Background
The authors write that "the notorious tropical bias problem in climate simulations of global coupled general circulation models (e.g., Mechoso et al., 1995; Latif et al., 2001; Davey et al., 2002; Meehl et al., 2005) manifests itself particularly strongly in the tropical Atlantic," and they say that "while progress towards reducing tropical climate biases has been made in the tropical Pacific over the past decades (e.g., Deser et al., 2006), little or no progress has been made in the tropical Atlantic (Breugem et al., 2006; Richter and Xie, 2008; Wahl et al., 2009)." In fact, they state that "the climate bias problem is still so severe that one of the most basic features of the equatorial Atlantic Ocean -- the eastward shoaling thermocline -- cannot be reproduced by most of the Intergovernmental Panel on Climate Change (IPCC) assessment report (AR4) models," citing Richter and Xie (2008).

What was done
Wan et al., as they describe it, "show that the bias in the eastern equatorial Atlantic has a major effect on sea-surface temperature (SST) response to a rapid change in the Atlantic Meridional Overturning Circulation (AMOC)." This they do by (1) exemplifying the problem "through an inter-model comparison study of tropical Atlantic response to an abrupt change in [the] AMOC using the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Climate Model (CM2.1) and the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM3)," and by (2) dissecting the oceanic mechanisms responsible for the difference in the models' SST responses.

What was learned
The four researchers say their results demonstrate that the different SST responses of the two models "is plausibly attributed to systematic differences in the simulated tropical Atlantic ocean circulation."

What it means
The ultimate implication of Wan et al.'s findings is, in their words, that "in order to accurately simulate past abrupt climate changes and project future changes, the bias in climate models must be reduced." But if "little or no progress" on this problem has been made in the tropical Atlantic "over the past decades," as noted by the four of them, the outlook is not very promising for such a positive development any time soon.

References
Breugem, W.P., Hazeleger, W. and Haarsma, R.J. 2006. Multimodel study of tropical Atlantic variability and change. Geophysical Research Letters 33: 10.1029/2006GL027831.

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.

Deser, C., Capotondi, A., Saravanan, R. and Phillips, A.S. 2006. Tropical Pacific and Atlantic climate variability in CCSM3. Journal of Climate 19: 2451-2481.

Latif, M., Sperber, K., Arblaster, J., Braconnot, P., Chen, D., Colman, A., Cubasch, U., Cooper, C., Delecluse, P., Dewitt, D., Fairhead, L., Flato, G., Hogan, T., Ji, M., Kimoto, M., Kitoh, A., Knutson, T., Le Treut, H., Li, T., Manabe, S., Marti, O., Mechoso, C., Meehl, G., Power, S., Roeckner, E., Sirven, J., Terray, L., Vintzileos, A., Voss, R., Wang, B., Washington, W., Yoshikawa, I., Yu, J. and Zebiak, S. 2001. ENSIP: the El Niņo simulation intercomparison project. Climate Dynamics 18: 255-276.

Mechoso, C.R., Roberston, 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.

Meehl, G.A., Covey, C., McAvaney, B., Latif, M. and Stouffer, R.J. 2005. Overview of the coupled model intercomparison project. Bulletin of the American Meteorological Society 86: 89-93.

Richter, I. and Xie, S.-P. 2008. On the origin of equatorial Atlantic biases in coupled general circulation models. Climate Dynamics 31: 587-595.

Wahl, S., Latif, M., Park, W. and Keenlyside, N. 2009. On the tropical Atlantic SST warm bias in the Kiel Climate Model. Climate Dynamics 33: 10.1007/s00382-009-0690-9.