Possible damping model of the 6 year oscillation signal in length of day

A significant 6 year oscillation signal exists in the observed length-of-day (LOD) series. We recently used normal Morlet wavelet transform method to recover this oscillation signal. The result indicates that the amplitude of this oscillation has been decreasing for the over past 50 years. Using the simulation analysis, this study further demonstrates that the above result is reliable. However, the geophysical mechanism responsible for this decrease is less clear. Here, we develop a temporal-decaying function to characterize the secular attenuation of the oscillation signal. Using the least squared method, we obtain the corresponding quality factor Q value (51.6 +/- 0.4) and the damping relaxation time tau(99.2 +/- 0.8 years). We find the attenuation of the 6 year oscillation signal and its Q value can be explained by previous theoretical prediction, providing constraints on the related physical parameters of the lowermost mantle. The dissipative effect of electromagnetic coupling at the core-mantle boundary is likely to be a primary factor to cause the decaying of the 6 year oscillation signal. (C) 2017 Elsevier B.V. All rights reserved.