Abstract We use the method of wavelet transform to analyze the time series of the Earth's rotation rate of the EOP (IERS) C04. The result shows that the seasonal (annual and semiannual) variation of the length-of-day (LOD) has temporal variability in its period length and amplitude. During 1965.0–2001.0, the periods of the semiannual and annual components varied mainly from 175-day to 190-day and from 360-day to 370-day, respectively; while their amplitudes varied by more than 0.2 ms and 0.1 ms, respectively. Analyzing the axial component of atmospheric angular momentum (AAM) during this period, we have found that time-variations of period lengths and amplitudes also exist in the seasonal oscillations of the axial AAM and are in good consistency with those of the seasonal LOD change. The time variation of the axial AAM can explain largely the change of the LOD on seasonal scales.
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