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El Nino Reduces Atlantic Hurricane Number, Intensity

August 2009

By WeatherBug's Stephen Baxter


The developing El Nino in the tropical Pacific will affect weather all over the globe, including Atlantic hurricanes. Its tendency to reduce the number and strength of Atlantic tropical cyclones comes as a major relief to residents and businesses along the Gulf Coast and Atlantic Seaboard who are significantly affected by hurricanes.

Many hurricane seasons over the past 15 years have seen an increase in the number and intensity of tropical storms, largely due to large-scale, natural climate variability. The record 2005 hurricane season that featured damaging hurricanes Katrina and Rita, as well as record-strength Wilma, as well as the busy 2006 and 2007 seasons are fresh on the minds of those who work and live in hurricane-prone regions along the Gulf of Mexico and Atlantic Ocean. However, this year`s developing El Nino will help keep hurricane numbers lower than previous seasons.

El Nino reduces the number and intensity of Atlantic hurricanes by creating unfavorable weather conditions. One ingredient necessary for the formation of tropical cyclones is a very small amount of vertical wind shear, which is the change in speed or direction of the wind with height. El Nino increases wind shear by increasing westerly winds high above the tropical Atlantic, over the common easterly trade winds found near the surface. This discourages tropical storm development by shearing systems apart.

Therefore, it is no surprise that El Nino seasons tend to experience fewer tropical storms, hurricanes, and major hurricanes. An average Atlantic hurricane season, which runs from June 1 to November 30, produces a median of 11 named tropical storms, six hurricanes, and two major hurricanes. During El Nino seasons, those numbers drop to seven named storms, four hurricanes, and one major hurricane.

One method used by meteorologists to analyze hurricane activity is Accumulated Cyclone Energy (ACE). The ACE index takes into account the strength and longevity of tropical cyclones, rather than the actual storm number. The median ACE index since 1950 for Atlantic tropical storms is 88, while El Nino seasons drop that number to 65, a 26 percent decrease. In fact, there have only been four hurricanes seasons since 1950 in which there was not a major hurricane; all were El Nino seasons.

Despite the historical tendency for decreased hurricane activity during El Nino years, residents should continue normal preparations for hurricanes whether or not El Nino conditions are present. There are notable exceptions to the lower El Nino-related hurricane activity. Four El Nino seasons (1951, 1963, 1969, and 2004) had significantly above normal hurricane activity. The 2004 hurricane season saw 15 named storms and Hurricanes Charley, Frances, and Ivan all making U.S. landfalls. The El Nino season of 1992 had only one major hurricane, but it was Hurricane Andrew that devastated extreme southern Florida.

WeatherBug meteorologists expect an active hurricane season despite the slow start, but less active than last year partially due to El Nino. Keep checking your WeatherBug for the latest on the hurricane season and how it can affect you.

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