Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 4:34 PM GMT on January 16, 2006
La Niña is back. In their January 12 dicussion, NOAA's Climate Prediction Center declared that the cooler waters and increased easterly winds off of the western coast of South America that have developed the past two months have met the official criteria to be called a weak La Niña event. As seen in Figure 1 below, sea surface temperatures along the equator between the South American coast and the Date Line (180 degrees longitude) are about .5 C cooler than normal, which is the threshold for a La Niña event. The last La Niña occurred in 2000-2001 and was a weak event.
La Niña events can have a strong effect on the climate across the Pacifc, North and South America, and the Atlantic. In particular, La Niña winters see a more northerly jet stream over the U.S., which leads to drier weather over southern half of the country. Major droughts have accompanied two recent major La Niñas in the Midwest (1988-89) and Southern Plains (1995-96). The Pacific Northwest tends to be wetter and colder during La Niña. The Northeastern U.S. sees little change in precipitation and slightly increased maximum temperatures. Tornado and severe storm activity tends to shift more to the north in Spring due to the more northerly location of the jet stream.
Up until 1975, La Niña events and El Niño events used to alternate fairly regularly with a period of 2-7 years. Between 1950 and 1976 there were seven El Niño events and seven La Niña events. Since 1976, El Niño events have been approximately twice as frequent as La Niña events, with ten El Niño events and only six La Niñas. Some researchers have speculated that this is due to the effects of global warming causing a new "resonance" in the climate system. If so, this is one way in which global warming may end up causing a decrease in Atlantic hurricane activity over the coming decades, since the increased wind shear over the Atlantic during EL Niño events greatly reduces the number and intensity of these storms.
Effect of La Niña on hurricane season
It is well known that La Niña conditions enhance tropical storm formation in the Atlantic. The winds associated with La Niña tend to decrease the amount of wind shear in the tropical Atlantic, allowing more storms to form, and more major hurricane to occur. During La Niña more hurricanes form in the deep Tropics from African easterly waves, and these systems have a much greater likelihood of becoming major hurricanes that threaten the U.S. and Caribbean islands. However, the current La Niña is a weak one, and about 80% of the computer models used to forecast La Niña predict that it will no longer be around this Fall. Neutral El Niño/La Niña are expected for the coming hurricane season--which is what we had during the record-breaking Hurricane Season of 2005.
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