Three‐Dimensional Convection Planforms With Internal Heat Generation

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Abstract
Thermal convection planforms in an infinite Prandtl number, isoviscous fluid layer with basal and internal heating are determined using three‐dimensional finite difference calculations. With basal heating only, at Rayleigh number RaT=1.5×105 the planform consists of a weakly time‐dependent spoke pattern, with connected networks of equally buoyant up‐ and down‐wellings. With volumetric heat sources the planform is strongly time variable. It consists of negatively buoyant flow concentrated in thermal trenches [discrete, arcuate sheets] and spots [cylindrical columns], separated by diffuse, nonbuoyant upwelllngs. Rapid planform changes occur primarily by trench propagation, as observed experimentally by Weinstein and Olson [this issue]. The analogy between thermal trenches in internally heated convection and convergent plate boundaries on Earth suggests that the global pattern of subduction may evolve by propagation of Wadati‐Benioff zones along the strike of the slab.