INTENSE LOCAL TOROIDAL MOTION GENERATED BY VARIABLE VISCOSITY COMPRESSIBLE CONVECTION IN 3-D SPHERICAL-SHELL

Strong dynamical effects arising from lateral viscosity variations are found near the heads of fast-upwelling plumes approaching the top bou ndary for unsteady 3-D compressible convection in a spherical geometry at an average Rayleigh number of O(10(6)). The temperatures at the ce nter of fast-upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature-dependence of the viscosity. Although the laterally averaged ratio of toroidal to p oloidal energy varies only around 10%, the local toroidal velocities c an reach about 40% of the poloidal velocities. The strongest toroidal motions always take place right above or close to the fast upwelling p lume heads and behave as vortical flows with time. This finding may ex plain the origins of microplate rotation in regions with hotspot activ ity (e.g. Easter Island), which shows no direct relation to the transf orm-faulting system near the mid-ocean ridge.