  The dynamics, structure and stability of zonal jets in planetary flows are
still poorly understood, especially in terms of coupling with the small-scale
turbulent flow. Here, we use an experimental approach to address the questions
of zonal jets formation and long-term evolution. A strong and uniform
topographic $\beta$-effect is obtained inside a water-filled rotating tank
thanks to the paraboloidal fluid free upper surface combined with a
specifically designed bottom plate. A small-scale turbulent forcing is
performed by circulating water through the base of the tank. Time-resolving PIV
measurements reveal the self-organization of the flow into multiple zonal jets
with strong instantaneous signature. We identify a subcritical bifurcation
between two regimes of jets depending on the forcing intensity. In the first
regime, the jets are steady, weak in amplitude, and directly forced by the
local Reynolds stresses due to our forcing. In the second one, we observe
highly energetic and dynamic jets of width larger than the forcing scale. An
analytical modeling based on the quasi-geostrophic approximation reveals that
this subcritical bifurcation results from the resonance between the directly
forced Rossby waves and the background zonal flow.
