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The Equatorial Cold Tongue Bias in CGCMs: Its Impact on ENSO

Vanniere, B., Guilyardi, E., Madec, G., Doblas-Reyes, F.J. and Woolnough, S. 2013. Using seasonal hindcasts to understand the origin of the equatorial cold tongue bias in CGCMs and its impact on ENSO. Climate Dynamics 40: 963-981.
In the words of Vanniere et al. (2013), "the cold equatorial SST [sea surface temperature] bias in the tropical Pacific that is persistent in many coupled OAGCMs [Ocean-Atmosphere Global Climate Models] severely impacts the fidelity of the simulated climate and variability in this key region, such as the ENSO [El Niño-Southern Oscillation] phenomenon." More specifically, they note that "the seasonal equatorial cold tongue extends too far west, is too cold in the east Pacific and is associated with too strong trade winds," citing Davey et al. (2001), AchutaRao and Sperber (2006) and Lin (2007). In addition, they say that "a warm SST bias is observed near the coast of South America," which is "associated with a lack of low clouds and deficient winds." And they indicate that "mean state biases relevant to ENSO also include too strong easterlies in the west Pacific and the double ITCZ [Inter-Tropical Convergence Zone] syndrome," citing Lin (2007) and de Szoeke and Xie (2008).

In attempting to unscramble and resolve these many problems, Vanniere et al. used seasonal re-forecasts or hindcasts to "track back" the origin of the major cold bias, so that "a time sequence of processes involved in the advent of the final mean state errors can then be proposed," applying this strategy to the ENSEMBLES-FP6 project multi-model hindcasts of the last decades."

When all was said and done - and there was really a lot that was said and done - the five researchers discovered that "the models are able to reproduce either El Niño or La Niña close to observations, but not both [itlaics added]." Thus, "more work is needed to understand the origin of the zonal wind bias in models," and they indicate, in this regard, that "understanding the dynamical and thermodynamical mechanisms that drive the tropical atmosphere is required both to alleviate OAGCM errors and to describe the full extent of the atmosphere's role in tropical variability, such as ENSO."

Additional References
AchutaRao, K. and Sperber, K. 2006. ENSO simulations in coupled ocean-atmosphere models: are the current models better? Climate Dynamics 27: 1-16.

Davey, M., Huddleston, M., Sperber, K., Braconnot, P., Bryan, F., Chen, D., Colman, R., Cooper, C., Cubash, U., Delecluse, P., DeWitt, D., Fairhead, L., Glato, G., Gordon, C., Hogan, T., Ji, M., Kimoto, M., Kitoh, A., Knutson, T., Latif, M., Le Treut, H., Li, T., Manabe, S., Mechoso, C., Meehl, G., Power, S., Roeckner, E., Terray, L., Vintzileos, A., Voss, R., Wang, B., Washington, W., Yoshikawa, I., Yu, J., Yukimoto, S. and Zebiak, S. 2001. STOIC: a study of coupled model climatology and variability in tropical ocean regions. Climate Dynamics 18: 403-420.

de Szoeke, S.P. and Xie, S.P. 2008. The tropical Eastern Pacific seasonal cycle: assessment of errors and mechanisms in IPCC AR4 coupled ocean atmosphere general circulation models. Journal of Climate 21: 2573-2590.

Lin, J.L. 2007. The double-ITCZ problem in IPCC AR4 coupled GCMs: ocean-atmosphere feedback analysis. Journal of Climate 20: 4497-4525.

Archived 9 July 2013