By
OurAmazingPlanet
updated 1/18/2011 12:47:58 PM ET 2011-01-18T17:47:58

The bar that burgeoning storms must hurdle to become full-blown hurricanes is rising, a new study finds.

Hurricanes need hot ocean water to fuel the convection that drives their fearsome circulation. When seawater is hot enough, tropical cyclones — the generic term for hurricanes, tropical storms and typhoons — can start swirling.

Due to rising ocean temperatures caused by global climate change, the threshold for how hot these waters need to be is moving up the temperature scale, according to the new study. As a result, hurricanes and other severe storms could become more intense, though the number of storms is not expected to change.

"It is too difficult to say how our results relate precisely to the number of hurricanes, but our results do suggest that rising sea surface temperatures alone should not appreciably affect the number of tropical cyclones," said Nat Johnson, a study team member at the International Pacific Research Center of the University of Hawaii at Manoa. "However, most tropical cyclone studies do project that the total number of tropical cyclones will change little but the number of intense tropical cyclones will increase."

Ocean surface waters generally must be between 78 and 82 degrees Fahrenheit (26 to 28 degrees Celsius) to ignite deep convection (rapidly rising air that forms the thunderstorms that power the tropical cyclones ). This hot water threshold has been rising at the same rate as that of the tropical oceans under global warming — about 0.18 degrees Fahrenheit (0.1 degrees C) per decade over the last 30 years, the study found.

"The correspondence between the two time series is rather remarkable," Johnson said. "The convective threshold and average sea surface temperatures are so closely linked because of their relation with temperatures in the atmosphere extending several miles above the surface."

Global climate models project that the threshold will continue to rise in tandem with the tropical average sea surface temperature, Johnson found. If true, this warming, along with other tropical cyclone ingredients, could muddle future hurricane forecasts.

"Other factors such as changes in the winds high in the atmosphere or changes in the regions where moisture converges could impact the number of hurricanes," Johnson told OurAmazingPlanet.

The study was detailed in the Nov. 7 online edition of the journal Nature Geoscience.

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