El Nino and Hurricanes
EL NIÑO NOT THE DRIVING FORCE BEHIND NORTH PACIFIC HURRICANES
COLUMBUS, Ohio -- El Niño may be responsible for severe weather conditions across North America, but an Ohio State University study has revealed that El Niño weather systems dont always spawn severe hurricanes in the North Pacific.
The current El Niño is different than most, however, and this difference may help researchers solve the mystery behind some curious atmospheric trends, like why the average intensity of North Pacific hurricanes is increasing over time.
Jay Hobgood, associate professor of geography at Ohio State, and graduate student Luke Whitney reviewed 31 years of weather data and found that hurricanes in the 7 years during which a severe El Niño visited the North Pacific were not much more intense than hurricanes during the other 24 years. Hurricanes during El Niño years averaged 49.3 percent of the maximum possible intensity, versus 48.6 percent in non-El Niño years. The work appeared in a recent issue of the Journal of Climate.
Hobgood explained that the warm surface water caused by El Niño off the west coast of South America heats the air and gives rise to stormy weather over the Pacific as far north as Southern California. The researchers expected that El Niño would intensify hurricanes in that area, but it didnt.
We were surprised, said Hobgood. But then we realized that El Niño also may generate more wind shear. In the Atlantic, wind shear keeps the storms down. We think that in the Pacific the warm water and the wind shear may cancel each other out.
In fact, wind shear is the major reason that most hurricanes only reach 40 to 80 percent of their maximum possible intensity.
But this El Niño is different, said Hobgood. Most El Niños start around Christmas, but this El Niño started in March of 1997. The warming was also greater than anything weve seen this century, and during the hurricane season it didnt create much wind shear to weaken storms over the Eastern North Pacific. On the average, El Niños arent going to have much of an effect on hurricanes, but this one has.
The continued warm surface temperatures and low winds aloft from this El Niño nurtured Hurricane Linda in September of 1997, allowing it to become the strongest storm ever observed in the East Pacific, with winds reaching 190 miles per hour.
Hobgood said this work represents a natural next step in the research thats been going on for decades at the National Hurricane Center (NHC), part of the National Centers for Environmental Prediction in Miami. The work may one day help scientists predict hurricane strength.
For the last 30 years most hurricane research has focused on prediction of where hurricanes are going to hit, and weve gotten pretty good at it, said Hobgood. We can warn people about 2 days in advance that a storm may hit their area. Now were trying to learn how to predict the strength of hurricanes.
NHC scientists were already investigating hurricane strength in the Atlantic, so Hobgood and Whitney decided to look at the Pacific. When they did, they noticed that hurricanes in the North Pacific have on the average grown more intense between 1963 and 1993.
While the average intensity fluctuated year to year, it gradually increased over the 31 years. The minimum intensity of 32.6 percent occurred in 1964, then grew to about 50 percent in 1973, remained relatively stable until 1987 when it began to increase again, and then reached a maximum of 63.7 percent in 1990.
Hobgood admits that the gradual increase in intensity may be at least partially due to an overall increase in ocean surface temperature, which may be due to global warming.
Im not convinced that the change is entirely due to global warming, he said. Its probably a combination of things, possibly including global warming.
Hobgood was quick to point out that as satellite technology has improved over the years, analytical techniques have become more systematic, which could account for some of the increase in recorded intensity.
Now that Hobgood and Whitney have investigated ocean surface temperature, they plan to examine the influences of other atmospheric conditions on hurricanes. Currently they are studying data collected at Socorro, a tiny volcanic island off the west coast of Mexico, an area of much hurricane activity. They are examining weather balloon information such as wind speed, humidity, and temperature, all of which contribute to hurricane incidence and strength. Hobgood presented his preliminary analyses of the Socorro data at the March 1998 meeting of the Association of American Geographers meeting in Boston.
This work was supported by the Air Force Institute of Technology at Wright-Patterson Air Force Base in Ohio.#