Stationary eddy response to surface boundary forcing: Idealized GCM experiments

A set of atmospheric general circulation model (AGCM) experiments under idealized conditions is performed to investigate atmospheric response to surface boundary forcing by tropical sea surface temperature (SST), extratropical land-sea contrast, and large-scale orography. Stationary eddies forced by the extratropical land-sea distribution are strongest in the high latitudes, where their amplitudes slightly exceed internal chaotic variability. By contrast, the stationary response to zonal variations in tropical SST is strong in both the subtropics and mid-latitudes. These SST-forced stationary waves are trapped in the troposphere, while those induced by orography show a strong vertical propagation into the stratosphere. Analysis of transient eddies indicates that orography is most e! ective in generating a zonally localized storm track while extratropical land-sea contrast has little e! ect on the zonal variation of storm activity. A vorticity budget analysis is carried out to understand the tropical SST forcing of extratropical stationary eddies. In the subtropics, the dominant balance is between the vortex stretching and zonal advection. North of the tropical warm water pool, a subtropical anticyclone forms in the upper troposphere in response to the divergence of the locally enhanced Hadley circulation. We further show that this subtropical response to tropical SST variations is nonlinear in both its amplitude and zonal phase.