Excitation of polar thermal convection in a rotating spherical shell

The structure of thermal convection of a Boussinesq fluid with Prandtl number of 1 in a rotating spherical shell of inner-to-outer radius ratio of 0.5 is investigated numerically for the Rayleigh number Ra up to 90 times the critical value Rac at the Taylor number of 1.6×106. As long as Ra is close to Rac from above, a convective flow is driven only outside the polar regions. At Ra≃5Rac, the convection starts also inside the polar regions where high temperature blobs are created, that enhance the heat transport. The convection in the polar regions is more active than that in the other region for Ra>10Rac. The relative strength of convective motions in the two regions attains the largest value at Ra≈20Rac, beyond which the convection field becomes more isotropic as Ra increases.