Patterns of centennial to millennial Holocene climate variation in the North American mid-latitudes

Noise in Holocene paleoclimate reconstructions can hamper the 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 years. To do so, ensembles of temperature and moisture reconstructions were compared across four different spatial scales: multi-continent, regional, sub-regional, and local. 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 5600-5000 yr BP that increased mean effective moisture and produced temperature anomalies of different signs in different regions. The changes shifted the north-south temperature gradient in mid-latitude North America with a pattern similar to that of the North Atlantic Oscillation (NAO). Second, correlated multi-century (similar to 350 years) variations produce a distinct spectral signature in temperature and hydroclimate records along the western Atlantic margin. Both patterns differ from random variations, but they express distinct spatiotemporal characteristics consistent with separate controlling dynamics.