Low-Frequency Variations, Chandler and Annual Wobbles of Polar Motion as Observed Over One Century

Abstract

Abstract. Polar motion data is available from the mid-19th century to the present. Based on time series with a variety of sampling intervals (monthly, 0.05-year, 5-day and daily), we have separated the low-frequency terms by low-pass filtering and the Chandler and annual terms by recursive band-pass filtering of the pole coordinates. Using a simple unweighted least-squares fit to the filtered low-frequency terms, the linear trends of the rotation pole were estimated. Assessing the estimates based on intercomparisons, the most reliable trend estimate was found. Using a Fast Fourier Transform, we have computed the prograde, retrograde and total amplitude spectra of the low-frequency part of polar motion in order to reveal the long-periodic signals. The characteristics and time evolution of the Chandler and annual wobbles are described by changes in their parameters (radii, directions and period lengths) over one century.

On the double-peak spectrum of the Chandler wobble

“…Some authors assumed or proved the frequency to be invariant (e.g., Ooe 1978;Kuehne and Wilson 1996;Vicente and Wilson 1997;Gross et al 2003;Liao et al 2003;Seitz et al 2004). Some authors suggested a variable frequency (e.g., Melchior 1954Melchior , 1957Sekiguchi 1972Sekiguchi , 1976Guinot 1972;Carter 1981;Nastula et al 1993;Liu et al 2000;Schuh et al 2001;Höpfner 2003Höpfner , 2004. Jochmann (2003) argued that, as the CW is a proper mode of the Earth, the variation of its frequency should depend on the variation of the dynamical structure of the Earth, and showed that the variation due to changes in the structure of the Earth is insignificant.…”

Variable Chandler and Annual Wobbles in Earth’s Polar Motion During 1900–2015

“…Although CW has been under investigation for more than a century, its excitation mechanism has remained elusive (Lenhardt and Groten 1985;King and Agnew 1991;Gross 2000;Höpfner 2004;Malkin and Miller 2010). Up to now, various hypotheses on CW excitation have been proposed, such as atmospheric and oceanic processes (Brzeziński et al , 2012Bizouard et al 2011;Brzeziński and Nastula 2002;Salstein 2000;Gross et al 2003;Zotov and Bizouard 2015), wind and surface pressure variations (Wahr 1982;Gross et al 2003), groundwater impulses, changes in snow cover, interaction at the boundary between the core and mantle, and earthquakes (Dahlen 1971(Dahlen , 1973Höpfner 2004;Smylie et al 2015).…”

“…The prograde part of AW has a significant change in the period ranging between 356 and 376 days (Höpfner 2004). The excitation of the AW has been a hot topic.…”

“…Gross et al (2003) compared the contributions to the annual wobble such as atmospheric, wind, surface pressure, oceanic and currents and found that the effects of winds and currents on exciting the annual wobble have nearly the same retrograde amplitude and a prograde amplitude that differs by about 30 %, and the effect of currents on exciting the annual wobble is about 2/3 of the effect of bottom pressure variations. Höpfner (2004) considered the excitations of the AW to come from the seasonal displacement of air and water masses.…”

“…Wavelet transform was used to determine and analyze the periods and amplitudes of the CW and AW (Schuh et al 2001). Zero-phase digital filters were applied to filter out the CW and AW from several homogeneous PM time series, distinguished by a variety of sampling intervals the polar motion time series, such as POLE2001 (JPL), EOP (IERS) C01, OA00 (AICAS), EOP (IERS) C04, and SPACE2001 (JPL) (Höpfner 2004). Furthermore, in resent years, Malkin and Miller (2010) applied the singular spectrum analysis (SSA) and Fourier transform to extract the CW and then examined the CW amplitude and phase variations by means of the wavelet transform and Hilbert transform, their analysis had shown that besides the well-known CW phase jump in the 1920s, two other large phase jumps have been found in the 1850s and 2000s, and as in the 1920s, these phase jumps occurred with a sharp decrease in the CW in the 2000s; Zotov and Bizouard (2015) utilized the time domain excitation in Chandler frequency band which was extracted by Panteleev's filtering method to investigate the evolution of the regional atmospheric influence on CW; Smylie et al (2015) applied the maximum entropy method (MEM) to remove the AW, and leaving a pure CW and secular polar shift; hence, it overcomes the usual problem of how to avoid mixing the AW and CW which is beneficial to reveal details of the effect of earthquakes on the polar motion.…”