Variations in the Earth's oblateness during the past 28 years

Analysis of satellite laser ranging (SLR) data indicates that the Earth's dynamic oblateness (J(2)) has undergone significant variations during the past 28 years. The dominant signatures in the observed variations in J(2) are (1) a secular decrease with a rate of approximately -2.75x10(-11) yr(-1), (2) seasonal annual variations with a mean amplitude of 2.9x10(-10), (3) significant interannual variations with timescales of 4-6 years, and (4) a variation with period of similar to21 years and an amplitude of similar to1.4x10(-10) with minimum in December 1988. Two large interannual variations are related to the strong El Nino-Southern Oscillation events during the periods of 1986-1991 and 1996-2002, and it appears that another interannual cycle may have started in late 2002. The superposition of the decadal variation on the interannual signal makes the J(2) fluctuation appear to be anomalously large during the 1996-2002 period. Contemporary models of the mass redistributions in the atmosphere, ocean, and surface water can explain a major part of the 4- to 6-year fluctuations. However, the cause of the decadal variation remains unknown.