In situ strength measurements on natural upper-mantle minerals

Using in situ strength measurements at pressures up to 10 GPa and at room temperature, 400, 600, and 700°C, we examined rheological properties of olivine, orthopyroxene, and chromian-spinel contained in a mantle-derived xenolith. Mineral strengths were estimated using widths of X-ray diffraction peaks as a function of pressure, temperature, and time. Differential stresses of all minerals increase with increasing pressure, but they decrease with increasing temperature because of elastic strain on compression and stress relaxation during heating. During compression at room temperature, all minerals deform plastically at differential stress of 4–6 GPa. During subsequent heating, thermally induced yielding is observed in olivine at 600°C. Neither orthopyroxene nor spinel shows complete stress relaxation, but both retain some stress even at 700°C. The strength of the minerals decreases in the order of chromian-spinel ≈ orthopyroxene > olivine for these conditions. This order of strength is consistent with the residual pressure of fluid inclusions in mantle xenoliths.     

Similitude of the hydraulic model experiment for tidal mixing

The treatment of diffusion due to the tidal current in the near-shore ocean, and the similitude in the hydraulic model experiment are studied.In broad and shallow tidal bays and in the coastal seawaters near irregular boundaries, horizontal eddy-currents induced geometrically and turbulence caused by their cascading have predominant effect on dispersion of river and waste waters. These turbulent diffusion processes are similarly reproduced in the Froude models of turbulent resume, by adding the similitude for the self-similar structure of the spectral density of turbulence, or the eddy diffusivity. The similitude means to take the scale ratios of the time and the vertical length as the two-thirds power of the scale ratio of the horizontal length.Similitudes are also derived for the system of the gravitational circulation, the stratification and the salt-mass transport in partially and well-mixed estuaries. Generally, the vertical eddy diffusivity must be exaggerated by a half power of the model distortion externally by some methods of agitation. When the tidal bay is broad and very shallow, the Rayleigh number and the Hansen number are small, and the effect of density current and stratification on the flushing is small. Instead the effect of local eddies, geometrically induced tidal residual circulations become predominant. In this special case, there is no need to satisfy the similitude for density difference and vertical shear effects on dispersion.     

Effect of boundary geometries on tidal currents and tidal mixing

The effects of the coastal boundary geometries on the tidal currents and the tidal mixing are studied mainly on the basis of hydraulic model experiments. Mizushima-Nada Sea, which is located in the central part of Seto Inland Sea, and the whole Seto Inland Sea were chosen as the prototypes.Currents and eddies geometrically induced in the tidal currents and in the ocean currents have significant effects on the water exchange, from small scales to meso scales, in the bays and near-shore regions of the ocean.As to small scale phenomena near irregular coastal boundaries such as river mouths, headlands and harbors, tidal currents produce organized eddy currents of the width scale. They are important to the temporary flushing and the local redistribution of the river water or the waste water.As to phenomena of larger scales, the tidal currents produce rather steady residual circulations in each part of the Inland Sea, due to the non-linear effects of the oscillating component. They are controlled by the geometry of the sea as separated by a narrow strait. These horizontal circulations of about 20 km in scale become the main mechanism of the water exchange in the Inland Sea. The one-dimensional dispersion coefficient due to these circulations is proportional to the product of the diameter and the current velocity of the circulations. The proportional constant takes the value of 0.40.5.     

The instability of viscous two-layer oscillatory flows

The instability of oscillatory flows in a two-layer fluid where the two layers differ in density and viscosity has been analysed using a perturbation method for long waves with special interest on effects of viscosity, time scale, density and depth of the fluid. The flow of a fluid with homogeneous density can be unstable, when the kinematic viscosity of the upper fluid layer is different from that of the lower one. Viscosity stratification results in unstable oscillatory flows. Two limiting cases of single-layer flow are also considered.     

Decadal Variability of Subsurface Temperature in the Central North Pacific

Decadal variability of subsurface temperature in the North Pacific has been investigated. Two dominant regions were found; the central subarctic region (CSa) and the north-eastern subtropical region (NESt). In CSa, cooling (warming) of wintertime subsurface temperature corresponds to the large (small) temperature gradient and southward (northward) shift of subsurface temperature front, associated with the increase (decrease) of positive wind stress curl and the southward (northward) shift of curl τ zero line with 2 years delay. It is suggested that the relocation of subtropical-subarctic boundary plays an important role. In NESt, importance of heat flux through the sea surface and heat divergence in the Ekman layer is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection of systematic errors in XBT data and their correction

Comparison experiment between XBT of T-7 probe and CTD was conducted at 15 stations in the sea area centered on 29°N, 135°E in December 1985. There were systematic errors in XBT temperature profiles in comparison with CTD temperature profiles. The main cause of errors was attributed to an error in the free-fall speed of the XBT probes which was provided by the XBT maker. A previous equation for depth correction proposed by Heinmilleret al. (1983) could not give effective correction for our data. A new equation between the probe depth and the elapsed time from landing of the probe on the water was obtained by the method of adjusting temperature gradients of XBT profiles to those of CTD profiles. This equation agreed with the theoretical result given by Seaver and Kuleshov (1982) much better than that of Heinmilleret al. (1983). Systematic errors due to a scatter of values of the reference resistance and variation of B-constant of thermistors used in XBT also seemed to exist. After an adjustment using the temperature difference between XBT and CTD in the mixed layer with depths of about 100 m, the standard deviation of temperature difference between XBT and CTD from the surface to the depth of 750 m was 0.14°C.     

