Combined utilization of GPS and bottom pressure gauge in landfast ice region around Antarctica

To monitor sea level and sea state change, measurements of the vertical displacement of the landfast ice at Nisi-no-ura Cove in Lutzow-Holm Bay, Antarctica, were made between April and December 1998 using GPS. Bottom pressure measurements were made simultaneously at the sea floor beneath the GPS station with a bottom pressure gauge (BPG). Seasonal variations were detected in the ice surface elevation and bottom pressure time series. It was shown that the difference between the potential sea level and ice surface elevation revealed a realistic freeboard. To postulate that the GPS and BPG observations are given, sea ice thickness was now predicted from the sea ice surface elevation and bottom pressure, assuming constant ice and water densities. The derived thickness agreed well with the observed one with an RMS error of 0.06 m. Thus the combined GPS-BPG system can provide a useful estimate for the sea ice thickness. Seasonal variation of the "sensitivity ratio" (the conversion coefficient from bottom pressure to sea level) was detected in the sea ice surface elevation and bottom pressure time series. The sensitivity ratio increased until September-October, and then decreased, consistent with thickening of the sea ice during the winter. Thus it ensures the obvious facts that the sensitivity ratio at a certain time should be used with the information of density stratification and the GPS can provide a height reference for correcting the intrinsic drift nature of the BPG Therefore, the combined use of GPS and BPG will enable us to monitor long-term sea level change more precisely in landfast ice-covered regions