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In the late 1970s, the ENSO cycle exhibited frequency change. The oscillation period increased from 2-4 yr (high frequency) during 1962-75 to 4-6 yr (low frequency) during 1980-93. Observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled ENSO mode. In comparison with the high-frequency regime, the structure of the coupled mode in the low-frequency regime shows three distinctive features during the warm phase of ENSO: the eastward shift of the westerly anomalies, the meridional expansion of the westerly anomalies, and the weaker intensity of the easterly anomalies in the eastern Pacific.
An et al. (Thu,) studied this question.
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