis was conducted using data from the last 15 years after a 33-year spin-up period. In IV_Qnet, the spin-up period was 25 years under the same conditions as those in the Control Run, and then from the 26th year, surface heat flux with interannual variation was applied from 1993 to 2015. Due to a discontinuity between seasonal and interannual forcings, the results from 1993 to 2000 were not used in this study. Consequently, IV_Qnet was analyzed using data from 2001 to 2015.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDesign of numerical experiments depending on the applied external forcings. \u0026ldquo;Seasonal forcing\u0026rdquo; means that climatological mean data were applied.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExperiment\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eVolume transport\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eWind stress\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSurface heat flux\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eIntrinsic-Exp\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eIV_Qnet\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003eInterannual variation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eIV_Wind\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eInterannual variation\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSeasonal forcing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eVariability in the surface heat flux\u003c/b\u003e \u003c/p\u003e \u003cp\u003eWhen conducting numerical experiments in a marginal sea such as the East Sea, the heat in the East Sea exchanges through the three straits (the KTS, Tsugaru, and Soya Straits) and the heat released into the atmosphere must be balanced. To achieve heat balance in the East Sea, the surface heat flux applied in this study was based on the Barnier type (Barnier et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Noh et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), which is expressed as follows:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\text{Q}\\text{n}\\text{e}\\text{t}=\\:{Q}^{*}+\\:\\rho\\:{C}_{p}\\varDelta\\:{Z}_{1}({T}^{*}-{T}_{1})/\\tau\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Q}^{\\text{*}}\\)\u003c/span\u003e\u003c/span\u003e is the surface net heat flux obtained from JRA-55 data, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}\\)\u003c/span\u003e\u003c/span\u003e is the seawater density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{k}\\text{g}\\:{m}^{-3}\\)\u003c/span\u003e\u003c/span\u003e), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{p}\\)\u003c/span\u003e\u003c/span\u003e is the specific heat of seawater at constant pressure. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{Z}_{1}\\)\u003c/span\u003e\u003c/span\u003e is the thickness of the first layer in the RIMAMOM, and the restoring time scale (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\tau\\:}\\)\u003c/span\u003e\u003c/span\u003e) is 10 days. The annual mean surface heat flux (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{Q}\\text{n}\\text{e}\\text{t}\\)\u003c/span\u003e\u003c/span\u003e in Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)) was approximately \u0026minus;\u0026thinsp;65.39 W m\u003csup\u003e-2\u003c/sup\u003e, indicating that heat was released from the ocean to the atmosphere. This was characterized by heat release predominantly in winter and the opposite occurred in summer. The monthly variability in the surface heat flux is over 60 W m\u003csup\u003e-2\u003c/sup\u003e in winter, which was greater than that in summer (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The temporal variability of the surface heat flux in the northern region can represent the temporal variability of the surface heat flux over the entire East Sea (correlation coefficient: 0.90). In this study, we focused on the interannual variability in winter surface heat flux used in December to February and categorized years into strong cooling (\u0026lt; -330 W m\u003csup\u003e-2\u003c/sup\u003e) and weak cooling (\u0026gt;\u0026thinsp;280 W m\u003csup\u003e-2\u003c/sup\u003e) for analysis based on approximately 1 standard deviation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). For example, in this study, strong cooling (weak cooling) periods were defined in winter 2001, 2006, 2012 and 2013 (2002, 2004, 2007, 2009, and 2015).