Angular momentum fluctuations within the Earth's liquid core and torsional oscillations of the core-mantle system

Abstract

Summary Non-steady differential rotation is one of the main characteristics of the buoyancy-driven flow within the Earth's liquid metallic outer core that generates the main geomagnetic field by magnetohydrodynamic (MHD) dynamo action. Concomitant fluctuations in angular momentum transfer both within the core and between the core and the overlying ‘solid’ mantle are investigated by assuming that average departures from ‘isorotation’ on coaxial cylindrical surfaces are negligibly small. The interval of time covered by the analysis is the 15 decades from 1840 to 1990, for which requisite determinations are available of decadal fluctuations of the length of the day (LOD) and also of flow velocities just below the core–mantle boundary (CMB), as inferred from geomagnetic secular variation (GSV) data. Core angular momentum (CAM) fluctuations are most pronounced in the mid-latitudes, where they are generally out of phase with those occurring in equatorial regions. They are roughly in phase with decadal LOD fluctuations, especially after about 1870, with a dominant variability period of approximately 65 years, in keeping with previous analyses based on GSV and/or LOD data. The largest positive correlations (0.8 when data before 1867.5 are excluded) are found in the mid-latitudes, with a maximum at zero lag and with secondary peaks at 67 years and at −64 years, again implying a mode of approximately 65 years. Propagation of CAM anomalies from the equatorial to polar regions is evident in both the time-latitude dependence of CAM and its latitudinal correlation with length of day fluctuations. Future work on excitation mechanisms should establish the connection, if any, between the dominant timescale of approximately 65 years seen in the data and the gravest of the theoretically possible subseismic modes of torsional MHD oscillations of the core.