Kazuko Shibuya

Evidence for tides in the subglacial Lake Vostok, Antarctica

The phenomenon of lake tides is investigated for the subglacial Lake Vostok, East Antarctica. A model computation of equilibrium lake tides leads to a maximum lake level variation of about 18mm. To verify the hypothesis of really existing lake tides data from two different observational techniques are analysed. The re-evaluation of old tidal gravity data from Vostok Station as well as satellite synthetic aperture radar interferometry (InSAR) reveal specific signals which are explained as lake tides. The results of gravimetric tidal observations at Vostok Station from 1969 [Schneider, 1971; Schneider and Simon, 1974] show systematic deviations from the expected solid earth tide signal which correspond to a vertical tidal displacement of 12 and 2 mm amplitude for the waves K1 and M2, respectively. The InSAR data show the areal pattern of a vertical motion at the southern lake tip which is analogous to the grounding zone of ice shelves.

Observed Gravity Change at Syowa Station Induced by Antarctic Ice Sheet Mass Change

Continuous observations with superconducting gravimeters (SG) TT-70 #016 and CT#043 have been on-going since 1993 to monitor Earth tides and Earth’s free oscillations at a gravity observation hut in Syowa Station, Antarctica. We obtained gravity residuals from the SG CT#043 data by subtracting Earth tides, effects of atmospheric pressure changes and polar motion, and instrumental drift from the original record. The smoothed gravity residuals obtained by taking a running mean of 33 days reveal variations from –5 to +5 μgal (10–8m/s2).

Determination of geoid height at Breid Bay, east Antarctica

By the combined observation of satellite Doppler positioning, Global Positioning System (GPS) relative carrier phase measurement, and ocean tide observation, we obtained the geoid height at Breid Bay (70°12′S, 23°47′E) as 16.8 m above the World Geodetic System 1984 (WGS84) Earth ellipsoid. The broadcast ephemeris satellite Doppler positioning was made, and the ellipsoidal height at L0 point on the ice sheet of Breid Bay is estimated to an accuracy of ±4 m from the accepted 95 Navy Navigation Satellite System (NNSS) satellite passes. GPS relative carrier phase measurement was made between L0 point and the deck of the icebreaker Shirase (S point). The height difference between L0 point and S point is determined to an accuracy of ±0.3 m from the analysis of 15-min carrier phase data from four satellites by the doubly differenced phase method. The recording of sea level variation was made using the sea bottom pressure-transducer water level recorder for 4 days, together with monitoring of ships attitude and meteorological data. The separation of S point from the local mean sea level is determined to an accuracy of ±0.3 m. The standard deviation of the obtained geoid height can be considered to be (42 + 0.32 + 0.32)½ ∼ 4 m. Thus obtained geoid height at S point is 3.4 m smaller than the WGS84 model geoid (18 order truncation) and 8 m smaller than the OSU-86D geoid of 250 order harmonics. Though the discrepancy of a few meters may be explained by local geoid highs in Breid Bay, 3–5 m systematic depression of geoidal contours may be required in the region concerned (20°E to 40°E) in order to explain geoid heights at both S point and Syowa Station. The method we applied here is operationally simple and appropriate for dense installation of geoid height control stations along circum-Antarctic coastal zone.