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e \u003cb\u003eVariability in upper layer circulation\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe main features of upper layer circulation in the East Sea have investigated through observations and numerical experiments in previous studies (Kawabe \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Yoon \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1982a\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003eb\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003ec\u003c/span\u003e; Hase et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Park et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). One of the main currents, the TWC inflowing through the KTS, bifurcates into two branches: the EKWC flowing northward along the Korean coast and the Nearshore Branch flowing northeastward along the Japanese coast. In addition, a cyclonic gyre appears in the northern part (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The model results clearly reproduce distinct features of the upper layer circulation. The distributions of the intrinsic variability (Intrinsic-Exp) were large around meandering and eddy activities regions in the southern part of the East Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), similar to previous studies (Trusenkova, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Choi et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Based on the Control Run, the IV_Qnet experiment revealed that the variability in upper layer circulation increases, especially in the Yamato Basin, but does not significantly increase in the Ulleung Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The surface heat flux plays an important role in the formation of cold water in the northern region. The correlation between the surface heat flux and nonseasonal SST, with a 1-month time lag in the northern region was approximately 0.44. This means that the effects of the surface heat flux on the upper ocean have a time lag of approximately one month. Therefore, to analyze the hydrographic conditions of the upper ocean caused by surface heat flux, winter data such as temperature and density data, were averaged from January to March.\u003c/p\u003e \u003cp\u003eIt is necessary to consider how interannual variations in the winter surface heat flux in the northern region could influence the variability in the upper layer circulation in the southern region through which physical processes occur. The surface heat flux not only contributes to cold water formation in the northern region but also enhances ventilation. Consequently, the planetary (topographic) beta effects strengthen (weaken), leading to the formation of the EKWC, one of the TWC branches (Kim et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Considering to the role of surface heat flux in the East Sea based on previous researches, two possibilities can be considered for inducing variability in the upper layer circulation due to interannual variation of the surface heat flux: 1) the changes in the meandering path due to the formation and expansion of the cold water in the northern region and 2) how it affects the separation latitude of the EKWC, although the variability in the Ulleung Basin did not increase significantly because of interannual variations in the surface heat flux.\u003c/p\u003e \u003cp\u003eAs mentioned earlier, the water temperature and density at a depth of 100 m were composited for periods of strong and weak cooling periods, and the spatial distributions during these periods were analyzed (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). During strong cooling in winter, cold water corresponding to temperatures less than 1\u0026deg;C was distributed over the Japan Basin at a depth of 100 m. Furthermore, the cold water extended to the entrance of the Yamato Basin, leading to meandering of the TWC and development of the subpolar front between the southern and northern region. In contrast, during weak cooling, cold water was limited to the eastern Japan Basin. When the 6\u0026deg;C isotherms (white lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, b), which represent the northern boundary of the main TWC at a depth of 100 m, with strong cooling and weak cooling conditions were compared, significant differences in the meandering phase of the TWC appeared in the Yamato Basin. In the water density distribution at a depth of 100 m, the 27.13 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{\\theta\\:}\\)\u003c/span\u003e\u003c/span\u003e isopycnal, corresponding to the center of the ESIW, extended distinctly into the Yamato Basin (white dashed line in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, d). In these cases, the buoyancy frequency (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:N=\\:\\sqrt{-(g/{\\rho\\:}_{0})(\\:\\partial\\:\\rho\\:/\\partial\\:z)}\\)\u003c/span\u003e\u003c/span\u003e) at