How do El Nino and La Nina influence the Atlantic and Pacific hurricane seasons?
Posted by
Doorman on 4/12/2011, 10:39 pm
The change in winds with height is referred to as vertical wind shear. Hurricane formation requires the winds to be fairly uniform throughout the atmosphere, meaning that they require low vertical wind shear. Hurricanes cannot form if the vertical wind shear is too high (above about 8 ms-1).
Dr. William Gray at the Colorado State University has pioneered research efforts leading to the discovery of El Niño and La Niña impacts on Atlantic hurricane activity.
El Niño contributes to more eastern Pacific hurricanes and fewer Atlantic hurricanes. La Niña contributes to fewer eastern Pacific hurricanes and more Atlantic hurricanes.
El Niño produces westerly wind departures at upper levels of the atmosphere and easterly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are opposite those normally seen in the region, and results in lower vertical wind shear. The eastern Pacific hurricane season is typically more active during El Niño because of the expanded area of low vertical wind shear in which hurricanes can form.
Across the tropical Atlantic, these same wind departures increase the total vertical wind shear, often to levels far too high for hurricanes to form. There tend to be fewer Atlantic hurricanes during El Niño because of this expanded area of high vertical wind shear.
La Niña produces easterly wind departures at upper levels of the atmosphere and westerly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are in phase with those normally seen in the region, resulting in higher vertical wind shear. The eastern Pacific hurricane season is typically less active during La Niña because of the expanded area of high vertical wind shear.
Across the tropical Atlantic these same wind patterns are opposite to those normally observed, and result in lower vertical wind shear. There tend to be more Atlantic hurricanes during La Niña because of this expanded area of low vertical wind shear.
El Niño and La Niña also influence where the Atlantic hurricanes form. During El Niño fewer hurricanes and major hurricanes develop in the deep Tropics from African easterly waves. During La Niña more hurricanes form in the deep Tropics from African easterly waves. These systems have a much greater likelihood of becoming major hurricanes, and of eventually threatening the U.S. and Caribbean Islands.
The chances for the continental U.S. and the Caribbean Islands to experience a hurricane increase substantially during La Niña, and decrease during El Niño.
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml#HURRICANES
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