Interdecadal change of the structure of the ENSO mode and its impact on the ENSO frequency

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. To test the robustness of the relationship between the oscillation period and the structure of the coupled mode, the authors designed empirical atmospheric models based on observations and coupled them with the ocean model of Zebiak and Cane. Numerical experiments demonstrate that the ENSO period is sensitive to changes in the wind anomaly pattern in a way much like the observed ENSO frequency-structure relation. The increase of the ENSO period after 1980 is mainly due to the eastward shift of the zonal wind stress with respect to the SST anomalies. Physical explanations of the dependence of ENSO frequency on the structure of the coupled mode are provided by diagnosing the relative contributions of the thermocline feedback and zonal advection feedback on ENSO evolution.