A nonlinear aspect of the El Niño—Southern Oscillation (ENSO)--its regulatory
effect on the background state (the climatological state)--is described. In particular, it is shown
that ENSO acts as a basin-scale heat “mixer” that prevents any significant increase from occurring
in the time-mean difference between the warm-pool SST (Tw) and the temperature of
the thermocline water (Tc). When this temperature contrast is forced to increase, the amplitude
of ENSO increases—El Niño becomes warmer and La Niña becomes colder. A stronger La
Niña event results in more heat transported to the subsurface of the western Pacific. A
stronger El Niño event then warms the eastern Pacific and cools the western Pacific. The
effect of a stronger La Niña event does not cancel the effect from a stronger El Niño event.
The long-term mean effect of ENSO—the recurrent occurrence of El Niño and La Niña
events--is to mix heat downward across the equatorial Pacific and prevent the time-mean
difference between Tw and Tc from exceeding a critical value.
The results have implications for several climatic issues and these implications are discussed.
In particular, it is noted that our existing paradigm to understand the response of ENSO to
global warming needs to be modified. It is emphasized that it is the tendency in the stability
forced by an increase in the greenhouse effect, not the actual changes in the time-mean climate,
that ENSO responds to. Changes in the latter—changes in the mean climate—are a
residual between the effect of the changes in the radiative forcing and the effect of the changes
in the ENSO behavior.