Earth's dynamic oblateness (J2) has been decreasing due to postglacial rebound (PGR). However, J2 began to increase in 1997, indicating a pronounced global-scale mass redistribution within Earth's system. We have determined that the observed increases in J2 are caused primarily by a recent surge in subpolar glacial melting and by mass shifts in the Southern, Pacific, and Indian oceans. When these effects are removed, the residual trend in J2 (-2.9 x 10(-11) year-1) becomes consistent with previous estimates of PGR from satellite and eclipse data. The climatic significance of these rapid shifts in glacial and oceanic mass, however, remains to be investigated.

Subdecadal changes in Earth's rotation rate, and hence in the length of day (LOD), are largely controlled by variations in atmospheric angular momentum. Results from two oceanic general circulation models (OGCMs), forced by observed wind stress and heat flux for the years 1992 through 1994, show that ocean current and mass distribution changes also induce detectable LOD variations. The close similarity of axial oceanic angular momentum (OAM) results from two independent OGCMs, and their coherence with LOD, demonstrate that global ocean models can successfully capture the large-scale circulation changes that drive OAM variability on seasonal and shorter time scales.

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Fluctuations in Earth rotation over time scales of 2 years or less are dominated by atmospheric effects; spectral analyses of length-of-day (LOD) and atmospheric angular momentum (AAM) datashow significantly increased variability in the 40-50 day band. LOD and AAM fluctuations on the 40-50 day time scale have previously been linked to tropical, convectively driven waves of the type first described by Madden and Julian (1971) (referred to as MJ hereinafter). A significant spectral peak centered at 42 days has also been found, however, in the AAM of a 3-year (1120-day) perpetual-January simulation of the global atmosphere, performed using a version of the University of California at Los Angeles (UCLA) general circulation model (GCM) which does not give rise to MJ oscillations in the tropics. In the present work the 40-50 day oscillation is studied using the 12-year overlap between two records: (1) AAM data, compiled from the National Meteorological Center (NMC); and (2) LOD variation from the Jet Propulsion Laboratory Kalman-filtered Earth rotation series. We analyze the NMC records by latitude belts, in light of the UCLA GCM results, in order to identify possibly distinct sources of the AAM oscillation in the mid-latitudes and the tropics. Results suggest that two intraseasonal oscillations exist in the Earth-atmosphere system: a tropical, 50-day oscillation associated with the convectively driven MJ wave and a mid-latitude, 40-day oscillation associated with the interaction of nonzonal flow with topography.

Irregular length of day (LOD) fluctuations on time scales of less than a few years are largely produced by atmospheric torques on the underlying planet. Significant coherence is found between the respective time series of LOD and atmospheric angular momentum (AAM) determinations at periods down to 8 days, with lack of coherence at shorter periods caused by the declining signal-to-measurement noise ratios of both data types. Refinements to the currently accepted model of tidal Earth rotation variations are required, incorporating in particular the nonequilibrium effect of the oceans. The remaining discrepancies between LOD and AAM in the 100- to 10-day period range may be due to either a common error in the AAM data sets from different meteorological centers, or another component of the angular momentum budget.