Hydrological excitations of polar motion, 1993-2002

Abstract

SUMMARY Changes in continental water storage obtained from an advanced land data assimilation system (LDAS) are employed to compute hydrological excitations of polar motion for the 10-yr period of 1993‐2002. The results are compared with observed excitations of polar motion and contributions from the atmosphere and ocean, and with results from previous studies. There is remarkably good agreement between LDAS-derived results and remaining observed excitations, i.e. after atmospheric and oceanic effects are removed from the observations. LDAS-predicted hydrological excitations play a major role in closing the excitation budget for prograde and retrograde annual wobble, especially for the prograde component, and show considerably better agreement with observations than previous results. LDAS-predicted results show significantly larger interannual variability and also agree well with observed excitations not accounted for by the atmosphere and ocean. As demonstrated in this study, LDAS shows major improvement in modelling large-scale change in land water storage at both seasonal and interannual timescales. 1I N T R O DUCTION Under the conservation of angular momentum, any change in the Earth’s inertia tensor due to redistribution of mass will cause changes in the Earth’s rotation. Variations of atmospheric angular momentum (AAM) or oceanic angular momentum (OAM) will also change the Earth’s rotation via exchange of angular momentum between the solid Earth and its geophysical fluid envelope. At interannual or shorter timescales, redistribution of air and water mass and movement within the Earth system, including the atmosphere, ocean and hydrosphere (plus snow/ice sheet), are primary driving forces behind changes in the Earth’s rotation, while redistribution of mass within the core and mantle plays a more important role at decadal or longer timescales.