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Abstract

The variations of the Earth’s length of day (LOD) exhibit several interannual oscillations. The first detected one, with a period of 5.9-year, is better seen after removing the atmospheric contribution. More recently was isolated an oscillation of period ~8.5-year, distinct from the previous one. Torsional waves could explain both signals, although recently discovered non-axisymmetric magneto-Coriolis waves may also be involved. With a much smaller amplitude, a 7.3-year fluctuation was also detected in LOD data but no physical mechanism has been proposed. We show using a continuous wavelet transform analysis of synthetic oscillators embedded into a random noise, the limits of isolating damped signals with nearby periods in time series of limited duration. In particular, we emphasize the possibility that the 7.3-year oscillation could originate from an artifact due to the restricted length of the LOD series. Finally, we perform a wavelet coherence analysis between geodetically observed LOD variations and predicted LOD changes from geomagnetically inferred core flow models. Observed coherence at 5.9-year and ~8.5-year periods most probably confirms the fluid outer core origin for these two oscillations. Two coherent oscillations around 3.5 and 5-year periods are also conspicuous. Torsional Alfvén waves and/or Quasi-geostrophic magneto-Coriolis waves are natural explanations to the presence of various quasi-periodic signals in the LOD at interannual time-scales.