Rectification of ENSO as a Mechanism for Climate Variability on the Decadal and Longer Time-scales
The importance of the tropical Pacific sea surface temperature (SST) in predicting climate variability over North American and the world at larger has been vividly demonstrated in our rich experience with ENSO (Philander 1990). The apparent regimelike shift in the tropical Pacific SST from about 1976 has underscored another fact about tropical Pacific SST: it also varies on decadal time scales (Wang and Ropelewski 1995; Zhang et al. 1997; Fedorov and Philander 2000). As the society increasingly needs a climate outlook beyond the time scale of ENSO, understanding the mechanisms that give rise to the decadal variability in the background state of ENSO (or the time-mean state relative to the time scale of ENSO) has become a forefront issue facing climate research community (Meehl et al. 2010).
Fig. 1. (a) SST differences between two epochs: 1977-2003 and 1950-76. (b) Niño3 SST time series. Niño3 SST (anomalies) (in color). The black solid line is the variance of Niño3 SST anomalies obtained by sliding a moving window of a width of 16 yr. Note that the epoch 1977-2003 has a higher level of ENSO activity than the previous period 1950-76. (SST data used are from the Hadley Center for Climate Prediction and Research) (Rayner et al. 1996).
"This regime shift shown in Fig. 1a is accompanied by 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 paper finds:
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.
"Among the many milestones marking the conceptual advances in our understanding of the origin of natural climate variability, we find that the study by Kessler and Kleeman (2000) stands out as distinctly as that by Hasselmann (1976) in their originality of pointing out that climate variability of one time scale can be an important cause or significant contributor for climate variability of a different time scale. Specifically, the study by Kessler and Kleeman (2000) points out that the Madden--Julian oscillation (MJO)--an intraseasonal climate signal in the surface winds--can be converted to a sea surface temperature (SST) anomaly in the tropical Pacific that resembles what is normally associated with the El Nino–Southern Oscillation (ENSO), an interannual climate signal. The underlying mechanism for this conversion is provided by nonlinear ocean dynamics and is termed “rectification” by Kessler and Kleeman (2000). A question that naturally follows up on the study of Kessler and Kleeman (2000) is whether this upscale conversion stops with MJO and ENSO. Can an ENSO signal in the surface winds be converted to SST anomalies resembling the observed decadal signal, that is, the tropical Pacific decadal variability (Zhang et al. 1997)? This paper deals with this question in a manner that is analogous to that of Kessler and Kleeman (2000). This study is also meant to complement earlier studies that use a hybrid coupled model (Sun and Zhang 2006; Sun 2007) as well as an analytical model (Liang et al. 2012) in delineating the time-mean effect of ENSO events."----Opening paragraph from
Sun, D.-Z., T. Zhang, Y. Sun, and Y. Yu, 2014: Rectification of El Nino-Southern Oscillation into Climate Anomalies of Decadal and Longer Time-scales: Results from Forced Ocean GCM Experiments.
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.