Inter-hemispheric synchrony of forest fires and the El Nino-Southern Oscillation

Fire histories were compared between the southwestern United States and nor thern Patagonia, Argentina using both documentary records (1914-87 and 1938 -96, respectively) and tree-ring reconstructions over the past several cent uries. The two regions share similar fire-climate relationships and similar relationships of climatic anomalies to the El Nino-Southern Oscillation (E NSO). In both regions, El Nino events coincide with above-average cool seas on precipitation and increased moisture availability to plants during the g rowing season. Conversely, La Nina events correspond with drought condition s. Monthly patterns of ENSO indicators (southern oscillation indices and tr opical Pacific sea surface temperatures) preceding years of exceptionally w idespread fires are highly similar in both regions during the 20th century. Major fire years tend to follow the switching from El Nino to La Nina cond itions. El Nino conditions enhance the production of fine fuels, which when desiccated by La Nina conditions create conditions for widespread wildfire s. Decadal-scale patterns of fire occurrence since the mid-17th century are highly similar in both regions. A period of decreased fire occurrence in b oth regions from c. 1780-1830 coincides with decreased amplitude and/or fre quency of ENSO events. The interhemispheric synchrony of lire regimes in th ese two distant regions is tentatively interpreted to be a response to deca dal-scale changes in ENSO activity. The ENSO-fire relationships of the sout h-western USA and northern Patagonia document the importance of high-freque ncy climatic variation to fire hazard. Thus, in addition to long-term trend s in mean climatic conditions, multi-decadal scale changes in year-to-year variability need to be considered in assessments of the potential influence of climatic change on fire regimes.