We demonstrate a theoretically expected behavior of the tropical sea surface temperature probability density function (PDF) in future and past (Eocene) greenhouse climate simulations. To first order this consists of a shift to warmer temperatures as climate warms, without change of shape of the PDF. The behavior is tied to a shift of the temperature for deep convection onset. Consequently, the threshold for appearance of high clouds and associated radiative forcing shifts along with temperature. An excess entropy coordinate provides a reference to which the onset of deep convection is invariant, and gives a compact description of SST changes and cloud feedbacks suitable for diagnostics and as a basis for simplified climate models. The results underscore that the typically skewed appearance of tropical SST histograms, with a sharp drop-off above some threshold value, should not be taken as evidence for tropical thermostats.


Manuscript Received: 3 August 2009

Manuscript Accepted: 13 October 2009

Version of record online: 7 November 2009

Williams, I. N., R. T. Pierrehumbert, and M. Huber (2009), Global warming, convective threshold and false thermostats, Geophysical Research Letters, 36, L21805, doi:10.1029/2009GL039849.


convection, cloud, feedback

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