Enso and pdo variability affect drought-induced fire occurrence in Rocky Mountain subalpine forests

Understanding the effect of variation in climate on large-fire occurrence across broad geographic areas is central to effective fire hazard assessment. The El Nino-Southern Oscillation, (ENSO) and the Pacific-Decadal Oscillation (PDO) affect winter temperature and precipitation regimes in western North America through mid-latitude tele-connections. This study examines relationships of ENSO and the PDO to drought-induced fire occurrence in subalpine forests of three study areas across the Rocky Mountains: Jasper National Park (JNP, northern Rockies), Yellowstone National Park (YNP, central Rockies) and Rocky Mountain National Park (RMNP, southern Rockies) over the 1700-1975 period. Large-scale climatic anomalies captured by ENSO (NINO3) and PDO indices had differential effects on large-fire occurrence across the study areas. Superposed epoch analysis (SEA) showed that large fires in RMNP occurred during extreme La Nina years, while the PDO, although predominantly negative during fire years, did not depart significantly from the mean. In YNP and JNP, neither ENSO nor PDO indices were significantly different from the mean during large-fire years, although fires tended to occur during El. Nino and positive PDO years. Constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the PDO were significantly associated with large-fire occurrence across the Rockies, even though these large-scale climatic anomalies were not significant when considered singly in SEAs. Combined warm phases (positive PDO during El Nino) co-occurred with large fires in the central and northern Rockies, while the combined cool phases (negative PDO during La Nina) appeared to promote large fires in the southern Rockies. Almost 70% of large fires in RMNP burned during La Nina events that coincided with a negative PDO, although these phases co-occurred during only 29% of the 1700-1975 period. Spatial teleconnection patterns between drought, PDO and ENSO across western North America independently support the sign and strength of relationships between these climatic anomalies and subalpine fire occurrence along a broad north-south gradient of the Rockies. Forecasts of ENSO that are dependent on the expected PDO phase suggest promise for fire hazard prediction across the West.