Why Eddy Momentum Fluxes are Concentrated in the Upper Troposphere
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 and then turns meridionally, eventually being 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.
Additional Information© 2015 American Meteorological Society. Manuscript received 26 August 2014, in final form 28 November 2014. This work was supported by the U.S. National Science Foundation Grants CCF-1048575 and CCF-1048701. We thank Brad Marston for useful discussions about quasi-linear approaches and for suggesting investigating the role of barotropic triads (section 4e). We also thank Freddy Bouchet and Cesare Nardini for useful discussions about the Orr mechanism at the Kavli Institute for Theoretical Physics (KITP) summer 2014 program on wave–flow interaction in geophysics, climate, astrophysics, and plasmas.
Published - jas-d-14-0243.1.pdf