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CMIP5 Climate Model Biases
Volume 17, Number 32: 6 August 2014

In a recent study published in Nature Climate Change, Wang et al. (2014) note that the IPCC Fifth Assessment Report "largely depends on simulations, predictions and projections by climate models," but they note that most of the models upon which they rely "have deficiencies and biases that raise large uncertainties in their products." And, therefore, they say that over the past several decades, "a tremendous effort has been made to improve model performance in the simulation of special regions and aspects of the climate system." So what is the current status of the most up-to-date CMIP5 models in this regard?

Wang et al. forthrightly acknowledge that sea surface temperatures or SSTs "simulated by CMIP5 models generally show too low values in the Northern Hemisphere and too high values in the Southern Hemisphere," and they say that these "annual-mean SST error magnitudes can be several degrees Celsius." In addition, they note that "misrepresentation of local processes and/or ocean-atmosphere interactions has caused some of the biases," giving as examples the facts that in the models there is (1) "excessive heat flux into the ocean under insufficient coverage by stratocumulus clouds (Mechoso et al., 2007; Huang et al., 2007)," as well as (2) "insufficient cooling by ocean transients from the upwelling regions along the eastern coasts (Colas et al., 2012)."

Continuing, the five researchers write that "the cold SST bias in the equatorial and tropical southwestern Pacific has been associated with [1] an excessive westward extension of the cold tongue from the eastern equatorial Pacific in association with [2] difficulties in the representation of surface winds and ocean mixing processes (Mechoso et al., 2007; Davey et al., 2002)." And they say that according to the study Hwang and Frierson (2013), "cloud errors over the Southern Ocean may be responsible for the generation of a spurious intertropical convergence zone south of the Equator in most CMIP5 models."

As for their own work, Wang et al. linked SST biases for different regions to simulations of the Atlantic meridional overturning circulation (AMOC), finding that (1) "improving climate models cannot be reduced to improved representation of regional processes," that (2) "much is to be done for a better understanding of the global teleconnections that ultimately affect climate model performance," and that (3) "an improvement of the simulated AMOC in climate models is needed for better climate predictions and projections."

In closing, however, Wang et al. warn that even if the AMOC strength is correctly simulated, if it is too shallow, "the associated northward heat transport could be too weak," which possibility, in their words, is readily supported by a well-known deficiency in level coordinate models that suggests that "North Atlantic Deep Water is too shallow," as indicated by the work of Yeager and Danabasoglu (2012). Clearly, therefore, the world's climate modelers still have a long and winding road to travel, in order to get to the point where they, or anyone else, can place any real confidence in their "simulations, predictions and projections."

Sherwood, Keith and Craig Idso

Colas, F., McWilliams, J.C., Capet, X. and Jaison, K. 2012. Heat balance and eddies in the Peru-Chile current system. Climate Dynamics 39: 403-420.

Davey, M.K., Huddleston, M., Sperber, K., Braconnot, P., Bryan, F., Chen, D., Colman, R., Cooper, C., Cubasch, U., Delecluse, P., DeWitt, D., Fairhead, L., Flato, G., Gordon, C., Hogan, T., Ji, M., Kimoto, M., Kitoh, A., Knutson, T., Latif, M., LeTreut, 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. 2002. STOIC: a study of coupled model climatology and variability in tropical ocean regions. Climate Dynamics 18: 403-420.

Huang, B., Hu, Z.-Z. and Jha, B. 2007. Evolution of model systematic errors in the tropical Atlantic basin from the NCEP coupled hindcasts. Climate Dynamics 28: 661-682.

Hwang, Y.-T. and Frierson, D.M.W. 2013. Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean. Proceedings of the National Academy of Sciences USA 110: 4935-4940.

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 general circulation models. Monthly Weather Review 123: 2825-2838.

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

Yeager, S. and Danabasoglu, G. 2012. Sensitivity of Atlantic meridional overturning circulation variability to parameterized Nordic Sea overflows in CCSM4. Journal of Climate 25: 2077-2103.