point A\u0026acute; peaks at approximately 80 m depth under strong cooling conditions, which is approximately 50 m shallower than that under weak cooling conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). This indicates that, under strong cooling conditions, the cold water widely formed in the northern part extends into the southern part and penetrates beneath the warm TWC at relatively shallower depths compared to weak cooling conditions. At shallow depths, cold water subduction can induce baroclinic instability. The intensity of baroclinic instability can be estimated by the eddy growth rate (Stammer \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Zhai et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), which is expressed as follows:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:EGR\\:\\left(eddy\\:growth\\:rate\\right)=\\:\\frac{f}{N}\\:\\frac{dU}{dz}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:f\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:N\\)\u003c/span\u003e\u003c/span\u003e indicate the Coriolis parameter and the buoyancy frequency, respectively. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:du/dz\\)\u003c/span\u003e\u003c/span\u003e represents vertical shear. In this study, monthly mean EGR was calculated from the sea surface to a depth of 100 m in the Yamato Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), where the large variability in the upper layer circulation appears to be due to the extension of the cold water in the northern region. At this time, the EGR during strong cooling periods was greater than that during weak cooling periods, especially in February. Qiu and Miao (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) suggested that variations in the Kuroshio path were influenced by baroclinic instability leading to the development of meandering system. In the East Sea, increased baroclinic instability led to the development of the meandering TWC path (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). However, over time, the difference in the EGR between the strong and weak cooling periods decreased gradually. By August, the EGR was similar in both cases, and the main path of the TWC was similar.\u003c/p\u003e \u003cp\u003eAnother possibility for increasing variability in the southern part of the East Sea is variability in the separation latitude of the EKWC due to interannual variation in surface heat flux. Previous studies have analyzed numerical experiments on wind forcings as the major atmospheric forcings affecting the separation latitude of the EKWC (Kim and Yoon, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Yoon et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, this study focused on the surface heat flux, highlighted as a significant external forcing influencing the formation of the EKWC, as demonstrated by Kim et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). We analyzed how interannual variation in surface heat flux affects variability in upper layer circulation, especially the separation latitude of the EKWC. Seung and Nam (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1991\u003c/span\u003e) and Ito (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) reported that during strong cooling, cold water in the northern part of the East Sea extended southward along the Korean coast, influencing the separation latitude of the EKWC. However, in the results from IV_Qnet, during strong cooling, the ESIW in the northern region was widely distributed and the cyclonic eddy called the Dok cold eddy is simulated west of the Yamato Rise, but the separation latitude of the EKWC is not significantly different from that under weak cooling conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, c). Because Ito (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) analyzed hydrographic conditions under strong and weak winter cooling conditions using observational data, it was difficult to separate the signals related to wind and surface heat flux independently. Combining these numerical experiments with previous research, it is inferred that only surface heat flux does not influence the separation latitude of the EKWC. However, if a positive wind stress curl acts along with strong winter cooling conditions, cold water in the northern part of the East Sea could extend to the Korean coast, thereby affecting the separation latitude of the EKWC.