We describe the temporal variability of the zonal jets of Jupiter and Saturn from a long-term systematic observation of motions at the upper cloud level. Globally, the pattern is highly stable but significant changes on the strongest eastward jets of each planet have been detected. The first one is the speed variability in the 180 m/s jet stream in Jupiter's north temperate latitude. The second one is the change by about 40 percent in the speed of the equatorial jet of Saturn. We discuss the possible nature of these changes and how they can be used to constrain the models of the giant planets general circulation.
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