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Climate Cycles

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.

during La Niña, the easterly trade winds strengthen and cold upwelling near along the equator and the west coast of South America intensifies leading to below normal SSTs.
During La Niña, the easterly trade winds strengthen and cold upwelling near along the equator and the west coast of South America intensifies leading to below normal SSTs.

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.


The Pacific Decadal Oscillation (PDO) (above) and precipitation anomalies for Arizona (below). Arizona is typically in-phase with PDO. Jagged black line in the bottom graph is the smoothed out average of precipitation anomalies. Adapted from National Oceanic and Atmospheric Administration (NOAA)."Pacific Decadal Oscillation (PDO)." http://ww2.wrh.noaa.gov/climate_info/PDO_page.htm 5/5/03.

J.G.

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References

Grissino-Mayer, H. D., and T. W. Swetnam. 2000. Century-scale climate forcing of fire regimes in the American Southwest. The Holocene 10(2): 207-214. Relationship between climate and fire occurences in the Southwest, based on data from El Malpais, NM.

Kitzberger, T. T. W. Swetnam, and T. T. Veblen. 2001. Inter-hemispheric synchrony of forest fires and the El Niño-Southern Oscillation. Global Ecology and Biogeography 10:315-326.

National Oceanic and Atmospheric Administration (NOAA)."Pacific Decadal Oscillation (PDO)." http://ww2.wrh.noaa.gov/climate_info/PDO_page.htm 5/5/03.

National Oceanic and Atmospheric Administration (NOAA). "What are El Niño and La Niña?" http://www.wrh.noaa.gov/Flagstaff/science/az_nino.htm 5/5/03.

National Oceanic and Atmospheric Administration (NOAA). "What is the Pacific Decadal Oscillation?" http://www.wrh.noaa.gov/Flagstaff/science/pdo.htm 5/5/03.

Pyne, S.J., Andrews, P.L., and Laven, R.D., 1996. Introduction to wildland fire. John Wiley and Sons, 769 pp.

Simard, A.J., D.A.Haines and W.A. Main, 1985. Relations between El Niño/Southern oscillation Anomalies and wildland fire activity in the United States. Agric. Forest Meteorology 36(2): 93-104.

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.

Swetnam, Thomas W. and J.L. Betancourt. 1990. Fire-Southern Oscillation relations in the Southwestern United States. Science 249:1017-1021.

Other resources

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.

University of Arizona Climate Assessment of the Southwest (CLIMAS) Program. "Recent History." http://www.ispe.arizona.edu/climas/learn/fire/recent_history.html 5/13/03.

 

Last edited June 6, 2003

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