In the late 1970s, the ENSO cycle exhibited frequency change. The oscillati
on 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 cha
nge 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 featur
es during the warm phase of ENSO: the eastward shift of the westerly anomal
ies, the meridional expansion of the westerly anomalies, and the weaker int
ensity of the easterly anomalies in the eastern Pacific.
To test the robustness of the relationship between the oscillation period a
nd the structure of the coupled mode, the authors designed empirical atmosp
heric 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 ob
served ENSO frequency-structure relation. The increase of the ENSO period a
fter 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 E
NSO frequency on the structure of the coupled mode are provided by diagnosi
ng the relative contributions of the thermocline feedback and zonal advecti
on feedback on ENSO evolution.