Numerical simulations in a simple atmospheric model show that a realistic extratropical tropopause structure, marked by a strong change both in meteorological variables such as lapse rate and in transport characteristics, may be achieved solely through the stirring effects of baroclinic eddies acting against a smooth thermal relaxation. The flow self-organizes to give a transport structure that has a sharp transition in the vertical. Below the transition level the eddies stir across entire isentropic surfaces. The transition level corresponds to the extratropical tropopause height. Above the transition level there is a midlatitude transport barrier, corresponding to the subtropical transport barrier in the real atmosphere, with eddy stirring on poleward and equatorward sides. Similar self-organizing mechanisms are likely to be relevant in the stratosphere and in oceanic flows. ¿ 2001 American Geophysical Union