Tropical cyclone Usagi intensity shifting poleward

Colour-enhanced infrared satellite image of Typhoon Usagi as it moved northwestward toward Hong Kong while explosively intensifying to a Category-5 storm. 

Usagi threatened Taiwan, the Northern Philippines, and mainland China, ultimately making landfall in eastern Guangdong province where it caused substantial flooding and more than 30 deaths. 

Credit: NOAA Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison

Powerful, destructive tropical cyclones are now reaching their peak intensity farther from the equator and closer to the poles, according to a new study co-authored by an MIT scientist.

The results of the study, published today in the journal Nature, show that over the last 30 years, tropical cyclones, also known as hurricanes or typhoons, are moving poleward at a rate of about 33 miles per decade in the Northern Hemisphere and 38 miles per decade in the Southern Hemisphere.

"The absolute value of the latitudes at which these storms reach their maximum intensity seems to be increasing over time, in most places," says Kerry Emanuel, an MIT professor and co-author of the new paper.

"The trend is statistically significant at a pretty high level."

And while the scientists who conducted the study are still investigating the atmospheric mechanisms behind this change, the trend seems consistent with a warming climate.

"It may mean the thermodynamically favourable conditions for these storms are migrating poleward," adds Emanuel, the Cecil and Ida Green Professor of Earth and Planetary Sciences at MIT.

The implications are serious, since the movement of peak intensity means regions further north and south of the equator, which have not previously had to face many landfalls by violent cyclones, may now have greater exposure to these extreme weather events.

That, in turn, could lead to "potentially profound consequences to life and property," the paper states. "Any related changes to positions where storms make landfall will have obvious effects on coastal residents and infrastructure."

Hadley Cells

"We think, but have not yet been able to establish, that this is connected to independently observed poleward expansion of the Hadley circulation," Emanuel says, referring to a large-scale pattern of global winds, which in recent years has also moved further poleward.

The paper notes the potential impact of vertical wind shear, which inhibits cyclone formation; data suggests a decrease in wind shear in the tropics and an increase at higher latitudes.

More information: Paper: "The Poleward Migration of the Location of Tropical Cyclone Maximum Intensity," was co-written by Emanuel, James P. Kossin of the University of Wisconsin, and Gabriel A. Vecchi of the National Oceanic and Atmospheric Administration (NOAA). dx.doi.org/10.1038/nature13278