The dynamic interaction of the solid Earth with its fluid envelope (atmosphere, oceans, and hydrology) provides one of the major sources for the manifold spatio-temporal variations in Earth rotation. The observed fluctuations in the motion of our planet are classically divided into three components: variations of the rotation speed, reckoned in changes in length of day (LOD), the motion of the spin axis in a reference frame tied to the Earth is known as polar motion, and changes in the orientation of this spin axis in a space-fixed reference frame are referred to as precession-nutation. Specifically, subdecadal and non-tidal changes in LOD are almost entirely related to atmospheric dynamics, while polar motion variability at periods from a few days to several years is mainly driven by the atmosphere, the oceans, and hydrology to a lesser extent. Earth's nutational motion relates mostly to the gravitational interaction with other celestial bodies, but can be affected by quasi-diurnal atmospheric and oceanic excitation at the level of 0.1 mas (milliarcseconds).

Comparison of observed changes in length of day (black line) and axial atmospheric angular momentum (greeen line) for the time span 1980 to 2010. (a) Initial time series after elimination of secular trend, decadal variations and the effect of solid Earth tides. (b) Annual and semiannual signal components obtained by Wavelet filtering. (c) Residual time series. Figure extracted from Schindelegger et al. (2013b). |

When studying the influence of the atmosphere on Earth rotation, it has become common practice to use the effective atmospheric angular momentum (AAM) functions, which can be directly calculated from globally-gridded meteorological data. The AAM consists of two components, usually referred to as matter and motion terms (or pressure and wind terms). The matter term describes the influence of atmospheric mass redistributions on the Earth's inertia tensor, whereas the motion term corresponds to the relative angular momentum of the atmosphere with respect to the mean rotating reference system. In its most practical formulation, the two AAM components are estimated from surface pressure data and from the global fields of zonal and meridional wind velocities.

The general goal of the AAM part of GGOS Atmosphere was to rigorously investigate atmospheric effects on Earth rotation on all time scales, with particular attention being paid to diurnal and sub-diurnal variations. Some of the specific objectives were:

- Determination of AAM pressure and wind terms from operational ECMWF data at 6-hourly intervals and 3-hourly forecasts consistent with the determination of the other target quantities (especially loading and gravity).
- Estimation of the total atmospheric mass and the cartesian coordinates of the global atmospheric center of mass—also on an operational basis at 6-hourly intervals. In the same manner, the height of the center of mass for each atmospheric column is provided.
- Crosschecking the AAM quantities on all time-scales with the torques exerted by the solid Earth on the atmosphere. The analytical equivalence between these two types of excitation measures is formulated by the AAM budget equation, which provides information on the reliability of the underlying atmospheric models (see Schindelegger et al., 2013a). These investigations have also laid the groundwork for project ASPIRE.
- Computation of AAM terms at the frequencies of atmospheric radiational tides and verification of the pressure portion to independent estimates, such as the pressure terms constructed from globally distributed barometric in-situ observations.

M. Schindelegger. Atmosphere-induced short period variations of Earth rotation, Geowissenschaftliche Mitteilungen, Schriftenreihe der Studienrichtung Vermessung und Geoinformation, Wien, Heft. Nr. 96, 2014.

M. Schindelegger, D. Salstein, J. Böhm. Recent estimates of Earth-atmosphere interaction torques and their use in studying polar motion variability, J. Geophys. Res., Vol. 118, 8, pp. 4586-4598, doi:10.1002/jgrb.50322, 2013a.

M. Schindelegger, S. Böhm, J. Böhm, H. Schuh. Atmospheric effects on Earth rotation, in J. Böhm and H. Schuh (eds): Atmospheric Effects in Space Geodesy, Springer Verlag, pp. 181-231, doi:10.1007/978-3-642-36932-2_6, 2013b.

M. Schindelegger, J. Böhm, D. Salstein. Seasonal and intraseasonal polar motion variability as deduced from atmospheric torques, J. Geod. Geoinf., Vol. 1, Iss. 2, pp. 89-95, doi:10.9733/jgg.231112.1, 2013c.

M. Schindelegger, J. Böhm, D. Salstein, H. Schuh. High-resolution atmospheric angular momentum functions related to Earth rotation parameters during CONT08, J. Geod., Vol. 85, pp. 425-433, doi:10.1007/s00190-011-0458-y, 2011.