Isocontours and equatorial cut of vorticity along the axis of rotation ω_z = (∇ × u)_z for different density stratification in thick shells (η = 0.2, upper panels) and in thin shells (η = 0.6, lower panels). These models correspond to the cases displayed in Figs. 7, 8. Positive (negative) values are rendered in red (blue). For each model, the white sphere corresponds to the inner radius.