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eDistribution of the temperature from the observation data\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe characteristics of the hydrographic conditions in the southern part of the East Sea, depending on the strong cooling and weak cooling periods selected from JRA-55, were analyzed via ship observations from the National Institute of Fisheries Science (NIFS) and Japan Meteorological Agency (JMA) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Owing to the rarity of both institutions observing data concurrently, we defined winter as the months of January, February, and March. Since the characteristics of the hydrographic conditions derived from observation data reflect the combined influence of both surface heat flux and wind stress, analyzing the effects of only surface heat flux is difficult. Nevertheless, even in observational data, during strong cooling periods, the 2\u0026deg;C isotherm extends as far south as 40\u0026deg;N near the Yamato Basin. In contrast, during weak cooling periods, cold water does not extend southward beyond 40\u0026deg;N.\u003c/p\u003e \u003cp\u003eAccording to observational data, the cold water extending southward along the Korean coast also expanded much further south during strong cooling periods than during weak cooling periods. These results are consistent with those of previous studies (Seung and Nam \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Ito \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). As mentioned earlier, cold water extending southward along the Korean coast was considered to be influenced not only by surface heat flux but also significantly by wind stress. Therefore, in the next section, we discuss how wind stress affects the separation latitude of the EKWC.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eWind stress response to the latitude of the EKWC\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAccording to the results of IV_Qnet in this study, the variability in the separation latitude of the EKWC seems to be unrelated to the interannual variation in the surface heat flux. Although cold water forms in the northern part of the East Sea during strong winter cooling, if the cyclonic gyre does not develop significantly, cold water cannot expand southward along the Korean coast. Therefore, to analyze the relationship between the interannual variation in wind stress and the separation latitude of the EKWC, an additional experiment, which applied the interannual variation in wind stress (IV_Wind), was conducted. Similar to the IV_Qnet experiment, during the analysis period, winter wind stress curls exceeding 1 standard deviation were classified as strong and weak positive wind stress curl conditions. The time series of the winter wind stress curl in the northern part of the East Sea is similar to that in the entire East Sea (not shown here). From winter to summer, under strong positive wind stress conditions, cold water below 2\u0026deg;C was widely distributed at a depth of 100 m, and the separation latitude of the EKWC was farther south than that under weak positive wind stress conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The difference in the separation latitude of the EKWC between the two conditions distinctly appears in summer. In the vertical structures along the 130\u0026deg;E line in August, during strong positive wind stress conditions, the thickness between 1\u0026deg;C and 2\u0026deg;C isotherms is prominently thickened and the subpolar front is located further south, because cold water in the northern part flows southward in the subsurface (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). In other words, when the upper layer circulation in the northern part of the East Sea strengthens due to a positive wind stress curl rather than strong cooling conditions, the 2\u0026deg;C isotherm in the northern part prominently extends further southwestward. At a result, the northward flow of the EKWC was impeded, but the isotherm was not clearly different in the Yamato Basin.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe total variability of the upper layer circulation in the East Sea consists of intrinsic variability and forced variability. In the East Sea, the variability of the upper layer was largely distributed in the southern region, especially in the Ulleung and Yamato Basins (Trusenkova, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Choi et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Kim et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). This study analyzed how interannual variations in surface heat flux, which play an important role in the TWC branches, affect the variability in upper layer circulation in the East Sea through physical processes. In the experiment (IV_Qnet) with only interannual variations in the surface heat flux, the variability was greater in the Yamato Basin and along the coast of Russia and off Vladivostok than the intrinsic variability (Intrinsic-Exp).