Formation mechanism of the cold-water belt formed off the soya warm current

The cold-water belt (CWB) is frequently formed off the Soya Warm Current (SWC) during summer and autumn. The detailed distribution of the flow and temperature fields observed by the R/V Sinyo-maru in the summer of 2001 captured the structures of the SWC and the CWB. The temperature and density distributions showed that the vertical distribution of the CWB is associated with the upwelling formed off the SWC. Numerical experiments using a two-layer model with realistic bottom topography have been performed to understand the formation mechanism of CWB and the upwelling structure off the current. In the experiment, the sea level difference between the Japan Sea and the Okhotsk Sea, and baroclinic flow assuming the Tsushima Warm Current were given along the open boundary. The numerical model well reproduces the current system of the SWC and upwelling region off it. The upwelling region is formed at the Soya Strait first, and then it spreads on the offshore side along the SWC as a developing current system. Analysis of the model data indicated that the geostrophic balance mainly dominates in the current system, while convergence of the bottom Ekman transport due to the SWC forms the upwelling region as the secondary circulation. In addition, the advection effect due to the SWC is found to strengthen the upwelling.     

Characteristics of coastal-trapped waves induced by typhoon along the southeast coast of Honshu,Japan

Mooring observations using ADCP, electromagnetic current meters and thermometers were performed to clarify the vertical and horizontal structure of coastal-trapped waves (CTWs) on continental shelf and slope on the eastern side of Sagami Bay, Japan, in August and September 2003. A strong inflow associated with CTW caused by Typhoon 0315 (CTW15) was observed with remarkable downwelling. The maximum current due to CTW15 was over 100 cm s⁻¹, confined to the upper layer shallower than 90 m. The CTW (CTW10) induced by Typhoon 0310, was associated with the coastal upwelling and maximum outflow was 33 cm s⁻¹; the currents were extended near the bottom at 230 m depth. Remarkable discrepancies were found between the current structures of CTWs. CTW15 was explained by superposing the second CTW mode on the first CTW mode, whereas CTW10 was explained by the first CTW mode. The generation and propagation processes of both CTWs were reproduced by numerical experiments using a three-dimensional level model. The model results indicated that the difference of modal characteristics between CTW15 and CTW10 already exists in the CTW generation region and are due to difference of the wind direction, i.e., the typhoon’s path.     

The structure of the nearshore branch of the Tsushima current on the shelf off the San'in coast in summer

Current measurements were made at five moored stations over the continental shelf off the San'in coast of the Japan Sea for a month in the summer of 1980 to study the vertical structure of the nearshore branch of the Tsushima Current. The time-mean current for the observational period is 20 to 25 cm sec^–1 eastward near the surface and about 10 cm sec^–1 westward near the sea bottom except at the shallowest station. The time-mean current,i.e. the nearshore branch of the Tsushima Current is mainly due to the baroclinic modes. The currents are less variable in the first half of the observational period, but fluctuate with a several-day period in the latter half. The obtained current data were decomposed into barotropic and baroclinic modes to investigate the detailed characteristics of the fluctuations. In the latter half, the current fluctuations of the two modes with about a 5-day period are well correlated with each other, as the baroclinic mode lagging behind the barotropic mode by 12 hr. The barotropic current fluctuation is correlated to the sea level, with the former leading the latter by about 12 hr. The baroclinic current is correlated to the temperature at the subsurface layer with a shorter time lag.