4:15 PM - 4:30 PM
[SGD01-09] Analysis of the atmospheric contribution to the recent absence of the Chandler wobble
Keywords:Earth orientation, Polar motion, Chandler wobble, Excitation function, Atmospheric angular momentum
The Chandler wobble (CW) is one component of the Earth’s polar motion and one of its normal modes excited by the mass redistribution of the atmosphere, ocean, and land water and their motion relative to the solid Earth (Gross, 2015). It has been absent since 2015, which was revealed by modelling the polar motion with a simple least-squares method (Yamaguchi & Furuya, 2024). Zotov et al. (2022) also mentioned the absence using singular spectrum analysis. The presence or absence of the CW will affect the polar motion prediction because the CW, if present, will generate a freely damping term that should ease the forecast. Yet, it remains uncertain how and why the CW has not been excited recently.
There are several possible scenarios: for instance, (1) the power levels of the excitations in total have been accidentally cancelled, (2) all power levels of the excitations have been reduced. To test these scenarios, we examine the contribution of each excitation source. Demonstrating that the atmospheric contributions have become small since 2015. Yamaguchi & Furuya (2024) suggested that the cancellation scenario is less likely. Here we aim to clarify the cause of the absence of the Chandler wobble since 2015 and to seek a mechanism for the excitation and damping of the Chandler wobble.
In this study, atmospheric angular momentum (AAM) was calculated using JRA-55 reanalysis data provided by the Japan Meteorological Agency (JMA). For comparison, we also analysed a combination of the AAM based on ECMWF reanalysis data, oceanic angular momentum (OAM) based on the MPIOM data, and the hydrospheric angular momentum (HAM) based on the LSDM data provided by the ESMGFZ (Dobslaw et al., 2018) and a one of the AAM based on the NCEP data and OAM based on the ECCO data provided by the Paris Observatory (Gross et al., 2003). We adopt an integration approach (Furuya et al., 1997); each geophysical excitation was integrated from 1976 to April 2022, assuming the Chandler period to be 432 days and the Q-value to be 25 and 50. Although Chao (1985) pointed out a flaw in the integration approach, we can circumvent its shortcomings by starting the integration with zero after removing the seasonal excitations.
The JRA-55 AAM was divided into every 15-degree latitudinal and longitudinal zone to estimate the CW excited by each of them. The result shows that for the matter term, the effect of atmospheric pressure, the contribution is relatively large in the latitudinal and longitudinal zones where the ocean occupies a large area. For the motion term, the effect of wind, the contribution was found to be larger than that of the matter term, with a particularly large contribution in the mid-latitudes. There are also several latitudinal zones where the amplitude of the estimated CW has increased or decreased since 2015. The Z components, which correspond to the length-of-day are also analysed in detail for the calculated AAM to investigate whether any changes have occurred since 2015.
There are several possible scenarios: for instance, (1) the power levels of the excitations in total have been accidentally cancelled, (2) all power levels of the excitations have been reduced. To test these scenarios, we examine the contribution of each excitation source. Demonstrating that the atmospheric contributions have become small since 2015. Yamaguchi & Furuya (2024) suggested that the cancellation scenario is less likely. Here we aim to clarify the cause of the absence of the Chandler wobble since 2015 and to seek a mechanism for the excitation and damping of the Chandler wobble.
In this study, atmospheric angular momentum (AAM) was calculated using JRA-55 reanalysis data provided by the Japan Meteorological Agency (JMA). For comparison, we also analysed a combination of the AAM based on ECMWF reanalysis data, oceanic angular momentum (OAM) based on the MPIOM data, and the hydrospheric angular momentum (HAM) based on the LSDM data provided by the ESMGFZ (Dobslaw et al., 2018) and a one of the AAM based on the NCEP data and OAM based on the ECCO data provided by the Paris Observatory (Gross et al., 2003). We adopt an integration approach (Furuya et al., 1997); each geophysical excitation was integrated from 1976 to April 2022, assuming the Chandler period to be 432 days and the Q-value to be 25 and 50. Although Chao (1985) pointed out a flaw in the integration approach, we can circumvent its shortcomings by starting the integration with zero after removing the seasonal excitations.
The JRA-55 AAM was divided into every 15-degree latitudinal and longitudinal zone to estimate the CW excited by each of them. The result shows that for the matter term, the effect of atmospheric pressure, the contribution is relatively large in the latitudinal and longitudinal zones where the ocean occupies a large area. For the motion term, the effect of wind, the contribution was found to be larger than that of the matter term, with a particularly large contribution in the mid-latitudes. There are also several latitudinal zones where the amplitude of the estimated CW has increased or decreased since 2015. The Z components, which correspond to the length-of-day are also analysed in detail for the calculated AAM to investigate whether any changes have occurred since 2015.