Information on the movements of the water within the Earth’s system as provided by the GRACE/GRACE-FO measurements helps to understand disturbances in the Earth’s rotation caused by the global mass distribution and mass transport in atmosphere, oceans, and the terrestrial hydrosphere.
Dr. Justyna Śliwińska, Space Research Centre (Warsaw, Poland)
Why is the knowledge about the Earth's time-variable rotation so important?
Observations of Earth rotation variations are essential in various areas of science, technology and everyday life. The parameters determining the orientation of our planet in space and its rotation (Earth Orientation Parameters, EOP) are used for precise positioning and navigation on the Earth’s surface with satellite techniques and for orienting astronomical instruments. They are needed for planning the launch of space missions and for communication with probes far away from Earth like the Voyager spacecrafts. In essence, time-varying EOP are precisely connecting a terrestrial reference frame used for locating objects on the rotating planet Earth with a celestial reference frame as realized by the positions of remote radio stars.
Causes for Earth orientation changes
The Earth’s rotation is not strictly uniform due to the gravity attraction of celestial bodies, the internal Earth structure, and dynamic processes taking place both at the surface and within the solid Earth. This includes post-glacial rebound of the Earth's mantle due to ice loss in North America and Fennoscandia after the last ice age, atmospheric and oceanic circulations, and also changes in mass distribution in the continental hydrosphere including icesheets, icecaps, and glaciers. As soon as the distribution of mass on Earth changes – be it due to the rapid increase of terrestrial water from heavy precipitation or due to tectonic plate motion – it has an impact on the Earth's rotation. By tracking alterations in gravity over time as realized with the GRACE missions, scientists have greatly enhanced our understanding of how those shifts in mass precisely affect the orientation of the Earth. Regular monitoring of the Earth's orientation and determining EOP with the means of space geodetic techniques like GNSS, SLR and VLBI is an important task in contemporary geodesy, which is coordinated by the International Earth Rotation and Reference Systems Service (IERS).
Using GRACE/GRACE-FO data to interpret Earth's rotation disturbances
Any redistribution of surface mass affects the Earth's inertia tensor, leading to a tilt in the rotational axis towards the direction of the mass deficit. Atmospheric and oceanic circulations also affect the rotational velocity. Monitoring the transport of mass within and between the Earth’s geophysical fluid layers is therefore crucial for understanding the mechanisms that disrupt the Earth's rotation.
While the primary goal of the GRACE and GRACE-FO missions is to monitor Earth's gravity field, the data they provide have proven invaluable for studying how changes in Earth's mass distribution influence its rotation. Linear relations exist between the degree-2/order-1 coefficients of time-variable geopotential (ΔC21, ΔS21) and the equatorial components (χ1, χ2) of the EOP mass excitation functions that characterize the position of the Earth’s rotational pole. A similar relation exists between ΔC20 and the axial component (χ3) of the excitation function describing the spin of the Earth around the pole.
The studies of temporal variations in terrestrial water storage (TWS) obtained from the GRACE/GRACE-FO showed that mass transport within the terrestrial hydrosphere has a particular impact on annual oscillation in polar motion excitation, due to the seasonal patterns of rainfall in many regions of the world (Fig. 1).
Recent climate changes, particularly the rise in global average air temperatures, contribute to the intensified melting of polar ice caps and mountain glaciers. Meltwater is primarily transported horizontally the surface towards the oceans where it contributes to barystatic sea-level rise. GRACE observations have revealed a mass loss of approximately 140 gigatons per year in Western Antarctica between 2002 and 2016, while the much smaller Greenland icesheet even contributed 280 gigatons per year. The resulting trends in polar motion reach several miliarcseconds per year (Fig. 2).
It is also likely that the rapid melting of the ice sheets in Greenland and Antarctica is responsible for the observed direction change in the pole's drift direction around the year 2000. GRACE/GRACE-FO observations of mass transport within the Earth system indicated non-negligible impact of the ocean dynamics on polar motion variations. Ocean-induced excitation functions of polar motion can reach more than 10 miliarcseconds. The χ1 component of the excitation function, which due to the spatial distribution of continents is more sensitive to changes in mass within oceanic areas, also shows a clear positive trend (Fig. 3).
From geophysical process understanding to EOP prediction
The improved understanding of the various excitation mechanisms of EOP changes as enabled by GRACE/GRACE-FO have also triggered numerous improvements in numerical models that may subsequently also provide predictive capabilities. In a recently completed international comparison campaign for EOP predictions, it became clear that methods utilizing forecasted excitation functions from atmosphere, oceans and hydrosphere dynamics lead to particularly accurate predictions at time-scales from a few days to many weeks. Future research at the Space Research Center in Warsaw will be thus directed towards better understanding of observed EOP variability, the consistent combination of EOP estimates from different sources, and improved methods for EOP prediction at various time-scales.
Further information
Further reading
- Adhikari, S., & Ivins, E. R. (2016). ClimateIn contrast to weather, which refers to daily or very short-term events, climate refers to an average condition in the atmosphere over a longer period of 30 to 40 years. All processes such as average temperature, precipitation, wind direction, wind s...-driven polar motion: 2003-2015. Science Advances, 2(4), 1–11. https://doi.org/10.1126/sciadv.1501693
- Dill, R., & Dobslaw, H. (2019). Seasonal variations in global mean sea level and consequences on the excitation of length-of-day changes. Geophysical Journal International, 218(2), 801–816. https://doi.org/10.1093/gji/ggz201
- Göttl, F., Groh, A., Schmidt, M., Schröder, L., & Seitz, F. (2021). The influence of Antarctic ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry. Earth, Planets and Space, 73(1). https://doi.org/10.1186/s40623-021-01403-6
- Nastula, J., Wińska, M., Śliwińska, J., & Salstein, D. (2019). Hydrological signals in polar motion excitation – Evidence after fifteen years of the GRACE mission. Journal of Geodynamics, 124, 119–132. https://doi.org/10.1016/j.jog.2019.01.014
- Śliwińska, J., Nastula, J., Dobslaw, H., & Dill, R. (2020). Evaluating gravimetric polar motion excitation estimates from the RL06 GRACE monthly-mean gravity field models. Remote Sensing, 12(6), 1–29. https://doi.org/10.3390/rs12060930