Interannual atmospheric torque and El Niño–Southern Oscillation: Where is the polar motion signal?

In this paper, we investigate the atmospheric excitation of polar motion (PM) associated with the El Niño–Southern Oscillation (ENSO) phenomenon. ENSO effects on length‐of‐day due to changes in the axial component of atmospheric angular momentum (AAM) have long been recognized, but identification of PM excitation with ENSO‐induced equatorial AAM anomalies has proved more elusive. Here we use an appropriately modified form of the inverted barometer (IB) assumption to study ENSO‐related atmospheric torques arising from pressure loading on the Earth's ellipsoidal bulge and mountains and from frictional wind stress over the Earth's land‐ and ocean‐covered surface. The resulting dissipation torques, which accommodate adjustment of the oceanic mass distribution to time‐variable atmospheric loading, are found to be small. The ellipsoidal torques have the largest amplitude, reflecting the order‐of‐magnitude discrepancy between the height departures of the Earth's bulge (∼20 km) and its surface orography (∼2 km). Because of relatively uniform pressure covariances with the Southern Oscillation Index over the continents for the largely land‐based X component and the uniform IB response for the largely ocean‐based Y component; however, the ENSO‐related PM excitation arising from the ellipsoidal torques is reduced to an amplitude comparable with the sum of regional mountain torques from the individual continents. The largest of these are generated over Asia and Antarctica, arising from efficient coupling of ENSO‐related surface pressure anomalies with large‐scale orographic features. The geometrical mitigation of the ellipsoidal torques, classically expected to dominate equatorial AAM forcing, accounts for the lack of a detectable atmosphere‐driven polar motion response to ENSO.