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Tropical Sea Surface Temperature Biases in CMIP 3 & 5 Models
Reference
Li, G. and Xie, S.-P. 2012. Origins of tropical-wide SST biases in CMIP multi-model ensembles. Geophysical Research Letters 39: 10.1029/2012GL053777.

Background
The authors write that "state-of-the-art coupled ocean-atmosphere general circulation models (CGCMs) suffer from large errors in simulating tropical climate, limiting their utility in climate prediction and projection," while noting in this regard that the models' "sea surface temperature (SST) errors are comparable or larger in magnitude than observed interannual variability and projected change in the 21st century." The most well-known of these errors, as they continue, include "too weak a zonal SST gradient along the equatorial Atlantic (Davey et al., 2002; Richter and Xie, 2008), an equatorial cold tongue that penetrates too far westward in the Pacific (Mechoso et al., 1995; de Szoeke and Xie, 2008), too warm SSTs over the tropical Southeast Pacific and Atlantic, and a spurious double inter-tropical convergence zone (Lin, 2007)."

What was done
Reduced to the simplest of terms, Li and Xie analyzed the nature and sources of various offset errors in tropical SSTs in the Coupled Model Intercomparison Project (CMIP) phase 3 and 5 multi-model ensembles.

What was learned
The two researchers state that the SST biases they studied were of two types: "one with a broad meridional structure and of the same sign across all basins that is highly correlated with the tropical mean; and one with large inter-model variability in the cold tongues of the equatorial Pacific and Atlantic." They report that "the first type can be traced back to biases in atmospheric simulations of cloud cover, with cloudy models biasing low in tropical-wide SST," while they indicate that "the second type originates from the diversity among models in representing the thermocline depth," such that "models with a deep thermocline feature a warm cold tongue on the equator."

What it means
With Li and Xie noting that the errors they studied "have persisted in several generations of models for more than a decade [italics and bold added]," we can only hope that someone will eventually get around to determining how to eliminate or at least significantly reduce them.

References
Davey, M.K., Huddleston, M., Sperber, K.R., Braconnot, P., Bryan, F., Chen, D., Colman, R.A., Cooper, C., Cubasch, U., Delecluse, P., DeWitt, D., Fairhead, L., Flato, G., Gordon, C., Hogan, T., Ji, M., Kimoto, M., Kitoh, A., Knutson, T. R., Latif, M., Le Treut, H., Li, T., Manabe, S., Mechoso, C. R., Meehl, G. A., Power, S. B., Roeckner, E., Terray, L., Vintzileos, A., Voss, R., Wang, B., Washington, W. M., Yoshikawa, I., Yu, J.Y., Yukimoto, S. and Zebiak, S.E. 2002. 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.

Mechoso, C.R., Robertson, A.W., Barth, N., Davey, M.K., Delecluse, P., Gent, P.R., Ineson, S., Kirtman, B., Latif, M., Treut, H. Le, 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.

Richter, I. and Xie, S.P. 2008. On the origin of equatorial Atlantic biases in coupled general circulation models. Climate Dynamics 31: 587-598.

Reviewed 27 February 2013