  This paper presents a study of the interaction between Alfv\'en modes and
zonal structures, considering a realistic ASDEX Upgrade equilibrium. The
results of gyrokinetic simulations with the global, electromagnetic,
particle-in-cell code ORB5 are presented, where the modes are driven unstable
by energetic particles with a bump-on-tail equilibrium distribution function,
with radial density gradient. Two regimes have been observed: at low energetic
particles concentration, the Alfv\'en mode saturates at much higher level in
presence of zonal structures; on the other hand at high energetic particles
concentration the difference is less pronounced. The former regime is
characterized by the zonal structure (identified as an energetic particle
driven geodesic acoustic mode), being more unstable than the Alfv\'en mode. In
the latter regime the Alfv\'en mode is more unstable than the zonal structure.
The theoretical explanation is given in terms of a 3-wave coupling of the
energetic particle driven geodesic acoustic mode and Alfv\'en mode, mediated by
the curvature-pressure coupling term of the energetic particles.
