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. This revised version was published online in August 2006 with corrections to the Cover Date.     

渤海环流与输运季节变化的数值模拟

渤海的风和温度层结有明显的季节变化 ,因而其环流与输运亦有明显的季节信号。以季节平均的海面气象条件和开边界的潮波系统驱动三维斜压水动力模型———HAMSOM ,模拟了渤海冬、夏季的总环流。渤海环流冬强夏弱 ,表层风漂流常被下层逆风流所补偿。深度平均环流 ,即水柱内的输运 ,流型有显著的季节变化 :冬季在渤海中部沿逆时针方向旋转 ,辽东湾顶有一个顺时针流涡 ,阻碍了湾顶水与外海水的交换 ;夏季则为一个大的贴岸的顺时针流环 ,内嵌许多局地涡旋。这些与渤黄东海海洋水文图集中给出的多年观测的环流基本相同 ,同时也被水文要素分布及耐盐浮游动物的出现所佐证。风的季节变化决定了渤海大部分海区、特别是海峡附近环流的季节变化 ,但辽东湾东岸众多的岬角涡旋却不随季节变化 ,因为它们是由潮波系统与岬角岸型变化的非线性相互作用产生的。     

Volume transport of the Soya Warm Current revealed by bottom-mounted ADCP and ocean-radar measurement

The vertical structure of the Soya Warm Current (SWC) was observed by a bottom-mounted acoustic Doppler current profiler (ADCP) in the region of the SWC axis near the Soya Strait during a 1-year period from May 2004. The ADCP data revealed a marked seasonal variability in the vertical structure, with positive (negative) vertical shear in summer and fall (winter and spring). The volume transport of the SWC is estimated on the basis of both the vertical structure observed by the ADCP and horizontal structure observed by the ocean radars near the strait. The transport estimates have a minimum in winter and a maximum in fall, with the yearly-averaged values in the range of 0.94–1.04 Sv (1 Sv = 10⁶ m³ s⁻¹). These lie within a reasonable range in comparison to those through other straits in the Japan Sea.     

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

Numerical experiments were performed in order to investigate the effects of variations of the transport through the Korea/Tsushima Strait, an inlet of the Japan/East Sea, on the upper layer circulation in the JES based on a 10-month transport observation from May 1999 to March 2000 (Perkins et al., 2000). All external forcings to the model were annual mean fields, except the transport variation through the Korea Strait. In the experiments where the periodic variation of the transport repeated continuously sinusoidally by several periods, strong variability of sea surface height (SSH) was detected in the region extending from the Korea Strait to the Japanese coast due to the geostrophy of the buoyancy forcing at the Korea Strait. The region along the Korean coast is more sensitive to the long-term variations than the short-term (≤60-day period) ones. In two experiments forced by realistic and monthly mean transport, the difference of rms of sea surface height was largest at the Japanese coast and relatively large at the East Korean Warm Current separation region (128∼130°E, 39∼41°N) and to the east of Yamato Rise. The distribution of difference of eddy kinetic energy at 100 m depth between the two experiments was similar to that of the rms of SSH. In the distributions of mean SSH and mean kinetic energy at 100 m depth the realistic transport invokes eddy variability to interact with mean current resulting in the changes of the mean SSH and the mean kinetic energy at the East Korean Warm Current separation region, but it does not produce conspicuous changes in the mean fields of entire JES compared with the mean fields forced by the seasonal transport.     

A Large-Scale Seasonal Modeling Study of the California Current System

A high-resolution, multi-level, primitive equation ocean model has been used to investigate the combined role of seasonal wind forcing, seasonal thermohaline gradients, and coastline irregularities on the formation of currents, meanders, eddies, and filaments in the entire California Current System (CCS) region, from Baja to the Washington-Canada border. Additional objectives are to further characterize the meandering jet south of Cape Blanco and the seasonal variability off Baja. Model results show the following: All of the major currents of the CCS (i.e., the California Current, the California Undercurrent, the Davidson Current, the Southern California Countercurrent, and the Southern California Eddy) as well as filaments, meanders and eddies are generated. The results are consistent with the generation of eddies from instabilities of the southward current and northward undercurrent via barotropic and baroclinic instability processes. The meandering southward jet, which divides coastally-influenced water from water of offshore origin, is a continuous feature in the CCS, and covers an alongshore distance of over 2000 km from south of Cape Blanco to Baja. Off Baja, the southward jet strengthens (weakens) during spring and summer (fall and winter). The area off southern Baja is a highly dynamic environment for meanders, filaments, and eddies, while the region off Point Eugenia, which represents the largest coastline perturbation along the Baja peninsula, is shown to be a persistent cyclonic eddy generation region. This revised version was published online in July 2006 with corrections to the Cover Date.     

