PolyU research sheds light on century-old mystery — The excitation of Chandler Wobble

Prof. CHEN Jianli, Core Member of the Research Institute for Land and Space (RILS) and Chair Professor of Space Geodesy and Earth Sciences in the Department of Land Surveying and Geo-Informatics, together with a team of international experts, has recently published a ground-breaking study in The Proceedings of the National Academy of Sciences (PNAS).  The study provides the first observational evidence confirming the Chandler wobble (CW) excitation theory proposed by British geophysicist Harold JEFFREYS in 1940, which suggests that annual variability in polar motion is a significant mechanism for exciting the CW.

The Chandler wobble is an oscillation of the Earth’s rotational axis relative to its surface, with a period of approximately 433 days.  Discovered and named after American astronomer Seth CHANDLER in 1891, the CW is a resonant motion of the Earth’s rotational axis.  Without continuous geophysical excitations, the intensity of the CW naturally weakens over time due to energy loss caused by deformation of the Earth’s mantle material, and friction between the solid Earth, ocean and atmosphere.

The amplitude of the CW is variable, but generally falls within the range of 150–350 milliarcsecond (equivalent to 5-10 metres on the Earth’s surface).  However, the CW has experienced two periods of significant decrease (near-absence) since 1900: one in the 1920s and another more recently in the 2010s.  Although movements of water and air masses within the Earth’s climate system are believed to be responsible for maintaining the CW, the precise geophysical excitation mechanism has remained a mystery for over a century.  The driving forces behind the two unprecedented reductions in CW amplitude also remain unknown.

The team identified episodes of a near six-year oscillation (SYO) in prograde annual polar motion excitation amplitude over the past 125 years, which they traced to variability within the climate system.  Furthermore, they discovered that the SYO of annual polar motion plays a major role in CW excitation: the team identified SYO episodes in the 1920s and 2010s, which coincide with the reductions in CW amplitude.