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Spectral analysis within a movable time window is used to investigate the frequency and intensity of tropical oscillations with intraseasonal time scales. Surface and rawinsonde time series from several locations in the western Pacific and Indian oceans are used. Evidence is provided that the Madden–Julian Oscillation (MJO) in the Indian Ocean region shows a seasonal variation in its preferred period from 50 days during Northern Hemisphere winter to about 35 days during summer, though this evidence is weak and stretches the limits of our ability to define a precise period for the oscillation. In the western tropical Pacific during the September–December season, a pronounced spectral peak with a central frequency between 20 and 25 days is observed. This spectral peak appears to reveal an oscillation that is distinct from the MJO but may also result from coupling between the large-scale motion and latent heat release on much smaller scales. A relationship between the oscillation and the occurrence of typhoons is demonstrated.
Hartmann et al. (1992) studied this question.