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Northern Hemisphere Modes of Variability and the Timing of Spring in Western North America

Identifies spatial and temporal patterns of variability in spring onset across western North America using a spring index (SI) model based on weather station minimum and maximum temperatures (Tmin and Tmax, respectively)
Employs a principal component analysis which shows that two significant and independent patterns explain roughly half of the total variance in the timing of spring onset from 1920 to 2005
Finds these patterns of spring onset do not appear to be linear responses to the primary modes of variability in the Northern Hemisphere: the Pacific–North American pattern (PNA) and the northern annular mode (NAM)
Looks at the period when reanalysis data and the spring index model overlap (1950–2005) and finds the patterns of spring onset are local responses to the state of both the PNA and NAM, which together modulate the onset date of spring by 10–20 days on interannual time scales
States the PNA and NAM modulate regional spring onset by controlling the number and intensity of warm days
Finds a regionwide trend in spring advancement of about −1.5 days decade per decade from 1950 to 2005 and that trends in the NAM and PNA can only explain about one-third (−0.5 day per decade) of this trend

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