Observed variations in U.S. frost timing linked to atmospheric circulation patterns

Courtenay Strong,Gregory J Mccabe

doi:10.1038/ncomms15307

Abstract

Several studies document lengthening of the frost-free season within the conterminous United States (U.S.) over the past century, and report trends in spring and fall frost timing that could stem from hemispheric warming. In the absence of warming, theory and case studies link anomalous frost timing to atmospheric circulation anomalies. However, recent efforts to relate a century of observed changes in U.S. frost timing to various atmospheric circulations yielded only modest correlations, leaving the relative importance of circulation and warming unclear. Here, we objectively partition the U.S. into four regions and uncover atmospheric circulations that account for 25–48% of spring and fall-frost timing. These circulations appear responsive to historical warming, and they consistently account for more frost timing variability than hemispheric or regional temperature indices. Reliable projections of future variations in growing season length depend on the fidelity of these circulation patterns in global climate models.

Highlights

Several studies document lengthening of the frost-free season within the conterminous United States (U.S.) over the past century, and report trends in spring and fall frost timing that could stem from hemispheric warming
The results presented here confirm that the timings of spring and fall frost in four objectively identified regions are significantly correlated with hemisphericscale temperature indices conventionally used to represent greenhouse gas-driven warming, with the Central region during fall being the one exception
The eight frost timing indices analysed here were significantly correlated with more local indices of temperature constructed by averaging over the season and region corresponding to each frost index

Summary

Introduction

Several studies document lengthening of the frost-free season within the conterminous United States (U.S.) over the past century, and report trends in spring and fall frost timing that could stem from hemispheric warming. Objective-clustering algorithms and optimization techniques were used to discover the atmospheric circulations that regulate conterminous U.S frost timing for fall and spring during the period 1920–2012 The importance of these circulation patterns relative to effects from large-scale warming and regional seasonal mean temperature variability was assessed via statistical modelling. We find that these circulations appear responsive to historical warming, and they consistently account for more frost timing variability than hemispheric or regional temperature indices. Our results indicate that reliable projections of future variations in growing season length depend on the fidelity of the uncovered circulation patterns in global climate models

Methods

Results

Conclusion