Richard S. Gross

the excitation of the Chandler wobble

The Chandler wobble is an excited resonance of the Earths rotation having a period of about 14 months. Although it has been under investigation for more than a century, its excitation mechanism has remained elusive. Here, the angular momentum of the atmosphere computed from the products of a numerical weather prediction analysis system and the angular momentum of the oceans computed from a global oceanic general circulation model driven by observed surface winds and fluxes are used to show that during 1985.0–1996.0 the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean-bottom pressure fluctuations.

Earth Rotation Variations – Long Period

The solid Earth is subject to a wide variety of forces including external forces due to the gravitational attraction of the Sun, Moon, and planets; surficial forces due to the action of the atmosphere, oceans, and water stored on land; and internal forces due to earthquakes and tectonic motions, mantle convection, and coupling between the mantle and both the fluid outer core and the solid inner core. The solid Earth responds to these forces by displacing its mass, deforming its shape, and changing its rotation. Geodetic observing systems can measure the change in the Earths gravity caused by mass displacement, the change in the Earths shape, and the change in the Earths rotation. Consequently, geodetic observing systems can be used to study both the mechanisms causing the Earths shape, rotation, and gravity to change and the response of the solid Earth to these forcing mechanisms. As a result, geodetic observing systems can be used to gain greater understanding of the Earths interior structure and of the nature of the forcing mechanisms including their temporal evolution. In this chapter, the variations in the Earths rotation that occur on timescales greater than a day are discussed. The standard theory used to study the variations is reviewed, the techniques by which the variations are observed are described, and the causes of the observed variations are discussed.

Atmospheric and oceanic excitation of length-of-day variations during 1980-2000

[1] Although nontidal changes in the Earth’s length-of-day on timescales of a few days to a few years are primarily caused by changes in the angular momentum of the zonal winds, other processes can be expected to cause the length-of-day to change as well. Here the relative contribution of upper atmospheric winds, surface pressure, oceanic currents, and ocean-bottom pressure to changing the length-of-day during 1980–2000 is evaluated using estimates of atmospheric angular momentum from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project, estimates of the angular momentum of the zonal winds in the upper atmosphere from the United Kingdom Meteorological Office, and estimates of oceanic angular momentum from the Estimating the Circulation and Climate of the Ocean consortium’s simulation of the general circulation of the oceans. On intraseasonal timescales, atmospheric surface pressure, oceanic currents, and ocean-bottom pressure are found to be about equally important in causing the length-of-day to change, while upper atmospheric winds are found to be less important than these mechanisms. On seasonal timescales, the upper atmospheric winds are more important than the sum of currents and bottom pressure in causing the length-of-day to change and, except at the annual frequency, are even more important than surface pressure changes. On interannual timescales, oceanic currents and ocean-bottom pressure are found to be only marginally effective in causing the length-ofday to change. INDEX TERMS: 1223 Geodesy and Gravity: Ocean/Earth/atmosphere interactions (3339); 1239 Geodesy and Gravity: Rotational variations; 3319 Meteorology and Atmospheric Dynamics: General circulation; 4532 Oceanography: Physical: General circulation; KEYWORDS: Earth rotation, length-ofday, oceanic angular momentum

The effect of ocean tides on the earth's rotation as predicted by the results of an ocean tide model

The published ocean tidal angular momentum results of Seiler [1991] are used to predict the effects of the most important semidiurnal (M2, S2, N2), diurnal (K1, O1, P1), and long period (Mf, Mf′, Mm, and Ssa) ocean tides on the Earths rotation. The separate, as well as combined, effects of ocean tidal currents and sea level height changes on the length-of-day, UT1, and polar motion are computed. The predicted polar motion results reported here account for the presence of the free core nutation and are given in terms of the motion (within the rotating, body-fixed terrestrial reference frame) of the celestial ephemeris pole so that they can be compared directly to the results of observations. Outside the retrograde diurnal tidal band, the summed effect of the semidiurnal and diurnal ocean tides studied here predict peak-to-peak polar motion amplitudes as large as 2 mas. Within the retrograde diurnal tidal band, the resonant enhancement caused by the free core nutation leads to predicted polar motion (or, equivalently, nutation) amplitudes as large as 9 mas (at the unobservable retrograde K1 tidal frequency).

Astrometric and space‐geodetic observations of polar wander

Optical astrometric measurements of star positions taken during the past century have been recently re-reduced using the final Hipparcos star catalog in order to determine variations in the mean location of the Earths rotation pole with respect to the Earths crust. This newly available polar motion series, which is the longest homogeneous polar motion series currently available, allows the drift in the pole path to be newly estimated. During the 1900.0 to 1992.0 span of the smoothed Hipparcos polar motion series, the Earths rotation pole is observed to drift at a mean linear rate of 3.51±0.01 milliarcseconds/year (mas/yr) towards 79.2±0.2°W longitude. This new estimate for the observed trend in the pole path, which can be considered to be the present-day expression of true polar wander, is nearly the same as that estimated in previous studies using the homogeneous ILS polar motion series.

