El Niño and La Niña Related Climate and Agricultural Impacts over the Great Plains and Midwest

Seasonal climate forecasting depends implicitly on climate mechanisms that behave predictably over season-to-season or longer time scales. One mechanism that shows evidence of such behavior is the El Nino-Southern Oscillation (ENSO). Using U.S. Climate Division data, USDA-NASS yield data, and the sea-surface temperature (SST) index of Wright (1989), the effects of both El Nino and La Nina Southern Oscillation phases on the central USA are evaluated, with a specific focus on comparing climate and agricultural yield effects during summer and winter. The climate analyses presented here reveal evidence of significant shifts in seasonal precipitation and temperature over parts of the central USA, dependent on the state of ENSO-associated SST anomalies. A significant tendency to cool and wet conditions over portions of the Missouri River drainage basin is found during EI Nino July-August-September (JAS) periods. During JAS periods consistent with strong La Nina conditions a significant incidence of seasonal temperatures in the highest 25% of the historical record is found over Iowa, Illinois, and Indiana. El Nino conditions during northern winter are consistent with a higher-than-chance incidence of wet seasonal conditions over winter wheat (Triticum aestivum L.) growing regions of Texas, Oklahoma, and Kansas. Conversely, I a Nina conditions show a significant tendency to below median precipitation and extreme dryness. A general tendency for winter wheat yields to be increased (decreased) is found in harvests after periods of warm (cold) winter SST, and a similar effect on corn (Zea mays L.) yield is found immediately after summer seasons marked by anomalously warm and cold SSTs in the equatorial Pacific. However, while significant tendencies for above or below normal yields are the rule in the winter wheat analyses, effects on corn yield-while clearly evident in some state's yield records-are less significant overall. This contrast in yield effect, combined with evidence of stronger northern winter climate impacts and the fact that the ENSO mechanism favors northern winters, lead us to propose that the value of ENSO forecasts of opportunity in long-term agricultural management may be greater for winter wheat than for corn.

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