Note DetailHistory of Midwest El Nino Knowledge
1/9/2008 10:48:00 AM

History of Midwest El Nino Knowledge:  I began serious work on the impact of El Nino and La Nina on Midwest crops in 1978.  All the books of the day stated that there was no impact on Midwest weather.  Dr. Dennis Todey (South Dakota State Climatologist) took the project on as his PhD research project and by 1994 the team of Carlson, Todey, and Taylor showed that the impact on Midwest crops was substantial.  We identified that the statistically significant value of the Southern Oscillation Index (SOI) to have a crop impact was either + or – 0.8.  The SOI is the historically well known difference in atmospheric pressure between Darwin (northern Australia) and Tahiti (mid- equatorial Pacific).   Other studies related conditions to the 5-month average of the SOI; we found that to be true for the Midwest as well.  This was fine for historical data, you just look back two or three months, see what the SOI was and what the crop did.  We began using the 90-day SOI as an indicator of what the SOI was likely to do.  This was successful and soon the world in general adopted the values of plus and minus 0.8 for the 90-day as the key “trigger” points.  The Australians developed a daily service that also included their work on using the 60-day SOI for a 3-month world climate outlook.     We still have to look back to see if a La Nina has come into place but the 90-day SOI report provides a look that comes a little bit sooner.  It was Christmas day 2007 that we had an SOI value of +0.8 and it was January 4, 2008 that we confirmed La Nina to be in place according to the scientific definition.  
When is La Nina?  We became confident that La Nina would exist in 2008 when the 90-day SOI reached +0.8 on December 25, 2007.  The La Nina officially was confirmed January 4, 2008.  The historical record now shows that this La Nina event began October 21, 2007.  The US weather pattern typical of a La Nina (moist in the Ohio River valley, dry in the High Plains, and extreme temperatures in Midwest) was apparent by the end of November.    The historical El Nino began July 11, 2006, ended October 18, 2006, and La Nina began October 21, 2007. El Nino typically persists for 12 to 14 months and La Nina for 6-9 months.     -Elwynn-

Iowa State University Extension