\u003c/p\u003e \u003cp\u003eThe annual mean surface heat flux in the East Sea releases heat from the ocean to the atmosphere, with dominant cooling especially noticeable during winter. Under strong winter cooling, cold water formation in the northern region was facilitated, and the isotherms exhibited a trough-like shape in the Yamato Basin due to the southward extension of cold water. In contrast, weakening of southward cold water in the Yamato Basin resulted in the TWC flowing northeastward with a mainly straight path rather than a meandering path. In other words, it is inferred that the variability due to interannual variation in the surface heat flux was greater than the intrinsic variability in the Yamato Basin because the phase of the TWC main path was altered depending on the winter cooling conditions. In observational data, during strong winter cooling conditions, cold water also extends toward the Yamato Basin, and the subpolar front moves southward and becomes more pronounced. In other words, interannual variations in the surface heat flux affect local changes in the SST and the TWC meandering path due to increasing baroclinic instability.\u003c/p\u003e \u003cp\u003eHowever, the variability in the separation latitude of the EKWC was not directly related to only the winter cooling conditions. Instead, the interannual variation in the wind stress curl rather than the surface heat flux in winter distinctly influenced the northward latitudinal variations in the EKWC. In IV_Wind, because the cold water that formed in the northern region extended southwestward, the separation latitude of the EKWC and thickness of the ESIW (between 1\u0026deg;C and 2\u0026deg;C isotherms) were influenced.\u003c/p\u003e \u003cp\u003eIn the real ocean, the combined effects of two external forcings (the wind stress curl and surface heat flux) influence upper layer circulation through more complex physical processes. However, during the analysis period of this study, there are insufficient examples to analyze the variability in upper layer circulation by categorizing cases where the expansion of cold water is in-phase and/or out-of-phase due to these external forcings. Therefore, to analyze the effects of these external forcings on the variability of upper layer circulation in the East Sea, planning and analyzing new experimental methods are needed.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003eAcknowledgments\u003c/em\u003e\u003c/strong\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (NRF-2022M3I6A1086449)\u0026nbsp;and\u0026nbsp;the Korea Institute of Marine Science and Technology Promotion (KIMST)\u0026nbsp;funded by the Ministry of Oceans and Fisheries (KIMST-20220033). The\u0026nbsp;main calculations were performed by using the supercomputing resource of the Korea Meteorological Administration (National Center for Meteorological Supercomputer).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eAvailability of data and materials\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data analyzed in this study are available from public websites. The observation data (temperature) are freely available online at National Institute of Fisheries Science (NIFS; https://www.nifs.go.kr/kodc/index.kodc) and Japan Meteorological Agency (JMA; https://www.data.jma.go.jp/gmd/kaiyou/db/vessel_obs/data-report/html/ship/ship_e.php). For the atmospheric external forcings, JRA-55 reanalysis data were provided by the NCAR/UCAR Research Data Archive (accessible via http://rda.ucar. edu/ under dataset number ds628.0).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eAuthor contributions\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eD Kim conceived the idea, performed the analysis, and wrote the manuscript. HR Shin supported the idealized experiments and HR Shin and JH Moon continuously discussed the results. All authors participated in discussions during this study and contributed to writing and editing of the article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eCompeting interests\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBarnier B, Siefridt L, Marchesiello P (1995) Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analysis. J Mar Syst 6:363-380. https://doi.org/10.1016/0924-7963(94)00034-9\u003c/li\u003e\n\u003cli\u003eChoi BH, Kim KO, Eum HM (2002) Digital bathymetric and topographic data for neighboring seas of Korea. J Korean Soc Coastal Ocean Eng 14:41\u0026ndash;50.