  We use a high resolution cosmological zoom simulation of a Milky Way-sized
halo to study the observable features in velocity and metallicity space
associated with the dynamical influence of spiral arms. For the first time, we
demonstrate that spiral arms, that form in a disc in a fully cosmological
environment with realistic galaxy formation physics, drive large-scale
systematic streaming motions. In particular, on the trailing edge of the spiral
arms the peculiar galacto-centric radial and azimuthal velocity field is
directed radially outward and azimuthally backward, whereas it is radially
inward and azimuthally forward on the leading edge. Owing to the negative
radial metallicity gradient, this systematic motion drives, at a given radius,
an azimuthal variation in the residual metallicity that is characterised by a
metal rich trailing edge and a metal poor leading edge. We show that these
signatures are theoretically observable in external galaxies with Integral
Field Unit instruments such as VLT/MUSE, and if detected, would provide
evidence for large-scale systematic radial migration driven by spiral arms.
