Jong Hwan Yoon

Warming and structural changes in the east (Japan) Sea: A clue to future changes in global oceans?

The East (Japan) Sea has been in a warming trend during the last more than 40 years: a 0.1–0.5 degree of warming in the upper 1000 meters. The warming is accompanied by the deepening of the oxygen minimum layer by more than 1000 meters. The analysis of chemical tracers such as dissolved oxygen and CFCs clearly implies that the warming is associated with changes in deep water structures in the area, resulted from a replacement of the past bottom-water formation with an intermediate water formation in recent time. This shift has a remarkable resemblance to that anticipated to the ocean conveyor-belt system in coming century associated with recent global warming. In considering a rapid turn-over time of time scale less than 100 years, the East Sea may serve as a natural laboratory for global changes in the future.

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

Abstract Current structures across the Tsushima Straits are studied using results from long-term acoustic Doppler current profiler (ADCP) observations by a ferryboat between Hakata and Pusan conducted since February 1997. Two maxima of the northeastward current are observed in the central parts of the eastern and western channels, and the maximum velocity in the western channel is stronger than that of the eastern channel. Downstream of the Tsushima Islands, a southwestward countercurrent is observed associated with a pair of cyclonic and anticyclonic eddies. In the western channel, the deep countercurrent is observed pronouncedly on the bottom slope of the Korean side from summer to winter. The volume transport of the Tsushima Warm Current through the straits has strong seasonal variation with a minimum in January and two maxima from spring to autumn (double peaks). The spring peak of the volume transport through the eastern channel is more pronounced than the autumn peak, and the autumn peak of the west...

The Current Structure of the Tsushima Warm Current along the Japanese Coast

The branching of the Tsushima Warm Current (TWC) along the Japanese coast is studied based upon intensive ADCP and CTD measurements conducted off the Wakasa Bay in every early summer of 1995–1998, the analysis of the temperature distribution at 100 m depth and the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997). The first branch of TWC (FBTWC) exists throughout the year. It starts from the eastern channel of the Tsushima Straits, flows along the isobath shallower than 200 m along the Japanese coast and flows out through the Tsugaru Strait. The current flowing through the western channel of the Tsushima Straits feeds the second branch of TWC (SBTWC) which develops from spring to fall. The development of SBTWC propagates from the Tsushima Straits to Noto Peninsula at a speed of about 7 cm sec−1 following the continental shelf break with a strong baroclinicity. However, SBTWC cannot be always found around the shelf break because its path is influenced by the development of eddies. It is concluded that SBTWC is a topographically steered current; a current steered by the continental shelf break. Salient features at intermediate depth are the southwestward subsurface counter current (SWSCC) between 150 m and 300 m depths over the shelf region in 1995–1998 with the velocity exceeding about 5 cm sec−1, although discrepancies of the velocity and its location are observed between the ADCP data and the geostrophic currents.

Some features of winter convection in the Japan Sea

Historical Russian data provide indication of winter convection reaching down to about 1000 m depth near Vladivostok. However, this kind of convection does not occur every winter. Further data analysis suggests that the location of convection is driven offshore by the coastal buoyant water, which otherwise would be the coastal area. The coastal buoyant water is mostly cold fresh water but occasionally warm coastal water in the south. Due to the large extent of fresh coastal water in the northern part of the basin, the convection does not happen in this area despite the most intense surface cooling.

A Numerical Modeling of the Upper and the Intermediate Layer Circulation in the East Sea

Circulation in the upper and the intermediate layer of the East Sea is investigated by using a fine resolution, ocean general circulation model. Proper separation of the East Korean Warm Current from the coast is achieved by adopting the isopycnal mixing, and using the observed heat flux (Hirose et al., 1996) and the realistic wind stress (Na et al., 1992). The simulated surface circulation exhibits a remarkable seasonal variation in the flow patterns of the Nearshore Branch, the East Korean Warm Current and the Cold Currents. East of the Oki Bank, the Nearshore Branch follows the isobath of shelf topography from late winter to spring, while in summer and autumn it meanders offshore. The Nearshore Branch is accompanied by cyclonic and anticyclonic eddies in a fully developed meandering phase. The meandering and the eddy formation of the Nearshore Branch control the interior circulation in the Tsushima Current area. A recirculation gyre is developed in the region of the East Korean Warm Current in spring and grown up to an Ulleung Basin scale in summer. A subsurface water is mixed with the fresh surface water by winter convection in the northeastern coastal region of Korea. The well-mixed low salinity water is transported to the south by the Cold Currents, forming the salinity minimum layer (Intermediate Water) beneath the East Korean Warm Current water. The recirculation gyre redistributes the core water of the salinity minimum layer in the Ulleung Basin.

