Quasi-geostrophic Rayleigh–Bénard convection on the tilted bold italic f f $\boldsymbol{f}$ -plane

Source abstract

Rapidly rotating Rayleigh–Bénard convection on an f f $f$ -plane at colatitude theta Subscript f ϑ f $\vartheta _{\!f}$ is investigated numerically using an asymptotically reduced equation set valid in the limit of very rapid rotation. The equations provide a non-hydrostatic but quasi-geostrophic description in a non-orthogonal coordinate system. The tilt changes the structure of the large-scale barotropic condensate from large-scale vortices to zonal flows as the colatitude of the f f $f$ -plane increases, with bistable states present for certain parameter ranges, extending prior work to a geophysically significant parameter regime. This behaviour is understood through the impact of broken rotation symmetry on the barotropic source terms resulting from baroclinic vorticity stresses and baroclinic torque. As the tilt angle theta Subscript f ϑ f $\vartheta _{\!f}$ increases, global heat and momentum transport is reduced relative to upright-polar convection, a result that is explained through linear theory and nonlinear power maps both of which demonstrate increased attenuation of the domain of dynamically active spatial scales as the convective modes depart from a north–south alignment in the horizontal plane. A key finding is that the predominance of lateral thermal mixing allows for the maintenance of a persistent unstable mean temperature gradient that saturates at increasing forcing levels and remains insensitive to the colatitude.