Recent seismic tomographic models have revealed broad, low-velocity anomalies in the lower mantle beneath Africa and the central Pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics. We have considered the possible impact from radiative heat transfer, which can be described by a nonlinear temperature-dependent coefficient in the thermal conductivity, in 2D numerical simulations. Results for Rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior. This nonlinearity is responsible for producing a strong attractor in the mantle convective system, which greatly simplifies its time-dependent dynamics. The possible link of the simplified lower mantle time-dependent dynamics with polar wander is discussed. The main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long-term rotational dynamics.
Bibliographical noteFunding Information:
We thank Ond~ej Cadek for encouraging discussion and Paul J. Morin for the technical assistance. We also acknowledge the comments of Arie van den Berg and Barbara Romanowicz. We are indebted to David J. Stevenson who answered our question "why do the poles of the bipolar symmetry lie almost exactly on the equator?" with simply "because of the existence of polar wander!" and who discussed the mechanism during the workshop on global dynamics held in Pfstina, south Bohemia, from July 28 to July 31, 1993. Financial support to J.M. from the German BMFT and the Max-Planck-Institut fiir Chemie in Mainz is acknowledged. This study is based on work sponsored by the NSF (Geochemistry Programme), by NASA (Lageos-II Programme), by the U.S.-Czechoslovak Science and Technology Joint Fund in cooperation with the MSMT CR, USGS and NSF under project 93002, by Charles University grant 320, and by the Grant Agency of the Czech Republic under project 202/93/0450. We are also indebted to AGFA for their interest in this research.