Deep Sea Pressure Observation and Its Application to Pelagic Tide Analysis

The Meteorological Research Institute, J.M.A., installed a quartz crystal pressure gage on the sea bottom (2200-meter depth) about 110 km south-southwest of Omaezaki in August, 1978, to observe sea level variation of the tsunami. The observations began on August, 1978, and are now continued regularly. The pressure gage is confirmed to be of use as a pelagic tide gage because the comparative tidal analysis of the pressure records and the coastal sea level records at Omaezaki showed consistent relations between the two. The tidal residual spectrum of the deep sea pressure variations has an energy level about one order lower than the sea levels at Omaezaki in the frequency range lower than 0.5 cpd. The tidal residual spectrum at Omaezaki has a configuration such that the energy density decreases toward higher frequency in proportion to ƒ-5/3 with a local rise in the frequency band between 0.1 cpd and 0.4 cpd, which may be due to meteorological disturbances. The tidal residual spectrum of the deep sea pressure has also a slope depending on ƒ-5/3 on the lower frequency side but a rather gentle slope on the higher frequency side due to the high frequency noise caused by the insufficient accuracy of the pressure observations. Consequently, the tidal residual spectral energy of the deep sea pressure becomes about the same level as that of the sea level at Omaezaki in the frequency range higher than 0.08 cph. This leads to the conclusion that the accuracy of the deep sea pressure gage used here is maintained about the same level as the coastal tide gage in the frequency range analyzed here.