Long-term variations in the Earth's Figure axis from SLR

The mass redistribution within the Earth's system causes variations in both the polar motion and Earth's figure axis on time scales from sub-daily to decadal and longer. The Earth's principal figure axis is defined by the degree two and order one geopotential coefficients, C21 and S21, and have been determined from both Satellite Laser Ranging (SLR) and the GRACE gravity measurements. Previous studies indicate good agreement, during the GRACE mission period, between the annual variations in the C21/S21 determined by SLR, GRACE and the polar motion excitation. The analysis of the SLR data over the last 30 years has shown that the Earth's figure axis undergoes significant decadal variations, besides the variations at Chandler and annual frequencies. This paper will present a comparison of the decadal variations in C21/S21 from SLR and polar motion excitation, and excitations due to the atmosphere-ocean and surface water. Analysis of the long-term variations in both the figure axis and polar motion excitation can provide greater understanding of a wide variety of geophysical and geodynamical processes, and could provide a constraint on temporal variations of the inner core.