Warmer seas fuel stronger cyclones

The strongest tropical cyclones are getting stronger, with the greatest increase in the North Atlantic and northern Indian oceans. A statistical analysis of satellite data by researchers in the USA has shown an upward trend in maximum wind speeds per cyclone as the oceans warm, and revealed a subtle global pattern across different ocean basins.

A recent trend of increasing seas surface temperatures in the tropical Atlantic is believed to be responsible for an increase in the average strength of cyclones in the region — causing significant concern in the community about the potential increase in tropical cyclone activity as a consequence of climate change. Theoretical and modeling studies have suggested that as seas get warmer, the ocean has more energy that can be released as tropical cyclone wind. Stronger evidence has eluded scientists, however, owing to a lack of observational data and the fact that studies have focused only on the average intensity of cyclones.

Now James Elsner from Florida State University and his colleagues have provided strong evidence for this theory using observational data. They made use of a 25-year archive of satellite data to study trends in the upper quantiles of per-cyclone maximum wind speeds — the maximum intensities that cyclones achieve during their lifetimes. Their results are published in this week’s issue of Nature.

Using a statistical technique known as quantile regression to analyze the data, they have found that there is a detectable upward trend in maximum wind speeds of the strongest tropical cyclones — the stronger the cyclone, the greater the change. With the exception of the South Pacific Ocean, all tropical cyclone basins show this pattern, although not all are statistically significant; the greatest increases are seen in the North Atlantic and northern Indian oceans.

The authors calculate that an increase of one degree Celsius in sea surface temperature results in an increase in the global frequency of strong cyclones from 13 to 17 cyclones per year — an increase of 31%.

Elsner and colleagues conclude that the results are consistent with the heat-engine theory of cyclone intensity — as seas get warmer, the ocean has more energy that can be released as tropical cyclone wind. The researchers caution, however, that the uncertainty ranges are still large, and the study does not include other influential factors such as cyclone origin and duration, proximity to land, El Niño conditions and solar activity.

“It is necessary to control other factors such as changes in upper-tropospheric temperatures, shearing winds and proximity to land to better understand regions differences in these trends,” write the authors.