Geophysical Research Letters, 35, L23302, doi:10.1029/2008GL036059, 2008
Mantle convection with longest-wavelength thermal heterogeneity in a 3-D spherical model: Degree one or two?
Masaki Yoshida
Abstract
The formation of longest-wavelength mantle convection in the sluggish-lid regime is
investigated using a three-dimensional spherical model. The bottom Rayleigh number
is fixed at 107. Considering temperature-dependent rheology, degree-one dominant
thermal convection occurs for both purely basal heating and mixed (i.e., basal
and internal) heating modes. For the purely basal heating mode, degree-one
convection occurs when the viscosity contrast due to temperature-dependent
rheology is 103--104 in both Boussinesq and extended-Boussinesq fluids. However,
with extended-Boussinesq fluid, degree-one convection may only occur in the basal
heating mode: In the mixed heating mode, degree-one convection shifts to one with
high-degree modes, presumably because of enhanced viscous dissipation in the highly
viscous lid over up/downwelling plumes. The geophysically relevant degree-two
convection with sheet-like downwellings is not observed in this study.
The inclusion of visco-plastic rheology in the top part of the mantle breaks
down degree-one convection.