The instability of precession-driven flows in a stably stratified spherical shell

We investigate the instability of precession-driven flows in a stably stratified and rotating spherical shell using direct numerical simulations. Our results show that stable stratification can make precessional flows more unstable compared with the neutrally stratified fluid, when the Brunt–Väisälä frequency in the bulk is comparable to the rotation frequency. The instability arises from triadic resonances between the basic precessional flow and a pair of gravito-inertial waves. The excitation of gravito-inertial waves facilitates the angular momentum transport in the fluid interior, resulting in prominent differential rotation in stably stratified precessing fluids. Our numerical simulations suggest that mechanical forcings, such as precession, are possible to sustain complex flows and lead to the angular momentum transport in planetary fluid interiors, even if they are thermally stable after long-term cooling.