An Investigation of Recent Observed Changes in the Earth’s Oblateness

Beginning in 1998, large changes in the Earth’s oblateness have been recently detected using Satellite Laser Ranging (SLR). These changes are consistent with mass moving from the higher lat-itudes into the equatorial region, and are the largest such changes observed since SLR started track-ing the phenomena in the late 1970s. We have conducted a comprehensive investigation into the source of these changes. We have compared SLR results from several different research groups, and concluded there is no problem with the data analysis. We have considered a variety of possible geo-physical sources including tide modeling errors, atmospheric mass redistribution, and ocean mass redistribution. Of these, the ocean appears to be the most likely source of the observed oblateness changes. Specifically, the changes appear to be concentrated in the Pacific Ocean. The observed oblateness variations are highly correlated with the Pacific Decadal Oscillation (PDO). We investi-gated the mass variations in the oceans using two different sources: 1) the ECCO ocean model, and 2) TOPEX/Poseidon sea level measurements corrected for steric sea level change using buoy obser-vations. These results explain a significant fraction of the observed oblateness changes, but are somewhat smaller in magnitude. GSFC’s Analysis and Results The motivation for our work stemmed from the SLR data analysis performed by Cox an