  The extratropical eddy momentum flux (EMF) is controlled by generation,
propagation, and dissipation of large-scale eddies and is concentrated in
Earth's upper troposphere. An idealized GCM is used to investigate how this EMF
structure arises. In simulations in which the poles are heated more strongly
than the equator, EMF is concentrated near the surface, demonstrating that
surface drag generally is not responsible for the upper-tropospheric EMF
concentration. Although Earth's upper troposphere favors linear wave
propagation, quasi-linear simulations in which nonlinear eddy-eddy interactions
are suppressed demonstrate that this is likewise not primarily responsible for
the upper-tropospheric EMF concentration. The quasi-linear simulations reveal
the essential role of nonlinear eddy-eddy interactions in the surf zone in the
upper troposphere, where wave activity absorption away from the baroclinic
generation regions occurs through the nonlinear generation of small scales. In
Earth-like atmospheres, wave activity that is generated in the lower
troposphere propagates upward, then turns meridionally, eventually to be
absorbed nonlinearly in the upper troposphere. The level at which the wave
activity begins to propagate meridionally appears to be set by the typical
height reached by baroclinic eddies. This can coincide with the tropopause
height but also can lie below it if convection controls the tropopause height.
In the latter case, EMF is maximal well below the tropopause. The simulations
suggest that EMF is concentrated in Earth's upper troposphere because typical
baroclinic eddies reach the tropopause.
