Seasonal variations in length of day and atmospheric angular momentum

The seasonal variations of the Earth's rotation are still not sufficiently well explained in terms of their causes. Quantitative estimates of the variability of the oscillations in length of day (LOD) and atmospheric angular momentum (AAM) have been applied very seldom. Therefore, the problem is re-examined. In particular, the axial AAM component labelled chi(3) is related to changes in LOD. For this reason, the following series of data at one-day intervals is used in this study: (a) LOD from the series EOP (IERS) C04 from 1962 to 1996, (b) chi(3) (W), chi(3) (P) and chi(3) (P + IB) from the series AAM (NMC) from 1976 to 1995 and (c) chi(3) (W), chi(3) (P) and chi(3) (P + IB) from the series AAM (JMA) from 1983 to 1995. Here, chi(3) (W) is the wind term, chi(3) (P) the pressure term and chi(3) (P + IB) the pressure term with inverted-barometer response. First, the seasonal oscillations are separated from the various time-series by filtering. To illustrate their characteristics, the amplitudes, periods and phases of the annual and semi-annual oscillations are then derived and presented in terms of their temporal variability. The discrepancies between the magnitudes of the annual and semi-annual components of LOD without the tidal effects Sa and without Ssa and of chi(3) (W), chi(3) (W) + chi(3) (P) and chi(3) (W) + chi(3) (P + IB) show to what extent uncertainties are present in the data, which portions of AAM originate from chi(3) (W), chi(3) (P) and chi(3) (P + IB), and whether another excitation source contributes to seasonal LOD variations. At the annual frequency, the wind term from the upper stratosphere that is neglected is evidently responsible for the imbalance between the LOD and AAM data. However, at the semi-annual frequency, the discrepancy is not fully explained by the missing stratospheric wind term, and a contribution from the global surface water redistribution is likely.