Paper Reviewed
Zhou, Z.-Q. and Xie, S.-P. 2015. Effects of Climatological Model Biases on the Projection of Tropical Climate Change. Journal of Climate 28: 9909-9917.

Writing that "CMIP5 models are widely used for global climate predictions and projections," Zhou and Xie (2015) note "there are still large errors in the simulation of the mean state in these models," while adding that some of the biases they produce in tropical climate -- such as the double Intertropical Convergence Zone (ITCZ) and various cold tongue problems -- "have persisted for several model generations despite the continued effort to reduce these biases." And, therefore, they go on to explore this major problem in greater depth to see what might be able to be done about it.

First of all, the two researchers used an atmospheric general circulation model to study how biases in its mean state affect the projection of tropical climate change, especially when forced by a pattern of sea surface temperature (SST) increase derived from a coupled simulation of global warming that employed an SST climatology derived from either real-world observations or a coupled historical simulation. And what did they thereby learn?

Zhou and Xie report that during the February-April part of the year, when the climatological ITCZ displaces spuriously into the Southern Hemisphere, the model (1,2) "overestimates (underestimates) the projected rainfall increase in the warmer climate south (north) of the equator over the eastern Pacific." In addition, they note that (3) "the global warming-induced Walker circulation slowdown is biased weak in the project using coupled model climatology, suggesting that [4] the projection of the reduced equatorial Pacific trade winds may also be underestimated," which finding is related to the bias that (5) "the climatological Walker circulation is too weak in the model, which is in turn due to [6] "a too-weak mean SST gradient in the zonal direction."

Last of all, the two researchers note that (7,8) "some of the biases (e.g., the double ITCZ and cold tongue problems) in tropical climate have persisted for several model generations despite the continued effort to reduce these biases," adding that "recent studies suggest that in addition to the importance of local ocean-atmosphere feedbacks, the tropical biases might originate from extratropical errors," citing in this regard the studies of Hwang and Frierson (2013), Wang et al. (2014) and Li and Xie (2014)," and stating that one of the primary challenges facing today's climate modelers is to "translate such improved understanding to improved model performance," which feat is something that continues to evade them.

References
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 Science USA 110: 4935-4940.

Li, G. and Xie, S.-P. 2014. Tropical biases in the CMIP5 multimodel ensemble: The excessive equatorial Pacific cold tongue and double ITCZ problems. Journal of Climate 27: 1765-1780.

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.