  We present a Spitzer IRS study of variability in 14 T Tauri stars in the
Taurus and Chamaeleon star-forming regions. The sample is composed of
transitional and pre-transitional objects which contain holes and gaps in their
disks. We detect variability between 5-38 microns in all but two of our objects
on timescales of 2-3 years. Most of the variability observed can be classified
as seesaw behavior, whereby the emission at shorter wavelengths varies
inversely with the emission at longer wavelengths. For many of the objects we
can reasonably reproduce the observed variability using irradiated disk models,
particularly by changing the height of the inner disk wall by ~20%. When the
inner wall is taller, the emission at the shorter wavelengths is higher since
the inner wall dominates the emission at 2-8 microns. The taller inner wall
casts a larger shadow on the outer disk wall, leading to less emission at
wavelengths beyond 20 microns where the outer wall dominates. We discuss how
the possible presence of planets in these disks could lead to warps which cause
changes in the height of the inner wall. We also find that crystalline
silicates are common in the outer disks of our objects and that in the four
disks in the sample with the most crystalline silicates, variability on
timescales of 1 week is present. In addition to explaining the infrared
variability described above, planets can create shocks and collisions which can
crystallize the dust and lead to short timescale variability.
