The latitudinal gradient of zonally averaged potential vorticity (PV) in the mid-latitudes of the troposphere is examined in terms of characteristics of isentropic transport. Assuming that the change in PV is given by a relaxation type forcing, the meridional profile of PV is expressed as a weighted integral of the Lagrangian mean of the relaxation values of PV on backward trajectories. The evolution of the Lagrangian mean is studied in a kinematic model flow and in an atmospheric flow generated by a general circulation model. Important differences between the homogeneous chaotic mixing and the atmospheric flow are explained as effects of transport barriers, which are thought to be the tropopause in the atmospheric case. The undulations of the transport barriers have significant effect on the latitudinal gradient of zonally averaged PV. ¿ 2001 American Geophysical Union