The dispersion of a passive tracer in a two‐dimensional, spatially periodic stationary flow,
such as convection rolls, is studied in the large Peclet number limit. In the case where
injection, at time t= 0, is localized in one roll, two regimes exist. First, there is an anomalous
diffusion regime in which the number of invaded rolls grows like t 1/3. This regime is due to
the presence of separatrices between rolls that induce trapping of tracer within each roll. At a
later time, when t≫ T d (the diffusion time within a roll), the usual diffusion regime is …