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Moving from CMIP3 to CMIP5: How are the Models Progressing?
Bellenger, H., Guilyardi, E., Leloup, J., Lengaigne, M. and Vialard, J. 2014. ENSO representation in climate models: from CMIP3 to CMIP5. Climate Dynamics 42: 1999-2018.

The authors write that "the El Niņo-Southern Oscillation (ENSO) is the dominant mode of inter-annual climate variability," and that "it is characterized by large-scale sea surface temperature (SST) anomalies in the eastern Equatorial Pacific Ocean," the amplitude of which variations "is typically on the order of 1-3°C and is associated with a change in the oceanic thermal structure and a switch in atmospheric circulation and convective activity," which "is characterized by an irregular period ranging between 2 and 7 years."

What was done
Bellenger et al. analyzed the ability of older CMIP3 and more recent CMIP5 coupled ocean-atmosphere general circulation models (CGCMs) to simulate the tropical Pacific Ocean's mean state and its periodically-perturbed El Niņo-Southern Oscillation (ENSO) state, hoping to discover some significant progress in transiting from the older to the more recent set of models.

What was learned
First of all, the five researchers report that (1) "the CMIP5 multi-model ensemble does not exhibit a quantum leap in ENSO performance compared to CMIP3." And they go on to demonstrate that fact by noting that (2) fundamental ENSO characteristics, such as central Pacific precipitation anomalies, "remain poorly represented," that (3) "the wind-SST feedback is still underestimated by 20-50%," that (4) "shortwave-SST feedbacks remain underestimated by a factor of two," that (5) "ENSO termination tends to occur much more to the west than in observations in both CMIP3 and CMIP5," that (6) "simple metrics of the atmospheric feedbacks that are thought to play a major role in ENSO physics ... show no clear improvement from CMIP3 to CMIP5," that (7) "most CMIP5 models still underestimate the observed positive atmospheric Bjerknes feedback (on average by roughly 30%)," that (8) "the damping surface flux feedback also remains too diverse in CMIP5 models," due to (9) "the difficulty to represent the shortwave feedback and its nonlinearity," and that (10) "there are moreover discrepancies between modelled and observed αsw- (La Niņa conditions) and αsw+ (El Niņo conditions)."

What it means
In the words of Bellenger et al., "significant model development work is still needed to correctly represent the basic ENSO characteristics (amplitude, evolution, timescale, seasonal phase lock ...) and the fundamental underlying processes such as the atmospheric Bjerkness and surface fluxes feedbacks."

Reviewed 9 July 2014