Gu, Daifang, and S. G. H. Philander, 1995: Secular changes of annual and interannual variability in the tropics during the past century. Journal of Climate, 8(4), 864-76.

Abstract: Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870-1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885-1915, to be small during the period 1915-1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle. Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870-1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885-1915, to be small during the period 1915-1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle. On short timescales of 2-5 years, ENSO stronly influences the annual cycle in certain parts of the central and eastern tropical Pacific where the thermocline is shallow. The annual cycle is weak in warm El Niño years and is strong in cold La Niña years. This result suggests that the amplitude of the seasonal cycle is affected by interannual variations in the depth of the thermocline and in the intensity of the trade winds.