The Response of Stationary Planetary Waves to Tropospheric Forcing

Abstract

A steady-state, linear, quasi-geostrophic model of stationary waves on a sphere is employed to study the lower boundary forcing of airflow over topography and the internal forcing that results from the geographical distribution of diabatic heating. The lower boundary vertical motions forced by airflow over topography are shown to depend on the following: 1) whether or not consideration is made of the horizontal deflection of airflow around topographic features; 2) the level of the wind profile at which flow over topography is assumed to take place; and 3) the topographic data set that was used in the forcing formulation. Different methods of calculating the lower boundary vertical motions give rise to sizeable differences in the calculated planetary waves. Large uncertainties are also found in the modeled results depending on the choices made as to the vertical distribution of the forcing by diabatic heating. Given these uncertainties, the relative roles of topographic forcing and diabatic heating in forcing stationary planetary waves are explored in an alternative manner. The lower boundary forcing is taken to be given by the observed stationary planetary wave in lower boundary (900 mb) geopotential height, and the internal forcing is computed using the planetary wave propagation equation on the observed wave structure. Using this method, it is found that the lower boundary forcing generally accounts for the phase structure of the stationary planetary waves, and the response to the internal forcing generally acts to destructively interfere with the response from the lower boundary forcing. This interference is larger for wavenumber 2 in the stratosphere than for wavenumber 1.