We study the evolution of a passive scalar subject to molecular diffusion and advected by an incompressible velocity field on a 2D bounded domain. The velocity field is u=del perpendicular to H, where H is an autonomous Hamiltonian whose level sets are Jordan curves foliating the domain. We focus on the high P & eacute;clet number regime (Pe:=nu-1 >> 1), where two distinct processes unfold on well separated time-scales: streamline averaging and standard diffusion. For a specific class of Hamiltonians with one non-degenerate elliptic point (including perturbed radial flows), we prove exponential convergence of the solution to its streamline average on a subdiffusive time-scale T nu <