Tree rings record hydroclimatic fluctuations in the length of day

Analyzing variations in the mantle’s rate of rotation and the length of day (ΔLOD)—the deviations of duration of a day from the nominal 86400 s—are important problems in Earth and planetary sciences because they provide many crucial constraints on global and planetary change. Climate, and more specifically atmospheric dynamics, dominates ΔLOD up to interannual timescales (periods of approximately 2-5 years), through exchange of angular momentum with the solid mantle. Although modern atmospheric observations have suggested the significant influence of El Niño-Southern Oscillation (ENSO) on interannual ΔLOD, these estimates are limited to the past few decades, hindering the analysis of mechanisms responsible for ΔLOD over longer time periods. Here, for the first time, we use tree ring records in the range 900-2020 CE to infer ENSO-driven ΔLOD, with a temporal resolution of 1 year. We demonstrate the consistency of these estimates with ΔLOD inferred from modern ENSO observations and interannual atmospheric angular momentum (1979-2020). In addition, we reconcile our estimates with those inferred from the output of a paleoclimate model (900-2020 CE). Finally, by comparing our derived ΔLOD with the observed ΔLOD from astronomy (1800-2020) and space geodesy (1962-2020), we explain a substantial portion of the interannual signal within observational uncertainties, although there are important discrepancies that could be attributed to the fluid motion at the top of the Earth’s core and other geophysical and climatic processes. Hence, our findings show that ENSO has likely been the dominant contributor to interannual ΔLOD in the past 1120 years, which could improve our understanding of planetary and mantle rotation.