Modulation of the seasonal cycle in length-of-day and atmospheric angular momentum

Global warming, by definition, changes the atmospheric temperature field. This temperature change is not expected to be uniform, either geographically, or with height in the atmosphere. By the thermal wind equation, changes in the pole-to-equator temperature gradient will cause changes in the atmospheric zonal winds. Numerous previous studies have shown that observed length-of-day (LOD) variations on time scales of a few days to a few years are largely caused by atmospheric zonal wind fluctuations. In particular, seasonal variations in LOD have been previously shown to be dominantly caused by seasonal variations in the atmospheric zonal winds. Here, observed changes in the strength of seasonal LOD and wind-driven atmospheric angular momentum signals during 1962 to 2000 are analyzed and shown to be significantly correlated with each other and with the Southern Oscillation Index. This demonstrates that the observed seasonal LOD signal can be used as a proxy measurement for changes in the angular momentum of the seasonal zonal winds, thereby allowing changes in the seasonal zonal winds to be studied even when wind measurements are not available. In addition, the approach of studying decadal-scale changes in the strength of the seasonal cycle allows LOD measurements to be used in climate change studies, studies that cannot be undertaken directly due to uncertainties in modeling the dominant decadal-scale changes in LOD caused by core-mantle boundary processes.