PDO-ENSO interaction and effects on Midwest climate and corn yields

The effects of the El Nino/Southern Oscillation (ENSO) on climatic effects have become well documented and integrated into long range forecasts. Their effects on crop yields in the Midwest and throughout the United States have also been reviewed extensively because of the disruption caused by the phases of the ENSO. Other large-scale Pacific sea-surface temperature anomalies also have effects on temperature and precipitation anomalies in the Midwest and consequently affect crop yields. The Pacific Decadal Oscillation (PDO) has been reported to modulate sea level pressure patterns of the ENSO throughout North America. Consequently, district temperature and precipitation patterns show a marked response to the interaction of ENSO-PDO phases. Contrasting phases (El Nino-high phase PDO, La Nina-low phase PDO) tend to see enhanced effects while the opposite combinations tend to oppose effects.

Categorizing monthly climatic data by ENSO and PDO combination has produced composite responses in temperature and precipitation. Specific situations and regions of the country see large anomalies caused by the interaction of the SST anomalies. Particular to the Midwest is an El Nino-high phase PDO combination which leads to a large positive precipitation anomaly over a large part of the Corn Belt in the late summer. In contrast an El Nino-low phase PDO condition shows a very weak positive precipitation anomaly in this same area with dry anomalies just to the north. The former situation has a significant impact on corn by reducing moisture stress during a possible stress time. This can help to explain some of the variability in El Nino climatic anomalies seen throughout the Midwest. La Nina-low PDO tends to produce drought conditions in the early summer over much of the central portion of the United States while La Nina-high phase conditions see a much-reduced anomaly shifted farther to the west.

The most extreme anomalies contrasts are greater than 7.5 cm for precipitation and 2.5 C for temperature. While significant variability in responses is seen, this research does seem to explain some of the variability of effects seen in ENSO events and enhance the predictability of events.