"This regime shift shown in Fig. 1a is accompanied with the change in the level of ENSO variability—the variance of the interannual variability of the tropical Pacific SST (Fig. 1b). The level of ENSO activity during the epoch with a warmer time-mean SST in the eastern tropical Pacific is anomalously higher than the previous epoch with a colder time-mean SST in the eastern tropical Pacific. Is the change in the level of ENSO activity caused by the change in the time-mean state, or is the change in the time-mean state a consequence of the change in the level of ENSO activity? ---"
Liang, J., X.-Q. Yang, and D.-Z. Sun, 2012: The effect of ENSO events on the Tropical Pacific Mean Climate: Insights from an Analytical Model.
The study elucidates the role of ENSO events in shaping the tropical mean climate state and suggests that decadal warming in the recent decades in the eastern tropical Pacific may be more a consequence than a cause of the elevated ENSO activity during the same period. The results also provide a simple explanation for why it is difficult to detect an anthropogenically forced trend in the zonal SST contrast in the observations.
This study finds
(1) "the rectified effect of ENSO events has its maximum off equator"--a hallmark of the Tropical Pacific Decadal Variability(2) The rectified effect of ENSO has a complex spatial structure in the equatorial upper ocean (Figs. 6, 7): an overall reduction in the thermal contrast between the surface warm pool and the subsurface thermocline water is accompanied by a strengthening of the vertical stratification in the central equatorial Pacific. Thus, the present study may also potentially provide a path to understand the dynamics behind the suggestion from empirical studies that the transition (or change) from a weak ENSO regime to a strong ENSO regime (or vice versa) on decadal and longer time scales may be accompanied by a change in the dominance by the two types of El Niño events—the central Pacific El Niño (warm-pool El Niño or Modoki) and the eastern Pacific El Niño (3) The rectified effect of ENSO includes a substantial cooling to the warm pool. The cooling is much more profound, particularly at the subsurface level, than the traditional ENSO residual map had suggested. The present finding of a profound cooling to the western Pacific by the collective effect of El Niño events highlights a role for other factors in causing the observed warming in the western Pacific over the last few decades.