Patterns of Centennial-to-Millennial Holocene Climate Variation in the North American Mid-Latitudes
Abstract. Noise in Holocene paleoclimate reconstructions can hamper detection of centennial-to-millennial climate variations and diagnoses of the dynamics involved. This paper uses multiple ensembles of reconstructions to separate signal and noise and determine what, if any, centennial-to-millennial variations influenced North America during the past 7000 yr. To do so, ensembles of temperature and moisture reconstructions were compared across four different spatial scales: continental, regional, sub-regional, and local scales. At each scale, two independent multi-record ensembles were compared to detect any centennial-to-millennial departures from the long Holocene trends, which correlate more than expected from random patterns. In all cases, the potential centennial-to-millennial variations had small magnitudes. However, at least two patterns of centennial-to-millennial variability appear evident. First, large-scale variations included a prominent Mid-Holocene anomaly from 5600–4500 YBP that increased mean effective moisture and produced temperature anomalies of different signs in different regions. The changes steepened the north-south temperature gradient in mid-latitude North America with a pattern similar to the positive mode of the North Atlantic Oscillation (NAO). Second, correlated multi-century (~500 yr) variations produce a distinct spectral signature in temperature and hydroclimate records along the western Atlantic margin. Both patterns differ from random autocorrelated variations but expressed distinct spatiotemporal characteristics consistent with separate controlling dynamics.
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