|Courtesy AMSU Temps Website|
There are three lines of values on the chart- 2008 (purple), 2010 (orange), and 2012 (light green). I am using 2008 and 2010 as basically defining boundaries between an El Nino and La Nina, with El Nino seeing warmer 400mb temperatures, and a La Nina observing cooler than normal temperatures in the upper troposphere.
At this time, the upper troposphere is pretty much in the middle of both benchmarks at this point in time. If we want to see an El Nino really get going (which should happen soon), I would expect to see a warming trend in the 400 millibar level to prove that the atmosphere is prepared for an El Nino.
You may ask why I see these two as good benchmarks. If you look towards the beginning of the year, you will see 2012 values in 2008 level temperatures. However, as of recent, the 400mb level has warmed considerably, but not to the point that I am convinced that the atmosphere is primed for an El Nino.