Meteorological excitation of interannual polar motion by the North Atlantic Oscillation

Abstract

Variations of the atmospheric angular momentum (AAM) is responsible for almost all of the observed length-of-day (LOD) variation on time scales from several days to several years, and much of the observed polar motion on the intra-seasonal time scale. El Niño–Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO) have been singled out to successfully explain interannual LOD variations. However, excitation sources for the interannual polar motion, including the 14 month Chandler wobble, have not been positively identified. This paper studies the correlation of the observed polar motion Ψ with the North Atlantic Oscillation (NAO), an interannual, mid-latitude, north-south seesaw in the atmosphere-ocean system. An index (NAOI) used as a proxy to measure the strength of NAO is compared with the observed interannual polar motion excitation during 1964–1994. Cross-correlation functions and coherence spectra demonstrate significant correlations between NAOI and Ψ, especially in the x component as expected from NAOs location and general pattern. This constitutes a positive identification of excitation of interannual polar motion by a meteorological system. Furthermore, our corresponding results with respect to ENSO strongly suggest that a combination of meteorological systems will better explain the observed interannual polar motion and perhaps the Chandler wobble.