Background
The authors write that "the seasonal climate of the tropical Atlantic Ocean is notoriously difficult to simulate accurately in coupled models," noting that a long history of studies, including those of Zeng et al. (1996), Davey et al. (2002), Deser et al. (2006), Chang et al. (2007) and Richter and Xie (2008), "have linked the ultimate causes of the persistent model biases to problems in simulating winds and clouds by the atmospheric model component."

What was done
In an effort designed to "revisit" this unsolved problem, Grodsky et al. utilized the Community Climate System Model, version 4 (CCSM4; Gent et al., 2011), which is a coupled climate model that simultaneously simulates earth's atmosphere, ocean, land surface and sea ice processes. This they did by comparing 20th-century runs forced by time-varying solar output, greenhouse gas, volcanic and other aerosol concentrations for the period 1850-2005 with observed (real world) monthly variability computed from observational analyses during the 26-year period 1980-2005.

What was learned
In the enlightening words of the four researchers, (1) the "atmospheric component of CCSM4 has abnormally intense surface subtropical high pressure systems and abnormally low polar low pressure systems," (2) "in the tropics and subtropics, the trade wind winds are 1-2 m/sec too strong [and] latent heat loss is too large," (3) "sea surface temperature in the southeast has a warm bias [due in part to] erroneously weak equatorial winds," (4) "the warm bias evident along the coast of southern Africa is also partly a result of insufficient local upwelling," (5) "excess radiation is evident in the south stratocumulus region of up to 60 W/m²," (6) there is "excess precipitation in the Southern Hemisphere," and (7) "errors in cloud parameterization lead to "massively excess solar radiation in austral winter and spring in CCSM4."

What it means
Meet the new model. Same as the old model. And don't get fooled again.

References
Chang, C.-Y., Carton, J.A., Grodsky, S.A. and Nigam, S. 2007. Seasonal climate of the tropical Atlantic sector in the NCAR Community Climate System Model 3: Error structure and probable causes of errors. Journal of Climate 20: 1053-1070.

Davey, M.K., Huddleston, M., Sperber, K. R., 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. 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.

Deser, C., Capotondi, A., Saravanan, R. and Phillips, A. 2006. Tropical Pacific and Atlantic climate variability in CCSM3. Journal of Climate 19: 403-420.

Gent, P.R., Danabasoglu, G., Donner, L.M., Holland, M.M., Hunke, E.C., Jayne, S.R., Lawrence, D.M., Neale, R.B., Rasch, P.J., Vertenstein, M., Worley, P.H., Yang, Z.-L. and Zhang M. 2011. The Community Climate System Model version 4. Journal of Climate 24: 4973-4991.

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

Zeng, N., Dickinson, R.E. and Zeng, X. 1996. Climate impact of Amazon deforestation - A mechanistic model study. Journal of Climate 9: 859-883.