El Nino & Hurricanes: Devil May Be in Details
The traditional rap on El Nino, the periodic ocean-atmosphere cycle in the equatorial tropical Pacific Ocean that is infamous for altering weather patterns worldwide, is that it causes a quieter Atlantic hurricane season. For this reason, recent forecasts calling for a moderate El Nino to continue to develop during the next few months should be good news for those living in harm's way along the Eastern Seaboard and Gulf Coast, and for the Washington area, which as of now would likely have trouble handling a tropical deluge after all the rain during the first half of the year.
However, a new study, published last week in the journal Science, suggests that greater caution should be exercised before concluding that this year will be a quiet one in the Atlantic. The study, by Hye-Mi Kim, Peter J. Webster and Judith Curry of the Georgia Institute of Technology, found that the relationship between El Nino and the North Atlantic hurricane season might in fact be much more nuanced than previously thought.
Keep reading for more on the not-so-straightforward relationship between El Nino and hurricane season...
The study details a slightly different form of tropical Pacific Ocean warming that may previously have been mistaken for El Nino. This type of warming is referred to as central Pacific warming or El Nino "modiki" -- the Japanese word meaning "same, but different" -- and may actually juice up the Atlantic hurricane season. The study found that since the early 1990s, El Nino modiki events have been occurring with increasing frequency compared to traditional El Nino episodes, although the reasons for this are unclear.
A traditional El Nino event is characterized by abnormally warm water temperatures in the eastern equatorial Pacific, as well as altered air flow across the Pacific Ocean. El Nino modiki events feature unusually warm water located further to the west, in the central Pacific near the International Date Line. Although the two events are quite similar, the authors found that the difference in location of the warmer waters can have profound ramifications for residents in hurricane-prone areas of the United States, Caribbean and Central America.
By examining correlations between the North Atlantic hurricane season, both in terms of where storms tracked and the number of storms that formed in a given year, and the different types of Pacific Ocean warming events, the researchers found that central Pacific warming is associated with increased numbers of tropical storms and hurricanes. An analysis of hurricane damages by Roger Pielke Jr. of the University of Colorado at Boulder found that U.S. storm damages were greater in El Nino modiki years than in El Nino years.
"...we cannot assume that because there is a warming of the Pacific Ocean that there will be a decrease of tropical cyclones. It depends very much on the location of the warming," said co-author Peter J. Webster in an e-mail conversation.
Previous research had shown that El Nino events tend to cause wind shear -- winds blowing at different directions and/or speeds at different altitudes -- to increase over the areas where tropical storms and hurricanes typically form in the Atlantic. Strong wind shear can prevent a tropical storm or hurricane from developing or keep it from intensifying.
However, according to the new study, central Pacific warming events do not increase wind shear over the hurricane-formation regions of the Atlantic Ocean, and therefore are more favorable for storm development.
Webster said the study will "raise the bar" on making more accurate El Nino forecasts. "Not only need we forecast that there will be a warming but where in the Pacific the warming will take place," he said.
Webster said the study could lead to improved seasonal hurricane forecasts, in part because central Pacific warming showed greater predictability at longer lead times than did El Nino. Webster (and others) refers to the unreliable nature of springtime El Nino forecasts as the "spring predictability barrier," saying, "In the spring, the system just doesn't know if it is going to develop into an El Nino, La Nina or some average condition."
As for the hurricane season that is already off to a rather quiet start, Webster said "the probability is very high" that a Pacific warming event will take place. "The climate models seem to indicate an eastern Pacific warming [event] possibly becoming a central Pacific [one]," he said. "If this is the case then we might expect a quiet first half of the season and a quite active second half. We will know much more by the middle of the month."
| July 8, 2009; 10:45 AM ET
Categories: Freedman, Science, Tropical Weather
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