Orographic versus thermal forcing of stationary waves: the importance of the mean low-level wind

The amplitude of the linear, stationary response to low-level extratropical heating decreases as the magnitude of the low-level mean flow increases, while the amplitude of the orographically forced waves increases. As a result, linear theory predicts that the relative importance of thermal and orographic forcing for the extratropical stationary wave field is very sensitive to the magnitude of the zonal mean low-level winds. In the proces of illustrating this sensitivity, we also show how the dependence of the orographic response on the low level winds can be distorted by a numerical σ-coordinate model. -Authors