The Influence of ENSO on Zea Mays (Corn) Yields across the US Corn Belt Based on Historic Data
Olivia Kellner, PhD, Climate Impact Company Lead Research Scientist
As of December 14th, 2017, NOAA acknowledges that La Niña conditions have become established across the equatorial Pacific, predicting La Niña conditions to continue (~65%-75% chance) at least through the Northern Hemisphere winter 2017-18, followed by ENSO-neutral conditions beginning late spring 2018. The Australian Bureau of Meteorology also acknowledges (as of December 19th, 2017) that La Niña conditions are present across the equatorial Pacific; however, conditions are likely to be short-lived and end by early southern autumn (Australia’s fall season is March to May) 2018.
One of the most widely researched teleconnections, the El Niño-Southern Oscillation (ENSO) is widely known to impact major crop growing regions around the world. ENSO’s influence on weather patterns in the Northern Hemisphere is most strongly felt in the fall, winter, and spring months, outside of the primary growing season months in the United States Corn Belt (Figure 1).
Figure 1: States within the United States that comprise the US Corn Belt region are highlighted with a black outline.
However, there is a large body of research showing that there is a marked difference between corn yields harvested during an El Niño growing season (i.e. El Niño conditions are present during growing season months) and after a La Niña growing season. It is generally found that yields are less than average during La Niña growing seasons and slightly greater than average during an El Niño growing season. This is because of the way each ENSO phase (El Niño or La Niña) influences the jet stream over North America during growing season months, which generates different temperature and precipitation patterns. An El Niño typically brings seasonal, to slightly cooler than normal temperatures and near-normal to normal precipitation. A La Niña typically brings warmer than normal temperatures and drier conditions.
When temperatures reach 86°F or greater, the corn plant struggles physiologically to carry enough water through the plant to keep itself cool and begins to experience water stress (even if there is ample soil moisture). A Stress Degree Unit (SDU) is defined by taking the observed daily high temperature and subtracting from it 86°F. A Stress Degree Day (SDD) is a day during which a SDU (or SDUs) is recorded. The higher the SDUs, the more stress the plant experiences during the summer months. As the SDUs accumulate through time, the plant experiences more and more water stress, which in turn, reduces yield, especially if the temperatures occur during grain fill (roughly speaking, the last two weeks in July into the first two weeks of August). An example of cumulative SDUs during a La Niña summer and an El Niño summer for Des Moines, Iowa:
Loikith and Broccoli (2014) present the influence of the Pacific North American pattern (PNA), the Northern Annual Mode (NAM), and ENSO on extreme (90th percentile or greater) winter (DJF) and summer (JJA) temperatures. Since this discussion is focused on summer/growing season impacts of ENSO, Figure 2 below (as copied from Loikith and Broccoli 2014) shows maps of “extreme warm JJA maximum temperature (e),(f) days and (g),(h) months that occur when the Niño-3.4 index is in the upper or lower quartile.”
Figure 2: June, July, and August temperature relationships between El Niño events (left images, noted by the Niño+) and La Niña events (right images, noted by the Niño-) across North America (copied from Loikith and Broccoli, 2014). As noted in the paper’s figure, “all grid cells where the percentage is greater than expected by chance are shaded and grid cells where the percentage is significantly greater than expected by chance at the 5% level are highlighted. As seen in the images, the hottest days on record in June, July, and August occur during La Niña summers. Circled in red is the region of the US Corn Belt that experiences more day time high temperatures at or greater than the 90th percentile during June, July, and August.
Figure 3: This figure demonstrates what Figure 2 shows spatially across the United States. To make this graph, every daily observed maximum temperature on record at a given station is arranged from least to greatest, and a count of how many times each temperature is recorded is made. The resulting bar graph follows a bell-curve distribution, as represented by the bell curve here. Please note the data presented in this diagram is for general understanding only and are not accurate temperatures to any specific location. The El Niño (La Niña) box is yellow (blue) to represent the warm (cold) ocean temperatures during an El Niño (La Niña).
Historic SDUs and State Yields by El Niño/La Niña/ENSO Neutral Growing Seasons:
Six cities across the US Corn Belt (Chicago, IL, Indianapolis, IN, Kansas City, MO, La Crosse, WI, Minneapolis, MN, and Des Moines, IA) were selected and are presented below to better show evidence of the influence of ENSO phases on summer temperatures (as noted/found by Loikith and Broccoli) and the impact each phase has on crop production.
Growing seasons experiencing a La Niña during summer: 1985, 1988, 1999, and 2011.
Growing seasons experiencing an El Niño during summer: 1982, 1987, 1991, and 2015.
Growing season experiencing ENSO neutral conditions during summer: 1980 (1981 for Chicago, IL due to data availability), 1990, 1993, and 2017.
Graph 1: The cumulative stress degree units during La Niña summers (blue bars) are greater than those seen during El Niño summer (red bars). Note that if the stress degree units are negative, those during La Niña summers are closer to zero than those of El Niño summers indicating more SDUs overall than during an El Niño summer. ENSO neutral condition SDUs are noted to vary above and below El Niño summers. However, they are always less than La Niña summers except for Kansas City, MO. The data for Kansas City is largely influenced by one summer that was struck by a heat wave.
Graph 2: Average state yields by ENSO phase or during transition to an El Niño (as anticipated for the growing season 2018). Highest yields are seen during established El Niño events or during a transition to an El Niño during the growing season. The data for Kansas City is largely influenced by one summer that was struck by a heat wave. Note as well that the average for transition summers has two less years due to the limited occurrence of such events. This impacts the average value so that it is not as representative as an average determined with four reported years.
Looking ahead into the 2018 North American Growing Season, 3 possible scenarios for ENSO are plausible (presented below), with Climate Impact Company concluding Scenario 3 as the most likely (as also supported by statistical and dynamic models):
Producers across the US Corn Belt will likely experience drier conditions, hotter temperatures, and more SDUs than observed during an El Niño summer. Overall, this will likely result in slightly lower than average yields.
If the currently established La Niña weakens heading into the 2018 growing season and adjusts to an ENSO neutral state, other warm-season teleconnections will influence weather patterns and crop yields. Yield research for ENSO neutral years show that yields vary above and below normal.
Current statistical and dynamical ENSO forecast models collected by the International Research Institute for Climate and Society: