Sep 1, 1999

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The Effects of El Niño on Atlantic Hurricane Activity

Sept. 1, 1999: The 1997-98 El Niño was the largest this century, and it created an upheaval in the world's weather patterns. Some areas were flooded with too much rain, while other regions fought uncontrollable wildfires caused by drought. News reports throughout 1997 and '98 blamed El Niño for almost every climatic disaster, but El Niño had another, unacknowledged affect - it reduced the number of intense hurricanes that formed in the Atlantic basin.

Right: A false-color image of the Earth shows ocean temperatures. El Niño's warmer waters are represented by red and yellow.

"El Niño is a natural part of the climate system, not a beast in and of itself," says Robert M. Wilson of NASA's Marshall Space Flight Center. "It's an interaction between the ocean and the atmosphere that has global consequences. One of the consequences of El Niño is less hurricane development in the Atlantic."

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El Niño is just the warm phase of the larger El Niño-Southern Oscillation (ENSO) cycle, although many people simply refer to El Niño because of the warm phase's often devastating effects on global weather. El Niño events tend to occur every three to seven years. Just as a clock pendulum swings, or oscillates, back and forth between two extremes, ocean conditions vary between El Niño (warmer water), normal conditions, and La Niña (colder water).

Left: Hurricanes are ranked on a scale from 1 to 5 according to central pressure, maximum sustained wind speed, and storm surge. Intense hurricanes are classed 3 or higher. Atlantic basin hurricanes occur in the Atlantic Ocean, Caribbean Sea and Gulf of Mexico. The hurricane season lasts from June 1 to November 30.

Wilson has recently completed a statistical analysis of Atlantic basin hurricanes appearing between 1950 and 1998. His study, "Statistical Aspects of Major (Intense) Hurricanes in the Atlantic Basin During the Past 49 Hurricane Seasons: Implications for the Current Season," will appear in Geophysical Research Letters.

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Wilson found that when El Niño is present, the number of intense hurricanes in a season has never risen above three. In contrast, when El Niño is not present the number of intense hurricanes in a season has risen as high as seven. On average, El Niño seasons experience only one intense hurricane, while non-El Niño seasons experience two or more intense hurricanes.

Right: Hurricane Bonnie impacts the Eastern coast of the US on August 26, 1998. For a QuickTime movie of a 3 dimensional fly-by of Bonnie, click the image. The movie was prepared by Digital Radiance from satellite imagery.

"The 1998-99 La Niña appears to be winding down. Should El Niño return next spring or summer, one suspects that tell-tale signs of its impending occurrence will soon be manifested," Wilson explained. If that happens, then next year's count of major hurricanes should be less than average. However, if a long interlude follows the present La Niña, the next hurricane season may be another non-El Niño-related (NENR) season, thereby inferring 3 or more major hurricanes again, rather than an El Niño-related (ENR) season having fewer major hurricanes.

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"A major result of my study which relates to a higher than average number of intense hurricanes expected this season is the demonstration that the past 49 seasons can be simply divided into two regimes of relative activity," Wilson said.

"First is a 'less' active interval consisting of 25 consecutive seasons between the mid-1960s and the early 1990s when the annual frequency was always 3 or less. Second are portions of two 'more' active intervals, one prior to the mid-1960s and the other since the early to mid-1990s."

Left: Hurricane Elena. Credit: NASA

Hurricanes are one of Nature's most powerful and destructive forces. The vast damage they inflict make many view hurricanes as Nature gone out of control. But hurricanes result from very precise conditions - a combination of specific atmospheric pressures, ocean temperatures, and winds. By carefully studying these conditions scientists hope to be able to predict when and where hurricanes will occur.

ENSO is one factor in that prediction. "El Niño" originally referred to the regional surface warming of the Pacific Ocean off of South America near Peru and Ecuador. Local fishermen had noticed that the waters occasionally became warmer around Christmas time, hence the name "the Christ child." Although El Niño has been tracked back as far as 1525, meteorologists have only known since the 1960s that this regional event has worldwide effects.

Right: Click on image to watch a QuickTime movie of the El Niño-Southern Oscillation. White represents the warmest water, followed by red and yellow. Purple represents cold water. The images are derived from ocean height data collected by the Franco-American Ocean Topography Experiment (TOPEX) satellite. Credit: NASA and CNES

Besides ENSO, other factors that contribute to Atlantic hurricane activity are the strength and direction of winds in the troposphere and stratosphere, the barometric pressure in the Caribbean, rainfall and temperature conditions in West Africa, and perhaps even global warming.

Research by Prof. William Gray of Colorado State University suggests that ENSO suppresses Atlantic hurricane activity through its influence on the upper troposphere. The rise in ocean temperatures causes strong upper tropospheric winds to blow from the Pacific Ocean to the tropical Atlantic Ocean, and these strong winds can shear the tops off developing hurricanes. These winds can also block the westward motion of hurricanes that form off the coast of Africa.

The 1997-98 El Niño lasted from April 1997 to May 1998, but its effects carried over until mid-July - influencing the earliest portion of the 1998 hurricane season. Thanks to El Niño, the 1997 hurricane season witnessed only one intense Atlantic hurricane: Erika, a class 3 storm.

Left: Hurricane Mitch caused flooding and landslides in Nicaragua. Credit: Hurricane Mitch Response and Recovery.

The cold water of La Niña dominated recent weather events, including most of the 1998 hurricane season. That season produced three intense Atlantic hurricanes: Bonnie (class 3), Georges (class 4), and Mitch (class 5). Hurricane Mitch devastated the Central American countries of Nicaragua and Honduras, causing $10 billion in damage and killing over 9,000 people.

Web Links
Guide to Meteorology - University of Illinois
El Niño Theme Page - National Oceanic and Atmospheric Administration.
TOPEX/Poseidon satellite - false color images of ocean heights.
Tracking El Niño - NOVA's El Niño web site.
see caption
Left: The upper false-color globe shows El Niño (white represents the warmest water), while the lower globe shows evidence of La Niña (purple represents cooler water).

La Niña and El Niño seem to reflect each other - mirror opposites in their influence on the world's weather patterns. La Niña brings the same abnormal weather conditions to the Pacific region as El Niño does to the Atlantic. And while El Niño may suppress hurricane activity in the Atlantic, it seems to increase the number of hurricanes in the Pacific.

The warm, wet conditions El Niño creates in the Eastern Pacific are especially conducive to the formation of intense hurricanes, or 'cyclones,' as they are called in Australia and other parts of the Western Pacific. While the 1997 hurricane season only produced one intense Atlantic hurricane, there were seven intense hurricanes in the Pacific.

"Because El Niño won't be around this year, the 1999 Atlantic hurricane season should be an active one," predicts Wilson. "There will probably be about four intense hurricanes, but we may see even more."

Wilson's belief is supported by William Gray's predictions for the 1999 hurricane season. Gray has predicted 14 tropical storms, 9 hurricanes, and 4 intense hurricanes for the Atlantic basin.

Right: Hurricane Fran, as seen by GOES (Geostationary Operational Environmental Satellite). Credit: NASA.

Wilson's findings should help meteorologists make accurate predictions about hurricanes, an especially vital function since the storms are so destructive to both life and property. Statistically, another El Niño is not expected until next year or later. In the meantime, the destruction created by intense hurricanes may cause those living close to the Atlantic to see El Niño's next visit as a mixed blessing rather than solely as a curse.

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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Leslie Mullen
Curator: Linda Porter
NASA Official: Ron Koczor