3.2.2. Dry Summer Seasons

During the summer season (Figure 5C,D), the occurrence of more zonal NH flow regimes during both extreme and moderate drought and wet seasons was consistent with summer seasons overall. During all drought summers for the NA region, the flow was clearly even more zonal, and this result was significant at *p* = 0.01 (Figure 5C). While extreme drought summers were accompanied by larger maximum dry precipitation anomalies than moderate drought summers (−3.9 mm day−<sup>1</sup> versus −2.8 mm day−1) as anticipated, the absolute value of the maximum 500 hPa height anomalies were of similar strength (18.2 m versus 22.5 m). As the flow was more zonal the maximum 500 hPa height anomalies were not always positive height anomalies within the NA study region. This can be seen in Figure 8A,B.

**Figure 8.** As in Figure 6, except for the NA summer seasons. The contour interval is (**A**) 2.5 m and (**B**,**C**) 1.5 m.

In Figure 8A, the extreme dry years featured a positive 500 hPa height anomaly within the study region and the continuation of negative PNA values from the spring season (Figure 4A). This is accompanied by significantly fewer (*p* = 0.05) and weaker (*p* = 0.01)

blocking events in association with an anomalously strong Aleutian Low (Figure 3A). Additionally, note the troughs off each coast of North America which validates the results of References [23–25]. For moderate drought years (Figure 8B), the pattern changes to a positive PNA pattern (Figure 4B). The positive height anomaly is now over the Western NA region which strengthens the climatological ridge located there (not shown). A negative height anomaly is located over the Eastern USA (and study region). This configuration places the study region in the convergent region between a 500 hPa trough and ridge with high pressure at the surface. In fact, for extreme drought summers eight of ten were positive height anomalies while only six of 12 moderate drought years were similar. In both cases (Figure 8A,B), the dominant PNA pattern has a shorter wavelength for dry summers as posited by Reference [25]. Finally, there is a strong trough over the eastern 2/3 of USA for wet summers (Figure 8C). These results here support more strongly the results of Reference [29], which differentiated between wet and dry summers over the Midwest USA.

In the EE/WR study region (Figure 5D), the extreme dry years were more meridional over the entire NH, but more zonal during moderate drought and wet years (*p* = 0.01). Like the NA region, the maximum dry precipitation anomalies are larger during the extreme drought summers than during moderate drought summers (−3.1 versus −2.4 mm day<sup>−</sup>1). However, the maximum 500 hPa height anomalies for both drought groups (53.3 versus 26.0 m) were larger overall than over the NA region. This is consistent with the higher relative occurrence of meridional flow types (Figure 5C,D). Almost all of the dry years (13 of 16) were associated with positive 500 hPa height anomalies within the study region.

Several studies (e.g., References [47,49,55,58,78]) demonstrated that the EE/WR region has been associated with an increase in the occurrence of more meridional ECM or NH flow regimes, especially during the summer (Figure 5D). The study region has also been associated with an increase in atmospheric blocking (e.g., References [44–47,55,57,58]) and studies of drought associated with the extreme summer-season drought of 2010 over this region associated extreme dry summers with atmospheric blocking.

However, during the summer season (Figure 3D), extreme droughts were associated with more blocking events and days, and this is significant at *p* = 0.10, while moderate droughts were associated with fewer blocking events, blocking days, and weaker events, all significant at *p* = 0.10. These results are consistent with Section 3.1 and the occurrence of blocking versus ENSO in the Atlantic Region [62].

Furthermore, during extreme drought summers over the EE/WR region, the negative EU pattern of the spring season continued (Figure 9A versus Figure 7A) but is stronger, such that the largest positive 500 hPa height anomaly is located over the study region. In fact, the strongly negative EU pattern during extreme dry summer seasons is reminiscent of the quasi-stationary sub-seasonal and seasonal Rossby Wave trains that accompany the NH flow in the PNA region and other parts of the globe (e.g., References [56,81–85]). During wet summer seasons (Figure 9C), the EU pattern is positive. Moderate drought years are associated with a negative NAO, similar to that of the spring season, but the EU pattern becomes less organized.

Lastly, the discussion above implies that atmospheric dynamics are the primary drivers of summer-season drought in both study regions. In order to determine strength of the contributions of surface processes to these droughts, we examine the potential evaporation (E). For the NA region, the average P–E (Table 3) was −10.9 mm day−<sup>1</sup> for extreme dry summers and the average maximum precipitation anomaly was −3.9 mm day<sup>−</sup>1. Thus, the maximum potential evaporation anomaly was 7 mm day<sup>−</sup>1. For, moderate drought years the P–E anomaly was −6.5 mm day−1, and the maximum potential evaporation was 3.7 mm day<sup>−</sup>1. Thus, the extreme dry summer potential evaporation was larger in both an absolute sense and relative to the total P–E anomaly. Given the analysis here, this suggests that the extreme drought years were driven strongly by surface processes, as well. The same analysis for the EE/WR region produces mean P–E values of −9.9 and −6.4 mm day−<sup>1</sup> for extreme and moderate dry summers, respectively. Using the precipitation anomalies cited in Section 3.2.2 suggests a similar result (maximum potential evaporation of 6.4 and 4 mm day−1, respectively), that extreme dry years are driven to a greater extent by surface process. This is in spite of the fact that 50% of these summers were not preceded by a dry spring.

**Figure 9.** As in Figure 7, except for the EE/WR summer seasons. The contour interval is (**A**,**C**) 2.5 m and (**B**) 1.5 m.
