Shallow temperature inversions in the Pacific Ocean near Japan

The statistical properties of shallow temperature inversions in the Pacific Ocean near Japan were investigated using data obtained from. BT observations. In the sea east of Honshu, the Kuroshio front forms the southern boundary of the area where temperature inversions are abundant. Though the occurrence frequency of the temperature inversion layers is very low in the sea south of Honshu, the path of the Kuroshio influences its regional distribution in this region also, and the high occurrence area shifts offshore when the large cold water mass is present off Enshu-nada. The magnitude of the inversion temperature differences in the sea south of Honshu is considerably smaller than that in the sea east of Honshu. The magnitude of inversion thickness has a clear tendency to increase from south to north in the sea east of Honshu, reflecting the higher occurrence frequency of large-scale thick inversion layers in the northern part under the influence of the sub-arctic water mass. The frequency distribution of the inversion thickness in each sub-region (1° square area) in the sea south of Honshu is very similar to that in the region just south of the Kuroshio front in the sea east of Honshu, suggesting that the inversion layers may be generated by similar mechanisms in the sea south of the Kuroshio front.     

Mixing process on the northeast coast of Hokkaido in summer

Direct measurements using a free-falling micro-profiler were conducted on the northeast coast of Hokkaido in the summer of 2007 to clarify the mixing process in the Soya Warm Current (SWC) region in terms of microstructure. The distribution of the Turner angle (Tu) showed that these regions have a high potential for double diffusive convection, but direct measurements of the turbulent dissipation rate (ε) and dissipation of temperature variance ( $ \chi_{T} $ ) did not necessarily correspond to each other in the SWC region, especially in the offshore front of SWC and farther offshore. The mixing efficiency indicated that, even though the Turner angle (Tu) indicated a high potential for double diffusive convection, turbulent mixing was the main contributor to the mixing process in this region, and double-diffusive convection only contributed partially and sparsely, especially in the boundary off SWC water. The bottom mixed layer (BML) is known to thicken off the SWC. The vertical diffusivity coefficient was enhanced near the bottom (10^?4–10^?3 m² s^?1) off the SWC, and these results support that turbulence near the bottom off the SWC contributed to the thickening of the BML.     

Coastal current on the eastern shelf of Hidaka Bay

In order to examine seasonal variation in a coastal current and the dynamics of the current, we carried out a mooring current measurement near the coast on the eastern shelf of Hidaka Bay from December 2002 to July 2003. There seemed to be two current regimes during the observed period; one a southeastward current from December to March, and the other a northwestward current after April. Arrested topographic wave dynamics was used to understand along-shore steady momentum balance at the mooring site. It was found that the friction term was negligible during the former regime, that is, the wind stress term roughly counterbalanced the pressure gradient term. On the other hand, the contribution of each term to the momentum balance was sensitive to the resistance coefficient value during the latter regime. A numerical study showed that wind forcing alone could not reproduce the observed current velocity and momentum balance during the former regime. One possible interpretation of the observed results is superposition of the arrested topographic waves forced by the along-shore wind stress and downstream extension of the Coastal Oyashio. Numerical experiments using combined forcing of the wind stress and an inflow associated with the Coastal Oyashio supported the expected dynamics.     

Seasonal and Interannual Variation of DIC in Surface Mixed Layer in the Oyashio Region: A Climatological View

Eight-year observation results of DIC from 1996 to 2003 in the Oyashio region have been analyzed to obtain a climatological view of its seasonal variation and interannual variation. Data of DIC obtained by several institutes are synthesized to give a dataset with an uncertainty lower than 5 μmol/kg. The obtained climatology of NDIC seasonal variation in the Oyashio mixed layer shows a seasonal amplitude of 176 μmol/kg, with a maximum in January and a minimum in September. These features closely resemble those observed in the southern half of the western subarctic North Pacific (WSNP) including Station KNOT, although the timing of the NDIC maximum is slightly advanced in the case of the Oyashio. Analysis using a quasi-conservative tracer Cp0 (NDIC - 106NP) shows that among 176 μmol/kg of NDIC seasonal variation, only 16 μmol/kg is attributed to hydrographic processes while the remaining 160 μmol/kg is attributed to biological processes. The Cp0 value in the Oyashio mixed layer also resembles that of the WSNP mixed layer during the months May to November, suggesting further resemblance of the Oyashio water mass to that of WSNP in terms of carbon dynamics. The present results also suggest that a single data obtained in Oyashio mixed layer contains 30 μmol/kg of potential uncertainty for the representativity of this region, which leads to a note about a need to treat with caution results obtained by a single observation in this region.     

