Grey swan tropical cyclones


We define ‘grey swan’ tropical cyclones as high-impact storms that would not be predicted based on history but may be foreseeable using physical knowledge together with historical data. Here we apply a climatological–hydrodynamic method to estimate grey swan tropical cyclone storm surge threat for three highly vulnerable coastal regions. We identify a potentially large risk in the Persian Gulf, where tropical cyclones have never been recorded, and larger-than-expected threats in Cairns, Australia, and Tampa, Florida. Grey swan tropical cyclones striking Tampa, Cairns and Dubai can generate storm surges of about 6 m, 5.7 m and 4 m, respectively, with estimated annual exceedance probabilities of about 1/10,000. With climate change, these probabilities can increase significantly over the twenty-first century (to 1/3,100–1/1,100 in the middle and 1/2,500–1/700 towards the end of the century for Tampa). Worse grey swan tropical cyclones, inducing surges exceeding 11 m in Tampa and 7 m in Dubai, are also revealed with non-negligible probabilities, especially towards the end of the century.

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Figure 1: The 1921 Tampa hurricane compared with two grey swan TCs.
Figure 2: Estimated storm surge level as a function of return period for Tampa for the NCEP/NCAR reanalysis climate of 1980–2005, based on 7,800 synthetic events.
Figure 3: Estimated storm surge level as a function of return period for Tampa in the climate of 1980–2005 (based on 2,100 events), 2006–2036 (3,100 events), 2037–2067 (3,100 events), and 2068–2098 (3,100 events) projected using each of the six climate models for the IPCC AR5 RCP8.5 emission scenario.
Figure 4: Storm surge risk analysis for Cairns, Australia, based on 2,400 synthetic events in the NCEP/NCAR reanalysis climate of 1980–2010.
Figure 5: Storm surge risk analysis for Dubai, based on 3,100 synthetic events in the MERRA reanalysis climate of 1980–2010.


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We acknowledge the World Climate Research Program’s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output. We thank J. Westerink of the University of Notre Dame and C. Dietrich of North Carolina State University for their technical support on the ADCIRC model applied in this study for storm surge analysis. We also thank G. Holland of National Center for Atmospheric Science and J. Nott of James Cook University for their helpful comments. N.L. acknowledges support from Princeton University’s School of Engineering and Applied Science (Project X Fund) and Andlinger Center for Energy and the Environment (Innovation Fund). K.E. was supported by NSF Grant 1418508.

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K.E. performed numerical modelling of the storms. N.L. carried out storm surge simulations and statistical analysis. N.L. and K.E. co-wrote the paper.

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Correspondence to Ning Lin.

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Lin, N., Emanuel, K. Grey swan tropical cyclones. Nature Clim Change 6, 106–111 (2016).

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