False Alarm: Tropical Storm Intensity Not Increasing

Despite predictions that global warming would lead to more powerful storms, a new study finds that there has been no increase in the intensity of tropical storms.

Instead, the ongoing poleward migration of storms that has been apparent in recent decades and which acts to suppress storm intensity is offsetting the impact of increased global average temperatures which tends to increase storm intensity, the new research finds.

Fundamental physics means that warmer atmospheric and sea surface temperatures provide the energy to produce more powerful storms. But as cyclones drift poleward they move into cooler latitudes that are less capable of fuelling major storms, according to the research findings published in the Bulletin of the American Meteorological Society.

This conclusion is based on an analysis of 30 years of satellite data on tropical storms by Jim Kossin, an atmospheric scientist with the US National Climatic Data Center.

Kossin analysed satellite data to determine cloud top temperatures of tropical storms which are linked to the intensity of storms. Temperatures in the upper troposphere, near the tropopause, control what is known as the potential intensity of cyclones – a measure of the thermodynamically-based maximum strength that a tropical cyclone can attain.

Previous attempts to calculate potential intensity have run into problems because there have been significant variations in the results derived from different data sets. Kossin ruled out variations in sea surface temperature (SST) as a cause of these differences since all the data sets exhibited similar mean SST variability and trends.

Kossin was able to overcome the variability problem by using the storms themselves as thermometers and analysing the temperature of storm cloud tops as measured by satellite infra-red instruments. This analysis showed no increase in storm intensity over the last 30 years.

Kossin states in his paper that the “lack of a global trend in mean storm-local potential intensity suggests that there is no manifest theoretical expectation that global-mean tropical cyclone intensity has increased in the past 30 years, at least based on potential intensity theory. This is consistent with observation that mean tropical cyclone lifetime-maximum intensity exhibits no significant trend over this period”.

Kossin concludes that the “lack of trends in mean storm-local intensity is not due to a lack of temporal changes in the environment, but appears to be due to the offsetting effect of the poleward migration of tropical cyclones that has occurred over the same period”.

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