A bound on the mixing rate of 2d perfect fluid flows

Using the $H^{-1}$ norm as a measure of mixing, we prove that 2d Euler flows on the torus mix passive scalars at most exponentially. The mixing rate is bounded linearly by the BMO norm of the vorticity (and thus by its $L^\infty$ norm). We also give an analogous bound on the growth rate of scalar gradients.