Hurricanes, typhoons moving away from equator

Almost all structures in Guiuan, Eastern Samar, Philippines, were damaged or flattened by super typhoon Yolanda on Nov. 8, 2013. The photo was taken from a helicopter on Nov. 10. Typhoons and hurricanes are shifting further from the equator, according to a study released today.

Armed Forces of the Philippines Central Command

Tropical cyclones — the giant rotating storms commonly called hurricanes or typhoons — are moving further from the equator, and that is shifting the resulting danger zone.

Trends in tropical cyclones are difficult to study: A century ago, many made their presence known only if they crossed a populated coast. Things started to get much better with the advent of meteorological satellites in the 1960s.

But when James Kossin, a research meteorologist at the National Climatic Data Center, sifted through 30 years of satellite data, he and his colleagues calculated that the average tropical cyclone path was retreating from the equator at about 39 miles per decade, on average.

That translocation is likely to make a difference on the ground, Kossin says, shifting the danger north or south, while alleviating danger closer to the equator. Beyond the direct damage, Kossin says, “A lot of people rely on tropical cyclones for their fresh water supply.” In the American Southwest, for example, rainfall from spent Pacific hurricanes brings a major addition to the water supply. “You could imagine that a migration could lead to water deficits,” he says.

ENLARGE (impressive)

Typhoon Haiyan approaching the Philippines at 13:00 UTC on Nov. 7, 2013. This is a composite image incorporating data from the geostationary satellites of the Japan Meteorological Agency (MTSat 2) and EUMETSAT (Meteosat-7), overlaying NASA’s ‘Black Marble’ imagery.

Copyright 2013 JMA/EUMETSAT

Roots

As global warming adds energy to the earth system, scientists have tried to assess how that might change tropical cyclones. Their conclusion? These storms are unlikely to grow more common but could well gain strength.

Kossin, who works at the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison, says he was clued into the possibility of a track shift while focusing on air temperatures high above tropical cyclones. “I kept finding trends that I did not expect. I looked further and saw that it was because the hurricane tracks are moving poleward. Like most things in science, I stumbled on this.”

ENLARGE

Color-enhanced infrared satellite image of Typhoon Usagi while it was moving toward Hong Kong in 2013 and explosively intensifying to a Category-5 storm. Usagi threatened Taiwan and the Philippines, and caused substantial flooding and more than 30 deaths in Guangdong province, China. A shift in tropical cyclones paths in the northern hemisphere is moving the locus of disaster northward.

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

Despite the advent of satellite surveillance in the 1960s, hurricane scientists have struggled to pinpoint their strength and exact paths. To look at how hurricanes paths might be changing, Kossin sidestepped this problem by focusing on a much easier task — the point where the storm achieved maximum strength.

The study found differing shifts in different regions — from an extreme around New Zealand (208 kilometers per decade) to a significant increase in East Asia (50 to 70 kilometers per decade in practically no increase at all (7 kilometers per decade) in the North Atlantic. It’s not clear why the North Atlantic was such an exception to the global rule.

Change in hurricane tracks in northern hemisphere

ENLARGE

Graph shows the erratic but detectable northward movement of tropical cyclones in the northern hemisphere. Red shows best tracks from the data; blue is based on data that was, like much hurricane data, reanalyzed. Straight, sloping lines show trends.

Credit: Kossin et al¹

Kossin points to two major factors governing tropical cyclone intensity. The “potential intensity” measures how much energy can be transferred from the warm ocean to the storm. Other factors matter greatly, he says, “but the potential intensity sets the speed limit.”

Tropical cyclones, like some people we could name, don’t always reach their potential, and the most common reason is a difference in wind speed and/or direction at different altitudes, called vertical wind shear. “Storms like to be vertically stacked,” Kossin says. “When they exist in a sheared environment, they get pushed over and are not very efficient in using the available energy.”

Crystal unclear!

It’s unclear why the changes are taking place, but Kossin is focusing on the well-studied Hadley circulation, which causes warm moist air to rise in the tropics and then descend as cold, dry air toward the pole. “We found a trend of the poleward movement of tropical cyclones, and … the average is almost exactly the same as the movement in the Hadley cells. That could be a coincidence, but it seems compelling enough to search further.”