南海贯穿流季节变化数值模拟研究

本文采用美国伍兹霍尔研究所研发的海洋-大气-波浪-泥沙输运耦合模式COAWST(Coupled Dcean-Atmosphere-Wave-Sediment Transport)对南海及邻近海域进行了9 km分辨率的数值模拟研究。结果表明,南海贯穿流的季节变化再现了冬强夏弱的特征,在南海内部冬季呈现气旋环流结构,夏季呈现反气旋环流结构,尤其在冬季其流轴结构更为清晰和稳定,海水从吕宋海峡进入南海,从民都洛海峡、卡里马塔海峡、台湾海峡和巴拉巴克海峡流出,吕宋海峡断面流量与其他4个海峡流量合计在数量级上相当,保持南海海水总量不变。吕宋海峡、卡里马塔海峡、民都洛海峡的流量呈现明显相关性,吕宋海峡流量增大时,民都洛海峡和卡里马塔海峡的流量也相应增大,相关系数分别达到0.78和0.9。通过更适于分析中短期变化的简化绕岛环流理论,定量计算2019年吕宋海峡、黑潮和棉兰老流流量与北赤道流分叉点位置的关系,发现夏季北赤道流分叉点NECBL(North Equatorial Current Bifurcation Latitude)偏南,在13.6°N附近;冬季NECBL偏北,在15.6°N左右,同期黑潮流量减少,棉兰老流流量增加,作为南海贯穿流入流的吕宋海峡流量可达13.4 Sv。吕宋海峡输运补偿了北赤道流到达菲律宾海岸后的北向分支的流量,与棉兰老流的流量呈正相关,相关系数达到0.5361。     

Characteristic ichthyoplankton taxa in the separation zone of the East Australian Current: Larval assemblages as tracers of coastal mixing

Ichthyoplankton assemblages were compared between regions dominated by the oligotrophic East Australian Current (EAC) and the inner-shelf waters off southeastern Australia, to determine if the early life history of fish was related to the separation of the EAC from the coast, producing different water masses as well as characteristic taxa. Samples were collected at the surface and in sub-surface waters, at 50 and 100 m isobath stations, during two summer research voyages in November 1998 and January 1999. On both voyages the study region was characterized by coastal and EAC waters in the north (∼31°S), and in the south by topographically induced upwelling (∼31°S), associated with narrowing of the continental shelf and separation of the EAC from the coast. Among the 111 families of larval fish, we observed distinctive assemblages of ichthyoplankton associated with the two different water masses. A greater abundance of the Carangidae, Labridae, Lutjanidae, Microcanthidae, Myctophidae and Scombridae was associated with the nutrient poor EAC water mass, while the Callionymidae, Clupeidae, Platycephalidae and Sillaginidae were mostly found in the cooler and/or fresher inner-shelf water mass. We assessed these patterns with opportunistic samples from an unusual, wind-driven upwelling event in the north (∼31°S) earlier in the November voyage. The relative abundance of these 10 characteristic families distinguished this wind-driven upwelling event from the subsequent relaxation and predominance of the EAC assemblage at this location just 6 d later. Distinctive and abundant families such as larval clupeids, relative to larval carangids, could be a useful marker of inner-shelf, EAC and mixed water masses in the absence of robust hydrographic data. This and related studies indicate contrast in early life histories of Sardinops sagax and Trachurus spp., which appear to spawn respectively in the inner-shelf and outer-shelf waters. The post-flexion stages of S. sagax predominate in the outer-shelf and Tasman Front, while post-flexion Trachurus spp. predominate in inner-shelf water masses.     

Contrasting local retention and cross-shore transports of the East Australian Current and the Leeuwin Current and their relative influences on the life histories of small pelagic fishes

Transport between shelf and offshore environments supports a significant proportion of ocean primary productivity and is critical to the life cycle of many marine species. While fundamental differences in the underlying dynamics of eastern and western boundary currents have been recognized and studied for more than half a century, the implications for physical dispersal rates have received much less attention. In this study we explore how Australia’s two major boundary current systems, the East Australian Current and the Leeuwin Current, differ in their local retention and cross-shore transports in the upper water column and how these differences favor contrasting life histories of small pelagic fishes. The results suggest that the East Australian Current forms a partial barrier to onshore transport, but is effective in entraining shelf waters and transporting them offshore, particularly in the region where the current separates from the coast. Blue mackerel (Scomber australasicus) spawn on the outer-shelf in this separation region and may thereby maximize the dispersion of eggs and larvae in the mainly oligotrophic waters of the southern Coral Sea. In contrast, the Leeuwin Current system promotes onshore transport through the combined effects of mean onshore flow and eddy-induced mixing. In the Great Australian Bight, sardine (Sardinops sagax) and anchovy (Engraulis australis) may exploit the high coastal retention of the Leeuwin Current system by spawning on the inner-shelf during summer when the current is weakest and winds assist retention and enhance production through local upwelling.     

Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan: A Hypothesis on the Oxygen-Deficient Water Mass Formation

A hydrographic survey and a 25-hour stationary observation were carried out in the western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated the upper layer water from the bottom water over the observational area except for near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M₂ tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer water is controlled by the advection due to the M₂ tidal current. On the other hand, the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due to the M₂ tidal current. The steep thermocline and halocline were maintained throughout the observation period in spite of the rather strong tidal currents. This implies an active intrusion of the low temperature and high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a course of circulation.