Ecological theory asserts that the climate of a region exerts top-down controls on regional ecosystem patterns and processes, across space and time. To provide empirical evidence of climatic controls, it would be helpful to define climatic regions that minimized variance in key climate attributes, within climatic regions—define the periods and features of climatic regimes, and then look for concordance between regional climate and ecosystem patterns or processes. In the past, these steps have not been emphasized. Before we evaluated the recent climate of the northwestern United States, we established a Northwest climatic region by clustering time series of the Palmer Drought Severity Index (PDSI) for the period of 1675–1978, for the western United States. The background climatic regime and anomalies of the recent northwestern U.S. climate were then identified through temporal pattern analysis involving application of correspondence analysis to the same PDSI time series.
Our analysis distinguished 10 distinct periods and four unique types of regimes (climatic signals). Five of the 10 periods (79% of the ∼300-year record) were marked by mild and equitable moisture conditions (Pacific regime), the “background” climate of the Northwest. The remaining periods were anomalies. Two periods displayed a high-variance, mixed signal marked by switching between severe to extreme annual to interannual dry and wet episodes (High/Mixed regime; 9% of the record). Two more periods displayed a moderate-variance, mixed signal marked by switching between moderate to severe annual to interannual dry and wet episodes (Moderate/Mixed regime; 5%). Only one period was unidirectional and relatively low variance, marked by persistent yet mild to moderate drought (Low/Dry regime, 7%).
Our method distinguished decadal- to interdecadal-scale regimes, defined regime periods, and detected both mixed and unidirectional anomalies from the background climate. The ability to distinguish the variance, direction, and period of sequential climatic regimes provides a plausible basis for examining the role of past climate within terrestrial ecosystems of the Northwest. For example, we found concordance between the period of the Low/Dry anomaly and a period of tree establishment in the Olympic Mountains of Washington, close alignment between tree growth with the Moderate/Mixed and High/Mixed signals in Oregon, and a mixed fire response to mixed climatic signals in northeastern Oregon. Linking historical climatic regimes to particular ecosystem patterns and processes also aids in the prediction of future ecosystem changes by providing evidence of the kinds of interactions that may be anticipated.