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Still Seeking Answers to Basic Questions Regarding Earth's Climate System
Reference
Bellon, G., Le Treut, H. and Ghil, M.  2003.  Large-scale and evaporation-wind feedbacks in a box model of the tropical climate.  Geophysical Research Letters 30: 10.1029/2003GL017895.

Background
Bellon et al. begin their paper by noting that "observed tropical sea-surface temperatures (SSTs) exhibit a maximum around 30C," and that "this maximum appears to be robust on various timescales, from intraseasonal to millennial."  In light of this demonstrable fact of climate history, they say that "identifying the stabilizing feedback(s) that help(s) maintain this threshold is essential in order to understand how the tropical climate reacts to an external perturbation," which is itself essential in order to understand how the global climate reacts to perturbations such as those produced by solar variability or the ongoing rise in the air's CO2 content.

This contention is further substantiated by the study of Pierrehumbert (1995), which "clearly demonstrates," in the words of Bellon et al., "that the tropical climate is not determined locally, but globally."  In addition, they note that Pierrehumbert's work demonstrates that interactions between moist and dry regions are an essential part of tropical climate stability, which hearkens back to the adaptive infrared iris concept of Lindzen et al. (2001) and the controversy surrounding it (Chou et al., 2002).

What was done
Noting that previous box models of tropical climate have shown it to be rather sensitive to the relative areas of moist and dry regions of the tropics, the authors analyze feedbacks associated with this sensitivity in a four-box model of the tropical climate "to show how they modulate the response of the tropical temperature to a radiative perturbation."  In addition, they investigate the influence of the model's surface-wind parameterization in an attempt to shed further light on the nature of the underlying feedbacks that help define the global climate system that is responsible for the tropical climate observations of constrained maximum SSTs.

What was learned
Bellon et al.'s work, as they describe it, "suggests the presence of an important and as-yet-unexplored feedback in earth's tropical climate, that could contribute to maintain the "lid" on tropical SSTs," much like the adaptive infrared iris concept of Lindzen et al.  They also note that their demonstrated "dependence of the surface wind on the large-scale circulation has an important effect on the sensitivity of the tropical system," specifically stating that "this dependence reduces significantly the SST sensitivity to radiative perturbations by enhancing the evaporation feedback," which injects more heat into the atmosphere and allows the atmospheric circulation to export more energy to the subtropical free troposphere, where it can be radiated to space.

What it means
Clearly, the case is not closed on either the source or the significance of the maximum "allowable" SSTs of tropical regions; and, hence, neither is the case closed on the degree to which the planet as a whole may warm in response to continued increases in carbon dioxide and other greenhouse gases.

References
Chou, M.-D., Lindzen, R.S. and Hou, A.Y.  2002.  Reply to: "Tropical cirrus and water vapor: an effective Earth infrared iris feedback?"  Atmospheric Chemistry and Physics 2: 99-101.

Lindzen, R.S., Chou, M.-D. and Hou, A.Y.  2001.  Does the earth have an adaptive infrared iris?  Bulletin of the American Meteorological Society 82: 417-432.

Pierrehumbert, R.T.  1995.  Thermostats, radiator fins, and the local runaway greenhouse.  Journal of the Atmospheric Sciences 52: 1784-1806.


Reviewed 10 December 2003