  Millimetre-wavelength very long baseline interferometric (mm-VLBI)
observations of M87 by the Event Horizon Telescope (EHT) should provide a
unique opportunity to observe and characterize the origins of jet variability
already seen at longer wavelengths. Synchrotron spot models have been used to
model variability near black holes; this work extends these by allowing spots
to shear and deform in the jet velocity field. Depending on the position of the
spot, shearing forces can significantly alter the structure of the spot,
producing distinct signals in reconstructed images and light curves. The
maximum intensity of the shearing spot can vary by as much as a factor of five
depending on the spot azimuthal launch position, but the intensity decay time
depends most significantly on the spot radial launch position. Spots launched
by a black hole driven jet exhibit distinct arc structures in reconstructed
images, and exhibit brighter and shorter-lived enhancements of the light curve.
Spots launched by a wind-driven jet have exhibit much simpler structures in the
image, and longer-lived light curve enhancements than spots launched by a black
hole driven jet.
