Interdecadal Changes in the Structure and Frequency of ENSO Mode

In the late 1970's, ENSO cycle exhibits an evident frequency change. The oscillation period increased from 2-4 years (high frequency) during 1962-1975 to 4-6 years (low frequency) during 1980-1993. Using observations, we show 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.

The eigen analysis of a simple coupled ocean-atmosphere model is applied to show the dynamical linkage between the interdecadal changes of ENSO mode and the basic state changes during the two decades. Two major feedback mechanisms, the zonal advection and thermocline feedbacks, are effectively modified under the realistic decadal changes of basic state. These modifications lead to a regime change of the leading coupled mode. Moreover, the interdecadal changes of the frequency, growth rate, and the spatial pattern of ENSO mode are consistent with the observation.