1. Departamento Física Aplicada I, Escuela Superior de Ingenieros, Universidad del País Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain
2. Departamento Física Aplicada I, EUITI, Universidad del País Vasco, Plaza Casilla s/n, 48013 Bilbao, Spain
3. Department of Astronomy, Wellesley College, Wellesley, Massachusetts 02481, USA

Correspondence to: A. Sánchez-Lavega¹ Correspondence and requests for materials should be addressed to A.S.-L. (Email: wupsalaa@bi.ehu.es).

Abstract

The atmospheres of the giant planets Jupiter and Saturn have a puzzling system of zonal (east–west) winds alternating in latitude, with the broad and intense equatorial jets on Saturn having been observed previously to reach a velocity of about 470 m s^-1 at cloud level¹. Globally, the location and intensity of Jupiter's jets are stable in time to within about ten per cent^{2, 3}, but little is known about the stability of Saturn's jet system. The long-term behaviour of these winds is an important discriminator between models for giant-planet circulations^{4, 5, 6, 7, 8, 9}. Here we report that Saturn's winds show a large drop in the velocity of the equatorial jet of about 200 m s^-1 from 1996 to 2002. By contrast, the other measured jets (primarily in the southern hemisphere) appear stable when compared to the Voyager wind profile of 1980–81.