Time-Mean Flow and Variability in a Nonlinear Model of the Atmosphere with Orographic Forcing

Abstract

The variability and time-mean response to orographic forcing are examined in a nonlinear atmospheric model. Distinct signatures from both high-frequency (synoptic-scale) and low-frequency (periods greater than 10 days) transients are seen in the temporal variance and eddy fluxes. Downstream of the orography, in the region of the time-mean jet stream, high-frequency transients are organized into a storm track and exhibit baroclinic energy conversions. The low-frequency transients, while producing, greater variability in the same region as the storm track, exhibit significantly less baroclinic energy generation. The structure of the low-frequency transients downstream of the orography is similar to the observed PNA pattern. The time scale of the eddies in this region appears to be longer than typical time scales associated with stationary Rossby wave dispersion. These eddies exhibit large local barotropic conversion of mean kinetic energy due to the large zonal gradient of the mean zonal wind. These barotropic processes downstream of the mountain give the appearance of low-frequency waves propagating out of the tropics even though there is no low latitude forcing in this model. Midlatitude orography is shown to influence the tropical time-mean circulation; a weak easterly jet along the equator develops due south of the orography. The influence on the tropical variability is restricted to increased high-frequency variance with limited effects on the low-frequency transients.