Mechanisms of Changing Orographic Stationary Rossby Wave Forcing

It is not well understood how stationary Rossby waves, which are forced by topography, ocean heating, and transient momentum fluxes, respond to climate change. We investigate topographically forced stationary Rossby waves in an idealized GCM and simulate climate change by varying the amount of longwave absorption. Globally, tropospheric stationary-eddy kinetic energy (sEKE) behaves non-monotonically with climate change, first increasing, then decreasing in climates much warmer than modern. These changes are primarily driven by vortex stretching immediately over the topography, which is determined by the vertical profile of vertical velocity. Using the thermodynamic equation, we disentangle the many influences on the vertical velocity profiles such as static stability, horizontal temperature gradients, rotation, and the vertical profile of the zonal-mean wind. This analysis shows that topographically forced sEKE largely tracks the inverse of tropospheric static stability and thus behaves similar to transient-eddy kinetic energy with warming. For comparison, we find a significant decrease in annual-mean sEKE in the 20 - 50°N latitude band in the CMIP5 RCP8.5 scenario. We investigate whether this is related to increasing tropospheric static stability in agreement with the idealized GCM findings.