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Thursday, May 15, 2014

Hurricanes May Threaten Northern Cities Like Never Before as Tropics Expand, Study Finds

by Andrew Freedman, Mashable, May 15, 2014

In this December 5, 2012, file photo, men walk past a house damaged during Superstorm Sandy in the Belle Harbor section of the Queens borough of New York. IMAGE: KATHY WILLENS/ASSOCIATED PRESS
Hurricanes and tropical storms are reaching their peak intensity closer to the poles, migrating at about 30 miles per decade, according to a new study published Wednesday. If this shift continues, it could have major consequences for places like New York City, Tokyo, Japan and Brisbane, Australia, as well as other high latitude areas that don't normally see intense hurricanes.
The study, published in the journal Nature, is the first to detect this trend, and in doing so it reveals a somewhat indirect but tangible link between human activities and Mother Nature's most powerful storms.
In order to reach their conclusions, the researchers overcame a formidable obstacle that has long hindered research on the links between hurricanes and global warming. Changes in how we detect and measure the intensity of tropical cyclones over the years makes finding statistically significant trends difficult to near impossible.
Pacific Storms

Typhoon Francisco and Super Typhoon Lekima seen from a satellite on October 23, 2013, as they tracked northwestward toward China and Japan. IMAGE: NOAA
The new study examines a metric known as a storm's "lifetime-maximum intensity" during the period from 1982 to 2012, which is a time frame that is not complicated by changes in storm observations. The metric refers to the point where storms max out in strength.
By examining storm data using this metric, the researchers found a strikingly apparent poleward shift in the locations where storms are reaching their peak intensity. Although the changes varied from ocean basin to ocean basin, with the greatest migration seen in the western North Pacific Ocean, which is the most active area for tropical cyclones, the shift was found in both the Northern and Southern Hemispheres.
In the Northern Hemisphere, the lifetime-maximum intensity point is moving north at 33 miles per decade, whereas in the Southern Hemisphere, that point is moving south at 39 miles per decade.
Such a change is enough to alter the risk that a coastal location will be hit by a major storm, said lead author James Kossin of the National Climatic Data Center.
"There’s no doubt that a signal like this introduces the potential for a change in the risk,” Kossin said in an interview with Mashable. There's certainly a potential for decreased risk in some areas and increased risk in others at higher latitudes.”
The study itself says that if the observed trends continue, there could be "potentially profound consequences for life and property."
In other words, the risk of damage and fatalities from tropical storms and hurricanes could increase in northern latitudes, while declining somewhat at lower latitudes.
Super Typhoon Usagi

Color-enhanced infrared satellite image of Typhoon Usagi as it moved northwestward toward Hong Kong while explosively intensifying to a Category-5 storm. IMAGE: NOAA/COOPERATIVE INSTITUTE FOR METEOROLOGICAL SATELLITE STUDIES, UNIVERSITY OF WISCONSIN-MADISON.
In addition, parts of the tropics that depend on rainfall from tropical storms and hurricanes (collectively referred to as tropical cyclones) to provide water resources may be at risk for lower water availability as the storms move away from them.
One caveat from the study is that there hasn't been a detectable poleward shift in the maximum intensity point of Atlantic tropical cyclones. However, this may be due to other factors that are masking this movement.
"The Atlantic is fairly unique in the last 30 years in how it’s been behaving,” Kossin says. "We kind of suspect that this global signal that we uncovered is just kind of getting muddled up with strong regional effects.
“The takeaway from that is just because we have not seen a trend there in the last 30 years doesn’t mean this effect is not present there, it’s just being masked by other things and it won’t necessarily be masked forever.”
Worldwide Tropical Cyclones

Tracks of all tropical cyclones which formed worldwide from 1985 to 2005. IMAGE: WIKIMEDIA COMMONS
Chris Landsea, a meteorologist at the National Hurricane Center in Miami, Florida, who was not involved in the study, told Mashable: "This is an important, very well researched paper that uncovers something that was unknown previously."
He continued, "Such changes in where storms are peaking are somewhat unexpected (at least to me) but are apparently due to global scale changes in the atmosphere, perhaps tied to anthropogenic [man-made] global warming."

Expanding tropics

The study ties the migration of storms' peak intensity points to a gradual expansion of the tropics, which in turn has been linked to manmade factors. The expansion has to do with the widening of what is known as the Hadley Cell, which is a pattern of air circulation that causes air to converge near the Equator, sparking thunderstorms with heavy rain throughout the tropics. (This helps explain the typical location of rain forests worldwide.)
As air spills out of the Hadley cell and descends, it dries and warms, which is why the subtropics at the edges of the Hadley Cell are often home to deserts.
Studies of how emissions of planet-warming greenhouse gases will affect the tropics have consistently shown that the tropics will expand poleward in both hemispheres, which will push the dry subtropics poleward as well. This is part of the reason why the Southwest U.S. is poised to become drier as the climate warms, since the subtropics are starting to encroach on that area.
Hadley Cell

Diagram of tropical air circulation, including the Hadley Cell.
According to Kossin, the expansion of the tropics has also been tied to emissions of aerosols, which are small particles in the atmosphere, from factories and natural sources like desert dust, as well as manmade depletion of the stratospheric ozone layer. It is not yet clear which factor is the biggest contributor to the expansion of the tropics, however, Kossin said.
If greenhouse gases are driving the expansion of tropical atmospheric circulation, "then we won’t be seeing any stop to this in the near future," Kossin says.
However, if the main driver is the depletion of the ozone layer, the phaseout of ozone-depleting pollution should slow this expansion over time.
In addition to the growing tropics, two other environmental changes have taken place that affect storm intensity. The first concerns vertical wind shear. This refers to the change in wind speed or direction (or both) with height. Vertical wind shear has increased in the tropics, but decreased farther away from the tropics, closer to the poles. Vertical wind shear can make it difficult for tropical storms and hurricanes to form and intensify, since they tear apart the thunderstorms that comprise the inner core of such storms.
The second is that potential intensity, which is how strong a storm can get given sea surface temperatures and the temperature and moisture content of the upper atmosphere, is also changing. “In the tropics the potential intensity is decreasing, and it's increasing outside of the tropics,” Kossin said.
Hugh Willoughby, a hurricane researcher at Florida State University, told Mashable in an email that the study's results match his understanding of how air circulation is changing in and around the tropics. "My take on the situation is that shear attributable to the subtropical jet will increase as the planet warms; whereas the shear attributable to the middle latitude jet will decrease as high latitudes warm more than the tropics," he said. Willoughby was not involved in the new study.
The link between global warming and tropical cyclone behavior has been a source of controversy in the past, but this study appears to present clear, and unexpected, evidence that storms are already responding to the changing climate.
The big question is what it will mean for coastal residents around the world, but early hints are not encouraging for higher latitude cities.

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