Warmer ocean waters could mean fewer Atlantic hurricanes striking the United States, according to new findings by climate scientists at the National Oceanic and Atmospheric Administration (NOAA).

In a report published last week in Geophysical Research Letters, the NOAA scientists said warming of global sea surface temperatures is associated with a sustained long-term increase of vertical wind shear in the main development region for Atlantic hurricanes. The increased vertical wind shear coincides with a downward trend in U.S. landfalling hurricanes.

"We looked at U.S. landfalling hurricanes because it is the most reliable Atlantic hurricane measurement over the long term," said Chunzai Wang, a physical oceanographer with NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami. "Using data extending back to the middle nineteenth century, we found a gentle decrease in the trend of U.S. landfalling hurricanes when the global ocean is warmed up. This trend coincides with an increase in vertical wind shear over the tropical North Atlantic and the Gulf of Mexico, which could result in fewer U.S. landfalling hurricanes."

Observations from 1854 to 2006 show a warming of sea surface temperature occurring almost everywhere over the global ocean, with large warming in tropical regions of the Pacific, Atlantic and Indian oceans. Warmer waters in the tropical Pacific and Indian oceans increase vertical wind shear in the Atlantic hurricane main development region, while warming in the tropical North Atlantic decreases vertical wind shear. Overall, warming in the Pacific and Indian oceans is of greater impact and produces increased levels of vertical wind shear that suppresses Atlantic hurricane activity, NOAA said.

Earlier this month, WSI Corp. forecasters said a continuation of warmer-than-normal Atlantic Ocean temperature anomalies into the summer and fall and the likelihood of a favorable or neutral wind shear environment on the heels of the current La Nina event will bring an active 2008 Atlantic hurricane season (see NGI, Jan. 7). WSI's forecast called for 14 named storms and seven hurricanes, including three intense hurricanes (Category 3 or greater) during the coming Atlantic hurricane season. The forecast numbers are all larger than the 1950-2007 averages of 9.7 named storms, 5.9 hurricanes and 2.3 intense hurricanes. Hurricane season runs from June 1 to Nov. 30.

Last month weather forecasters at Colorado State University (CSU) predicted a somewhat above-average 2008 hurricane season, calling for 13 named storms and seven hurricanes, including three major hurricanes (see NGI, Dec. 10, 2007). The CSU forecasters said their analysis was based on a new statistical forecast technique that explains a considerable amount of hurricane variability in hindcasts issued from 1950 to 2007.

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