dc.description.abstract | The variation of Earth’’s rotation, by definition a three-dimensional vectorial quantity, can be conveniently separated into two parts: polar-motion (X,Y) and the length of day (LOD). In the first part, we employ wavelet analysis on Earth’’s rotation series to find the components of periodic signals and compare them to the excitation sources. The variable rotation of the Earth is produced and maintained by mass redistribution and movement within the Earth system in terms of angular momentum predominately by hydrological circulations such as atmosphere and ocean. The wavelet time-frequency spectrum has been proven as a powerful tool to reveal nonstationary periodicities in time series. Our results exhibit good consistency with previous studies as expected. After using least-square fits to remove the contributions due to tides and atmospheric angular momentum, the unknown signals, e.g. ENSO-like, quasi-biennial and some decadal oscillations are extracted. For the two strong periodic signals locate around 6 and 13 years, we are trying to confirm if they are associated to ENSO cycle. In the another part, the works primarily aim to retrieve the major amplitude and phase variations of Chandler wobble (CW) which is the normal-mode free oscillation of the rotating Earth, continuously excited by mass transports in Earth’’s interior, surface, atmosphere, and ocean. It is well known that some close spectral peaks with comparable amplitudes exist in the CW band in the observed polar motion spectrum during the first half of the last century, which can actually be attributed to the unusual ~180◦ phase reversal of the CW during the 1920s–1930s. Although it was argued that the latter may be a manifestation of some dynamic changes of the Earth, it lacks specific evidences and geophysical interpretation. However, contrarily, we assert that the apparent phase reversal was simply a consequence of erratic excitation during a time when the amplitude happens to be rather small and hence easily altered. We simulate this assertion numerically by synthesizing long segments of polar motion, which are formed by convolving the free CW with Gaussian random noise meant to represent the external excitation. We do statistical examinations and indeed confirm our assertion above, indicating that the observed CW phase reversal during 1920-30 was nothing extraordinary.
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