The El Niño – La Niña climate cycle has been shown to have an influence on fuel load buildup and fire potential. During El Niño years, vegetation, particularly fire-prone grasses, tend to thrive and expand. Big fire years in the West tend to occur when wet El Niño years are followed by dry La Niña conditions.
The most extensive fire years tended to be dry years following two or three wet years. 1997-98 were strong El Niño years, followed by two years of La Niñas. So it was no surprise to climatologists that the summer of 2000 was such a severe fire year in the Southwest.
The Pacific-Decadal Oscillation (PDO) describes an oscillation in northern Pacific sea surface temperatures (SSTs) between above normal northern Pacific SSTs (negative phase) and below normal SSTs (positive phase) (see figure below). PDO has been described as a long-lived El Niño/La Niña-like pattern of Pacific climate variability because the two climate oscillations have similar relationships regarding deviations in SSTs. However, PDO operates at something on the order of a 20- to 30-year time scale, while El Niño/La Niña events are typically on the order of 6 to 18 months in length. Precipitation anomalies in the American Southwest tends to be in phase with PDO.
What this suggests for the Southwest is that, when the PDO is in its positive phase, as it was from 1977 to at least 1995, we tend to experience wetter El Niño winters, but closer to normal precipitation during La Niña winters. If, however, as some climatologists believe, conditions have shifted to the negative phase of the PDO, we may experience drier La Niña winters, and El Niño winters that are closer to our normal precipitation than the excessive precipitation they typically provide. This would shift our long-term average precipitation downward, and give us more opportunities for below normal precipitation winters than we have experienced in the 1977-1995 period. Thus, the onset of a negative PDO phase could cause the Southwest to experience generally drier conditions over the next several decades. It is uncertain how long or how dry conditions could get. Most of the region experienced a prolonged and severe drought during the 1950s, in the midst of the last PDO negative phase.
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Swetnam, Thomas, W. "Fire and Climate History in the Western Americas From Tree Rings." Laboratory of Tree-Ring Research, The University of Arizona, Tucson, AZ. http://www.ltrr.arizona.edu/~tswetnam/graphics/PAGES/PAGES3.pdf 4/10/03
Swetnam, Thomas W., and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11:3128-3147.
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Garfin, Greg and B. Morehouse. 2001. Climate Assessment of the Southwest (CLIMAS) 2001 Fire and Climate Workshop Proceedings. Available online at http://www.ispe.arizona.edu/climas/conferences/fire2001/fire2001.pdf 5/13/03.
University of Arizona Climate Assessment of the Southwest (CLIMAS) Program. "Overview of Fire Research." http://www.ispe.arizona.edu/climas/learn/fire/index.html 5/13/03.
University of Arizona Climate Assessment of the Southwest (CLIMAS) Program. "Fire History Analysis Using Tree Rings." http://www.ispe.arizona.edu/climas/learn/fire/history.html 5/13/03.
University of Arizona Climate Assessment of the Southwest (CLIMAS) Program. "Connection With Climate." http://www.ispe.arizona.edu/climas/learn/fire/connection.html 5/13/03.
University of Arizona Climate Assessment of the Southwest (CLIMAS) Program. "Ecological Effects." http://www.ispe.arizona.edu/climas/learn/fire/ecological.html 5/13/03.
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Last edited June 6, 2003