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 Niño-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 Niño and La Niña 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 El Niño July–August– September (JAS) periods. During JAS periods consistent with strong La Niña conditions a significant incidence of seasonal temperatures in the highest 25% of the historical record is found over Iowa, Illinois, and Indiana. El Niño 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, La Niña 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. Research QuestionThe El Niño-Southern Oscillation (ENSO) mechanism is an important and potentially predictable source of variability in the Earth's climate system. However, its effects over the grain producing regions of the Great Plains and Midwest are not well resolved, particularly with regard to where agriculturally important climate effects occur and which growing seasons are most strongly affected. Important issues addressed here are the relative frequency and strength with which ENSO affects winter and summer climate over the central USA, and the relative degree to which those climate effects affect winter wheat and corn yields. Literature SummaryEarly research indicated that both the El Niño and La Niña phase of the ENSO mechanism affected precipitation during the winter and early spring over southern portions of the central USA. Subsequent work presented evidence of climate effects over the Corn Belt and the Northern Plains during summer. Most studies concentrating on ENSO-related yield effects have focused on summer crops such as corn, even though the strongest climate impacts over North America typically occur during the northern winter. Study DescriptionThe climate analysis method used here compares the distribution of seasonal climate values sampled during periods consistent with ENSO conditions against a null hypothesis that assumes random sampling. This approach first samples cumulative rainfall and mean temperature values of U.S. Climate Division Data during 3 mo seasonal periods marked by El Niño and La Niña sea-surface temperatures (SSTs) in the eastern equatorial Pacific. Next, the incidence of those sampled values above or below a season's historical median, and in the highest and lowest 25% of the historical record, is tested for deviation inconsistent with random sampling at an approximate 90% confidence level. After the identification of statistically significant and relevant seasonal climate effects, related effects on subsequent crop yield were evaluated through a similar statistical analysis of USDA-NASS per-acre corn and winter wheat yields. Applied QuestionsWhat significant climate effects were apparent? During El Niño July–August–September (JAS) periods, a significant incidence of relatively cool and wet growing conditions was found over the lower Missouri River drainage region. During JAS seasons consistent with strong La Niña conditions, a tendency toward seasonal temperatures in the uppermost 25% of historical values was found over the Corn Belt states of Iowa, Illinois, and Indiana. During El Niño winters (December–January–February) a significant tendency for precipitation above the median and in the highest 25% is indicated over important winter wheat growing regions of Texas, Oklahoma, Kansas, and Nebraska. Conversely, during La Niña November–December–January periods, a significant tendency for seasonal precipitation below the median and in the lowest 25% was found over portions of Texas, Oklahoma, Kansas, and Nebraska. How do yield effects on corn compare with yield effects on winter wheat? At a 90% confidence level or better, winter wheat yields in all five wheat producing states considered (Texas, Oklahoma, Kansas, Nebraska, and Colorado) showed a significant tendency to below normal values after November– December–January periods marked by cold La Niña SSTs. Conversely, a significant tendency to above normal yields was found after El Niño December–January–February periods in four of those five states. Thus a significant yield effect linking anomalously cold (warm) winter SSTs to subsequent decreases (increases) in wheat yield was found in four of the five wheat producing states. However, of the five corn producing states considered here (Illinois, Indiana, Iowa, Nebraska, and Minnesota), a comparable linear yield effect in corn harvests immediately after El Niño and La Niña summers was found only in Illinois and Indiana. Generally speaking, the effects of both ENSO phases appeared more consistent in winter wheat yields than in corn yields. What do these results imply for long-term agricultural management? The ability to forecast the state of SST, combined with evidence of significant SST-climate effects and plausible subsequent yield effects, implies the potential for forecasting those effects. The potential for adjusting management strategy in response to anticipated shifts in growing conditions is also implied. However, the fact that the ENSO mechanism is an intermittent feature of inter-annual climate will restrict such forecasts to occasional forecasts of opportunity. As a result, we make an analogy between the use of ENSO forecasts in long-term agricultural management and being given occasional access to loaded dice in an ongoing gambling situation; i.e., agriculture's year-to-year gamble on seasonal growing conditions. Noting the ENSO mechanism's preference for northern winters and the more pronounced effects on winter climate and winter wheat yield found here, we extend the analogy. That is, we suggest that producers of winter wheat may be offered such dice more frequently in the long term, and that they may be more heavily weighted towards certain climate and yield out comes. Stated more directly, we propose that the value of ENSO forecasts of opportunity in long-term agricultural management may be greater for winter wheat producers than for corn producers.