  We investigate the onset and evolution of zonal flows in a growing convective
layer when a stably-stratified fluid with a composition gradient is cooled from
above. This configuration allows the study of zonal flows for a wide range of
values of the Rayleigh number, $Ra$, and aspect ratio of the convection zone
within a given simulation. We perform a series of 2D simulations using the
Boussinesq approximation, with \edit{aspect ratio of the computational domain}
between $1$ and $5$, and Prandtl number $Pr = 0.1$, 0.5, 1, and $7$. We find
that for square domains zonal flows appear when the aspect ratio of the
convective layer is smaller than two, and the evolution of the system depends
on the Prandtl number. For $Pr\leq 1$, the fluid experiences bursts of
convective transport with negligible convective transport between bursts. The
magnitude and frequency of the bursts are smaller at low $Pr$, which suggests
that the bursting regime is stronger in a narrow range around $Pr=1$, as
observed in previous studies of thermal convection. For $Pr=7$, the structure
of the flow consists of tilted convective plumes, and the convective transport
is sustained at all times. In wider domains, the aspect ratio of the convective
zone is always much larger than two and zonal flows do not appear. These
results confirm and extend to fluids with stable composition gradients previous
findings on thermal convection. The fact that zonal flows can be avoided by
using computational domains with large aspect ratios opens up the possibility
of 2D studies of convective overshoot, layer formation and transport properties
across diffusive interfaces.
