The blurb that FPizz posted was mine from a Phillywx post. I included with my post the MEI values since 1979, so I will include them with this post. You can see from the MEI values, ALL strong events were greater than 1 at the May/June reading with 97/98 at 2.4. I haven't seen a graph for the RONI, and maybe I'm reading your post incorrectly, but the MEI this year clearly does not stack up with past super el nino events thru May/June. In fact, this year's MEI matches closest to 02/03 & 09/10 to my aged eyes. I agree with your definition/purpose of the MEI, so maybe there's a lag, but it sure is odd that the readings so far have been far from previous strong events and much closer to weak Niños.
Fyi, if anyone is looking for a 1 stop site for Enso modeling/conditions, this would be it:
http://www.stormsurf.com/page2/links/ensocurr.html