In contrast, there is a strong and highly significant trend ( P ‐value = 0.02) in the production of NAWTD events, with the six leading patterns exhibiting an increase of 0.61 percentage points per year in the fraction of pattern occurrences that produce NAWTD events (Figure 4 m, red line).
We find that the seasonal‐scale west‐east temperature difference exhibits an increasing trend (0.03 K/yr; P ‐value = 0.19) over the period of observation, primarily as a consequence of increasing temperatures (0.03 K/year; P ‐value = 0.05) in the western domain and slightly decreasing temperatures (−0.01 K/year; P ‐value = 0.75) in the eastern domain (Figure 2 a).
Collectively, the frequency of occurrence of these six “leading” atmospheric patterns has a positive trend corresponding to an increase of 5 days per season over the 36 year period, with 2013–2014 and 2014–2015 exhibiting among the highest seasonal occurrences of these patterns on record (Figure 4 m, black line).