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

Area of Science:

• Climatology
• Atmospheric Science
• Agronomy

Background:

• Observed lengthening of the frost-free season in the U.S. over the past century is documented.
• Trends in spring and fall frost timing are linked to hemispheric warming, but circulation anomalies also affect frost timing.

Purpose of the Study:

• To objectively partition the U.S. into regions and identify atmospheric circulations driving frost timing.
• To clarify the relative importance of atmospheric circulation versus warming on frost timing.

Main Methods:

• Objective partitioning of the conterminous United States into four distinct regions.
• Analysis of atmospheric circulation patterns and their correlation with spring and fall frost timing.

Main Results:

• Identified specific atmospheric circulations that account for 25-48% of spring and fall frost timing variability.
• These identified circulations appear responsive to historical warming.
• Circulation patterns explained more frost timing variability than regional or hemispheric temperature indices.

Conclusions:

• Atmospheric circulation plays a more significant role in U.S. frost timing than previously established by temperature indices alone.
• Accurate projections of future growing season length rely on the fidelity of these circulation patterns in global climate models.