Variations in the Earth's gravity field caused by torsional oscillations in the core

Abstract

S U M M A R YWe investigate whether a component of the flow in the Earth's fluid core, namely torsional oscillations, could be detected in gravity field data at the surface and whether it could explain some of the observed time variations in the elliptical part of the gravity field (J 2 ). Torsional oscillations are azimuthal oscillations of rigid coaxial cylindrical surfaces and have typical periods of decades. This type of fluid motion supports geostrophic pressure gradients, which produce deformations of the core-mantle boundary. Because of the density discontinuity between the core and the mantle, such deformations produce changes in the gravity field that, because of the flow geometry, are both axisymmetric and symmetric about the equator. Torsional oscillations are thus expected to produce time variations in the zonal harmonics of even degree in the gravity field. Similarly, the changes in the rotation rates of the mantle and inner core that occur to balance the change in angular momentum carried by the torsional oscillations also produce zonal variations in gravity. We have built a model to calculate the changes in the gravity field and in the rotation rates of the mantle and inner core produced by torsional oscillations. We show that the changes in the rotation rate of the inner core produce changes in J 2 that are a few orders of magnitude too small to be observed. The amplitudes of the changes in J 2 from torsional oscillations are 10 times smaller than the temporal changes that are observed to occur about a linear secular trend. However, provided the mechanism responsible for these changes in J 2 is identified and that this contribution is removed from the data, it may be possible in the future to detect the lowest harmonic degrees of the torsional oscillations in the gravity field data. We also show that torsional oscillations have contributed to the linear secular change in J 2 by about −0.75 × 10 −12 per year in the last 20 years. Finally, the associated change in the vertical ground motion at the surface of the Earth that is predicted by our mechanism is of the order of 0.2 mm, which is too small to be detected with the current precision in measurements.

Core–Mantle Interactions

“…When the mantle responses elastically to the pressure change, the surface displacement can be a few millimeters. The corresponding change in the gravity field is within current the limits of detection (Dumberry and Bloxham, 2004;Greff-Lefftz et al, 2004). Recent interest in this process is motivated by observations of decadal changes in the elliptical part of the gravity field (Cox and Chao, 2002).…”

“…For example, fluctuations in pressure produce observable changes in both gravity and surface topography (Dumberry and Bloxham, 2004;Fang et al, 1996; Greff- Lefftz et al, 2004). A typical pressure of 10 3 Pa changes on timescales of several decades as the flow evolves at the CMB.…”

Earth Rotation Variations – Long Period

“…Other mechanisms that may cause a linear trend in LOD over the last or next century include continental drift (Dickman, 1979), tectonic processes taking place under nonisostatic conditions (Sabadini and Vermeersen, 2004;Vermeersen and Vlaar, 1993;Vermeersen et al, 1994), plate subduction (Alfonsi and Spada, 1998;Greff-Lefftz, 2011;Ricard et al, 1993;Spada et al, 1992), mantle convection (Greff-Lefftz, 2011), upwelling mantle plumes (Greff-Lefftz, 2011), deformation of the mantle caused by pressure variations acting at the core-mantle boundary that are associated with the motion of the fluid core (Dumberry and Bloxham, 2004;Fang et al, 1996;Greff-Lefftz et al, 2004), earthquakes (Chao and Gross, 1987;Gross and Chao, 2006), and climate change (Abarca del Rio, 1999;de Viron et al, 2002;Huang et al, 2001;Landerer et al, 2007;Räisänen, 2003;Rosen and Gutowski Jr., 1992;Rosen and Salstein, 2000;Winkelnkemper et al, 2009). The fluctuation in LOD of 1500-year period found by Stephenson and Morrison (1995) and Morrison and Stephenson (2001) is currently of unknown origin.…”

Degree one loading by pressure variations at the CMB

“…In a separate investigation Dumberry and Bloxham (2004) argue that the CMB pressure variation not only perturbs the mantle but also perturbs the entire Earth including the core itself. This model in essence is to treat the applied CMB pressure variation as external sources completely foreign to the Earth.…”

“…Observational evidences for tangible decadal variation in the core motion is associated with the flow at the upper reach of the outer core, which have been established based on the similarities between the decadal variation of the west drift of the magnetic field and the decadal variation of the Earth's rotation rate commonly known as variation in the length of day (Dumberry and Bloxham, 2004;Jackson et al, 1993). By "frozen flux" approximation (e.g., Roberts and Scott, 1965), the magnetic lines of force move along with the fluid parcels at the top reach of the outer core underneath the core mantle boundary (CMB).…”