ENSO-driven oceanic excitation of polar motion

The El Niño–Southern Oscillation (ENSO) is the dominant mode of variability in the atmosphere—ocean system. It is characterised by circulation anomalies in the tropical Pacific that affects other regions of the planet through teleconnections. Studies over the last decades have demonstrated that ENSO exerts a strong control on zonal atmospheric angular momentum and consequently changes in length-of-day, but a comparable effect on polar motion remains to be quantified. Here, we test the hypothesis that part of the ENSO imprint on polar motion excitation is embedded in oceanic angular momentum (OAM) changes.

To this end, we analyse output from four CMIP6 (Coupled Model Intercomparison Project 6) climate models using lagged regression analysis, with a particular focus on monthly ocean bottom pressure (p_b) changes over a 165-year period. The regression of the p_b fields against each model’s ENSO index reveals prominent anomalies in the Bellingshausen Basin and a large-scale bipolar pattern between the Pacific and Indian oceans, which is also evident in satellite gravimetry data. The OAM changes implied by these p_b anomalies excite polar motion primarily along 90°E, showing amplitudes of ±4 mas during recently observed El Niño/La Niña events (e.g., 1997/98, 2006/07, 2009/10). After removal of known geophysical fluid effects, the ENSO-related oceanic excitation accounts for ~40%–50% of the variance in observed polar motion excitation. However, as these fluctuations co-occur with other broadband (oceanic) excitation signals, polar motion observations may provide only limited insight into the variability of ENSO itself.