A three-dimensional off-line simulation of the transport of the long-lived
source gas N2O in the summer stratosphere has been made with a global model
. We found long-lived westward-propagating patterns resulting from the slow
advection of coarsely-mixed structures of ex-vortex air and midlatitude ai
r. The mixing of the air masses with these two different characteristics is
a slow process that lasts from spring until late summer. In the regime of
summer easterlies above 20 km, we found evidence of long-lived, "fossil" de
bris from the polar vortex even in August.