A new study suggests that the strongest Atlantic tropical cyclones are in fact becoming stronger and that this is related to rising sea temperatures as a result of global warming.

Previous work has shown that as sea surface temperatures (SST) rise, the warmer oceans have more energy to drive cyclones. This is known as the 'heat-engine theory of cyclone intensity'. It is therefore expected that there will be more intense storms in the future under a warming climate.

This study found that there is already evidence of increased intensity of storms as sea surface temperatures rise. The researchers say the results were consistent with the heat-engine theory, although not proof of it. The researchers examined satellite records of the wind speeds of tropical cyclones in six ocean basins between the years 1981-2006. Good records exist for the North Atlantic, where SSTs have been correlated with global near-surface temperatures. These explain the trend for tropical cyclones of greater intensity seen in the North Atlantic.

Previous studies, however, have relied on unreliable and incomplete observational records for other basins. By using global satellite data, as opposed to observational records, and focusing on the highest wind speeds of the strongest tropical cyclones, the researchers could demonstrate the trends predicted by the theory and models. The greatest wind speeds found in the strongest cyclones are a better indicator of the association between cyclone strength and climate than average wind speeds which have been used in previous studies.

The researchers found that at the highest levels of wind intensity, the global number of stronger cyclones increased. The numbers rose from 13 to 17, or an increase of 31 per cent per year for each one degree rise in surface sea temperature. In addition, researchers found that the trend for increases in the highest wind speeds of the strongest tropical cyclones were found in all tropical basins, except the South Pacific Ocean. They were especially strong for cyclones in the North Atlantic and northern Indian oceans.

The results also showed that, within the global upward trend, there were noticeable regional trends. The researchers expect that the warming of the coolest oceans should show the greatest increases in intensity of tropical cyclones. A correlation between warming of the coolest oceans, the North Atlantic, the eastern Pacific and the southern Indian Ocean, and the upward trend of more intense tropical cyclones was seen, although other environmental factors affecting the development of tropical storms need to be considered when understanding the regional differences.

Source: Elsner, J.B., Kossin, J.P., Jagger, T.H. (2008). The increasing intensity of the strongest tropical cyclones. Nature. 455: 92-95. Contact: jelsner@fsu.edu Theme(s): Climate change and energy, Natural hazards