Patterns, Causes and Consequences of Spring Onset Timing Variations and Trends in the U.S.

Julio Betancourt, Senior Scientist with the U.S. Geological Survey in Reston VA will present.
Seasonal timing has myriad impacts on plants and animals, biospheric processes, and human systems, and is critical for formulating adaptive responses to both climate variability and change. In the U.S., and especially, the timing of seasonal transitions varies widely from year to year and is also changing directionally, yet the climatic drivers, patterns, and consequences of these variations are not well understood. This presentation will discuss day-of-year (DOY) metrics that define spring onset in the U.S. These DOY metrics exhibit secular trends consistent with both natural variability and greenhouse warming, with abrupt advances spring for most regions clustered in the mid-1980’s and abrupt delays in fall clustered in the mid-1990. Exceptions include “warming holes,” with delayed spring onset abruptly ~1958 and advanced autumn onset in the High Plains gradually since the 1950’s. In the West, both snowmelt and accumulated heat needed to bring plants out of winter dormancy track Pacific Ocean variability. In the atmosphere, spring onset variations also appear linked to the Pacific North American (PNA) pattern and the Northern Annular Mode (NAM). By contrast, last spring frost, first fall frost, and the duration of the growing season in the coterminous U.S. follows Indian and North Atlantic Ocean variability.  The presentation will reconcile different interpretations of large-scale drivers, and discuss opportunities for long-range forecasting.