Wavelet transforms (WLT) and waveform transforms (WFT) are effective tools that reveal temporal structure of nonstationary time series. The authors discuss principles and practical aspects of their geophysical applications. The WLT can display variance as a continuous function of time and frequency, but the frequency (time) locality reduces at the high (low) frequency bands. The WFT, on the other hand, provides a sharp view of the locality in both time and frequency, but presents variance by discrete base functions. The two techniques are complementary. The authors use both Morlet WLT and Gabor WFT to analyze temporal structure of the Southern Oscillation (SO).
The principal period of the SO has experienced two rapid changes since 1872, one in the early 1910s and the other in the mid-1960s. The dominant period was 3-4 years in the earliest four decades (1872-1910), 5-7 years in the ensuing five decades (1911-1960, except the 1920s), and about 5 years in the last two decades (1970-1992). The SO also exhibits noticeable amplitude changes. It was most energetic during two periods: 1872-1892 and 1970-1992, but powerless during the 1920s, and 1930s, and 1960s. The powerless period is dominated by quasi-biennial oscillation. Excessively strong cold phases of the El Niņo-Southern Oscillation cycle enhance annual variation of SST in the equatorial eastern and central Pacific. The enhancement, however, appears to be modulated by an interdecadal variation.
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