  We construct long wavelength asymptotically locally AdS_5 spacetimes with
slowly varying (background) gauge fields which are solutions to the U(1)^n
Einstein-Maxwell-Chern-Simons system. These bulk spacetimes are dual to
(3+1)-dimensional fluid flows with n anomalous currents in the presence of
external electromagnetic fields. We utilise the area form on the outer horizon
to holographically compute an entropy current for the dual fluid to first order
in boundary derivatives. Our resulting expression contains additional terms
proportional to the vorticity and magnetic field and thus provides holographic
confirmation of the entropy current calculated by Son and Surowka for
hydrodynamics with triangle anomalies. We then restrict our bulk metric to
describe the fluid/gravity model of the chiral magnetic effect (CME) and again
holographically obtain the entropy current. As expected, our calculation
replicates the result produced using standard hydrodynamic/thermodynamic
arguments.
