by David A. Widmar and Brent A. Gloy
One of the top stories in agriculture for 2015 will likely be the impact the current El Nino has on production. Earlier this spring an official El Nino was declared and is expected to last until 2016. It appears the event predicted for last summer has finally arrived.
There is great interest, and concern, over exactly how El Nino conditions could impact the summer growing season for corn and soybeans in the U.S. Much work has been done looking at the temperature and precipitation patterns of El Nino events, but these are often very general. This led us to look into how historic El Nino conditions have overlapped with corn and soybean yields.
About El Nino
An important influencer of global weather is the El Nino-Southern Oscillation (ENSO) cycle. Based on the sea surface temperature of the Pacific Ocean, periods of warm temperatures are often associated with El Nino event. Conversely, cool periods are often La Nina events.
While the general warm/cool relationship of the ENSO cycle seems straightforward, actually declaring an official El Nino can be tricky as several factors must be considered; measurements of sea surface temperatures from various regions, observations of trade wind patterns, and considerations for atmospheric pressure. Arguably the most important indicator of the ENSO cycle is the Oceanic Nino Index (ONI). The ONI reports a 3-month running average of sea surface temperatures for a specified region of the Pacific Ocean. After 5 consecutive observations of the 3-month running average more than (less than) 0.5 degrees Celsius above (below) normal, conditions are prime for an El Nino (La Nina).
The most current report of the ONI is the 3-month average for March, April, and May recorded at +0.7, or 0.7 degrees Celsius above normal. This marked the seventh consecutive report of warm sea surface temperatures. And while other factors have gone into the current El Nino event, the warm ONI index alone concludes conditions are strongly favoring El Nino.
To look at the relationship between El Nino and crop yields, ONI data were used to evaluate trend-adjusted corn and soybean yield data provided by the USDA. Data from 1950 to 2014 were evaluated. For this work, only the ONI data for the growing season were considered (the 3-month means for May-June-July, July-August-September, August-September- October were considered).
El Nino and U.S. Yields
In Figure 1 the relationship between El Nino and corn yields is presented. The bars represent the difference between the trend adjusted yield and the mean trend yield. The shading of the bar corresponds to the observed ONI conditions for the growing season (red for warm, blue for cool, black for neither).
At first glance, recent yield extremes were not associated with a warm or cool period as strong corn yields in 2014 and drought-plagued yields of 2012 both occurred with neutral ONI conditions during the growing season. Also eye-catching are the yields that are most above average (2004, +17.4 bpa; 1994, +14.8 bpa; 1972, +15.2 bpa) occurred during warm, El Nino favoring conditions. However, a warm ONI index has also been associated with below average yields. (1991, 2002).
Across 65 years of data, warm ONI conditions were observed 21 times. During these El Nino favoring conditions, above trend yields occurred 57% of the time. For growing seasons with cool sea surface temperatures (La Nina favoring conditions), above trend corn yields occurred only 43% of the time.
Figure 1. Departure of Trend-Adjusted Corn Yields from Average (1950-2014) and ONI Conditions for the Growing Season (Warm, El Nino favoring in red; Cool, La Nina favoring in blue; Neutral in black). Data Sources: USDA NASS, National Weather Service’s Climate Prediction Center.
In figure 2, the difference of the trend adjusted mean from the average trend yield is shown for soybeans with shading, again, to represent the ONI index for the growing season. Several years with below trend yields occurred when the ONI index was neutral (bars in black). Also, in 1994 the highest above-trend yield was observed (+5.5 bpa) and occurred during a warm, El Nino favoring growing season.
Across 65 years of data, above trends yields occurring during 76% of the warm, El Nino favoring growing seasons. For cool, La Nina favoring conditions, above trend yields occurred only 45% of the time.
Figure 2. Departure of Trend-Adjusted Soybean Yields from Average (1950-2014) and ONI Conditions for the Growing Season (Warm, El Nino favoring in red; Cool, La Nina favoring in blue; Neutral in black). Data Sources: USDA NASS, National Weather Service’s Climate Prediction Center.
El Nino’s Impact’s Vary
When considering state-level data, the share of historic El Nino conditions that have resulted in above trend yields can vary. From figure 3, Minnesota and Wisconsin have experienced above trend yields for 81% of past El Nino favoring growing seasons. Elsewhere in the Corn Belt, these conditions have resulted in above trend yields 67% of the time (Iowa, Illinois, Indiana, Ohio, Missouri, and Michigan). In the Southern Plain and in the South, however, the El Nino conditions resulted in above trend yields a much lower share of the time.
Figure 3. Percent of Times Yields were Above Average in El Nino Conditions, by State. Data Sources: USDA NASS, National Weather Service’s Climate Prediction Center.
For soybeans, different trends emerge (Figure 4). In Wisconsin soybean yields were above trend for only 57% of El Nino conditions; 67% for Wisconsin. Meanwhile the Southern Plains (Texas) and Missouri experienced above trend yields a large share of the time that El Nino has occurred. In Missouri, 17 out of the 21 years with El Nino conditions during the growing season resulted in above trend yields. In the South, soybean yields have been above trend much less frequently, especially in Georgia and Louisiana.
Figure 4. Percent of Times Yields were Above Average in El Nino Conditions, by State. Data Sources: USDA NASS, National Weather Service’s Climate Prediction Center.
Wrapping it Up
As speculation about the potential size of the U.S. corn and soybean crop begins, much emphasis and attention will be placed on the current El Nino conditions. It is important to keep in mind that while El Nino conditions have been associated with some of the best U.S. yields in history it does not guarantee above trend yields. For both corn and soybeans, El Nino favoring conditions have also been associated with below trend yields.
El Nino favoring conditions have resulted in above trend corn yields 57% of the time and 76% of the time for soybeans; suggesting El Nino conditions may be more favorable for soybeans. It’s worth nothing, however, that over this time frame yields overall were above trend about 54% of the time for corn and 58% of the time for soybeans. For corn, above trend yields were only slightly more common during El Nino conditions.
At the state level, the impacts can be more dramatic. For instance, in Minnesota and Wisconsin above trend corn yield occurred more frequently with the presence of El Nino. In the case of soybeans states, Texas and Missouri appeared to benefit from El Nino. Given the soybean planting problems in Missouri this will bear watching as the summer unfolds.
How the forecasted El Nino for the 2015 growing seasons progresses will be carefully watched. When evaluated the historic relationship between the ONI index and yields, El Nino conditions are not a guarantee for bin-busting yields. Of course the development of El Nino is an important factor but there are also lots of other factors that come into play when determining whether final yields will be above or below trend.
Photo by Johnny Klemme
 The ONI data were considered warm or cool if any observed 3-month rolling
mean temperatures were +/- 0.5 degrees Celsius for 5 consecutive overlapping periods. This is to say that if any of the periods (JJA-JAS-ASO) were part of at least 5 consecutively warm (cool) periods, the growing season was considered warm (cool).