Global Loading and Deformation at Tidal Timescales

The periodic motion of the world's oceans (the tide) is a well-known everyday phenomenon, arising from the variation of the gravitational forces due to the Moon and Sun as their distance to the Earth changes. Such periodic (tidal) motion is actually composed of many different constituents, which summed together give the total tidal effect. The eight dominant constituents have periods close to 12 and 24 hours, which is why in many parts of the world, roughly two high and two low waters occur each day, but the level of high water (and low water) varies over time. The resulting change in distribution of water results in periodic 'ocean tide loading' on the surface of the Earth, causing it to displace by more than 10 cm in around 6 hours, in some parts of the world. Ocean tide loading can be difficult to predict, largely because the tidal movements of the oceans are themselves hard to model due to uncertainties in the movement of the water masses near to complex coastlines and in shallow seas. A less well known effect is that the solid Earth also deforms due to the same gravitational attractions of the Moon and Sun, and with the same periods as the ocean tides. Close to the Equator, the solid Earth's surface moves through a range of nearly 40 cm in around 6 hours. This 'solid Earth tide' can be predicted from the very well known astronomy of the Moon and Sun, combined with models of the physical structure of the Earth. Such models describe whether the Earth may be treated, at these tidal periods, as elastic (i.e. it returns to its original shape when the deforming gravitational force no longer acts), or whether it exhibits a variation from elastic behaviour at such tidal periods (i.e. it has anelastic properties, which result in dissipation of energy and changes in the Earth's rotation rate). The inner Earth's physical behaviour is expected to be frequency-dependent, and is well studied at seismic frequencies (periods of seconds to minutes) using the travel times of vibrations transmitted by earthquakes, and by studying changes in the Earth's rotation known as the Chandler wobble (which has a period of ~14 months). However, it is less well observed at intermediate periods such as those of the tides (~12 hours to 1 year, although small longer-period tides also occur). Recent developments in the measurement of the Earth's shape using Global Positioning System (GPS) satellites allow us to measure tidal movements of the solid Earth with high precision. The International GPS Service maintains a freely-available archive of GPS data from a steadily growing number of global observatories, in some cases going back to the early 1990s. This archive has now reached sufficient duration and spatial coverage to allow a reliable global study using GPS, in a way not possible with previous satellite or astronomical techniques. By selecting sites where the ocean tide loading effect is well calibrated, we will use GPS observations to infer the degree of anelastic behaviour of the solid Earth at tidal timescales. Conversely, our observations of the ocean tide loading effect at other sites will allow us to discriminate between existing ocean tide models, and to identify required augmentations to these models. Our work will have important consequences, not only for our knowledge of the Earth as a planet, but also for the measurement of key aspects of present-day climate change from space. In particular, the changes in the Earth's gravitational attraction due to the ocean tides can interfere with GRACE satellite measurements of longer-term climatic movements of water and ice, and the ocean tide loading-related variations in Earth's shape impede our ability to measure changes in ice sheet thickness using satellites such as ICESat/CryoSat.