Degree One Loading by Pressure Variations at the CMB

Hemispherical asymmetry in core dynamics induces degree-1 pressure variations at the core mantle boundary (CMB), which in turn deforms the overlaying elastic mantle, at the same time keeps center of mass of the whole Earth stationary in space. We develop a systematic procedure to deal with the degree-1 CMB pressure loading. We find by direct calculation a surprisingly negative load Love number h1=-1.425 for vertical displacement. Further analysis indicates that the negative h1 corresponds to thickening above the positive load that defies intuition that pressure inflation pushes overlaying material up and thins the enveloping shell. We also redefine the pressure load Love numbers in general to enable comparison between the surface mass load and the CMB pressure load for the whole spectrum of harmonic degrees. We find that the gravitational perturbations from the two kinds of loads at degrees n>1 are very similar in amplitude but opposite in sign. In particular, if the CMB pressure variation at degree 2 is at the level of ~1 hpa/yr (1 cm water height per year), it would perturb the variation of Earth’s oblateness, known as the J2, at the observed level. INTRODUCTION Advances in analyzing teleseismic waves and normal modes have shown evidences of EasternWestern hemispherical asymmetry in material properties at the top of the Earth’s inner core (e.g., Monnereau et al., 2010; Irving et al., 2009; Niu and Wen, 2001). There are two competing hypotheses in the attempt to explain this hemispherical asymmetry. The “lopsided growth” model (Monnereau et al., 2010) This study was supported in part by NASA (No. NNX09AK 70G). *Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin