ENSO relationships to Summer drought in the United States Midwest
Buhta, Michelle A.
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Drought affects many livelihoods, especially those in the agribusiness. Therefore, being able to develop a seasonal drought forecasting tool becomes very important to decision makers. The goal of this research was to determine if a link could be made between summer drought in the Midwest and 1) El Nino and La Nina events and 2) Pacific Decadal Oscillation (PDO) regimes during the period 1896-1995. Summer drought was identified by climate division for Minnesota, Wisconsin, Iowa, Illinois, and Indiana as the 20 driest years out of 100. The 20 “extreme” El Nino and La Nina events occurring in the winter and spring seasons were evaluated to determine whether they were related to summer droughts. Summer drought frequency was assessed during four PDO regimes. 1896-1924, 1925-1946, 1947-1976, and 1977- 1995. Results showed that it is less likely for a summer drought to occur after an extreme El Nino event in the northern part of the study area (Minnesota and Wisconsin), while it is more likely that an extreme El Nino event would precede summer drought, especially when examining strong spring El Nino events, in the southern part of the area (southern Illinois and Indiana). The pattern reverses for extreme La Nina events, with a greater chance of experiencing an extreme La Nina event prior to summer drought in the northern sections of the study area, and less likely in the south. The patterns that related extreme ENSO events and summer drought did not vary much when considering either winter or spring as the forecast period. When exploring the possibility that the PDO could be an indicator of drought, it was found that no consistent pattern of drought prevailed. Earlier PDO regimes generally experienced a greater frequency of drought while the most recent period exhibited the fewest summer droughts.