电阻率/激发极化率数据的二维反演程序

本程序在目前日本流行的电阻率／激发极化法二维反演程序［４］基础上,将电导率和极化率分块均匀变为分块连续变化,并在目标函数中加入了更多的先验信息,使反演结果的唯一性和真实性大大增加。在有限元正演模拟中,又采用三角单元剖分,使实测数据在反演以前不需要进行地形改正。本文介绍了该方法的基本原理和程序的使用方法,并给出了几个理论算例和野外实例     

Dilution of North Pacific Intermediate Water studied with chlorofluorocarbons

Assessment was made of residual ratio of North Pacific Intermediate Water (NPIW) produced in subpolar region of the North Pacific using chlorofluorocarbons, CFC-11 and CFC-12 (CCl₃F and CCl₂F₂), along 175°E. NPIW on density horizons less than 26.80 remained more than 80% north of 30°N. It was suggested that new NPIW laterally spreads over the northern North Pacific without hardly being diluted by the surroundings. For density horizons greater than 26.80 north of 30°N, NPIW remained less than 60%. The difference in the residual ratio between <26.80 and >26.80 north of 30°N suggests that NPIW is produced on density horizons less than 26.80, which contacts the atmosphere in the subpolar region, and that NPIW is diluted by upwelling deep water on density horizons greater than 26.80 in high latitude of the North Pacific. NPIW on a density horizon of 26.80 remained about 50% south of 30°N. The decrease in the horizontal distribution of the residual ratio of NPIW suggests that half the new NPIW produced in the subpolar region is laterally spread over the North Pacific with the southward movement of NPIW.     

U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt,northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.     

A Late Winter Hydrography in Hidaka Bay, South of Hokkaido, Japan

Hydrographic observations in Hidaka Bay, south of Hokkaido, Japan were carried out in late winter 1996 and 1997 to examine the spatial distributions and circulation features of two different water masses, i.e., Coastal Oyashio Water (COW) and Tsugaru Warm Water (TWW), and their modifications. It is known that COW is mostly composed of cold and low-salinity water of the melted drift ice coming from the Okhotsk Sea and flows into Hidaka Bay from winter to spring and TWW with high-salinity continuously supplies from the Tsugaru Strait to the North Pacific. Cold surface mixed layers (<26.2σ_θ, 0–100 m depth) were found mainly over the shelf slope, confirming that anti-clockwise flow of COW was formed. TWW was relatively high in salinity and low in potential vorticity, and had some patch-like water masses with a temperature and salinity maximum in the limited area in the further offshore at the deeper density levels of 26.6–26.8σ_θ. The fine structure of vertical temperature and salinity profiles appeared between TWW and COW is an indication of enhanced vertical mixing (double-diffusive mixing), as inferred from the estimated Turner angles. At a mouth of the Tsugaru Strait in late winter 1997, a significant thermohaline front between TWW and the modified COW was formed and a main path of TWW spreaded south along the Sanriku coast, probably as the bottom controlled flow. Hence, the patch-like TWW observed in late winter is isolated from the Tsugaru Warm Current and then rapidly modified due to a diapycnal mixing. This revised version was published online in August 2006 with corrections to the Cover Date.     

A numerical study on the formation and variation of a clockwise-circulation during winter in the Yellow Sea

In the Yellow Sea, the north-westerly wind dominates in winter and the existence of horizontal clockwise circulation has been suggested (Yanagi and Takahashi, 1993). The formation and variation mechanisms of this clockwise circulation is investigated using the wind forced numerical model which has a simplified basin configuration of the Yellow Sea. The model results show that two vortices (an anticlockwise vortex off Chinese coast and a clockwise vortex off Korean coast) are generated by the uniform north-westerly wind. Both vortices propagate along the shelf slope as the first mode shelf waves. An anti-clockwise vortex can not grow because it does not balance to the wind forcing. On the other hand, a clockwise vortex can grow and it reaches to the equilibrium condition at the northern part of the Yellow Sea, because this circulation can balance to the wind forcing. The time scale to become into the equilibrium condition is about 2 days. From this fact, it is ascertained that a clockwise circulation in the basin is generated periodically according to the variable wind forcing with 4 days period. The steady part of the current field exists with the fluctuating one which is induced by the periodical north-westerly wind.