  Supermassive black holes in active galactic nuclei (AGN) are known to launch
relativistic jets, which are observed across the entire electromagnetic
spectrum and are thought to be efficient particle accelerators. Their primary
radiation mechanism for radio emission is polarized synchrotron emission
produced by a population of non-thermal electrons. In this Letter, we present a
global general relativistic magnetohydrodynamical (GRMHD) simulation of a
magnetically arrested disk (MAD). After the simulation reaches the MAD state,
we show that waves are continuously launched from the vicinity of the black
hole and propagate along the interface between the jet and the wind. At this
interface, a steep gradient in velocity is present between the mildly
relativistic wind and the highly relativistic jet. The interface is, therefore,
a shear layer, and due to the shear, the waves generate roll-ups that alter the
magnetic field configuration and the shear layer geometry. We then perform
polarized radiation transfer calculations of our GRMHD simulation and find
signatures of the waves in both total intensity and linear polarization,
effectively lowering the fully resolved polarization fraction. The tell-tale
polarization signatures of the waves could be observable by future Very Long
Baseline Interferometric observations, e.g., by the next-generation Event
Horizon Telescope.
