Andrea Thompson, LiveScience.com Yahoo News 3 Sep 08;
Strong hurricanes are getting stronger, likely thanks to global warming, a new study finds.
The news comes as Tropical Storms Hanna, Ike and Josephine march almost in a line across the Atlantic basin, and just days after Hurricane Gustav slammed into the Louisiana coast.
Scientists have previously predicted that as global warming further heats up the ocean, hurricanes could become more frequent, more intense or both. The new work is in line with some of those previous studies but projects that it is the strongest of these storms - the Katrinas and Andrews - that will suck up this extra heat energy and become even stronger.
The research is detailed in the Sept. 4 issue of the journal Nature.
Heat engine
The theory behind the formation of tropical cyclones (the term that encompasses hurricanes and other tropical storms) is that the warm, moist air over the ocean surface fuels the storm's convection. The warmer the ocean surface, the more energy is available to fuel a storm's ferocious winds.
James Elsner of Florida State University and his colleagues studied the wind-speed data from a 25-year satellite record of storms across the globe. They found that warmer ocean temperatures matched up with an increase in the highest wind speeds achieved by storms.
Weaker storms have other factors besides sea surface temperature influencing them: wind shear (which can stifle hurricane formation); interaction with other storms (this happened with Gustav and Hanna); and travel over land (which weakens a storm because it is cut off from its fuel source, the warm ocean water).
Stronger storms generally develop because these weakening factors aren't in play, making it easier to see the boost they get from warmer oceans, Elsner said. However, weaker storms would also probably show a slight strengthening due to warmer waters with all other factors being taken into account, he said.
The North Atlantic and north Indian Ocean basins showed the strongest intensification signals in wind speed with increasing ocean temperature. Elsner said that this is expected because these basins are colder and would show a stronger response to any warming, whereas basins that are already warm are optimized for storm formation.
"We speculate that it has to do with the fact that, you know, there is kind of a saturation point," Elsner told LiveScience. "You can't just keep getting strong hurricanes as the ocean warms up."
Globally, the researchers found a 31 percent increase in strong storms (those in the top fifth in a ranking of storms by their intensities), from 13 to 17 strong cyclones for a 1.8 degree Fahrenheit (1 degree Celsius) rise in ocean temperature.
Speed limit?
Other researchers have looked into whether there might be a cap on wind speeds.
According to a 1998 calculation by MIT climatologist Kerry Emanuel, the estimated maximum potential for hurricanes is about 190 mph. Emanuel and other scientists have predicted that wind speeds - including maximum wind speeds - should increase about 5 percent for every 1 degree Celsius increase in tropical ocean temperatures.
[Using observed data to arrive at actual wind speeds for this, Elsner and his colleagues found, for the same 1-degree C ocean temperature increase, wind speed increases of around 4.5 mph (2 meters per second) for storms in the top fifth of the intensity ranking, and wind speed increases of around 14.5 mph (6.5 meters per second) for storms in the top tenth.]
However, Typhoon Nancy in 1961, in the Northwest Pacific Ocean, was said to have maximum sustained winds of 215 mph, according to the World Meteorological Organization's Commission on Climatology, a clearinghouse for climate records set up at Arizona State University to settle the many disputes on weather and climate extremes. (A typhoon is the same thing as a hurricane, just in a different part of the world.)
There are known records for wind speeds that outstrip anything ever measured in a hurricane. The fastest "regular" wind that's widely agreed upon was 231 mph, recorded at Mount Washington, New Hampshire, on April 12, 1934. During a May 1999 tornado in Oklahoma, researchers clocked the wind at 318 mph.
Meanwhile, Hanna is expected to turn northward over the next 24 hours, away from Haiti, and to strike the Bahamas before possibly becoming a hurricane again and hitting the U.S. Southeast coast. Ike was expected to become a hurricane Wednesday as it heads westward into the Caribbean. Josephine could strengthen some, but will likely stay out at sea for the time being, according to the latest forecasts from the National Hurricane Center.
Forecasters recently said that warm ocean surface temperatures will make September a busy month of Atlantic storms.
Warming boosts strongest storms
Richard Black, BBC News website 3 Sep 08;
The strongest tropical storms are becoming even stronger as the world's oceans warm, scientists have confirmed.
Analysis of satellite data shows that in the last 25 years, strong cyclones, hurricanes and typhoons have become more frequent in most of the tropics.
Writing in the journal Nature, they say the number of weaker storms has not noticeably altered.
The idea that climate change might be linked to tropical storms has been highly controversial.
A few years ago, it was claimed that hurricanes would become more frequent as well as more common in a warming world.
The swirling winds pick up energy from a warm ocean.
But recent research has suggested they would occur less frequently, though likely to pack a more powerful punch each time.
James Elsner from Florida State University in Tallahassee, US and colleagues believed the link might become clearer if they analysed data according to the strength of storms.
"We're seeing a signal, and it's telling us that the strongest effect (of rising ocean temperatures) is on the strongest storms," he told BBC News.
"At average or median wind speeds, about 40m/s, we don't see a trend; but when we get up to 50 or 60m/s we do see a trend."
A hurricane featuring winds of 40m/s (89mph) is a Category One storm according to the often-used Saffir-Simpson scale.
At about 60m/s (134mph) it enters Category Four, the strength at which Hurricane Gustav recently hit Cuba before weakening to Category One over the US coast.
Tropical trends
Hurricanes, typhoons and cyclones are different terms used in different regions of the world for the same phenomenon.
However, the bulk of the scientific work on possible links to climate change has featured North Atlantic hurricanes, largely because of the relatively good historical records contained in the US.
The new analysis, using satellite data acquired by US, European and Japanese programmes, shows up different trends across the tropics.
The increase in strong storms shows up most markedly in the North Atlantic and Indian oceans, and is absent in the South Pacific.
"We're looking at different ocean basins, and some are already pretty warm," said Professor Elsner.
"So there, an increase in temperature isn't going to produce as strong an increase as in basins where the the temperatures are only marginally supportive of cyclones."
The researchers believe weaker storms are not affected so much because the factors that prevent them developing to their full potential, notably wind shear - abrupt changes in wind speed and direction that prevent the cyclone fuelling itself with ocean heat - are not related to ocean temperatures.
Globally, a rise of 1C in sea surface temperature would increase the occurrence of strong storms by about one third, the researchers calculate.
Apart from human-induced climate change, the incidence of tropical storms is determined by natural cycles such as El Nino that affect surface temperatures in various parts of the oceans.
The damage they do is affected far less by their strength than by where they hit land, and by how able a society is to withstand the winds and rain.
