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 χ3 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) χ3 (W ), χ3(P) and χ3 (P + IB) from the series AAM (NMC) from 1976 to 1995 and (c) χ3 (W ), χ3 (P) and χ3 (P + IB) from the series AAM (JMA) from 1983 to 1995. Here, χ3 (W ) is the wind term, χ3 (P) the pressure term and χ3 (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 χ3 (W ), χ3(W ) + χ3 (P) and χ3 (W ) + χ3 (P + IB) show to what extent uncertainties are present in the data, which portions of AAM originate from χ3(W ), χ3 (P) and χ3 (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.

Abstract. In this study, we examine the major components of polar motion, focusing on quantifying their temporal variability. In particular, by using the combined Earth orientation series SPACE99 computed by the Jet Propulsion Laboratory (JPL) from 1976 to 2000 at daily intervals, the Chandler and annual wobbles are separated by recursive band-pass filtering of the ¢ ¡ ¤ £ and ¦ ¥ § £ components. Then, for the trigonometric, exponential, and elliptic forms of representation, the parameters including their uncertainties are computed at epochs using quarterly sampling. The characteristics and temporal evolution of the wobbles are presented, as well as a summary of estimates of different parameters for four epochs.

Abstract. Polar motion data is available from the mid-19th century to the present. Based on time series with a variety of sampling intervals (monthly, 0.05-year, 5-day and daily), we have separated the low-frequency terms by low-pass filtering and the Chandler and annual terms by recursive band-pass filtering of the pole coordinates. Using a simple unweighted least-squares fit to the filtered low-frequency terms, the linear trends of the rotation pole were estimated. Assessing the estimates based on intercomparisons, the most reliable trend estimate was found. Using a Fast Fourier Transform, we have computed the prograde, retrograde and total amplitude spectra of the low-frequency part of polar motion in order to reveal the long-periodic signals. The characteristics and time evolution of the Chandler and annual wobbles are described by changes in their parameters (radii, directions and period lengths) over one century.

Abstract.Variations of annual and semiannual oscillations in rotation parameters have been investigated on the basis of length-of-day (LOD) as well as atmospheric-angular-momemtum (AAM) time series. These oscillations were determined using band-pass filters. In order to show the character of variations of seasonal oscillations, amplitudes, phases and periods were computed by a least-squares adjustment with the method of modified harmonic analysis at quarterly intervals. In addition, the seasonal imbalances in LOD and AAM budgets were determined and analysed in a similar way. These discrepancies were corrected for tidally excited effects. The non-atmospheric oscillations without the annual tide effect Sa and the semiannual tide effect Ssa have changeable amplitudes between 0.02 and 0.10 ms.

Abstract. The annual and semiannual residuals derived in the axial angular momentum budget of the solid Earthatmosphere system reflect significant signals. They must be caused by further excitation sources. Since, in particular, the contribution for the wind term from the atmospheric layer between the 10 and 0.3 hPa levels to the seasonal variations in length of day (LOD) was still missing, there is the need to extend the top level into the upper stratosphere up to 0.3 hPa. Under the conservation of the total angular momentum of the entire Earth, variations in the oceanic angular momentum (OAM) and the hydrological angular momentum (HAM) are further significant excitation sources at seasonal time scales. Focusing on other contributions to the Earth's axial angular momentum budget, we use the following data in this study: Axial atmospheric angular momentum (AAM) data derived for the 10-0.3 hPa layer from 1991 to 1997 for computing the missing wind effects, axial OAM functions as generated by oceanic general circulation models (GCMs), namely for the ECHAM3 and the MICOM models, available from 1975 to 1994 and 1992 to 1994, respectively, for computing the oceanic contributions to LOD changes, and, concerning the HAM variations, the seasonal estimates of the hydrological contribution as derived by Chao and O'Connor (1988). Using the vector representation, we show that the vectors achieve a close balance in the global axial angular momentum budget within the estimated uncertainties of the momentum quantities on the seasonal time scales.