Study shows tropical cyclone intensity shifting poleward

(Press-News.org) MADISON, Wis. — The latitude at which tropical cyclones reach their greatest intensity is gradually shifting from the tropics toward the poles at rates of about 33 to 39 miles per decade, according to a study published today (May 14, 2014) in the journal Nature.

The new study was led by Jim Kossin, a National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center scientist stationed at the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies. The research documents a poleward migration of storm intensity in both the Northern and Southern Hemispheres through an analysis of 30 years of global historical tropical cyclone data. The term "tropical cyclone" describes a broad category of storms that includes hurricanes and typhoons, large and damaging storms that draw their energy from warm ocean waters.

The findings are important, says Kossin, because they suggest that some areas, including densely populated coastal cities, could experience changes in risk due to large storms and associated floods and storm surges. Regions closer to the equator, he notes, could experience a reduced risk, and places more distant from the equator could experience an increased risk. The trend observed by Kossin and his colleagues is particularly important given the devastating loss of life and property that can follow in the wake of a tropical cyclone.

Conversely, equatorial regions where people depend on tropical cyclone rainfall to replenish sources of fresh water may experience water shortages.

According to Kossin, there isn't a global trend in the frequency of tropical cyclones for the 30-year study period. However, the data record shows a distinct poleward trend in the observed latitude where storms are the most intense. While estimates of storm intensity vary in the data, the latitude at which tropical cyclones reach their maximum intensity, Kossin explains, is a more reliable assessment of changes in the way tropical cyclones behave.

"We've identified changes in the environment in which the deep tropics have become more hostile to the formation and intensification of tropical cyclones and the higher latitudes have become less hostile," Kossin explains. "This seems to be driving the poleward migration" of storm intensity.

From a global climate perspective, Kossin says, "the more compelling aspect is that the rate of migration fits very well into independent estimates of the observed expansion of the tropics." That phenomenon has been widely studied by other scientists and is attributed, in part, to increasing greenhouse gases, stratospheric ozone depletion, and particulate pollution, all by-products of human activity.

Whether the observed movement of tropical cyclone maximum intensity toward the poles is a result of the expansion of the tropics and its links to human activity requires more and longer-term investigation, says Kossin. Both phenomena, however, exhibit very similar behavior over the past 30 years, lending credence to the idea that the two are linked.

INFORMATION: Co-authors on the Nature paper are Kerry A. Emanuel, Program in Atmospheres, Oceans and Climate at the Massachusetts Institute of Technology, and Gabriel A. Vecchi, NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey. —Jean Phillips, 608-262-8164, jeanp@ssec.wisc.edu