Fourth Symposium on Fire and Forest Meteorology

Thursday, 15 November 2001: 11:10 AM
A novel approach to distinguishing Regime-scale climate signals (Formerly Paper Number 7.1)
Paul F. Hessburg, USDA Forest Service, Wenatchee, WA; and E. E. Kuhlmann and T. W. Swetnam
At any moment in time, climate regimes may differ among adjacent regions, and through time, regimes may differ within a region both in temporal scale and in overall signal. Despite extraordinary advances in our understanding of regional climate forcing and spatial controls, there has been relatively little effort to distinguish the collection of unique features (signal) of regional climate regimes, or to characterize the temporal patterns and boundaries of regimes. We adapted methods from community ecology to characterize regime signals for a single climate region, the northwest United States (US). For historical records of drought conditions, we used existing reconstructions of the Palmer Drought Severity Index (PDSI, 1675-1978) developed from a gridded network of tree-ring width chronologies. Using nine time series to represent the northwest US, we applied two-way indicator species analysis (TWINSPAN) to group years that shared the most similar moisture conditions. Pattern analysis of PDSI conditions yielded 4 unique climate signals recurring over 10 periods. Repeated cross-validation showed that each signal and period was robust to prediction. To verify the occurrence and period boundaries of the signals, we stratified the nine PDSI time series using the 10 periods and compared the average PDSI range and the amplitude and duration of positive and negative PDSI. We found that the 4 signals were readily separated on that basis. Our method effectively distinguished the temporal pattern and boundaries of climate regime periods, revealed the unique features of each signal (amplitude, duration, and mix), and minimized background noise inherent in the original time series. The ability to distinguish features and temporal patterns of climate regimes enables the evaluation of integrated research questions that were previously inaccessible. For example, it has been tacitly assumed that fire regimes have remained unchanged for several centuries, but shifting climate regime may produce concordant shifting in fire frequency, intensity, or spatial extent. A plausible stratification of recent climate history may enable fire researchers and dendroclimatologists to detect and explain regionally synchronous disturbance events or changes in fire regimes and plant biogeography that relate to shifting climate.

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