\u003c/li\u003e\n\u003cli\u003eChoi BJ, Cho SH, Jung HS, Lee SH, Byun DS, Kwon K (2018) Interannual variation of surface circulation in the Japan/East Sea due to external forcings and intrinsic variability. Ocean Sci J 53 (1):1-16. https://doi.org/10.1007/s12601-017-0058-8\u003c/li\u003e\n\u003cli\u003eDong S, Kelly KA (2004) Heat budget in the Gulf Stream region: the importance of heat storage and advection. J Phys Oceanogr 34(5):1214-1231. https://doi.org/10.1175/1520-0485(2004)034\u0026lt;1214:HBITGS\u0026gt;2.0.CO;2\u003c/li\u003e\n\u003cli\u003eGamo T, Nakayama N, Takahata N, Sano Y, Zhang J, Yamazaki E, Taniyasu S, Yamashita N, (2014) The Sea of Japan and its unique chemistry revealed by time-series observations over the last 30 years. Monogr Environ Earth Planets 2(1):1-22. https://doi.org/10.5047/meep.2014.00201.0001\u003c/li\u003e\n\u003cli\u003eHase H, Yoon JH, Koterayama W (1999) The current structure of the Tsushima Warm Current along the Japanese coast. J Oceanogra 55:217-235. https://doi.org/10.1023/A:1007894030095\u003c/li\u003e\n\u003cli\u003eIto M (2014) Seasonal and interannual variations of the Tsushima Warm Current paths in the southwestern part of the Japan Sea. Dissertation, Nagoya University.\u003c/li\u003e\n\u003cli\u003eJapan Meteorological Agency (2013) Japanese 55-year Reanalysis, daily 3-hourly and 6-hourly data. National Center for Atmospheric Research Computational and Information Systems Laboratory Research Data Archive, https://doi.org/10.5065/D6HH6H41\u003c/li\u003e\n\u003cli\u003eKara AB, Rochford PA, Hurlburt HE (2003) Mixed layer depth variability over the global ocean. J Geophys Res:Oceans 108:3079. https://doi.org/10.1029/ 2000JC000736\u003c/li\u003e\n\u003cli\u003eKawabe M (1982) Branching of the Tsushima Current in the Japan Sea, Part I. Data analysis. J Oceanogr Soc Japan 38:95\u0026ndash;107. https://doi.org/10.1007/BF02110295\u003c/li\u003e\n\u003cli\u003eKim CH, Yoon JH (1996) Modeling of the wind-driven circulation in the Japan Sea using a reduced gravity model. J Oceanogr 52:359-373. https://doi.org/10.1007/BF02235930\u003c/li\u003e\n\u003cli\u003eKim D, Shin HR, Kim CH, Hirose N (2020) Characteristics of the East Sea (Japan Sea) circulation depending on surface heat flux and its effect on branching of the Tsushima Warm Current. Con Shelf Res 192:104025. https://doi.org/10.1016/j.csr.2019.104025\u003c/li\u003e\n\u003cli\u003eKim D, Shin HR, Kim CH, Kim J, Hirose N (2024) Intrinsic low-frequency variability in the upper-layer circulation of the East Sea (Sea of Japan). J Phys Oceanogr 54(3):697-716. https://doi.org/10.1175/JPO-D-23-0030.1\u003c/li\u003e\n\u003cli\u003eKim YG, Kim K (1999) Intermediate waters in the East/Japan Sea, J Oceanogr 55:123-132. https://doi.org/10.1023/A:1007877610531\u003c/li\u003e\n\u003cli\u003eKim T, Yoon JH (2010) Seasonal variation of upper layer circulation in the northern part of the East/Japan Sea. Con Shelf Res 30:1283-1301. https://doi.org/10.1016/j.csr.2010.04.006\u003c/li\u003e\n\u003cli\u003eLee E, Noh Y, Qiu B, Yeh SW (2015) Seasonal variation of the upper ocean responding to surface heating in the North Pacific. J Geophy Res:Ocean 120:5631-5647. https://doi.org/10.1002/2015JC010800\u003c/li\u003e\n\u003cli\u003eLee HC (1996) A numerical simulation for the water masses and circulations of the Yellow Sea and East China Sea. Dissertation, Kyushu University.\u003c/li\u003e\n\u003cli\u003eLee HJ, Park JH, Wimbush M (2011) Tidal effects on intermediate waters: A case study in the East/Japan Sea. J Phys Oceanogr 41:234-240. https://doi.org/10.1175/2010JPO4510.1\u003c/li\u003e\n\u003cli\u003eNoh Y, Jang CJ, Yamagata T. Chu PC, Kim CH (2002) Simulation of more realistic upper-ocean processes from an OGCM with a new ocean mixed layer model. J Phys Oceanogr 32:1284-1307. https://doi.org/10.1175/1520-0485(2002)032\u0026lt;1284:SOMRUO\u0026gt;2.0.CO;2\u003c/li\u003e\n\u003cli\u003ePark KA, Park JE, Choi BJ, Byun DS, Lee EI (2013) An oceanic current map of the East Sea for science textbooks based on scientific knowledge acquired from oceanic measurements (in Korean with English abstract). J Korean Soc Oceanogr 18(4):234-265, https://doi.org/10.7850/jkso.2013.18.4.234\u003c/li\u003e\n\u003cli\u003eQiu B, Miao W (2000) Kuroshio path variations south of Japan: Bimodality as a self-sustained internal oscillation. J Phys Oceanogr 30(8):2124-2137. https://doi.org/10.1175/1520-0485(2000)030\u0026lt;2124:KPVSOJ\u0026gt;2.0.CO;2\u003c/li\u003e\n\u003cli\u003eSeung YH, Nam SY (1991) Effects of winter cooling on subsurface hydrographic conditions off Korean coast In the East (Japan) Sea. In: Takano K.