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We investigate the onset and evolution of zonal flows in a growing convectivelayer when a stably-stratified fluid with a composition gradient is cooled fromabove. This configuration allows the study of zonal flows for a wide range ofvalues of the Rayleigh number, $Ra$, and aspect ratio of the convection zonewithin a given simulation. We perform a series of 2D simulations using theBoussinesq approximation, with aspect ratio of the computational domain between$1$ and $5$, and Prandtl number $Pr = 0.1$, 0.5, 1, and $7$. We find that forsquare domains zonal flows appear when the aspect ratio of the convective layeris smaller than two, and the evolution of the system depends on the Prandtlnumber. For $Pr\leq 1$, the fluid experiences bursts of convective transportwith negligible convective transport between bursts. The magnitude andfrequency of the bursts are smaller at low $Pr$, which suggests that thebursting regime is stronger in a narrow range around $Pr=1$, as observed inprevious studies of thermal convection. For $Pr=7$, the structure of the flowconsists of tilted convective plumes, and the convective transport is sustainedat all times. In wider domains, the aspect ratio of the convective zone isalways much larger than two and zonal flows do not appear. These resultsconfirm and extend to fluids with stable composition gradients previousfindings on thermal convection. The fact that zonal flows can be avoided byusing computational domains with large aspect ratios opens up the possibilityof 2D studies of convective overshoot, layer formation and transport propertiesacross diffusive interfaces.

Shear flows and their suppression at large aspect ratio. Two-dimensional simulations of a growing convection zone