Changes in the sea surface temperature threshold for tropical convection

Journal name:
Nature Geoscience
Volume:
3,
Pages:
842–845
Year published:
DOI:
doi:10.1038/ngeo1008
Received
Accepted
Published online

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.

At a glance

Figures

  1. Time series of tropical mean SST and the SST threshold for convection.
    Figure 1: Time series of tropical mean SST and the SST threshold for convection.

    Thirty-year time series of annual tropical mean (20°S to 20°N) SST (black diamonds) and two estimates of the SST threshold for convection (blue squares and red stars). Linear trend lines are also shown. The linear trends with 95% confidence intervals for the tropical mean SST, the P=2mmd−1 SST threshold estimate and the linear P fit SST threshold estimate are 0.088±0.057,0.083±0.076 and 0.080±0.113°C per decade, respectively. The effective degrees of freedom in the 95% confidence interval calculations account for the lag-1 autocorrelation in the residual time series.

  2. The relationship between the SST threshold for convection and upper-tropospheric temperature in global climate models.
    Figure 2: The relationship between the SST threshold for convection and upper-tropospheric temperature in global climate models.

    a, Scatter plot of SST threshold versus tropical mean 300hPa temperature with regression lines for each of ten CMIP3 models under emissions scenario A1B in simulations of the twenty-first century. b, Ensemble mean tropical mean SST (black), SST threshold for convection (red) and tropical mean 300hPa temperature (blue) anomaly time series for the CMIP3 models. Anomalies are relative to the 1961–1990 climatology. The left y axis corresponds to the tropical mean SST and SST threshold, whereas the right y axis corresponds to the 300hPa temperature. The scaling for the 300hPa temperature (right y axis) corresponds approximately to that of MALR adjustment of the tropical mean SST (left y axis).

  3. Twenty-first-century changes in rainfall rate and SST frequency distributions.
    Figure 3: Twenty-first-century changes in rainfall rate and SST frequency distributions.

    a,b, Ensemble mean rainfall rate as a function of SST (a) and SST frequency distribution (b) for 2001–2020 (blue, solid) and 2081–2100 (red, dashed) for the ten CMIP3 models of Fig. 2. SST is expressed as the deviation from the 20-year tropical mean.

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Affiliations

  1. International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

    • Nathaniel C. Johnson
  2. International Pacific Research Center and Department of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

    • Shang-Ping Xie

Contributions

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.

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The authors declare no competing financial interests.

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