Exploring the existence of quasi-500-day Wobble in polar motion inspired by gravity

Recent geodetic observations reveal a prominent oscillatory component in Earth’s polar motion (PM) with a period of approximately 500 days, hereafter referred to as the quasi-500-day wobble (quasi-500 dW). This signal is considerably more pronounced than other minor periodic components, such as the quasi-biennial and near-decadal terms, yet its excitation mechanism remains unresolved. By applying the standardized normal time–frequency transform (NTFT), we quantitatively characterize the period, amplitude, and phase of the quasi-500-day component in both PM and gravity observations, thereby providing observational constraints on internal Earth processes at this timescale. To investigate its excitation mechanism, we first assess contributions from external geophysical processes, including atmospheric, oceanic, and hydrological mass redistribution and angular-momentum exchange. The results indicate that these processes lack sufficient energy near the 500-day band and cannot account for the observed signal, implying an internal origin. Previous studies show that core–mantle coupling predominantly excites variability on interannual to decadal scales and is unlikely to provide a persistent contribution at ~500 days. Meanwhile, quasi-500-day periodicity has also been reported in several inner-core–related geophysical phenomena, such as geomagnetic activity, solar-wind interaction, and auroral variability, offering indirect observational support for an internal source. Motivated by this evidence, we test the hypothesis that internal excitation should produce signals of comparable periodicity in gravity. Analysis of superconducting gravimeter records from multiple globally distributed stations reveals significant quasi-500-day signals, with spatially variable amplitudes and phases that are broadly consistent with those extracted from polar motion. Taken together, the PM and gravity observations support the presence of a quasi-500-day dynamical process within Earth’s interior, which serves as the primary excitation source of the quasi-500 dW.