The six-year cycle in atmospheric angular momentum: robustness, zonal-wind structure, and implications for Earth rotation

Abstract. Variability near a 6-yr period has been reported in the length of day, motions within the Earth’s fluid core, several climatic parameters, and atmospheric angular momentum. Here we demonstrate the robustness of a quasi-6-yr oscillation in atmospheric angular momentum using several independent atmospheric reanalysis products over 1980–2020. This signal is highly significant, consistent across datasets, and accounts for up to about 25 % of atmospheric angular momentum variance at interannual time scales. Its expression in the atmospheric zonal wind circulation exhibits a coherent vertical structure throughout the troposphere, with maximum amplitudes near the tropopause in the tropical belt. In addition, the 6-yr oscillation in zonal winds is in phase across from southern to northern latitudes.  This structure distinguishes the 6-yr signal from the annual cycle and from ENSO-related variability, and points to a large-scale, organized component of the atmospheric circulation, consistent with alternating phases of weaker and stronger atmospheric super-rotation relative to the solid Earth. While the origin of the length-of-day 6-yr cycle is relatively well established and attributed to exchange of angular momentum  from the core to the mantle, the process underlying the 6-yr variability in the zonal wind circulation remains to be elucidated.