Abstract
Many climate models project increases in the Northern Annular Mode (NAM) and an associated northward shift in the storm track over the 21st century. Here we show that the Pacific storm track is displaced to the north during springs following high index winters, and that this shift is accompanied by a south-to-north redistribution of precipitation across the western United States. In addition, higher NAM indices are associated with warmer temperatures, particularly in the Southwest. The combination of decreasing precipitation and warmer temperatures commonly signals the start of spring throughout much of the West. An earlier spring onset may have significant impacts on ecological systems, and spring NDVI is, in fact, reduced over much of the West following high index winters. This suggests that a better understanding of projected changes in the seasonality of climate will be necessary for predicting ecological impacts and may aid in identifying climate shifts attributable to human activity.
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