Latitudinal and Longitudinal Oscillations of Cloud Features on Neptune

Voyager observations suggest that three of Neptune's major cloud features oscillate in latitude by 2° to 4° and that two of them simultaneously oscillate in longitude by 7.8° and 98° about their mean drift longitudes. The observations define most convincingly the two orthogonal oscillations of the second dark spot (near 53° south). These oscillations have similar periods near 800 hours and approximately satisfy a simple advective model in which a latitudinal oscillation produces a phase-shifted longitudinal oscillation proportional to the local wind shear. The latitudinal motion of the Great Dark Spot can be fit with an oscillation period of about 2550 hours, whereas its dominant longitudinal motion, if oscillatory at all, has such a long period that it is not well constrained by the Voyager data.