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 mu gal (10(-8)m/s(2)). The Ice, Cloud, and Land Elevation Satellite (ICESat) was launched into high polar orbit in January 2003, and the Geoscience Laser Altimeter System (GLAS) the satellite is equipped with enables us to measure temporal volume changes across the Antarctic ice sheet. We evaluated ice sheet volume changes from changes in the topography observed by ICESat/GLAS and reduced them to mass changes by multiplying surface densities obtained by in situ measurements. We calculated gravity changes induced by the ice sheet mass changes for 11 operation periods of ICESat/GLAS from the beginning of 2003 through the beginning of 2007. The expected gravity difference due to the ice sheet mass change is approximately 4.8 mu gal in magnitude when we employ mass changes of all grid elements. The magnitude of observed gravity residuals is almost double that of the expected changes. The appearance of expected gravity changes, such as a trough pattern in 2003 is consistent with the observed changes.