A 30–60 Day Oscillation in Length-of-Day and Atmospheric Angular Momentum: Extratropical Origin?
Fluctuations in Earth rotation over time scales of two years or less are dominated by atmospheric effects; spectral analyses of length-of-day (LOD) and atmospheric angular momentum (AAM) data show significantly increased variability in the 30–60 day band, LOD and AAM fluctuations on the 30–60 day time scale have previously been linked to equatorially-trapped waves of the type first described by Madden and Julian (1971: MJ hereafter). A significant spectral peak centered at 42 days has also been found, however, in the AAM of a three-year (1120-day) perpetual-January simulation of the global atmosphere, performed using a version of the UCLA General Circulation Model (GCM) which does not give rise to MJ oscillations in the tropics. In the present work, this oscillation is studied using the 12-year overlap between two records: (i) AAM data, compiled from the National Meteorological Center (NMC), and (ii) LOD variation from the JPL Kalman-filtered Earth-rotation series. We analyze the NMC records by latitude belts, in light of the UCLA GCM results, in order to identify possibly distinct sources of the AAM oscillation in the mid-latitudes and the tropics. Results suggest that two 30–60 day oscillations exist in the Earth-atmosphere system: a tropical oscillation associated with the equatorially-trapped, convectively-driven MJ wave and a mid-latitude oscillation, associated with the interaction of non-zonal flow with topography.
KeywordsGeneral Circulation Model Earth Rotation Lunar Laser Range Latitude Belt Atmospheric Angular Momentum
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