1.5‐Year Periodic Variations in the Equatorial Components of the Earth's Figure Axis and Their Dynamical Response to Major Climate Modes

Abstract Temporal variations in the equatorial components of the Earth's figure axis, represented by the second‐degree, first‐order Stokes coefficients (Δ C 21 and Δ S 21 ), reflect large‐scale mass redistribution within the Earth system and are key to understanding Earth rotation dynamics. However, the research on the dominant periodic features of the figure axis and their responses to major climate modes based on multiple observational sources, including satellite laser ranging, satellite gravimetry, Earth orientation parameters, and effective angular momentum data sets, remains relatively insufficient. Here, we apply the Multichannel Singular Spectrum Analysis (MSSA) to systematically investigate the principal periodic characteristics of Δ C 21 and Δ S 21 derived from multiple sources and to identify their underlying drivers. The results reveal significant annual, semiannual, ∼120‐day, and ∼1.5‐year periodic signals across all data sets, with the origin and climatic linkage of the ∼1.5‐year signal elucidated for the first time. Quantitative analyses show that the ∼1.5‐year signal is dominated by oceanic angular momentum (OAM), with smaller contributions from hydrological (HAM) and sea‐level (SLAM) angular momentum, whereas coherence results consistently indicate the strongest coupling with OAM, followed by SLAM and weaker coherence with HAM. Comparisons with major climate indices, including the El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Meridional Mode (AMM), reveal clear correlations with distinct lag times. Specifically, the ∼1.5‐year ENSO component leads Δ C 21 by 5–8 months and Δ S 21 by 2–4 months, with correlations up to 0.8 for Δ C 21 . IOD and AMM also contribute, indicating coupled influences of ENSO‐, IOD, and AMM‐driven mass redistribution on figure axis variation.