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Rossby, Carl-Gustaf (Arvid)

winds westerly waves upper

(1898–1957) Swedish–US meteorologist: discovered large-scale waves in the upper atmosphere, and the jet stream.

Rossby was educated at the University of Stockholm and at the Bergen Geophysical Institute. In 1926 he emigrated to America, where he subsequently held professorships at the Massachusetts Institute of Technology and the University of Chicago. In 1940 Rossby demonstrated that large-scale undulatory disturbances exist in the uniform flow of the westerly winds in the upper atmosphere, developing in the zone of contact between cold polar air and warm tropical air; such waves are inherent features of a rotating fluid with a thermal gradient. There are usually three to five such Rossby waves in each hemisphere, with wavelengths of up to 2000 km. Rossby also showed that the strength of the westerly winds has an important influence on global weather, either allowing the normal sequence of cyclones and anticyclones to develop when the westerlies are strong or allowing cold polar air to sweep south when they are weak. He is further credited with the discovery of the jet stream, the broad ribbon of upper westerly winds travelling at about 45 m s –1 in the mid-latitudes. Rossby waves – the mechanisms of development of high-altitude westerly winds. He set up a weather service for airways in 1927 in California; and his work has been important in numerical weather prediction, especially since computers became available.

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