Using U.S. Climate Division data, USDA-NASS agricultural yield data, and the SST Index of Wright (1989), the effects of both El Niņo and La Niņa Southern Oscillation phases on the Great Plains and Midwestern regions of the U.S. are evaluated, with an emphasis on comparing climate and agricultural yield effects during summer and winter periods.
The climate analyses presented here demonstrate that seasonal precipitation and temperature over parts of the central United States can be significantly skewed from climatology, conditional on the state of ENSO-associated sea-surface temperature (SST) anomalies. Of particular interest from an agricultural standpoint are apparently non-linear effects on rainfall and temperature over the Missouri River drainage region during July-August-September (JAS), and a semi-linear effect on November-April precipitation over winter wheat growing regions of the Southern Plains. A significant tendency to cool and wet conditions over portions of the Missouri River drainage basin is found during El Niņo JAS periods. During JAS periods consistent with strong La Niņa conditions a significant incidence of 4th quartile seasonal temperature is found over Iowa, Illinois and Indiana. While El Niņo conditions during northern winter are consistent with a higher than chance incidence of wet seasonal conditions over winter wheat growing regions of Texas, Oklahoma, and Kansas, La Niņa conditions show a significant tendency to below median and 1st quartile precipitation.
Analysis of both corn and winter wheat yields after periods of anomalous SST reveal a general tendency for corn yields to be increased (decreased) after periods of warm (cold) JAS SST, and a similar effect on winter wheat yield is found after periods of warm (cold) November-December-January (NDJ) SST. However, while significant tendencies to 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 is primarily a feature of northern winter periods, lead us to propose that the value of ENSO forecasts of opportunity in long-term agricultural management may be greater for winter crops than for summer crops.