Background
The authors write that "ENSO behaviors in coupled models have been widely evaluated," citing Neelin et al. (1992), Delecluse et al. (1998), Latif et al. (2001), Davey et al. (2002), AchuataRao and Sperber (2002, 2006), Capotondi et al. (2006), Guilyardi (2006) and Zhang et al. (2010); yet they say that "coupled models still exhibit large biases in modeling the basic features of ENSO [italics added]," citing Guilyardi et al. (2009), among which biases is "a sea surface temperature (SST) anomaly (SSTA) too tightly confined to the equator (e.g., Stockdale et al., 1998; Kang et al., 2001)." More specifically - and recently - they say "it was shown that the ENSO meridional width in the models participating in Phase 3 of the Coupled Model Inter-comparison Project (CMIP3) is only about two thirds of what is observed," citing Zhang et al. (2012).

What was done
Seeking the anticipated fruits of progress, Zhang and Jin ask the obvious question: "Does the systematical narrow bias in ENSO width still exist in current models developed for Phase 5 of the CMIP (CMIP5)?" And they then go on to answer their own question by assessing the ENSO meridional widths simulated by 15 CMIP5 models and 15 CMIP3 models for the period 1900-1999, comparing the results of both groups against observation-based monthly SST data from the Hadley Center Sea Ice and Sea Surface Temperature (HadISST) data of Rayner et al. (2003).

What was learned
The two researchers report that "a systematic narrow bias in ENSO meridional width remains in the CMIP5 models," although they state that the newest results represent "a modest improvement over previous models."

What it means
One question yet remains to be answered with respect to this particular aspect of model performance: Is a modest improvement good enough? ... especially when considering the time, money and scientific effort put into the program that produced it, and knowing, in the words of Zhang and Jin, that "models with a better performance in ENSO width tend to simulate the precipitation response to ENSO over the off-equatorial eastern Pacific more realistically."

References
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AchutaRao, K. and Sperber, K.R. 2006. ENSO simulation in coupled ocean-atmosphere models: are the current models better? Climate Dynamics 27: 1-15.

Capotondi, A., Wittenberg, A. and Masina, S. 2006. Spatial and temporal structure of tropical Pacific interannual variability in 20th century coupled simulations. Ocean Modeling 15: 274-298.

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Guilyardi, E. 2006. El Niņo mean state-seasonal cycle interactions in a multi-model ensemble. Climate Dynamics 26: 329-348.

Guilyardi, E., Wittenberg, A., Fedorov, A., Collins, M., Wang, C., Capotondi, A., Jan, G., Oldenborgh, V. and Stockdale, T. 2009. Understanding El Niņo in ocean-atmosphere general circulation models: Progress and challenges. Bulletin of the American Meteorological Society 90: 325-340.

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Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

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Zhang, W., Li, J. and Zhao, X. 2010. Sea surface temperature cooling mode in the Pacific cold tongue. Journal of Geophysical Research 115: 10.1029/2010JC006501.

Zhang, W., Jin, F.-F., Li, J. and Zhao, J.-X. 2012. On the bias in simulated ENSO SSTA meridional widths of CMIP3 models. Journal of Climate: org/10.1175/JCLI-D-12-00347.1.