There are various long-period signals in the polar motion record, with some falling in the category of quasi-decadal oscillations if their periods are at the order of decades. Kiani Shahvandi, et al. (2024a) have demonstrated that the causes of quasi-decadal oscillations in the polar motion record are rooted in the barystatic and core dynamics processes.

Specifically, among barystatic processes Terrestrial Water Storage (TWS) is the dominant contributor to the variations in decadal polar motion, although as mentioned there is a secondary contribution due to the core dynamics.  Kiani Shahvandi, et al. (2024a) argued that this contribution represents the dynamic response of the core to surface mass variations, a profound and striking discovery that can further our understanding of the climate and more broadly, the Earth system as a whole.

Kiani Shahvandi, et al. (2024b) also showed that the TWS induces decadal oscillations in the length of day and coefficient of ellipticity. Although the amplitude of these oscillations is less than one tenth of a milliseconds, they are projected to increase in the coming decades. By contrast, the role of core dynamics on the quasi-decadal oscillations observed in the length of day record is much more pronounced.

References

Kiani Shahvandi, M., Adhikari, S., Dumberry, M., Modiri, S., Heinkelmann, R., Schuh, H., Mishra, S., Soja, B. (2024a). Contributions of core, mantle and climatological processes to Earth’s polar motion. Nature Geoscience, 17: 705-710, https://doi.org/10.1038/s41561-024-01478-2

Kiani Shahvandi, M., Adhikari, S., Dumberry, M., Mishra, S., Soja, B.  (2024b). The increasingly dominant role of climate change on length of day variations. Proceedings of the National Academy of Sciences, 121: e2406930121, https://doi.org/10.1073/pnas.2406930121