**4. Summary and Conclusions**

Drought is a difficult topic to study, as it is typically challenging to define precisely the timescales and space scales for this phenomenon. It is also an important topic because of the impact on agricultural and economic activity. Here summer-season drought was examined for the agriculturally important regions of the Central USA and Eastern Europe and Western Russia during the late 20th and early 21st centuries. Here summer-season drought was defined by the seasonal mean composite precipitation minus potential evaporation anomalies. This criterion was developed to separate extreme dry summers from those of moderately dry summers and then compared to wet summers. The criterion was successful in identifying major drought summers that time-series indexes have identified as impactful droughts. This study produced the following results.

Summer-season drought within the agriculturally sensitive regions of NA and EE/WR occurred more often during the 1970s and the 2010s, with minima during the 1990s and 2000s (NA) and 1980s (EE/WR).

Summer seasons defined as extreme drought were accompanied by larger maximum precipitation deficits and greater maximum potential evaporation. The extreme-drought years were accompanied also by potential evaporation values that were a greater percentage of the maximum precipitation minus evaporation total.

In the NA region, extreme dry summers occurred more often during LN years, while moderate drought occurred more often during EN years. The opposite was found within the EE/WR region. The only result supported by statistical testing was the distribution of extreme summer drought in the NA region, but this result was weak.

Examining the synoptic–dynamic character of the NA drought summers demonstrated that these occurred in association with the more frequent occurrence of zonal NH flow regimes or ECMs (significant at *p* = 0.01). Extreme dry summers were separated from moderate dry summers by significantly fewer and weaker blocking events (*p* = 0.05 and stronger) and a negative PNA regime. For extreme dry summers, the positive 500 hPa anomaly was located above the study region rather than to the west as for moderate drought summers.

The synoptic–dynamic character of extreme drought within the EE/WR region showed a very strong negative EU teleconnection pattern similar to quasi-stationary long period Rossby Wave Trains found in other regions of the world. These summers were accompanied by significantly more blocking, although not necessarily stronger blocking events, as well as relatively more meridional NH flow regimes or ECMs. For moderate drought years, there was significantly weaker and less blocking, as well as a significantly more zonal NH flow.

In both regions, extreme dry summers were a continuation of the atmospheric flow regimes from the spring season, whereas, for moderate dry summers, the spring season flow regime was different from that of the summer season. In addition, for both regions, wet summer seasons displayed synoptic–dynamic characteristics that were either opposite in terms of the teleconnections, blocking character, or associated with more meridional (NA) or zonal (EE/WR) ECMs.

In summary, this study demonstrated the utility of the criterion used and the results provide recognizable and distinct atmospheric circulation patterns that could be used to identify possible drought summers. Thus, these results would have use for seasonal and sub-seasonal forecasting application.

**Author Contributions:** Conceptualization, N.K.K. and A.R.L.; methodology, all co-authors; software, A.R.L.; validation, all co-authors; formal analysis, all co-authors; investigation, all co-authors; resources, all co-authors; data curation, N.K.K. and A.R.L.; writing—original draft preparation, A.R.L.; writing—review and editing, A.R.L., I.G.S. and M.G.L.; visualization, all co-authors; supervision, not applicable; project administration, not applicable; funding acquisition, not applicable. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The analyzed data can be found on the computers within the Global Climate Change Laboratory at the University of Missouri. Other information, such as the archive for the ECM or blocking, can be found in the reference section.

**Acknowledgments:** The authors acknowledge, with deep gratitude, the contribution of the two anonymous reviewers whose time and effort in providing comments has improved this paper substantially.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

