The Role of ENSO in Regulting its Background State

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 cli-mate, 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.