  We develop a theory describing how a convectively unstable active field in an
open flow is transformed into absolutely unstable by local mixing. Presenting
the mixing region as one with a locally enhanced effective diffusion allows us
to find the linear transition point to an unstable global mode analytically. We
derive the critical exponent that characterizes weakly nonlinear regimes beyond
the instability threshold and compare it with numerical simulations of a full
two-dimensional flow problem. The obtained scaling law turns out to be
universal as it depends neither on geometry nor on the nature of the mixing
region.
