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[SGD01-02] Can we reproduce the post-2015 absence of the Chandler wobble?
Keywords:Earth orientation, Polar motion, Chandler wobble, Excitation function, Atmospheric angular momentum, Oceanic angular momentum
The Chandler Wobble (CW) is one component of the Earth’s polar motion. The CW has been considered as a free motion excited by mass redistribution of the atmosphere and ocean and their motion relative to the solid Earth (Gross, 2015). In addition, the observed Chandler period (P) is 433.7 ± 1.8 days (e.g., Furuya and Chao, 1996), and has been regarded as a single and time-invariable constant.
By fitting the polar motion with the “standard” model containing the CW, the annual wobble (AW), and the long-term motion and the model excluded the CW, we found that the CW started to be weaker in 2005 and almost disappeared in 2015. The estimated amplitude of the CW is about 20 mas, we interpret these results in both excitation and wobble domains as the absence of CW for the first time in the observation history (Yamaguchi and Furuya, JpGU 2022).
In order to understand the cause of the CW disappearance, we analysed two types of available geophysical excitation function data: the combination of the atmospheric angular momentum (AAM) based on the National Centers for Environmental Prediction (NCEP) reanalysis data and the oceanic angular momentum (OAM) based on the Estimating the Circulation and Climate of the Ocean (ECCO) model driven by the same atmospheric data provided by the Paris Observatory(Salstein et al., 1993), and the combination of the AAM based on the European Centre for Medium-Range Weather Forecasts (ECMWF) model, the OAM based on the Max Plank Institute Ocean Model (MPIOM) driven by the ECMWF AAM, and the hydrospheric angular momentum (HAM) based on the Hydrological Land Surface Discharge Model (LSDM) provided by the Deutsches GeoForschungsZentrum (GFZ) Potsdam (Dobslaw et al., 2010). Moreover, we newly calculated and analysed the AAM based on the JRA-55 data, which is a reanalysis data provided by the Japan Meteorological Agency (JMA).
Assuming P as 432 days and Q as 25, 50, and 100, we integrated the geophysical excitation function for the period 1976-2021 (up to 2018 only when the ECCO OAM are included) to examine whether the CW after 2015 can be reproduced. First, because the pre-2015 CW cannot be explained by the AAM alone, the OAM and HAM are needed. Possibilities for the CW absence could be, for example, due to a coincidental cancellation of the sum of AAM, OAM, and HAM, or all angular momentum has become smaller. The results show that the amplitude of the CW by the integration of the ECMWF AAM and JRA-55 AAM has become smaller since 2015, whereas the estimated CW based on the other AAM, OAM, or HAM has not. This result suggests the influence of the atmosphere on the post-2015 CW absence. However, because the two OAM contributions are not consistent, further analysis is needed on the contribution of OAM and HAM to explain completely. When Q is assumed 50 or more, the amplitude of the estimated CW become larger and closer to the observed value before the 2000s, while it is too larger than the observed value after 2015. On the other hand, when Q is assumed as 25, the CW absence is reproduced, while it has a smaller amplitude than the observed value before the 2000s. Although no assumption of Q can reproduce the behaviour of the CW completely, the result suggests that Q close to 50 is proper. In addition, the result of the integration of the JRA-55 AAM showed that the estimated amplitude of the CW gets larger and closer to the observed CW when the topography is considered in the AAM than when it is not.
By fitting the polar motion with the “standard” model containing the CW, the annual wobble (AW), and the long-term motion and the model excluded the CW, we found that the CW started to be weaker in 2005 and almost disappeared in 2015. The estimated amplitude of the CW is about 20 mas, we interpret these results in both excitation and wobble domains as the absence of CW for the first time in the observation history (Yamaguchi and Furuya, JpGU 2022).
In order to understand the cause of the CW disappearance, we analysed two types of available geophysical excitation function data: the combination of the atmospheric angular momentum (AAM) based on the National Centers for Environmental Prediction (NCEP) reanalysis data and the oceanic angular momentum (OAM) based on the Estimating the Circulation and Climate of the Ocean (ECCO) model driven by the same atmospheric data provided by the Paris Observatory(Salstein et al., 1993), and the combination of the AAM based on the European Centre for Medium-Range Weather Forecasts (ECMWF) model, the OAM based on the Max Plank Institute Ocean Model (MPIOM) driven by the ECMWF AAM, and the hydrospheric angular momentum (HAM) based on the Hydrological Land Surface Discharge Model (LSDM) provided by the Deutsches GeoForschungsZentrum (GFZ) Potsdam (Dobslaw et al., 2010). Moreover, we newly calculated and analysed the AAM based on the JRA-55 data, which is a reanalysis data provided by the Japan Meteorological Agency (JMA).
Assuming P as 432 days and Q as 25, 50, and 100, we integrated the geophysical excitation function for the period 1976-2021 (up to 2018 only when the ECCO OAM are included) to examine whether the CW after 2015 can be reproduced. First, because the pre-2015 CW cannot be explained by the AAM alone, the OAM and HAM are needed. Possibilities for the CW absence could be, for example, due to a coincidental cancellation of the sum of AAM, OAM, and HAM, or all angular momentum has become smaller. The results show that the amplitude of the CW by the integration of the ECMWF AAM and JRA-55 AAM has become smaller since 2015, whereas the estimated CW based on the other AAM, OAM, or HAM has not. This result suggests the influence of the atmosphere on the post-2015 CW absence. However, because the two OAM contributions are not consistent, further analysis is needed on the contribution of OAM and HAM to explain completely. When Q is assumed 50 or more, the amplitude of the estimated CW become larger and closer to the observed value before the 2000s, while it is too larger than the observed value after 2015. On the other hand, when Q is assumed as 25, the CW absence is reproduced, while it has a smaller amplitude than the observed value before the 2000s. Although no assumption of Q can reproduce the behaviour of the CW completely, the result suggests that Q close to 50 is proper. In addition, the result of the integration of the JRA-55 AAM showed that the estimated amplitude of the CW gets larger and closer to the observed CW when the topography is considered in the AAM than when it is not.