Space geodesy monitors mass transports in global geophysical fluids

Large-scale mass transports in the Earth system produce variations in Earths rotation, gravity field, and geocenter. Although relatively small, these global geodynamic effects have been measured by space geodetic techniques to increasing, unprecedented accuracy, opening up important new avenues of research that will lead to a better understanding of global mass transport processes and the Earths dynamic responses. To take full advantage of these advances, the International Earth Rotation Service (IERS), the organization that monitors the rotational motions of the Earth and related properties, saw the need in 1998 to create an infrastructure to facilitate the link between the space geodetic measurement and the geodynamic “global change” research communities [Dehant et al., 1997]. Hence was born the IERS Global Geophysical Fluids Center (GGFC).

Accuracy of the International Terrestrial Reference Frame origin and Earth expansion

The International Terrestrial Reference Frame (ITRF) is a fundamental datum for high?precision orbit tracking, navigation, and global change monitoring. Accurately realizing and maintaining ITRF origin at the mean Earth system center of mass (CM) is critical to surface and spacecraft based geodetic measurements including those of sea level rise and its sources. Although ITRF combines data from satellite laser ranging (SLR), Very Long Baseline Interferometry (VLBI), Global Positioning System (GPS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), its origin is currently realized by the single technique of SLR. Consequently, it is difficult to independently evaluate the origin accuracy. Also, whether the solid Earth is expanding or shrinking has attracted persistent attention. The expansion rate, if any, has not been accurately determined before, due to insufficient data coverage on the Earths surface and the presence of other geophysical processes. Here, we use multiple precise geodetic data sets and a simultaneous global estimation platform to determine that the ITRF2008 origin is consistent with the mean CM at the level of 0.5 mm yr?1, and the mean radius of the Earth is not changing to within 1? measurement uncertainty of 0.2 mm yr?1.

Considerations concerning the non-rigid Earth nutation theory.

This paper presents the reflections of the Working Group of which the tasks were to examine the non-rigid Earth nutation theory. To this aim, six different levels have been identified: Level 1 concerns the input model (giving profiles of the Earths density and theological properties) for the calculation of the Earths transfer function of Level 2; Level 2 concerns the integration inside the Earth in order to obtain the Earths transfer function for the nutations at different frequencies; Level 3 concerns the rigid Earth nutations; Level 4 examines the convolution (products in the frequency domain) between the Earths nutation transfer function obtained in Level 2, and the rigid Earth nutation (obtained in Level 3). This is for an Earth without ocean and atmosphere; Level 5 concerns the effects of the atmosphere and the oceans on the precession, obliquity rate, and nutations; Level 6 concerns the comparison with the VLBI observations, of the theoretical results obtained in Level 4, corrected for the effects obtained in Level 5.Each level is discussed at the state of the art of the developments.

Degree‐2 harmonics of the Earth's mass load estimated from GPS and Earth rotation data

A fluid, mobile atmosphere and oceans surrounds the solid Earth and upon its land surface lays a continually changing distribution of ice, snow, and ground water. The changing distribution of mass associated with the motion of these surficial fluids changes the Earths rotation by changing its inertia tensor and changes the Earths shape by changing the load on the solid Earth. It has recently been demonstrated that large-scale changes of the Earths shape, and hence of the mass load causing the Earths shape to change, can be measured using the global network of GPS receivers. Here, the degree-2 mass load coefficients determined from GPS data are compared with those obtained from Earth orientation observations from which the effects of tides, winds, and currents have been removed. Good agreement is found between these two estimates of the degree-2 mass load, particularly at seasonal frequencies.

The Gravitational Signature of Earthquakes

The static displacement field generated by an earthquake redistributes the Earth’s mass and consequently causes the Earth’s rotation and global gravitational field to change. Although the coseismic effect of earthquakes on the Earth’s rotation and global gravitational field has been modeled in the past, no unambiguous observations of this effect have yet been made. However, the Gravity Recovery And Climate Experiment (GRACE) satellite, which is scheduled to be launched in 2001, will measure time variations of the Earth’s gravitational field to high degree and order with unprecedented accuracy. Here, the coseismic effect of earthquakes on the Earth’s global gravitational field will be modeled and compared with the expected accuracy of the GRACE measurements. It is shown that the coseismic effects of great earthquakes such as the 1960 Chilean or 1964 Alaskan events can cause global gravitational field changes that are large enough to be detected by GRACE. However, the coseismic effects of the largest earthquakes that have occurred during the past 35 years cause global gravitational field changes that are probably too small to be detected by GRACE.