Upper Tropical Tropospheric Temperature: Simulations vs. Reality

According to Fu et al. (2011), a "pronounced feature in GCM (general circulation model)-predicted climate change in the 21st century is the much enhanced maximum warming in the tropical upper troposphere near ~200 hPa (IPCC, 2007)," and they state that this feature has important implications for climate sensitivity, because of its influence on "water vapor, lapse rate and cloud feedbacks (e.g., Colman, 2001; Hartmann and Larson, 2002)," as well as its significant impact on the atmosphere's circulation, as described by Held (1993) and Butler et al. (2010). In light of these facts, they state that it is critically important to "observationally test the GCM-simulated maximum warming in the tropical upper troposphere," which is the focus of the authors' paper. Specifically, Fu et al. examined trends of temperature differences between the tropical upper- and lower-middle troposphere based on satellite microwave sounding unit (MSU) observations, after which they compared them with AR4 GCM simulations for the period 1979-2010.

The three researchers report, first of all, that "AR4 GCMs overestimate the warming in the tropics for 1979-2010," noting that the tropical surface temperature trend from the multi-model ensemble mean is more than 60% larger than that derived from observations. Second, although they report that the satellite observations generally support GCM results with respect to tropical deep-layer tropospheric warming being greater than surface warming, they still find that the AR4 GCMs "exaggerate the increase in static stability between the tropical middle and upper troposphere during the last three decades."

Fu et al. conclude their paper by writing that "in view of the importance of the enhanced tropical upper tropospheric warming to the climate sensitivity and to the change of atmospheric circulations, it is critically important to understand the causes responsible for the discrepancy between the models and observations." And so it is that in regard to the ability of state-of-the-art climate models to correctly foretell the planet's climatic future, there is still a long way to go before they get there.

Additional References
Butler, A.H., Thompson, D.W.J. and Heikes, R. 2010. The steady-state atmospheric circulation response to climate change-like thermal forcings in a simple general circulation model. Journal of Climate 23: 3474-3496.

Colman, R.A. 2001. On the vertical extent of atmospheric feedbacks. Climate Dynamics 17: 391-405.

Hartmann, D.L. and Larson, K. 2002. An important constraint on tropical cloud-climate feedback. Geophysical Research Letters 29: 10.1029/2002GL015835.

Held, I.M. 1993. Large-scale dynamics and global warming. Bulletin of the American Meteorological Society 74: 228-241.

IPCC. 2007. Climate Change 2007: The Physical Science Basis. Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.). Cambridge University Press, Cambridge, United Kingdom.