The Formation and Circulation of the Intermediate Water in the Japan Sea

In order to clarify the formation and circulation of the Japan/East Sea Intermediate Water (JESIW) and the Upper portion of the Japan Sea Proper Water (UJSPW), numerical experiments have been carried out using a 3-D ocean circulation model. The UJSPW is formed in the region southeast off Vladivostok between 41°N and 42°N west of 136°E. Taking the coastal orography near Vladivostok into account, the formation of the UJSPW results from the deep water convection in winter which is generated by the orchestration of fresh water supplied from the Amur River and saline water from the Tsushima Warm Current under very cold conditions. The UJSPW formed is advected by the current at depth near the bottom of the convection and penetrates into the layer below the JESIW. The origin of the JESIW is the low salinity coastal water along the Russian coast originated by the fresh water from the Amur River. The coastal low salinity water is advected by the current system in the northwestern Japan Sea and penetrates into the subsurface below the Tsushima Warm Current region forming a subsurface salinity minimum layer.

Offshore Detachment Process of the Low-Salinity Water around Changjiang Bank in the East China Sea

Abstract A patchlike structure of low-salinity water detached from the Chanjiang “Diluted Water” (CDW) is frequently observed in the East China Sea (ECS). In this study, the offshore detachment process of CDW into the ECS is examined using a three-dimensional numerical model. The model results show that low-salinity water is detached from the CDW plume by the intense tide-induced vertical mixing during the spring tide period when the tidal current becomes stronger. During the spring tide, thickness of the bottom mixed layer in the sloping bottom around Changjiang Bank reaches the mean water depth, implying that the stratification is completely destroyed in the entire water column. As a result, the offshore detachment of CDW occurs in the sloping side of the bank where the tidal energy dissipation is strong enough to overcome the buoyancy effect during this period. On the other hand, the surface stratification is retrieved during the neap tide period, because the tidal current becomes substantially weaker ...

Tidal Currents in the Tsushima Straits Estimated from ADCP Data by Ferryboat

Tidal currents in the Tsushima Straits have been analyzed using measurements obtained since February 1997 by an acoustic Doppler current profiler (ADCP) mounted on the ferryboat Camellia. Tidal current constituents (M2, S2, K1, O1) are dominant among the ten tidal current constituents (Q1, O1, P1, K1, N2, M2, S2, K2, MSf, Mf), and generally 1.4–2.1 times stronger at the western channel of the straits than those at the eastern channel. The ratio between amplitude of M2, S2, K1 and O1 averaged along the ferryboat track is 1:0.45:0.59:0.51. The major axis directions of tidal current ellipses are generally SW to NE, exceptionally in the vicinity of the Tsushima Islands. Approaching the Tsushima Islands from the Korean Peninsula side, the major axis gradually rotates clockwise. At the western channel, the M2 and K1 constituents change the rotation direction of current vectors from clockwise to counterclockwise at about 90–130 m depth. The contributions of the tidal currents to the mean kinetic energy and the mean eddy kinetic energy along the ferryboat track are, on average, 0.56 and 0.71, respectively. This suggests that tidal current activities are generally more dominant than the mean current activities and much more dominant than eddy activities. The only region where the eddy activities are comparable to the tidal current activities is located on the east side of the Tsushima Islands.

Comparison of RIAMOM and MOM in Modeling the East Sea/Japan Sea Circulation

The seasonal variations in the circulation of the water mass in the East Sea/Japan Sea have been simulated using a free surface primitive ocean model, RIAMOM (RIAM Ocean Model), comparing the results from GFDL-MOM1 (Geophysical Fluid Dynamics Laboratory Modular Ocean Model, version 1.1, hereafter MOM) with the GDEM (Generalized Digital Environmental Model) data. Both models appear to successfully reproduce the distinct features of circulation in the East Sea/Japan Sea, such as the NB (Nearshore Branch) flowing along the Japanese coast, the EKWC (East Korean Warm Current) flowing northward along the Korean coast, and the NKCC/LCC (North Korean Cold Current/Liman Cold Current) flowing southwestward along Korean/Russian coast. RIAMOM has shown better performance, compared to MOM, in terms of the realistic simulation of the flow field in the East Sea/Japan Sea; RIAMOM has produced more rectified flows on the coastal region, for example, the narrower and stronger NKCC/LCC than MOM has. There is however obvious differences between the model results and the GDEM data in terms of the calculation of the water mass; both models have shown a tendency to overpredict temperature and underpredict salinity below 50m; more diffusive forms of thermocline and halocline have been simulated than noted in GDEM data.

Surface Heat Flux in the East China Sea and the Yellow Sea

Climatological monthly mean variations of the surface heat fluxes over the East China Sea and the Yellow Sea are calculated by both a data analysis and a numerical simulation. The result of the data analysis based on the empirical/bulk method agrees well with the directly observed solar radiation and several previous studies of the surface heat fluxes. An adjustment in the formation of the Haney-type heat flux is presented by comparing to the result of the bulk method. The numerical simulation of these seas using an ocean general circulation model demonstrates the success of the improved Haney-type condition over the original one in simulating sea surface temperature. The surface heat flux simulated in the Yellow Sea is more reasonable than by the data analysis considering the total heat budget of this sea.