Changes in the sea surface temperature threshold for tropical convection

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Deep convection over tropical oceans is observed generally above a threshold for sea surface temperatures1,2,3,4, which falls in the vicinity of 26–28 °C. High-resolution models suggest that the related sea surface temperature threshold for tropical cyclones rises in a warming climate5,6. Some observations for the past few decades, however, show that tropical tropospheric warming has been nearly uniform vertically7,8, suggesting that the troposphere may have become less stable and casting doubts on the possibility that the sea surface temperature threshold increases substantially with global warming. Here we turn to satellite observations of rainfall for the past 30 years. We detect significant covariability between tropical mean sea surface temperatures and the convective threshold on interannual and longer timescales. In addition, we find a parallel upward trend of approximately 0.1 °C/decade over the past 30 years in both the convective threshold and tropical mean sea surface temperatures. We conclude that, in contrast with some observational indications, the tropical troposphere has warmed in a way that is consistent with moist-adiabatic adjustment, in agreement with global climate model simulations.

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Figure 1: Time series of tropical mean SST and the SST threshold for convection.
Figure 2: The relationship between the SST threshold for convection and upper-tropospheric temperature in global climate models.
Figure 3: Twenty-first-century changes in rainfall rate and SST frequency distributions.


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We thank the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, for the CMAP, GPCP and ERSST Version 3b data, which were provided by their website at We acknowledge the modelling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP3 multi-model data set. Support of this data set is provided by the Office of Science, US Department of Energy. N.C.J. and S-P.X. were supported by grants from NOAA, NSF, NASA and JAMSTEC.

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Both N.C.J. and S-P.X. contributed to the central ideas presented in the paper. N.C.J. carried out the analysis and wrote the paper. S-P.X. offered guidance and contributed to the editing of the paper.

Correspondence to Nathaniel C. Johnson.

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