Three-Dimensional Velocity Field and Cross-Frontal Water Exchange in the Kuroshio Extension

A spread of warm water from the first crest of the Kuroshio Extension is periodically enhanced by northward warm water intrusions from the main current. The water type in the spread area was previously found to be the same as that in the Kuroshio front at depth. In looking for the possible mechanism responsible for the northward warm water intrusions, a dynamic analysis in the Kuroshio front was carried out by using CTD, ADCP, AVHRR and ARGOS buoy data, obtained in 1996 by the R.V. Hakuho Maru. Downstream, cross-stream and vertical velocities in the Kuroshio Extension were found by using a "stream coordinate system". The velocity field in the Kuroshio front at the first crest showed a double structure with two surface velocity maxima. In the inner part of the front, relatively high cross-stream (northward) and vertical (upward) velocities were found. Thus, this study suggests that while water particles flow downstream along the first stationary meander of the Kuroshio Extension, they also experience lateral and vertical movements which allow the deeper water from an upstream location to rise to the surface layer, and in certain locations to deflect northward. By assuming isopycnal movement and conservation of potential vorticity, it was found that in those locations where anticyclonic curvature of the meander increases, warm water is more likely to deflect northward. High ageostrophic components observed in the first 300 m of the water column are probably related to the relatively high cross-stream and vertical velocities in the inner part of the front.     

North Pacific Subtropical Mode Waters Observed in Long XBT Cross Sections along 32.5°N Line

The S/V Shoyo, of the Hydrographic Department, Japan Coast Guard, has conducted high-density expendable bathythermograph (XBT) measurements along the 32.5°N line in the North Pacific every year from 1990 to 1993 as a part of the Japanese-World Ocean Circulation Experiment (WOCE). These XBT data are analyzed here, focusing on year-to-year variations of the inventory and core layer temperature (CLT) of the North Pacific subtropical mode water (NPSTMW). Large year-to-year changes are found in the NPSTMW CLTs estimated in longitudes between 140°E and 160°E. CLT values were found of 17.4°C in 1990, 17.1°C in 1991, 17.3°C in 1992 and 17.6°C in 1993. Inspection of the wintertime westerlies over the formation area and sea surface temperature distribution revealed that this change in CLT can be qualitatively attributed to the strength of atmospheric cooling in the formation area in the previous winter. Although a large year-to-year variation of NPSTMW inventory was also found, it is hard to state any relationship between CLT and atmospheric forcing. There is a possibility that different observational seasons may affect the inventory. It has also been found that the thermocline depth in 1991 was shallower in the sea area east of 180° than in 1992 and 1993. Associated with this change, the North Pacific central mode water (NPCMW), characterized by thermostad with temperatures ranging from 14°C to 11°C, appears in the sea area east of 180° in the 1992 and 1993 cross sections. The 1993 cross section, which ranged from the Japanese coast to the west coast of North America, possessed another thermostad in the surface layer, with a temperature of about 17°C in the eastern part of the cross section, off California. This revised version was published online in August 2006 with corrections to the Cover Date.     

Further evidence for non-reemergence of winter SST anomalies in the North Pacific eastern subtropical mode water area

Following our previous study (Sugimoto and Hanawa, 2005b), we further investigate the reason why reemergence of winter sea surface temperature anomalies does not occur in the North Pacific eastern subtropical mode water (NPESTMW) area, despite its occurrence in the North Pacific subtropical mode water and North Pacific central mode water areas. We use vertical temperature and salinity profiles of the World Ocean Circulation Experiment Hydrographic Program and Argo floats with high vertical and temporal resolution, together with heat flux data through the sea surface. We point out first that one of the causes for non-occurrence of reemergence is that the thickness of NPESTMW is very thin. In addition to this basic cause, two major reasons are found: a vigorous mixing in the lower portion of NPESTMW and less heat input from the atmosphere in the warming season. Since, in the lower portion of NPESTMW and deeper, the stratification is favorable for salt-finger type convection to occur compared with the other mode water areas, vigorous mixing takes place. This is confirmed by both a large Turner Angle there and the existence of staircase structures in vertical temperature and salinity profiles. From the viewpoint of heat input, the NPESTMW area gradually gains heat in the warming season compared with other mode water areas. As a result, NPESTMW cannot be capped so quickly by the shallow summer mixed layer, and water properties of NPESTMW are to be gradually modified, even in the upper portion.     

A New Fall-Rate Equation for T-5 Expendable Bathythermograph (XBT) by TSK

The accuracy of the manufacturer’s fall-rate equation for the T-5 Model of expendable bathythermograph (XBT) has been investigated based on about 300 collocated pairs of XBT-CTD (Conductivity-Temperature-Depth profiler) measurements in various climatological regions. We found that the equation systematically overestimates depth by about 5% for the T-5 produced by Tsurumi Seiki, Co. Ltd. (TSK), but almost no bias is associated with the T-5 produced by Sippican, Inc., in USA. The cause of this difference is not clear, because the two manufacturers’ T-5 probes are reported to have identical shape and weight in water. We propose a new fall-rate equation for the TSK T-5: z(t) = 6.54071t - 0.0018691t ², where z(t) is depth in meters at time, t, in seconds.