Stationary Eddy Response to Surface Boundary Forcing: Idealized GCM Experiments

Abstract

A set of atmospheric general circulation model (AGCM) experiments under idealized conditions is performed to investigate atmospheric response to surface boundary forcing by extratropical land?sea contrast, large-scale orography, and tropical sea surface temperature (SST) distribution. Stationary eddies forced by the extratropical land?sea distribution are strongest in high latitudes, but their amplitudes are modest and comparable to internal chaotic variability. By contrast, the stationary eddy response to zonal variations in tropical SST is strong and robust in both the subtropics and midlatitudes. While these SST-forced stationary waves are trapped within the troposphere, those induced by orography show a strong vertical propagation into the stratosphere. Analysis of transient eddies indicates that orography is effective in generating a zonally localized storm track while extratropical land?sea contrast has little effect on the zonal variation of upper-level storm activity. A vorticity budget analysis is carried out to understand tropical SST forcing mechanism to set up extratropical stationary eddies. In the subtropics, the dominant balance is reached 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. The authors further show that this subtropical response to tropical SST variations has nonlinear characteristics in both its amplitude and zonal phase.