We have studied the dynamical effects of a stratified viscosity profile on time-dependent mantle circulations by using a 3D Boussinesq spherical-shell convection model. This particular viscosity profile, taken from the inversion of geoid data, has the distinct feature of having a large viscosity maximum in the middle lower mantle. Surface Rayleigh numbers ranging from 5 × 104 to 8 × 106 have been considered. There are several distinct differences in both the morphologies of the plumes and the sinking currents from constant viscosity models. In a viscously-stratified lower mantle, plumes are formed invariably only after the cold downwelling has reached the bottom. The mean mantle temperature is reduced and this allows for the sinking of very cold material into the deep mantle. The number of upwelling plumes is reduced and the overall circulation is stabilized, with well-organized triple junctions of cold sheets developed. The presence of internal heating increases the number of upwelling plumes and breaks up the network of downwelling sheets. These plumes become larger in size and have a hotter interior because of the nearly stationary character.
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