(ed.) Oceanography of Asian Marginal Seas. Ed.Takano K. Tokyo, p.163-178. \u003c/li\u003e\n\u003cli\u003eShin CW, Byun SK, Kim C, Lee JH, Kim BC, Hwang SC, Seung YH, Shin HR (2013) Seasonal variations in the low-salinity intermediate water in the region south of subpolar front of the East Sea (Sea of Japan). Ocean Sci J 48(1):35-47. https://doi.org/10.1007/s12601-013-0003-4\u003c/li\u003e\n\u003cli\u003eStammer D (1998) On eddy characteristics, eddy transports, and mean flow properties. J Phy Oceanogr 28(4):727-739. https://doi.org/10.1175/1520-0485(1998)028\u0026lt;0727:OECETA\u0026gt;2.0.CO;2\u003c/li\u003e\n\u003cli\u003eTakikawa T, Yoon JH, Cho KD (2005) The Tsushima Warm Current through Tsushima Straits estimated from ferryboat. J Phys Oceanogr 35:1154\u0026ndash;1168. https://doi.org/10.1175/JPO2742.1\u003c/li\u003e\n\u003cli\u003eTrusenkova OO (2014) Variability of eddy kinetic energy in the Sea of Japan from satellite altimetry data. Oceanology 54(1):8-16. https://doi.org/10.1134/S0001437014010111\u003c/li\u003e\n\u003cli\u003eYoon JH (1982a) Numerical experiment on the circulation in the Japan Sea, Part I. Formation of the East Korean Warm Current. J Oceanogr Soc Japan 38:43\u0026ndash;51. https://doi.org/10.1007/BF02110289\u003c/li\u003e\n\u003cli\u003eYoon JH (1982b) Numerical experiment on the circulation in the Japan Sea, Part II. Influence of seasonal variation in atmospheric conditions on the Tsushima current. J Oceanogr Soc Japan 38:81\u0026ndash;94. https://doi.org/10.1007/BF02110294\u003c/li\u003e\n\u003cli\u003eYoon JH (1982c) Numerical experiment on the circulation in the Japan Sea. Part III: mechanism of the nearshore branch of the Tsushima Current. J Oceanogr Soc Japan 38:125\u0026ndash;130. https://doi.org/10.1007/BF02110283\u003c/li\u003e\n\u003cli\u003eYoon JH, Abe K, Ogata T, Wakamatsu Y. (2005) The effects of wind-stress curl on the Japan/East Sea circulation. Deep Sea Res II 52:1827-1844. https://doi.org/10.1016/j.dsr2.2004.03.004\u003c/li\u003e\n\u003cli\u003eYoon JH, Kawamura H (2002) The formation and circulation of the Intermediate Water in the Japan Sea. J Oceanogr 58:197-211. https://doi.org/10.1023/A:1015893104998\u003c/li\u003e\n\u003cli\u003eYu L, Jin Z, Weller RA (2006) Role of net surface heat flux in seasonal variations sea surface temperature in the tropical Atlantic Ocean. J Clm 19(23):6153-6169. https://doi.org/10.1175/JCLI3970.1\u003c/li\u003e\n\u003cli\u003eZhai X, Greatbatch R.J., Kohlmann JD (2008) On the seasonal variability of eddy kinetic energy in the Gulf Stream region. Geophy Res Lett 35:L24609. https://doi.org/10.1029/2008GL036412\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"surface heat flux, cold water extension, current meandering, variability, baroclinic instability, East Korea Warm Current, wind stress curl","lastPublishedDoi":"10.21203/rs.3.rs-4760644/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4760644/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Intrinsic variability in the East Sea (Sea of Japan), excluding the influence of external forcings on interannual variation, is distributed mainly in the southern region, especially in the Ulleung and Yamato Basins. In this study, the effects of interannual variations in surface heat flux on upper layer circulation in the East Sea are analyzed via numerical experiments. The interannual variations in the surface heat flux amplify variability in the Yamato Basin, not in the Ulleung Basin. The variability in the water temperature in the northern region is highly correlated with the variability in the surface heat flux with one month time lag. Winter surface cooling facilitates cold water formation in the northern regions, and it extends toward the Yamato Basin. As the cold water region expands (contracts), the meandering (straight) path of the Tsushima Warm Current flows northeastward due to increasing baroclinic instability. However, the surface heat flux does not significantly impact the separation latitude of the East Korea Warm Current. Instead, the separation latitude of the East Korea Warm Current is influenced primarily by strong winter positive wind stress curl anomalies in the northern region and greatly affects the southward flow of cold water along the Korean coast.\u003c/p\u003e","manuscriptTitle":"Impact of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-09 08:53:10","doi":"10.21203/rs.3.rs-4760644/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"9f59d5e7-5c43-4fb4-a22c-c8dd0bcfdc0d","owner":[],"postedDate":"August 9th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-01-29T07:08:46+00:00","versionOfRecord":[],"versionCreatedAt":"2024-08-09 08:53:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4760644","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4760644","identity":"rs-4760644","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}