The atmosphere is one of the fluid portions of the Earth and its angular momentum shows significant temporal variations due to large-scale mass redistributions and changes in the pattern of winds. If one disregards external torques exerted by other solar bodies, the collective angular momentum of all fluid layers (atmosphere, ocean, core, …) and the solid Earth (crust and mantle) has to be conserved. This means that changes in the angular momenta of the solid Earth and the fluid layers are of equal size but otherwise opposite. Changes in the angular momentum of the solid Earth, however, are visible as fluctuations of the rotation of our planet. In this way, atmospheric processes are responsible for a certain part of geodetic polar motion and observed changes in length of day (LOD), see figure.
|Changes in length of day for 2009: observed change (grey line) vs. wind excitation (green line).|
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 were introduced by Barnes et al. (1983, Proc. R. Soc. Lond.) and 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 is to rigorously model atmospheric effects on Earth rotation for all time scales and improving the models which are involved in the estimation process. In detail, this comprises the following tasks:
Schindelegger M., Böhm J., Salstein D., Schuh H. High-resolution atmospheric angular momentum functions related to Earth rotation parameters during CONT08. Journal of Geodesy, 85(7), pp. 425-433, doi: 10.1007/s00190-011-0458-y, 2011.