Figure 4 : Circulation drivers of fall-frost timing.

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

Figure 4

(a) Correlation between the day of year with first fall frost in the North region (FN) and mean September–November 500-hPa geopotential height at each grid point (ZSON). The circulation mode that captures this ZSON pattern is denoted CFN, and is the second principal component of ZSON within the grey box. The CFN spatial pattern is contoured in black at 5 m per standard deviation of the CFN index with negative values dashed and the zero contour suppressed. On all maps, centres of action for the PNA are indicated by small filled white and black circles. (b) FN (black) and the CFN time series (blue). (c) Correlation between ZSON and the day of year with first fall frost in the Central region (FC). The associated circulation mode (CFC) is the second principal component of ZSON within the grey box. (d) FC (black) and the CFC time series (blue). (e) Correlation between ZSON and the day of year with first all frost in the South region (FS). The associated circulation mode (CFS) is the first principal component of ZSON within the grey box. (f) FS (black) and the CFS time series (blue). (g) Correlation between ZSON and the day of year with first fall frost in the West region (FW). The associated circulation mode (CFW) is the first principal component of ZSON within the grey box. (h) FW (black) and the CFW time series (blue).