Different transport regimes in a spatially-extended recirculating background

Passive scalar transport in a spatially-extended background of roll convection is considered in the time-periodic regime. The latter arises due to the even oscillatory instability of the cell lateral boundary, here accounted for by sinusoidal oscillations of frequency $\omega$. By varying the latter parameter, the strength of anticorrelated regions of the velocity field can be controled and the conditions under which either an enhancement or a reduction of transport takes place can be created. Such two ubiquitous regimes are triggered by a small-scale(random) velocity field superimposed to the recirculating background. The crucial point is played by the dependence of Lagrangian trajectories on the statistical properties of the small-scale velocity field, e.g. its correlation time or its energy.