Basic research on the gravity changes due to the effect of mass variations in the atmosphere and ocean

海底压力监测是研究海水质量变化对地球重力影响的必要手段，而地球重力又与气团运动有关。例如，Cox和Chao（2002）认为，太平洋的物质交换是最近地球引力J2项快速大变化的主要来源，这可能与1997年底结束的巨大ENSO事件有关。我们小组从海底压力（OBP）观测中发现了那次事件的实际压力变化（Fujimoto等人，Geophysical Research Letters, 2003）。卫星重力测量（CHAMP和GRACE）有望揭示海洋变率与重力变化之间的全球关系。自2001年以来，我们在西太平洋（三陆附近）的TOPEX/Poseidon （TIP）高度计轨迹交叉点开展了OBP观测，以研究OBP变化对重力的影响。我们将OBP-A（距海岸约100公里）和OBP-B（距海岸约350公里）两个站点的观测结果与三种不同类型的数据集进行了比较。它们是(1)基于T/P数据的四种海潮模式，(2)从T/P数据得到的海面高度（SSH）变化，(3)从ECCO模式得到的压力变化（Stammer et al., 2002）。我们的比较结果表明，对于以月测大地水准面精度为1 cm或更高为目标的卫星重力测量，仍然需要仔细研究海洋物质输送对卫星重力测量的影响，OBP观测是验证卫星重力测量（提交给GRL）的有用方法。以绝对重力仪校准的Esashi超导重力仪被用于检验潮汐模型和修正自由核共振参数（Sato et al., Journal Geodynamics, 2004，已接受）。

Seafloor pressure monitoring is an indispensable approach to study the effect of seawater mass changes on the Earth's gravity, which also link to the air mass motions. For example, Cox and Chao (2002) claimed the mass exchange in the Pacific Ocean as a major source of the recent rapid and large change in the J2 term of the Earth's gravity may related to the huge ENSO event terminated in the end of 1997. Our group detected the actual pressure changes at that event from the ocean bottom pressure (OBP) observations (Fujimoto et al., Geophysical Research Letters, 2003).Satellite gravimetry (CHAMP and GRACE) is expected to reveal the global relation between the ocean variability and the gravity change. Since 2001, we are carrying out OBP observations at the cross-over points of the TOPEX/Poseidon (TIP) altimeter tracks in the western Pacific Ocean (off Sanriku) in order to investigate the effect of OBP changes on the gravity. We have compared the observations at two sites of OBP-A (about 100 km from sea coast) and OBP-B (about 350 km) with the thee different kinds of data sets. These are (1) the four ocean tide models based on the T/P data, (2) the sea surface height (SSH) variations obtained from the T/P and (3) the pressure changes derived from the ECCO model (Stammer et al., 2002). Our comparison results suggest that it is still needed to carefully examine the effects of ocean mass transportations on the satellite gravimetry which aims at monthly geoid determination at an accuracy of 1 cm or better, and the OBP observation is a useful method for validation of the satellite gravimetry (submitted to GRL).The supper-conducting gravimeter at Esashi well calibrated with the absolute gravimeters have been used to check the tide models and to revise the free core resonance parameters (Sato et al., Journal Geodynamics, 2004, accepted).