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To clarify the Tsushima Warm Current (TWC) system around a downstream region of Noto Peninsula (NP) where Toyama Trough (TT) causes a discontinuity of the along shelf-current, numerical experiments were performed using two-layered ocean model with simplified bottom topography. When a current trapped by the continental slope representing an offshore branch of TWC (OB) encounters the NP and TT, the OB was trapped by the coast facing the TT. A clockwise lee-eddy developed between a current axis and a coastline over the TT, which results in a current path transition offshore (CPT). The OB finally adjusted to a current path which crosses TT by shifting from the discontinuity point of the continental slope. If a coastal current representing a coastal branch of the TWC (CB) developed under this situation, the CB also formed lee-eddy within the identical region of the OB's eddy, resulting in CPT. Period for the CPT of the CB was shorter with increase of the OB volume transport. These results indicate that the OB acted to accelerate CPT of the CB within the TT. To support these results, observational data comprising mooring current, tidal, and CTD measurements obtained in and around the TT, along with the output from high-resolution data-assimilated ocean model DR_C were analyzed. Although the lee-eddy of the CBs was generated in every summer, the CPT events did not fully develop when the OB was absent over the continental slope because of the slow growth rate of the lee-eddy. [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) can be influenced by both physical and biogeochemical conditions of the ocean and is hence an important index for predicting changes in ocean environments. However, the relationship between biogeochemical activities and the DO concentration has not been quantified clearly in the Japan Sea (JS). In this study, we developed a coupled physical-biogeochemical model and clarified the contributions of biological processes to the DO content in the upper JS. We conducted two numerical experiments: with and without the biogeochemical compartments. Taking the biogeochemical factors into account gave better estimations for the spring-to-autumn seasonal migration of the subsurface DO maximum. In the euphotic zone, high photosynthetic production increases the DO concentration from spring to autumn. The strongest positive effect of biological processes on DO concentrations appears in summer at a depth of 30 m and is more than 0.5 ml l−1. As summer progresses to autumn, the vertical pattern of DO concentration in the Tsushima Warm Current (TWC) region shows minima at depths between 50 and 100 m, which is caused by the intrusion of low DO water from the East China Sea. The strongest negative effect on DO concentrations mainly due to the decomposition of detritus occurs in the TWC region below 150 m depth because the water is poorly ventilated by the relatively weak winter convection. [ABSTRACT FROM AUTHOR]

A sudden strong coastal current called a 'kyucho' occurred in August 2013 in the eastern coastal waters of Wakasa Bay, Japan. This study examined its characteristics based on both observational data collected by intensive field investigations and the simulation results of a numerical model. The field investigations comprised moored buoy observations near the coast and voyages by a research vessels and fishing boats. The mooring observations indicated that a current speed exceeding 50 cm s occurred abruptly near the eastern coast of the bay, in association with a synchronous change in the current direction. Data collected by acoustic Doppler current profilers (ACDPs) mounted on the vessels showed that a clockwise eddy existed in the bay and that the current on the east side of the eddy generated the kyucho near the coast. Based on the results of the numerical model and the analysis of the ADCP data, it was considered that the clockwise eddy was generated by a strong current at the tip of the Tango Peninsula, in the western part of the bay. As the eddy propagated from west to east across the bay, it induced the kyucho in the coastal waters in the east of the bay. [ABSTRACT FROM AUTHOR]

The branches of the Tsushima Warm Current (TWC) are realistically reproduced using a three-dimensional ocean general circulation model (OGCM). Simulated structures of the First Branch and the Second Branch of the TWC (FBTWC and SBTWC) in the eastern Japan Sea are mainly addressed in this study, being compared with measurement in the period September–October 2000. This is the first numerical experiment so far in which the OGCM is laterally exerted by real volume transports measured by acoustic Doppler current profiler (ADCP) through the Tsushima Straits and the Tsugaru Strait. In addition, sea level variation measured by tide-stations along the Japanese coast as well as satellite altimeters is assimilated into the OGCM through a sequential data assimilation method. It is demonstrated that the assimilation of sea level variation at the coastal tide-stations is useful in reproducing oceanic conditions in the nearshore region. We also examine the seasonal variation of the branches of the TWC in the eastern Japan Sea in 2000. It is suggested as a consequence that the FBTWC is continuous along northwestern Honshu Island in summertime, while it degenerates along the coast between the Sado Strait and the Oga Peninsula in other seasons. On the other hand, a mainstream of the SBTWC exists with meanders and eddies in the offshore region deeper than 1000 m to the north of the Sado Island throughout the year. [ABSTRACT FROM AUTHOR]