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The Weak ENSO Asymmetry in CMIP5 Climate Models

Paper Reviewed
Sun, Y., Wang, F. and Sun, D-Z. 2016. Weak ENSO Asymmetry Due to Weak Nonlinear Air-Sea Interaction in CMIP5 Climate Models. Advances in Atmospheric Sciences 33: 352-364.

Introducing their study, Sun et al. (2016) write that "state-of-the-art climate models suffer from large errors in simulating the tropical Pacific mean state and ENSO variability," which problems, they say, "have existed for almost two decades," with the result that they "limit the skill of coupled models in simulating ENSO and its teleconnections," citing as a progression of examples the studies of Mechoso et al. (1995), Li and Xie (2012) and Wang et al. (2014) in this regard. And so the question arises: have things gotten any better throughout the course of the past two decades?

Determined to find the answer to this question, Sun et al. (2016) analyzed available ensemble runs of individual CMIP5 models of ENSO asymmetry in the 20th century. And what did they learn by so doing?

The three researchers report that (1) "CMIP5 climate models show a weaker positive nonlinear atmospheric response to ENSO-related SST changes than the observations," that (2) "excessive cold tongue biases in the mean state limit the skill of the air-sea coupling and thus lead to [3] the too weak non-linearity of air-sea interaction and [4] too weak ENSO asymmetry." And they say that the excessive cold tongue, which (5) "extends too far west in CMIP5 models," also (6) "pushes the deep convection to the western tropical Pacific warm pool region," while further noting that (7) "the cold tongue bias in the mean state is unfavorable for the development of deep convection over the central equatorial Pacific."

Continuing their description of their findings, Sun et al. note that (8) "the deep convection has difficulty in further moving to the eastern equatorial Pacific [EP] during El Niño events, especially extreme events, which [9] confines the westerly wind anomaly to the western Pacific." And they therefore report that (10) "strong EP El Niño events do not develop as easily as central Pacific El Niño events in the CMIP5 climate models," as has also been reported by Kim and Yu (2012). In addition, they note that this behavior (11) "weakens the EP El Niño events, especially extreme El Niño events, and thus [12] leads to weakened ENSO asymmetry in climate models."

And as if these twelve shortcomings were not enough to report, Sun et al. add, in concluding their paper, that (13) "CMIP5 models cannot reproduce the decadal variability over the tropical Pacific," nor can they adequately portray (14) "the decadal variability of ENSO asymmetry." Consequently, as the three researchers ultimately conclude, (15 and counting) "many problems remain regarding the simulation of ENSO asymmetry in climate models."

Kim, S.T. and Yu, J.Y. 2012. The two types of ENSO in CMIP5 models. Geophysical Research Letters 39: 10.1029/2012GL052006.

Li, G. and Xie, S.P. 2012. Origins of tropical-wide biases in CMIP multi-model ensembles. Geophysical Research Letters 39: 10.1029/2012GL053777.

Mechoso, C.R., Robertson, A. W., Barth, N., Davey, M. K., Delecluse, P., Gent, P. R., Ineson, S., Kirtman, B., Latif, M., Le Treut, H., Nagai, T., Neelin, J. D., Philander, S. G. H., Polcher, J., Schopf, P. S., Stockdale, T., Suarez, M. J., Terray, L., Thual, O., and Tribbia, J. J. 1995. The seasonal cycle over the tropical Pacific in coupled ocean-atmosphere general circulation models. Monthly Weather Review 123: 2825-2838.

Wang, C.Z., Zhang, L.P., Lee, S.K., Wu, L.X. and Mechoso, C.R. 2014. A global perspective on CMIP5 climate model biases. Nature Climate Change 4: 201-205.

Posted 15 June 2016