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Orographic Rossby waves: On planetary scales air flow over mountains is affected by the earth’s sphericity and rotation which sets up horizontal wave motions which can be likened to the Rossby waves generated in our simple laboratory experiment. Wavelengths of order 5000km are created with troughs located downstream of major mountain ranges in the westerly flow of middle latitudes. A trough at about 80W downstream of the Rocky mountains and another at about 130W downstream of the Himalayan and Tibetan plateaus are observed, as illustrated in the observations below. These features are most pronounced in wintertime.
Fig.1. January mean 500mb geopotential height (from NOAA/ESRL).