Hidden climate variability in complex terrain

It is generally recognized that long-term mean climate varies spatially over complex terrain, responding to factors such as elevation, aspect, and coastal proximity. It is also generally assumed that climatic variations in time respond less strongly to these factors, and are fairly consistent on a regional basis. For example, when one location has a warmer than normal winter, other nearby locations are expected to have had a similarly warm winter. The assumption of temporal synchrony of climate is made in every field study for which data from an off-site meteorological station are used to represent conditions at the location of interest. Most methods for downscaling climate change projections from coarse-grid general circulation models do so, as well.

This paper refutes the assumption of regional climatic synchrony in complex terrain, using temperature data collected from several stations at various elevations and topographic positions in the HJ Andrews Experimental Forest, Oregon. Even at the monthly time step, temperature trends and variations at sites just a few km or less apart can be completely different. A main culprit is the presence of cold air drainage and pooling in valley bottoms and other local depressions. In areas free from cold air drainage, such as hill slopes and ridge tops, temperatures respond strongly to changes in flow pattern in the upper atmosphere, but low-lying areas dominated by cold air drainage do not. This creates steep temperature response gradients among topographically different sites that ebb and flow with variations in flow pattern over time.

This is a “sleeper” issue that could (and should) significantly complicate the estimation of biotic and abiotic responses to climate change and variability. The good news is that our simple first efforts to model the effects were reasonably successful, at least in our study area. We hope that alerting the community to this issue will help spark new research and measurement programs designed to understand the complexities of climatic asynchrony in mountainous terrain. This, in turn, will reduce a potentially large source of error in scientific conclusions and management decisions regarding ecological and hydrological responses to climate change and variability.