Study Shows Tropical Storm Intensity Has Not Increased

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”.

Kossin co-authored research that appeared in Nature in May 2014 that showed tropical cyclone activity had been moving away from the tropics at a steady pace for 30 years. This Nature paper reported that in that time the location of peak storm intensity has shifted toward the North and South poles, at rates of 53 and 62 kilometres per decade, respectively. This migration may be linked to the observed expansion of the tropics, which has been attributed to contributions from human activity, the researchers stated.

As a reality check on their Nature paper results, Kossin and his colleagues looked for underlying changes in average vertical wind shear. Atmospheric physics suggests this should be consistent with the latitude shift: stronger shears being found closer to the equator (vertical shear can shred developing storms) and relaxed shears at higher latitudes. They found that the change in vertical wind shear was consistent with a warming-induced expansion of the general circulation of the tropical atmosphere.

Temperatures in the upper troposphere of the atmosphere, near the tropopause, play a key role in the evolution of tropical cyclones (TC) by controlling their potential intensity (PI), which describes the thermodynamically-based maximum TC intensity that the environment will support.

Accurately identifying past trends in PI is critical for understanding the causes of observed changes in TC intensity, but calculations of PI trends using different atmospheric reanalysis products can give very different results, due largely to differences in their representation of upper-tropospheric temperatures. Without a means to verify the fidelity of the upper tropospheric temperatures, PI trends calculated from these products are very uncertain.

Here, a method is introduced to validate the upper-tropospheric temperatures in the reanalysis products by using the TCs themselves as thermometers. Using a 30-year global dataset of TC cloud-top temperatures, and three widely-utilized atmospheric reanalysis products – MERRA, ERA-Interim, and NCEP/NCAR – it is shown that storm-local upper-level temperatures in the MERRA and ERA-Interim data vary similarly to the TC cloud-top temperatures on both interannual and decadal timescales, but the NCEP/NCAR data have substantial biases that introduce an increasing trend in storm-local PI not found in the other two products.

The lack of global storm-local PI trends is due to a balance between temporal increases in the mean state and the poleward migration of TCs into lower climatological PI, and has significant implications for the detection and attribution of mean TC intensity trends.

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