  We calculate the local current density in pristine armchair graphene
nanoribbons (AGNRs) with varying width, $N_\mathrm{C}$, employing a
density-functional-theory-based ab initio transport formalism. We observe very
pronounced current patterns (streamlines) with threefold periodicity in
$N_\mathrm{C}$. They arise as a consequence of quantum confinement in the
transverse flow direction. Neighboring streamlines are separated by stripes of
almost vanishing flow. As a consequence, the response of the current to
functionalizing adsorbates is very sensitive to their placement: adsorbates
located within the current filaments lead to strong backscattering, while
adsorbates placed in other regions have almost no impact at all.
