The Curious Case of 1959: Tree-ring Evidence of an Extreme Cold Event in Northwestern North America

Abstract

Tree-ring records are essential for identifying extreme climate at an annual or higher-resolution time scale in the past, and for assessing the climatic factors or forcings driving their occurrence. In northwestern Canada, a region particularly susceptible to intrusions of Arctic air masses, tree-ring records suggest a regionally significant cold event in the summer of 1959. Here, we analyze a large tree-ring network from northwestern North America, wood anatomical data from three sites, and climate reanalysis datasets, to decipher the spatiotemporal expression of the 1959 cold event. We found that the locus of inferred summer cold was centered around northern Northwest Territories and northern Yukon. In this region, 1959 was anomalous in the maximum latewood density and blue intensity records relative to the last three centuries, and a signature of reduced latewood cell wall thickness occurred in the outermost sectors of the annual ring. Despite limited archival evidence of an extreme cold event, we also identified a short-term (~2 week) period of record-low minimum temperatures in in early August of 1959 in northwestern Canada. This extreme, albeit brief, cold event was associated with a deep trough over the region, and might have resulted in a particularly early cessation of cell wall thickening in regional forests. These results highlight that short-term weather extremes at the appropriate time can have an important impact on tree-ring parameters traditionally used for assessing past summer climate variability, and illustrates the value of wood anatomical measures to reconstruct climate variability at a finer temporal resolution.