Most of the relative angular momentum of the atmosphere about the polar axis resides, on average, in the subtropical jet streams of the upper troposphere. Because the behavior of these jet streams is crucial to global weather and climate, the angular momentum framework has long been central to studies of large-scale atmospheric dynamics. In the past decade and a half, the realization has also grown that this framework is more generally applicable to understanding the dynamics of the complete, coupled atmosphere-ocean-solid Earth system. This viewpoint has emerged largely as a result of the ability of modern atmospheric and geodetic measurements to demonstrate that changes in the axial component of the atmosphere's angular momentum (AAM), derived mostly from the zonal-wind field, explain most of the non-tidal changes in the length-of-day (LOD) over a broad range of time scales up to the decadal. For the equatorial plane, in which changes in the solid Earth's angular momentum appear as motions of the pole, recent results [e.g., Ponte, 1995a] indicate that both atmospheric and oceanic motions, and the redistribution of mass within these fluids, are important in exciting polar motions on subseasonal time scales, suggesting anew the power of the planetary angular momentum framework for addressing issues about the coupling among the components of the global climate system.
Spurred on by improving geodetic measurements of LOD, meteorologists have turned to assessing how well their operational wind and pressure analyses, used to compute AAM, perform. Rosen  demonstrates that notable differences in zonal wind fields, and hence in the axial component of AAM, from two major operational weather centers do exist, but these differences have diminished with time. Currently, the accuracy of operational-based values of AAM seems to be at a plateau, and the coherence between such series from different centers is lower than expected at sub-weekly periods. A challenge facing EOS-era meteorological analyses, therefore, will be to improve upon the current level of performance with respect to AAM calculations.
Preliminary efforts in responding to this challenge at NASA Goddard Space Flight Center and NOAA/National Meteorological Center (NMC) where reanalysis projects herald future developments in analysis techniques, are encouraging. For example, reanalyzed series of AAM from NMC agree more closely with LOD values than do the operational analyses (Figure 2), a result being advanced by NMC as an indication of the quality of their reanalyses. This represents a clear instance in which geodetic measurements are proving valuable in testing the quality of meteorological products of the sort to emerge from EOS.
The angular momentum framework is also being used to test the ability of atmospheric general circulation models (GCMs) to simulate the current climate, a prerequisite to trusting these models' predictions of future climate change. On monthly and longer time scales, AAM is one of the most accurate of the presently observed indices for validating climate models. Its close relationship with LOD on these time scales bolsters confidence in its accuracy. In Hide et al. , we compare AAM values from 23 GCMs participating in the Atmospheric Model Intercomparison Project (AMIP) with observations for the decade 1979-88. Results reproduced in Figure 3 for the decadal mean values of the related component of axial AAM, for the models and the NMC-based observations, suggest that a wide range in model performance exists. Seasonal variations in AAM are reproduced well by the AMIP GCMs, with the median correlation coefficient between model and observed seasonal cycles equal to 0.95, but interannual signals in AAM are less successfully captured by the models. Importantly, there does not appear to be a relationship between a model's performance on one time scale and that on another, so that improving the skill of GCMs will require that attention be paid to their treatment of physical processes across a broad spectrum of frequencies.
Not only is AAM a useful index against which to judge the GCMs' representation of current climate, but also it can be used to measure the impact of doubling on future climate scenarios. Whereas most studies of global climate change have focused on surface parameters, AAM integrates the dynamical behavior of the entire atmosphere. Rosen and Gutowski  discover that changes in the tropical zonal wind fields of three GCMs run under doubled- conditions are comparable to the interannual variability in these fields; such changes, therefore, and concomitant ones in AAM and LOD, ought to become detectable at some stage. The prediction of an anthropogenic effect on Earth's rotation is still highly tentative but noteworthy nonetheless.