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Posted at 10:45 AM ET, 07/ 8/2009

El Nino & Hurricanes: Devil May Be in Details

By Andrew Freedman

* Where's Summer? Full Forecast | Fireworks Photo Spread *

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."

Stay tuned...

By Andrew Freedman  | July 8, 2009; 10:45 AM ET
Categories:  Freedman, Science, Tropical Weather  
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We certainly have had a quiet first few weeks of the current hurricane season.

Posted by: Bombo47jea | July 8, 2009 12:39 PM | Report abuse

One of the main things I learned while a forecaster at the Climate Prediction Center (CPC) - and reinforced since then - is that there are El Ninos and there are El Ninos. That is, there are several different "flavors" of El Nino in terms of both the location of maximum Pacific warming and impacts on weather and tropical cyclones affecting the eastern U.S. Moreover, even with a perfect forecast of El Nino (Pacific Water Temps), there is considerable variability (uncertainty) in the downstream (teleconnections) effects.

It appears now that the a traditional, rather than El Nino "modiki" is developing. Maximum surface and (more importantly) sub surface warm water is located east of the dateline (not central Pacific).

I'll post on the possible effects of ElNino on the coming winter weather in the fall.

Posted by: SteveT-CapitalWeatherGang | July 8, 2009 2:25 PM | Report abuse

The GIT study is interesting to say the least, but stay tuned.............

If the developing El Nino ultimately proves to be more west based, important impacts on next winter's weather as well as the tropical season may be witnessed, but as Steve pointed out, present factors favor a more east based El Nino.

The CFS ensemble is maintaining a +2.0 average for the coming winter, which is stronger than the preferred weaker condition for a snowy winter.

An example at BWI would be the DJF +2.3 during 97-98 which produced only 3.2 inches of snow compared to the DJF +1.2 during 02-03 which dumped 58.1 inches!

A +2.0 would favor a wet winter, but with a lack of cold air many of the storms would probably trend toward cold rain or a mix with heavy snow potential in the higher elevations s.w. of D.C.

It will be interesting to look forward to possibly an active second half in the tropics plus an El Nino winter after the past couple La Nina snow drought years.

Posted by: AugustaJim | July 8, 2009 3:44 PM | Report abuse

Andrew, will you write a post on the recent GoE global warming ceiling goal?

Posted by: --sg | July 9, 2009 7:07 PM | Report abuse

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