The mechanisms of interdecadal changes of El Ni¿o-Southern Oscillation (ENSO) modes are examined through an eigen analysis of a simple coupled ocean-atmosphere model. It is shown that the observed interdecadal climate shift can effectively modify the strength of two major coupled feedbacks for the ENSO mode, namely, the zonal advection and thermocline feedbacks. These modifications lead to quantitative changes of the leading coupled mode in its frequency, growth rate, and spatial pattern, which are consistent with the observation. ¿ 2000 American Geophysical Union