Retrograde equatorial surface flows generated by thermal convection confined under a stably stratified layer in a rapidly rotating spherical shell

Finite-amplitude thermal convection in a rapidly rotating spherical shell associated with a stably stratified layer placed near the outer surface is investigated. Systematic numerical experiments are performed with an Ekman number of E = 10-3, a Prandtl number of P = 1 and an inner/outer radius ratio of = 0.4, and the existence of a strongly stratified upper layer is shown to enhance the generation of equatorial surface retrograde flows when the Rayleigh number is approximately ten times larger than the critical value. The existence of the stable layer causes the bottom of the stable layer to behave as a virtual boundary for the convective motion underneath. Its effective dynamic condition varies from the free-slip condition to the no-slip condition as the Rayleigh number increases. The Reynolds stress of the convective vortices beneath the stable layer is weakened and is dominated by the transport of the planetary angular momentum. As a result, the latitudinal temperature gradient produced at the bottom of the stable layer induces the equatorial retrograde flow through the thermal wind balance. This diffuses through the stable layer by viscosity and produces the equatorial surface retrograde flow.