Mechanisms of Stationary Rossby Wave Change in Comprehensive and Idealized GCMs

Abstract:

Stationary Rossby waves are an important influence on the large-scale circulation of the atmosphere, especially in Northern Hemisphere winter. Changes in stationary waves with global warming have the potential to modify patterns of surface temperature and precipitation. We analyze 21st century stationary wave changes that are robust across comprehensive GCMs participating in the Coupled Model Intercomparison Project (CMIP5) and review the relevant mechanisms. We isolate mechanisms of changes in the orographic forcing of stationary Rossby waves in particular by performing a set of moist idealized GCM simulations with simplified midlatitude orography. We simulate a wide range of climates and allow latent heating and transient eddies to feed back on the stationary-eddy dynamics. In the idealized simulations, latent heating is found to damp orographic stationary waves, whereas transient eddies are found to reinforce them. As the climate warms, the damping by latent heating becomes more effective while the reinforcement by transient eddies becomes less effective, leading to an overall reduction in orographic stationary wave amplitude. These effects overwhelm the influences of a reduced meridional temperature gradient and increased dry static stability, both of which increase the sensitivity of the free troposphere to orographic forcing. Together with a reduction in the midlatitude meridional temperature gradient, the weakening of orographic stationary waves leads to reduced zonal asymmetry of temperature and precipitation in warm, moist climates of the idealized GCM.