Contribution of new AAM data source to Delta LOD excitation

Data sets of the changes of the length of day (Delta LOD) measured by space geodetic techniques and of the atmospheric angular momentum (AAM) derived from global meteorological data by the National Meteorological Center (NMC) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) are used to reanalyze and study the excita tions of Delta LOD, and to examine and compare the contribution of each AAM series to Delta LOD. The main results are as follows. 1. The AAM reanalyzed with the assimilated global meteorological data by NC EP/NCAR are more accurate and have lower noise than the original AAM derive d by NMC. The NCEP/NCAR-based AAM is more consistent with the changes of th e length-of-day series. 2. The NCEP reanalysed AAM data may better explain the non-tidal LOD variat ions on intraseasonal to interannual time scales, especially on the quasi-b iennial time scale. The atmosphere cannot, however, explain all LOD variati ons; other excitation sources are possible. 3. The effects of atmosphere on the estimated values of tides for Delta LOD components up to a month are weak. The preliminary results of the annual a nd semiannual tides can be estimated after removing the effects of atmosphe re from Delta LOD.