Zhang, T. and Sun, D.-Z. 2014. ENSO asymmetry in CMIP5 models. Journal of Climate 27: 4070-4093.
The El Niño-Southern Oscillation (ENSO) is a major source of inter-annual climate variability that affects weather and climate worldwide. Interestingly, however, its two phases - El Niño and La Niña - are not mirror images of each other; for authors Zhang and Sun (2014) state that "the strongest El Niño is stronger than the strongest La Niña," a fact that is referred to by Burgers and Stephenson (1999) as ENSO asymmetry, the cause of which, according to Zhang and Sun, is "not yet clearly understood."
In light of this lack of understanding, the two U.S. researchers purposed to evaluate "the ENSO asymmetry in CMIP5 models," along with "corresponding Atmospheric Model Intercomparison Project (AMIP) runs" in order to "gain more insight into possible causes of the bias in ENSO asymmetry." With respect to their findings, Zhang and Sun report the following, (1) "the underestimate of observed positive ENSO asymmetry measured by skewness is still a common problem in CMIP5 coupled models," (2) "all the models are also found to have a weaker ENSO asymmetry than observations," (3) "CMIP5 coupled models have a significant cold bias in the mean sea surface temperature," (4-6) "biases in zonal wind stress, precipitation and subsurface temperatures ... are also too symmetrical with respect to ENSO phases," (7) "sea surface temperature warm anomalies over the far eastern Pacific are found to be weaker in the coupled models than in observations," (8) "most models also have a weaker subsurface temperature warm anomaly over the eastern Pacific," (9) "most models have a weaker precipitation asymmetry over the eastern Pacific," (10) "most AMIP models have a stronger time-mean zonal wind over the equatorial central and eastern Pacific," and (11) they "underestimate the observed positive skewness of zonal winds in the central Pacific."
As time continues to progress, several elements of the climate-modelling juggernaut appear to be stuck in neutral.
Burgers, G. and Stephenson, D.B. 1999. The "normality" of El Niño. Geophysical Research Letters 26: 1027-1030.