Sensitivity to boundary conditions. Zonal-mean zonal velocity fields from simulations with doubly periodic boundary conditions (solid, “Double-P”) and with sponge layers that damp vorticity to zero at the northern and southern boundaries (dashed, “Sensitivity”). Results are shown for simulations with L_d → ∞, using ε = 5 × 10⁻⁷ and μ = 5 × 10⁻⁵ in (a) and (c) and ε = 10⁻¹⁰ and μ = 10⁻⁴ in (b) and (d).