Detections of a 6-year cycle in the Earth system

<p dir="ltr">A cycle of about 6 years has long been observed in the Earth&#8217;s magnetic field, length of day, dynamic oblateness, polar motions and surface displacements and attributed to dynamical processes occurring in the core and at the core mantle boundary. Recently, a 6-year cycle has also been detected in the rate of change of the global mean sea level and the ice-mass contributions from Greenland and continental glaciers. In this study, we report new observations of a 6-year cycle in the terrestrial water storage estimates based on the satellite gravity missions GRACE and GRACE-FO, consistent with precipitation and global hydrological models. The causes for such oscillations in the climate system are still unexplained, but raise the question of the respective contributions of the Earth&#8217;s deep interior and external surface fluid envelopes to the 6-year cycles reported in many geodetic variables. Indeed, while some of these 6-year fluctuations are convincingly attributed to Earth&#8217;s deep interior processes, for some other variables, climate-related processes occurring in the surface fluid envelopes or at the Earth&#8217;s surface may be more likely. This issue is exacerbated by an opposition of phase discovered between the angular momentum of the atmosphere and the length of day at around 6 years, suggesting that dynamical processes occurring in the Earth&#8217;s core induce a rotation of the solid Earth and the atmosphere as a single system. An overview of the 6-yr cycle observed in different variables of the Earth System may therefore help to better understand potential links between the solid Earth and climate.</p>