| Multi-Decadal Signal |
| (Weather Guru) |
| Thu May 24 2007 03:25 PM |
Hurricane Formation In Cool Earth Periods
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http://preview.tinyurl.com/ysu7ck
May 24, 2007 Copyright 2007 The New York Times
Study Finds Hurricanes Frequent in Some Cooler Periods
By ANDREW C. REVKIN
Over the last 5,000 years, the eastern Caribbean has experienced several periods, lasting centuries, in which strong hurricanes occurred frequently even though ocean temperatures were cooler than those measured today, according to a new study.
The authors, from the Woods Hole Oceanographic Institution, say their findings do not necessarily conflict with recent papers asserting a link between the region’s hurricane activity and human-caused warming of the climate and seas.
But, they say, their work does imply that factors other than ocean temperature, at least for thousands of years, appear to have played a pivotal role in shaping storminess in the region.
The study compared a 5,000-year record of strong storms etched in lagoon mud on the Puerto Rican island of Vieques with data on ocean temperatures and climate and storm patterns. The analysis is being published today in the journal Nature.
The Woods Hole team found that stormier spans, including one from 1700 until now, were associated with a relative paucity of El Niño warm-ups of the tropical Pacific Ocean and also with periods of heightened monsoon intensity in West Africa.
El Niño episodes tend to change wind patterns in ways that weaken Atlantic Ocean hurricanes, and Africa is a nursery for storm fronts that can drift westward and develop into hurricanes.
Storm records extracted from sediments on the Gulf Coast by other scientists, and near New York City by the Woods Hole team, show a similar pattern, implying that the shifts from quieter to stormier times are not just a local phenomenon, the authors said.
Jeffrey P. Donnelly, the lead author, said the findings pointed to the importance of figuring out an unresolved puzzle: whether global warming will affect the Niño cycle one way or the other. More intense or longer Pacific warm-ups could stifle Atlantic and Caribbean hurricanes even with warmer seas, Dr. Donnelly said.
“Warm sea-surface temperatures are clearly the fuel for intense hurricanes,†he said. “What our work says is that without sea temperatures varying a lot, the climate system can flip back and forth between active and inactive regimes.â€
He added that a disturbing possibility was a warming of waters while conditions in the Pacific and Africa are in their hurricane-nurturing mode.
“If you flip that knob and also have warming seas,†Dr. Donnelly said, “oh boy, who knows what could happen?â€
Judith A. Curry, an atmospheric scientist at Georgia Tech, said the new study, together with other recent research on warming and storms by her and others, added to a picture of rising risk and lagging government action on reducing vulnerability of coastal populations in the Atlantic and Caribbean hurricane zone.
“The bottom line is that we are in an unusually active period of hurricane activity, as a result of a combination of natural variability and global warming,†Dr. Curry said. “Analyses have been done, plans have been put on the table, but nothing seems to be happening.â€
(This post did not belong in the Storm Forum - the Storm Forum deals with active systems and seasonal forecasts.)