  We study the structure of accretion disks around supermassive black holes in
the radial range $30\sim 100$ gravitational radii, using a three dimensional
radiation magneto-hydrodynamic simulation. For typical conditions in this
region of Active Galactic Nuclei (AGN), the Rosseland mean opacity is expected
to be larger than the electron scattering value. We show that the iron opacity
bump causes the disk to be convective unstable. Turbulence generated by
convection puffs up the disk due to additional turbulent pressure support and
enhances the local angular momentum transport. This also results in strong
fluctuations in surface density and heating of the disk. The opacity drops with
increasing temperature and convection is suppressed. The disk cools down and
the whole process repeats again. This causes strong oscillations of the disk
scale height and luminosity variations by more than a factor of $\approx 3-6$
over a few years' timescale. Since the iron opacity bump will move to different
locations of the disk for black holes with different masses and accretion
rates, we suggest that this is a physical mechanism that can explain the
variability of AGN with a wide range of amplitudes over a time scale of years
to decades.
