Search for the 531-day-period wobble signal in the polar motion based on EEMD

Abstract

Abstract. In this study, we use a nonlinear and nonstationary time series analysis method, the ensemble empirical mode decomposition method (EEMD), to analyze the polar motion (PM) time series (EOP C04 series from 1962 to 2013) to find a 531-day-period wobble (531 dW) signal. The 531 dW signal has been found in the early PM series (1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977), but cannot be found in the recent PM series (1978-2013) using conventional analysis approaches. By virtue of the demodulation feature of EEMD, the 531 dW can be confirmed to be present in PM based on the differences of the amplitudes and phases between different intrinsic mode functions. Results from three sub-series divided from the EOP C04 series show that the period of the 531 dW is subject to variations, in the range of 530.9-524 days, and its amplitude is also time-dependent (about 2-11 mas). Synthetic tests are carried out to explain why the 531 dW can only be observed in recent 30-year PM time series after using EEMD. The 531 dW is also detected in the two longest available superconducting gravimeter (SG) records, which further confirms the presence of the 531 dW. The confirmation of the 531 dW existence could be significant in establishing a more reasonable Earth rotation model and may effectively contribute to the prediction of the PM and its mechanism interpretation.

Search for the 531 day-period wobble signal in the polar motion based on EEMD

“…Wahr (1983) implied that there exists an 18-month (∼ 547day) polar wobble (which can be considered as coinciding with the 531 dW signal mentioned in the present paper), which could be caused by oceanic excitation. Chen et al (2010) further demonstrated that the atmospheric excitation may give rise to the 531 dW signal with an amplitude 15 mas using the atmospheric angular momentum (AAM) data set spanning from 1948 to 2006. But, from the above results, those excited 531 dW signals almost disappear from the 1978-1994 and 1995-2013 series.…”

“…The spectra of synthetic series V (e) and VI (f); and the spectra of the IMF5 and IMF6 of synthetic series VI after using EEMD (f1-f3). and Chen et al (2010) suggested, the 531 dW can be excited by atmospheric/oceanic effects, and the excited signal by atmospheric or oceanic effects can be found in the PM se-ries only after convoluting with the CW term. However, the 531 dW signals excited by atmospheric and oceanic effects seem to have different phases; hence, the 531 dW cannot be found in the PM series (Chen et al, 2010).…”

“…and Chen et al (2010) suggested, the 531 dW can be excited by atmospheric/oceanic effects, and the excited signal by atmospheric or oceanic effects can be found in the PM se-ries only after convoluting with the CW term. However, the 531 dW signals excited by atmospheric and oceanic effects seem to have different phases; hence, the 531 dW cannot be found in the PM series (Chen et al, 2010). If we accept this, the 531 dW signal in gravity records comes from two different sources: indirectly from polar motion that is affected by the atmospheric/oceanic effects, and directly from the atmospheric/oceanic effects.…”

“…Recently, Na et al (2011) found a 500-day period component in the PM data with an average amplitude of 20 mas, and argued that, if assuming the existence of this component, the RMS error of the prediction of the PM could be reduced by 50 %, and suggested that this phenomenon should be caused by resonance of an unidentified oscillating mode of the Earth (possibly the Earth's inner core wobble). In addition, this wobble (or referred to as an 18-month wobble) was found in the analysis of the atmospheric angular momentum data by Wahr (1983) and Chen et al (2010). Furthermore, a signal with a period of about 1.5 years could also be found from the variation of length of day (LOD) by using the wavelet analysis as suggested by Chao et al (2014).…”

“…Clearly, when M = 0.5, the results from the frequency modulation of CW are consistent with the corresponding results in IMF6. If the 531 dW in IMF6 is really caused by the frequency modulation of CW with a modulation index M = 0.5, the 531 dW in IMF5 may be excited by some geophysical processes, such as the atmospheric-oceanic excitation (Wahr, 1983;Chen et al, 2010); then, we can appropriately explain the observations after using EEMD and the disappearance of the 531 dW in the recent PM series. Here, we would like to further demonstrate that EEMD can directly demodulate a frequency-modulated time series.…”