On the Influence of Orography on Large-Scale Atmospheric Flow

An investigation has been conducted of the steady response to orography as described by the linearized shallow-water equations on the sphere. Results have been obtained for both idealized and realistic climatological mean zonal flows when perturbed by simple isolated mountains. The response is interpreted in terms of the Rossby wavetrains and the energy dispersion ideas developed in Hoskins et al. (1977). Subsequent results from perturbing the several zonal flows by the earth orography are then readily understood. The surprisingly good comparison with observation suggests that the qualitative insight gained from the simple model is useful. The main difference from previous work is the emphasis on the two-dimensional nature of the horizontal propagation on the sphere. In particular, for a jet at 30° impinging on a mountain at the same latitude, there is a tendency to produce two wavetrains—one to the north and one to the south. At 60°–80° downstream these wavetrains are out of phase, giving a “blocking” region with a ridge to the north and a trough to the south. The southern train produces enhanced equatorial easterlies centered 30°–40° downstream. The results give interesting indications of the regions of influence of mountains and suggest that quantitative theories of the stationary waves must contain a full representation of the spherical domain.