Atmospheric Excitation of Length of Day Inferred From 21st Century Climate Projections

On seasonal timescales, atmospheric angular momentum, mainly constituted by wind but also pressure effects, is known as the most important driver of Earth rotation variations reflected in those of the length of day. However, in connection with long-term climatic changes, we additionally anticipate secular trends resulting from shifts or changes in the intensity of atmospheric circulation. We investigate potential atmospheric transitions and their consequences for length of day using historical and 21st century simulations of a CMIP6 multimodel ensemble. The future projections used rely on five scenarios of differing greenhouse gas emission strengths and societal development. In each scenario, the resulting mean in the atmospheric angular momentum ensemble trajectory aligns with that of the projected surface temperature. The two pathways characterized by sustainability do not indicate significant centennial trends in atmospheric angular momentum and the respective length of day excitation. The scenarios with medium, high, and very high-emission intensity project gradual increases in atmospheric angular momentum, dominated by strengthening subtropical westerly jet streams. For the most intense scenario, this would correspond to an atmosphere-induced slowdown of the Earth's rotation, with a 0.43 ms cy⁻¹ increase in length of day, which amounts to about one-fifth of the slowdown due to the effect of tidal friction. In contrast, we identify no clear trend regarding the long-term change in the amplitude of the annual oscillation. Our results emphasize that climate change can affect Earth's rotation rate or, equivalently, the length of day through secular variations in atmospheric angular momentum.