Journal of the Oceanographical Society of Japan

Th and Pa Isotopes in the Waters of the Western Margin of the Pacific near Japan: Evidence for Release of ²²⁸Ra and ²²⁷Ac from Slope Sediments

Yoshiyuki Nozakit, Han-Soeb Yang

1987 Volume 43 Issue 4 Pages 217-227
Published: 1987
Released on J-STAGE: June 17, 2011

DOIhttps://doi.org/10.1007/BF02109817

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Using the moored MnO₂-fiber method, we have obtained 38 determinations of Th and Pa isotope concentrations from 18 sites along the margin of the western North Pacific near Japan, from water depths of 1, 330 to 5, 873m. From our data, we are able to show that (1) ²²⁸Ra and ²²⁷Ac are being supplied to the seawater from the slope sediments of Honshu, Japan, (2) our ²³⁰Th and ²³¹Pa concentrations match those obtained by in situ pumping with a MnO₂-fiber adsorber in the Japan and Izu-Ogasawara trenches but are significantly higher than those from the Panama and Guatemala basins, and (3) our ²³²Th concentrations show a similar systematic decrease with depth as do those of trace metals like Mn, Al, Te and Bi whose concentrations are strongly controlled by particulate matter scavenging.
In contrast, our data fail to show (1) that enhanced removal of ²³⁰Th and ²³¹Pa by scavenging from the water column is taking place near the western margins and (2) that ²³¹Pa is being removed in preference to ²³⁰Th from the water column to the marginal sediments. This is probably due to rapid mixing of the deep waters as compared to the scavenging rates of ²³⁰Th and ²³¹Pa in the water column and at the sediment/seawater interface.

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Possible Subsurface Source of an Acoustic Tsunami Precursor

Shigehisa Nakamura

1987 Volume 43 Issue 4 Pages 228-236
Published: 1987
Released on J-STAGE: June 17, 2011

DOIhttps://doi.org/10.1007/BF02109818

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A hypothetical model is introduced to show that focused caustics can be the source of a boom which may be an acoustic precursor of a big tsunami. This model helps us have a more reasonable understanding about what was written in the old historical descriptions about booms followed by big tsunamis after the significant earthquake events. In this model, the fault line that includes the epicenter of an earthquake at the sea bed was assumed to be a source of sound. Some of the acoustic rays radiated from the source form an acoustic caustic between the sea surface and the axis of the sound channel. This caustics can be subsurface audible source of an acoustic tsunami precursor.

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A Proof and Applications of the Expansion Theorem for the Rossby Normal Modes in a Closed Rectangular Basin

Akira Masuda

1987 Volume 43 Issue 4 Pages 237-243
Published: 1987
Released on J-STAGE: June 17, 2011

DOIhttps://doi.org/10.1007/BF02109819

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An expansion theorem is derived for Rossby normal modes in a closed rectangular basin and the set of Rossby normal modes is proved to be complete. This theorem provides a general linear solution to the initial value problem as well as to the response problem. In particular, the Green's function is obtained for the instantaneous localized torque anywhere in the basin. Weakly nonlinear versions are solved also by the combination of the general linear solution with the asymptotic expansion in terms of small amplitude. Further, an application is suggested to the spectral method of numerical simulation based on Rossby normal modes relevant to the more nonlinear evolution equation on a β-plane, instead of sin functions or Chebyshev polynomials, which have been employed conventionally for this purpose.

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Seasonal Variations of Water Temperature and Salinity in Osaka Bay

Tetsuo Yanagi

1987 Volume 43 Issue 4 Pages 244-250
Published: 1987
Released on J-STAGE: June 17, 2011

DOIhttps://doi.org/10.1007/BF02109820

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The characteristics of seasonal variations of water temperature, salinity and density in the upper, middle and lower layers in Osaka Bay are described. Osaka Bay is considered to be an estuary, because the weak mixed state appears in spring and summer and the moderate mixed state in autumn and winter. Osaka Bay is divided into three areas, the eastern shallow area that has a large amplitude of seasonal variation of water temperature and low average salinity, the southwestern deep area which has small amplitude of seasonal variation of water temperature and high average salinity, and the northwestern deep area, which has small amplitude of seasonal variation of water temperature and low average salinity.

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A Comparison between Geostrophic and Observed Winds at A Japan Meteorological Agency Buoy in the East China Sea

Y. Hsueh, James H. Tinsman

1987 Volume 43 Issue 4 Pages 251-257
Published: 1987
Released on J-STAGE: June 17, 2011

DOIhttps://doi.org/10.1007/BF02109821

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Geostrophic winds (ν_q) calculated over the East China Sea from surface pressure maps are compared to observed winds (ν_o) from a Japan Meteorological Agency Ocean Data Buoy for the period of January-April 1986. For mean winds, the average counterclockwise veering angle from ν_q to ν_o is 32° and there is no difference in speed. For fluctuating winds, the Ekman veering and speed reduction from ν_q to ν_o are respectively 16° and 2%. Co-spectral analysis of the two time series indicates very high coherency and a slight lead in phase by the buoy observations. The phase difference is consistent with the general movement of fronts from west to east and the fact that the geostrophic winds are calculated for a point 155 km to the southeast of the buoy.

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