Breaking gravity waves are one way to create turbulent mixing. The region where deeper and shallower fluids meet can exhibit these phenomena, especially in the presence of strong velocity sheer, such as that found along the boundaries of a jet.

http://www.hawaii.edu/indo lang/surf.html

So where would you catch the wave in your 747? Below is a cross-section of one of these breaking waves (it’s going the other direction), showing where clear-air turbulence could be expected.

http://www.tpub.com/weather2/8-28.htm

Conclusion: Tropopause no barrier

While convective overshooting of the tropopause in equatorial regions is the most easily recognizable cause of mixing between tropospheric and stratospheric air, breaking gravity waves near tropopausal discontinuities and jet streams can mix air from the two regions in the mid latitudes.

This would make the meandering jet stream act a bit like the fins in an old fashioned ice-cream freezer.

This mechanism has been blamed for the unexpectedly large downwind fallout from nuclear tests during the Cold War era.

Bibliography

Cushman-Roisin, Benoit, Introduction to Geophysical Fluid Dynamics, Englewood Cliffs: Prentice-Hall, Inc.,1994

Dutton, John A., The Ceaseless Wind, Mineola, NY: Dover Publications, Inc., 1986

Pedlosky, Joseph, Geophysical Fluid Dynamics, New York: Springer-Verlag New York Inc.,1987

Wallace, John M., and Hobbs, Peter V., Atmospheric Science, San Diego: Academic Press, 1977