3.2.1. Preceding Spring Seasons

During the preceding spring seasons (Figure 5A,B), the occurrence of zonal versus meridional NH flows or ECM for extreme and moderate drought years, as well as wet years, was similar to those of spring seasons, overall, from 1970 to 2020. However, regionally, there are some strong differences in the flow field (Figure 6) between springs preceding summer drought or wet summers. In Figure 6A,B, the Pacific Region flow was clearly more zonal with an anomalously strong Aleutian Low present. This explains fewer Pacific Region blocking events (Figure 3A) and the negative PNA Index within NA region for drought springs (Figure 4A). However, for the springs preceding wet summers, the PNA Index is positive (Figure 4A) and there was significantly more blocking (Figure 3A). This is reflected in the strong positive 500 hPa height anomaly over the East Pacific (Figure 6C).

**Figure 3.** The character of blocking events for (**A**) NA spring, (**B**) EE/WR spring, (**C**) NA summer, and (**D**) EE/WR summer. The left-hand ordinate is mean block occurrence (number—leftmost bars) and mean block intensity (BI—middle bars), and the right-hand ordinate is mean blocking days (rightmost bars). Blue, green, and red bars stand for statistical significance at *p* = 0.1, 0.05, and 0.01, respectively. For each set of bars, the display represents extreme drought, moderate drought, and wet years from left to right.

**Figure 4.** The major teleconnection indexes for (**A**) NA spring, (**B**) EE/WR Spring, (**C**) NA Summer, and (**D**) EE/WR summer, where the left, middle, and right group of bars are the AO, NAO, and PNA indexes, respectively. The ordinate is the value of the index.

**Figure 5.** The fraction of the observed ECM (ordinate) that are zonal and meridional (left bars) and the information entropy (*H*(*x*)—ordinate) (right bars) for (**A**) NA spring, (**B**) EE/WR Spring, (**C**) NA Summer, and (**D**) EE/WR summer. For the left-hand bars, the black and orange colors represent the fraction of observed zonal versus meridional ECM.

Only during the spring preceding the extreme summer drought for the EE/WR study region was the NH flow more meridional relative to other spring seasons during the study period at a statistically significant level (*p* = 0.05) (Figure 5B). This is reflected in a strongly positive NAO (and AO) in Figure 4B, but also in Figure 7A. Lebedeva et al. [55] demonstrated the long-term correspondence between the NAO and AO, as these teleconnections were highly correlated. The correlations between the AO and NAO are similarly high here for spring and summer seasons and for each subsample in Figure 4 (*p* = 0.01). There is also a negative correlation between the AO and PNA at *p* = 0.05, but only in extreme and moderate dry springs and summers. For wet seasons, the PNA and AO do not correlate.

Within the EE/WR, the NAO is weakly negative for springs preceding moderate drought summers (Figures 4B and 7B). However, a negative (EU1—see Reference [56]) Eurasian (EU) teleconnection pattern emerges (e.g., References [79,80]) which is somewhat clear in Figure 7A also. A negative EU is characterized by a positive 500 hPa height anomaly near 20◦ E and 145◦ E and a trough near 75◦ E. Thus, the reversal of the NAO in spring may be an indicator of an extreme dry summer versus a moderately dry summer in the presence of a weakly negative EU. However, for spring seasons preceding wet summers, there is little signal in the NAO but a very distinctly positive EU (EU2—see Reference [56]) pattern (Figure 7C). Finally, there was no preference toward more or fewer blocking events in any of the spring seasons (Figure 3B); however, weaker blocking in the study region during the spring seasons preceding moderate drought summers and negative NAO phase is consistent with Reference [62].

**Figure 6.** The 500 hPa height anomalies (m) in the NA region during the spring season (March–May) preceding (**A**) extreme drought (Table 3), (**B**) moderate drought (Table 3), and (**C**) wet summer seasons (Table 5). The warm (cold) colors are positive (negative) height anomalies. The contour interval is (**A**,**C**) 2.5 m and (**B**) 5 m.

**Figure 7.** As in Figure 6, except for the EE/WR spring seasons in Tables 4 and 5. The contour interval in (**A**,**B**) is 2.5 m, and in (**C**), it is 5 m.
