EFFECTS OF STRONGLY TEMPERATURE-DEPENDENT VISCOSITY ON TIME-DEPENDENT, 3-DIMENSIONAL MODELS OF MANTLE CONVECTION

Numerical simulations of thermal convection in a wide (8x8x1) Cartesia n box heated from below with temperature-dependent viscosity contrasts of 1000, and Rayleigh number 105 show that boundary conditions and as pect ratio have an enormous effect on the preferred flow pattern. With rigid upper and lower boundaries, spoke-pattern flow with small (diam eter approximately 1.5) cells is obtained, consistent with laboratory experiments and previous numerical results. However, with the arguably more realistic stress-free boundaries, the flow chooses the largest p ossible wavelength, forming a single square cell of aspect ratio 8, wi th one huge cylindrical downwelling surrounded by upwelling sheets. Th e addition of stress-dependence to the rheology weakens the stiff uppe r boundary layer, resulting in smaller cells, though still with upwell ing sheets and downwelling plumes.