Chaotic oceanic excitation of low-frequency polar motion variability

Studies of Earth rotation variations generally assume that changes in non-tidal oceanic angular momentum (OAM) manifest the ocean's forced response to atmospheric pressure, wind, and heat and freshwater fluxes. However, on monthly and longer time scales, changes in OAM may also arise from chaotic intrinsic ocean variability, which has its origin in local non-linear (e.g., mesoscale) dynamics that can further map into mass fluctuations at basin scales. To examine whether or not such chaotic mass redistributions appreciably affect Earth’s polar motion, we compute monthly OAM anomalies from a 50-member ensemble of eddy-permitting global ocean/sea-ice simulations that sample intrinsic variability through a perturbation approach on model initial conditions. The resulting OAM (i.e., excitation) functions are compared both amongst each member and with Earth rotation data from 1995 to 2015. We find that intrinsic variability plays an important role in the excitation of the Chandler wobble, where it modulates the band-integrated excitation power due to forced OAM changes (2.5 mas² over 1995–2015, mas = milliarcseconds) by up to ±2 mas2. At interannual frequencies below the Chandler band, intrinsic signals (ensemble spread) account for 16–36% of the variability in the total equatorial oceanic excitation, with contributions being split almost equally between mass and motion terms. More than half of the variance in the mass term contribution is associated with one mode of intrinsic bottom pressure variability, which has opposite polarity between the Atlantic and the Southern Ocean and largest amplitudes around Drake Passage. Overall, chaotic oceanic excitation represents a factor to consider when interpreting low-frequency polar motion changes in terms of natural climate oscillations or core-mantle interactions.