THE PHASE DIFFERENCE BETWEEN LENGTH OF DAY AND ATMOSPHERIC ANGULAR-MOMENTUM AT SUBANNUAL FREQUENCIES AND THE POSSIBLE ROLE OF CORE-MANTLE COUPLING

We examine discrepancies in the angular momentum budget of the Earth o n subannual timescales. We find that for signals with long periods (> 30 days) the phase of the length of day (Delta LOD) significantly lead s that of the atmospheric angular momentum (AAM). The discrepancy has an amplitude of similar or equal to 15% of the observed AAM at periods of around 100 days. We show that if changes in the Delta LOD are driv en by the atmosphere, then this is diagnostic either of the importance of the oceans or of dynamical coupling between the core and mantle fo r these frequencies. We find that a simple model of core-mantle coupli ng can cause a phase lead of Delta LOD of the correct magnitude. We at tempt to fit the transfer function with a simple three-layer model of the Earth, treating the core as a rotating solid body coupled to the m antle by a specified function. Although our simple model can supply su fficient phase lead of the Delta LOD, the fit to the frequency depende nce is poor and it can not fit the magnitude of the transfer function. Nevertheless, this suggests that core-mantle coupling is a plausible explanation of the observed phase difference. Better fits might be obt ained with more realistic models of the core.