The Effect of Future Thermal Sea Level Changes on the Excitation of Earth Orientation Variations

We present a mechanism by which steric sea level rise leads to changes of the Earth's orientation parameters, e.g., polar motion (PM) and length-of-day (LOD) on decadal to centennial timescales. Steric sea level change is commonly considered unable to excite changes in Earth's orientation parameters. We show, however, that a causal link exists since thermal sea level rise leads to horizontal mass redistribution within ocean basins and hence to ocean bottom pressure changes. The projected changes of ocean angular momentum in the 21st and 22nd century are derived from simulations with the coupled climate models ECHAM5/MPI-OM and GFDL-CM2.1, forced with the IPCC-A1B emission scenario. The net effect is a mass transfer from the southern to the northern hemisphere, and also a net movement of mass closer towards Earth's axis of rotation. This, in turn, leads to a small negative LOD trend of about -0.002 ms per 10 millimeter of steric sea level rise. For polar motion, we project that ocean warming excits a movement of about 1.7 milli-arcseconds per 10 millimeter of steric sea level rise, nearly linearly polarized towards 150 degrees West. Because steric sea level rise was too small over the last 50 years, this mechanism cannot account for unexplained observed decadal fluctuations of PM and LOD during this period. However, as ocean warming and steric sea level rise are expected to accelerate in the decades to come, we conjecture that steric sea level rise could contribute increasingly more to Earth orientation variations in the near future.