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.
The analysis by Ning Lin of Princeton University and a colleague showed that surges generated by grey swans striking Tampa or Cairns could reach six metres (20 feet), and four metres for Dubai.
The damage caused by grey swans, as with all cyclones and hurricanes, is mostly from storm surges, which can be enhanced by higher water levels and tides.
Hurricane Katrina, which left 1,500 people dead and caused some $75 billion (68 billion euros) in damages in 2005, exemplified the destructive potential of such surges.
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 20 feet, 19 feet and 13 feet, 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 36 feet in Tampa and 23 feet in Dubai, are also revealed with non-negligible probabilities, especially towards the end of the century.