Background
The authors write that "the coldest level in the tropical tropopause layer, the cold-point tropopause (CPT), is known to play a crucial role in stratosphere-troposphere exchange (Holton et al., 1995)," and they say that "the CPT temperature largely determines the concentration of water vapor in the lower stratosphere (e.g., Mote et al., 1996), which serves as a key radiative constituent for surface climate (Forster and Shine, 2002; Solomon et al., 2010)."

What was done
As Kim et al. describe it, "the climatology, seasonality, and intra-seasonal to inter-annual variability of the temperature field near the cold-point tropopause are examined using the state-of-the-art climate models that participated in the Coupled Model Intercomparison Project phase 5 (CMIP5)."

What was learned
The three researchers report that "the models have several notable limitations" and "show non-negligible biases in several aspects." Enumerating a few of the most significant problems, they state that "(1) most models have a warm bias around the CPT, (2) large intermodal differences occur in the amplitude of the seasonal cycle in 100 hPa temperature, (3) several models overestimate lower stratospheric warming in response to volcanic aerosols, (4) temperature variability associated with the quasi-biennnial oscillation and Madden-Julian oscillation is absent in most models, and (5) equatorial waves near the CPT exhibit a wide range of variations among the models."

What it means
As a result of these and other problems, Kim et al. conclude that "the fine-scale processes that govern stratospheric water vapor and the CPT temperature are unlikely to be well represented in CMIP5 models," and that "the coarse vertical resolution of the models and their non-negligible biases in the climatology, seasonal cycle and variability of the tropical tropopause layer limit their accuracy in the assessment of past, present and future climates."

References
Forster, P. and Shine, K. 2002. Assessing the climate impact of trends in stratospheric water vapor. Geophysical Research Letters 29: 10.1029/2001GL013909.

Holton, J.R., Haynes, P.H., McIntyre, M.E., Douglass, A.R., Rood, R.B. and Pfister, L. 1995. Stratosphere-troposphere exchange. Reviews of Geophysics 33: 403-439.

Mote, P.W., Rosenlof, K.H., McIntyre, M.E., Carr, E.S., Gille, J.C., Holton, J.R., Kinnersley, J.S., Pumphrey, H.C., Russell, J.M. and Waters, J.W. 1996. An atmospheric tape recorder: The imprint of tropical tropopause temperatures on stratospheric water vapor. Journal of Geophysical Research 101: 3989-4006.

Solomon, S., Rosenlof, K.H., Portmann, R.W., Daniel, J.S., Davis, S.M., Sanford, T.J. and Plattner, G.-K. 2010. Contributions of stratospheric water vapor to decadal changes in the rate of global warming. Science 327: 1219-1223.