The
Southern Oscillation and the Colorado Plateau
The Southern Oscillation (a.k.a. ENSO or El Niņos and La Niņas) is the cyclic warming and cooling of the surface ocean of the central and eastern Pacific. This region of the ocean is normally colder than it's equatorial location would suggest, mainly due to the influence of northeasterly trade winds, the upwelling of cold deep water off the coast of Peru, and a cold ocean current flowing up the coast of Chile. It has been found that the combination of these factors creates an oscillation that leaves a distinctive fingerprint on sea level pressure. When the pressure measured at Darwin, Australia is compared with that measured at Tahiti, the difference between the two can be used to generate an "index" number. La Niņa conditions (or ocean cooling) generate a positive number, while the number is negative during El Niņo (or ocean warming).
The Southern Oscillation is only one of a number of factors that influence climate on the Colorado Plateau, and the impacts of El Niņo and La Niņa at these latitudes are most clearly seen in wintertime. During El Niņo years, temperatures in the winter are cooler than normal across the southwestern United States. Major El Niņo events during the winters of 1940-41, 1982-83, 1992-93 and 1994-95 also produced exceptionally wet weather in the region. But during more moderate El Niņo events (e.g., the winters of 1986-87, 1987-88), very dry conditions have been the rule. La Niņa conditions more predictably bring warmer and drier conditions to the region.
The Southern Oscillation has been clearly associated with drought cycles, fire frequencies and pulses of tree reproduction in the southwestern United States. Recent dry La Niņa conditions combined with record warm temperatures have led to unusual drought-related stress in biotic communities of the region. Episodes of catastrophic fire have also been tied to this cycle, but changed vegetative conditions in western forests are also clearly a factor.
Paleoecologists Thomas W. Swetnam and Julio L. Betancourt have evaluated both warm (El Niņo) and cold (La Niņa) episodes in a 300-year record of fire activity for the southwestern United States. Click here for an essay on their research.
Resources:
Betancourt, J. L., Pierson, E. A., Aasen-Rylander, K., Fairchild-Parks, J. A. and Dean, J. S. 1993. Influence of history and climate on New Mexico pinyon-juniper woodlands. Pp. 42-62 In: Aldron, E. F. and D.W.Shaw, editors. Managing pinyon-juniper ecosystems for sustainability and social needs: Proceedings of the symposium, Santa Fe, NM, April 26-30. General Technical Report RM-236. USDA Forest Service, Fort Collins, CO.
D'Arrigo, R. D. and Jacoby, G. C. 1991. A 1000-year record of winter precipitation from northwestern New Mexico, USA: a reconstruction from tree-rings and its relation to El Nino and the southern oscillation. The Holocene 1: 95-101.
Diaz, H. F. and Anderson, C. A. 1995. Precipitation trends and water consumption related to population in the southwestern United States: A reassessment. Water Resources Research 31.
Meko, D., C., Stockton, W. and Boggess, W. R. 1995. The tree-ring record of severe sustained drought. Water Resources Bulletin 31.
NOAA (National Oceanic and Atmospheric Administration). 2000. NOAA El Nino Page. <http://www.elnino.noaa.gov/> 9/7/00.
NOAA (National Oceanic and Atmospheric Administration). 2000. NOAA La Nina Page. <http://www.elnino.noaa.gov/lanina.html> 9/7/2000.
Stahle, D. W. and Cleaveland, M. K. 1993. Southern oscillation extremes reconstructed from tree rings of the Sierra Madre Occidental and Southern Great Plains. Journal of Climate 6: 129-140.
Swetnam, T. W. and Betancourt, J. L. 1990. Fire-Southern Oscillation relations in the southwestern United States. Science 249: 1017-1021.
Swetnam, T. W. 1990. Fire history and climate in the Southwestern United States. Pp. 6-17 In: Krammes, S. J., editor. Proceedings of Symposium on Effects on Fire in Management of Southwestern Natural Resources. General Technical Report RM-191. U.S. Forest Service.
Swetnam, T. W. and Betancourt, J. L. 1992. Temporal patterns of El Niņo/Southern Oscillation - wildfire teleconnections in the southwestern United States. Pp. 259-270 In: Diaz, H. F. and Markgraf, V., editors. Historical and Paleoclimatic Aspects of the Southern Oscillation. Cambridge University Press, New York, NY.