Variability in the Length of Frost-Free Periods in the Conterminous US during the Past Century

Variability and trends in the start, end, and duration of frost-free-conditions (i.e., the growing season) influence human activities and many physical and biological processes in natural and managed ecosystems at local to global scales. We used daily minimum temperature data (Global Historical Climatology Network (GHCN)) from 1920 to 2012 to determine last spring frost dates (LSFD), first fall frost dates (FFFD), and frost-free period length (FFPL) across the conterminous United States (CONUS). Analyses of variability and trends indicate a trend towards earlier LSFDs (after 1980), later FFFDs (particularly after 2000), and longer FFPLs in general for most locations in the CONUS. Changes to earlier LSFDs for most sites are highly correlated with increases in spring (April/May) temperature, whereas changes to later FFFDs for most sites are associated with increases in fall (September/October) temperature. There is little temporal correlation between LSFDs and FFFDs at individual sites; they are driven by different climatologies. LSFD variations appear to be associated with sea-surface temperature (SST) variability in the Indian Ocean, whereas FFFD variations are related to SST variability in the tropical North Atlantic. The effects of the El Niño/Southern Oscillation on LSFDs, FFFDs, and FFPL appear to be small. As noted by others, FFPL averaged over CONUS exhibits multidecadal variability that tracks the Atlantic Multidecadal Oscillation. Natural SST variability in the Indian Ocean and tropical North Atlantic SSTs may enhance or interfere with the effects of global warming on LSFDs, FFFDs, and FFPLs. Issues to be resolved include the role of advective versus radiative freezing in modulating the regional timing of LSFDs and FFFDs, and opportunities for long-lead forecasting of FFPLs.