10:00 AM - 10:15 AM
[SGD02-05] Calculation of OAM based on the OFES2 ocean model data
Keywords:Earth orientation, Polar motion, Chandler wobble, Excitation function, Oceanic angular momentum
The Chandler wobble (CW) is one component of the polar motion of the Earth, and it is normal modes excited by the mass redistribution and the motion relative to the solid Earth of the atmosphere, ocean, and land water (Gross, 2015). Although the amplitude of the CW had been around 100-200 milliarcseconds (mas), it was found to be around -30 mas after 2015, which means it was absent (Yamaguchi & Furuya, 2024). Yamaguchi & Furuya (2024) pointed out the atmospheric angular momentum (AAM) based on both ERA5 and JRA-55 data has been decreased consistently, and it suggested that one of the causes of the CW absence should be the contribution of the atmosphere. However, it found that the oceanic angular momentum (OAM) based on the MPIOM and ECCO oceanic data are inconsistent. Therefore, the cause of the CW absence is not completely clear. By integrating AAM based on the NCEP atmospheric data and OAM based on the output from ECCO model data, Xu et al. (2024) showed that the excitation of the atmosphere and the ocean cancel each other out, at the same time as the CW absence. However, as mentioned earlier, Yamaguchi & Furuya (2024) showed that the results of the OAM from ECCO model data are not consistent with the integrated values of the OAM from the MPIOM ocean model. Therefore, there is limited data available to discuss the effects of the ocean. Yamaguchi & Furuya (2024) also showed that NCEP AAM is not fully consistent with ERA5 AAM or JRA-55 AAM.
In this study, the OAM are calculated based on the OFES2 ocean model data (Sasaki et al., 2020). As OFES2 model uses the JRA-55 data as the driving source, we intend to compliment the JRA-55 AAM in Yamaguchi & Furuya (2024). The OAM are calculated for two terms: the matter term, which is the change of the product of inertia due to the mass redistribution of seawater, and the motion term, which is the change in relative angular momentum with the solid earth due to ocean currents. The matter term was calculated by first calculating the bottom pressure and then integrating it spatially. On the other hand, the motion term was calculated by integrating the sea current velocity on each isobath an then integrating it spatially.
By calculating the OAM based on OFES2, the estimation of the contribution of the ocean to the excitation of the CW can be expected.
In this study, the OAM are calculated based on the OFES2 ocean model data (Sasaki et al., 2020). As OFES2 model uses the JRA-55 data as the driving source, we intend to compliment the JRA-55 AAM in Yamaguchi & Furuya (2024). The OAM are calculated for two terms: the matter term, which is the change of the product of inertia due to the mass redistribution of seawater, and the motion term, which is the change in relative angular momentum with the solid earth due to ocean currents. The matter term was calculated by first calculating the bottom pressure and then integrating it spatially. On the other hand, the motion term was calculated by integrating the sea current velocity on each isobath an then integrating it spatially.
By calculating the OAM based on OFES2, the estimation of the contribution of the ocean to the excitation of the CW can be expected.