1. Progress on the Observations
The Japan Antarctic Research Expedition Project (JARE) started sea level observations in the 1960's at a site near the Syowa station shown in Figure 1, but its continuous record length was only a few months because its mechanical pressure tide gauge was frequently troubled by sea ice rush. Following these early trials, JARE succeeded in getting a continuous record over 1 year in 1981 using a pressure tide gauge (strain gauge) the data from which was outputed through a electric cable. However, this tide gauge aged in recent years. Then JARE started a project to renew the system and installed new pressure tide gauges with quartz oscillators in Jan. 1987 and in Jan. 1988, but both malfunctioned in Nov. 1989 and in Nov. 1988, respectively. In Jan. 1989, sea ice at Syowa Stn. was too thick to install a T.G. Finally, in Jan. 1990, JARE suceeded in the installation of the third pressure T.G. with a quartz oscillator. In 1991, the old T.G. tended to show a large fluctuation of sensitivity and reference level and stopped in July. Therefore a new T.G. was just in time to keep the tidal record continuous in 1990. These T.G. systems were described in the papers [1, 2].
The new T.G. measures the relative hydraulic pressure compensated for the atmospheric pressure in situ. This is remarkably different from the old T.G. which measured the total pressure of hydraulic and atmospheric pressures separately.
The old T.G., the drift of which was only 16.4cm for the zero point of the record for 10 years from 1981 to 1990, was working fairly well for a long time. The new T.G. installed in 1990 had been working well from 1990 to 1991. But in the checking of the T.G. in Jan. 1993, it was found that its compensating tube for the atmospheric pressure was choked in 1992 and its record was suspected to become wrong on the way. Now we are trying to check the data of 1992 in detail one more time.
Recently, the vertical displacement of sea ice accompanied by the ocean tide were measured at the Syowa tide station and compared with our tide gauge records by Sato et al [3]. According to the measurements of 6 times made in the 8 months from May to December 1993, it was concluded that the difference in amplitude between them were 5% at most. Thus their experiment has given another proof for the reliability of our tide gauge system.
2. Recent Monthly and Annual Mean Sea-Level
Zero point heights of old and new T.G.s were measured to be 3.952m and 3.381m under the Bench Mark 1040, respectively, on 23rd - 26th Jan. 1990. Therefore, the sea-level value by the new T.G. are systematically 57.1cm lower than those of the old T.G. Then we can connect the new data to the old data by adding the value 57.1cm. Figure 2 shows the recent fluctuation of the connecting monthly Mean Sea-Levels (MSL) from 1985 to 1989 by the old T.G. and from 1990 to 1991 by the new T.G.. Zero point heights of the new T.G. have been measured to be 3.511m in Jan. 1991 and 3.589m in Jan. 1992, respectively. These drifts are not corrected in Figure 2. The seasonal changes of 1990 and 1991 are recognized to be small compared to those of the former years. This difference may be due to the difference of the tide gauges measuring the absolute and the relative pressures.
The tendency of long term fall of annual MSL at Syowa Stn. appears from 1985 to 1989 in Figure 2, which is almost same as given in our former report [2] from 1981 to 1987. However the annual MSL in 1990 and 1991 by the new T.G. does not show such tendency of decending. Regrettably the present observation system is not completed even now so that we cannot make sure of the tendency by the new T.G. sufficiently. Then we shall continue to improve the system and to raise up its reliability furthermore.
REFERENCES
[1] Michida Y.: Japanese Activity on Sea Level Measurement in the Antarctic Region, Workshop on Sea-Level Measurement in Hostile Conditions Workshop Report No.54, UNESCO (1988).
[2] Odamaki M., Y. Michida, I. Noguchi, Y. Iwanaga, S. Ikeda and K. Iwamoto: Mean Sea-Level Observed at Syowa Station, East Antarctica, Proceedings of the NIPR Symposium on Antarctic Geosciences, No.5, 20-28, (1991)
[3] Sato T., K.Okano, T.Sawagaki and H. Enomoto : Tidal Displacements of Sea Ice Observed at Nishinoura Cove of The East Ongul Island, Antarctica, submitted to the Proceedings of the NIPR Symposium (1995).
FIGURE CAPTIONS
Figure 1: Location of Syowa and its tide station.
Figure 2: Recent Monthly Mean Sea-Level at Syowa Station, East Antarctica. The observing tide gauge was changed in Jan. 1990. The data are connected by correcting the difference of the zero point of T.G.s. Thick line means annual MSL.
For more information contact:
Minoru Odamaki, Maritime Safety Academy
Wakabacho 5-1, Kure-city, 737 JAPAN
Internet odamaki@msa.ac.jp
