The settling process of suspended particles in the turbulent flow was investigated by analyzing the Lagrangian motion of a large number of particles in the ocean mixed layer simulated by large eddy simulation ͑LES͒, focusing on the role of Langmuir circulation ͑LC͒. At the beginning, particles released near the surface are swept down rapidly following the downward jets in the presence of LC, contrary to the case of the horizontally uniform downward propagation in the absence of it. The strong vertical mixing by LC keeps the particle concentration almost uniform after the initial period in the presence of LC, while its mean concentration continues to decrease through particle settling. Under the influence of turbulence in the ocean mixed layer, the particle settling velocity W is always smaller than w s , and W / w s decreases with decreasing w s / u‫,ء‬ where w s is the terminal velocity of a particle in the still fluid and u‫ء‬ is the frictional velocity. The presence of LC causes a further decrease of W ultimately, and the difference between the cases with and without LC is the largest at w s / u ‫ء‬ ϳ 1. The analysis of LES data reveals that particles spend more time in upward flows and that more particles tend to accumulate in the high vorticity region in the presence of LC. It suggests that particle trapping is more likely when the length scale of vortices is larger, which may lead to the decrease of W in the presence of LC. It was also found that the preferential concentration occurs away from the regions of high horizontal vorticity and divergence when w s / u ‫ء‬ ϳ 1.

The seasonal variation of semidiurnal internal tides in the East/Japan Sea was investigated using 25 month long output from a real-time ocean forecasting system. The z coordinate eddy-resolving high-resolution numerical model, called the RIAM ocean model, incorporates data assimilation that nudges temperature and salinity fields together with volume transport through the Korea Strait to produce realistic oceanic currents and stratification. In addition to atmospheric forcing, it includes tidal forcing of 16 major components along open boundaries. The model generates energetic semidiurnal internal tides around the northern entrance of the Korea Strait. Energy conversion from barotropic to baroclinic (internal) tides varies seasonally with maxima in September (ranging 0.48-0.52 GW) and minima in March (ranging 0.11-0.16 GW). This seasonal variation is induced by the seasonality in stratification near the southwestern East/Japan Sea. The propagation distance of the internal tides is associated with generation intensity and wavelength. From late summer to early winter, the semidiurnal internal tides travel relatively far from the generation region due to stratification changes; its energy dissipates less as a result of longer wavelengths. Our results suggest that spatiotemporal variation of internal-tide-induced mixing due to the seasonality in the generation, propagation, and dissipation of internal tides should be considered for a more realistic simulation of water masses and circulation in models of the East/Japan Sea.

Large-eddy simulation of the oceanic mixed layer showed that Langmuir circulation (LC) is weakened under the surface heating and is ultimately broken down if the intensity of the surface heating becomes sufficiently strong. The critical condition for the breakdown of LC was mainly determined by the Hoenikker number Ho, and the transition occurs in the range Ho ∼ 1–2. The breakdown of LC leads to a drastic change in the characteristics of the oceanic mixed layer, such as the variation of the rms horizontal velocities with time, the ratio of the horizontal spectra of vertical velocity field, and the pitch. The stability condition for LC suggested by Leibovich was still observed in this simulation. Furthermore, it was found that LC is largely responsible for the formation of a thermocline and the maintenance of a well-mixed layer above it, and the depth of a thermocline was estimated in that case.

Dissolved oxygen (DO) is one of the most important oceanographic parameters measured for understanding various physicochemical processes in the ocean. This situation has been particularly true for the East Sea study ever since the first extensive investigation in the area during the 1930s (Uda, 1934). Uda found very high and uniform concentrations of DO, around 250 pM (5.6 ml/l), for waters below a few hundred meters over entire basins, and assumed that a veryfast ventilation system was operating in the East Sea. The Circulation Research of the East Asian Marginal Seas (CREAMS), Japan-Korea-Russia international cooperative studies on the East Sea have provided a unique opportunity to investigate the entire East Sea for the first time since Uda's study. A spectrophotometrically modified Winkler method (Pai et al., 1993) and a DO sensor (Sea Bird ModelSBE 13) were tested suc-cessfuUy during the CREAMS studies for improving the precision and accuracy of DO measurement. The study further confirmed an earlier observation by Gamo et al. (1986) that DO structures in the East Sea have been changing drasticaUy in such a way that the DO minimum depths have deepened by more than 1000 meters during the last 30 years. While the causes for these changes are not known at the present time, the analysis of DO profiles strongly suggests that the mode ofdeep water ventilation system in the East Sea has shifted from bottom water formation in the past to intermediate water formation at the present time . Studies of precise and accurate DO monitoring, along with other chemical tracers, deserve the highest priority for future research in the East Sea.

No abstract