Wrapping up the pleasantly weak 2006 Atlantic hurricane season, which officially came to an end Thursday, forecaster at the National Oceanic and Atmospheric Administration (NOAA) said the seasonal activity was lower than everyone expected due to the rapid development of El Nino -- a periodic warming of the ocean waters in the central and eastern equatorial Pacific, which influences pressure and wind patterns across the tropical Atlantic.
Earlier this month, William Gray and Philip Klotzbach of the Colorado State University forecast team used the same excuse for why their spring forecasts for a very active hurricane season proved very incorrect (see Daily GPI, Nov 20).
NOAA pointed out that the 2006 Atlantic hurricane season produced near-normal activity with a total of nine named storms, including five hurricanes, two of which became major hurricanes of Category 3 strength or higher. An average Atlantic hurricane season has 11 named storms, with six becoming hurricanes and two becoming major hurricanes. Unlike the past three seasons, the stronger hurricanes stayed well out at sea, sparing the Americas and the Caribbean islands from major hurricane damage this season.
Originally, NOAA had been calling for seven-to-10 hurricanes and three-to-five major hurricanes (see Daily GPI, Feb. 6; Aug. 9).
"The development of El Nino conditions by September helps explain why this Atlantic hurricane season was less active than predicted," said Gerry Bell, NOAA's lead forecaster on the Atlantic hurricane seasonal outlook team. "El Nino developed quickly and the atmosphere responded rapidly, reducing hurricane activity during an otherwise active era that began in 1995."
El Nino, combined with the large-scale weather patterns over the southeastern U.S., produced sinking air in the middle and upper atmosphere, along with higher than anticipated wind shear (the change in winds through the atmosphere) over the Atlantic Ocean, Gulf of Mexico and Caribbean Sea, NOAA said. These conditions minimized thunderstorm activity, which inhibited tropical storm and hurricane formation.
NOAA scientists linked El Nino's rapid development and intensification to a series of large subsurface ocean waves that affect ocean temperatures, which began in June. These waves produced a progressive warming of the tropical Pacific Ocean during the summer months. A particularly strong wave led to a significant warming of the entire eastern half of the equatorial Pacific in early September. This led NOAA in early September to report that an El Nino had developed. These warmer waters produced enhanced rainfall near the international date line, resulting in suppressed hurricane activity.
"Getting a quick handle on El Nino events, which rapidly intensify, is essential for predicting seasonal hurricane activity," said Bell. "The last time we had a rapidly developing El Nino was during the 2002 hurricane season, which also led to near-normal activity. NOAA continues to develop and improve climate models to better predict the onset of El Nino, its impacts on weather patterns in the United States and its effects on Atlantic hurricane activity."
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