Are you aware of a past archive for JMA forecasts/outlooks? I’ve looked for it before on the web and wasn’t able to find it. Therefore, it’s hard to compare forecasts with JMA so I can’t answer that part of your question... CPC has an archive of past outlooks here:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory… (click "Archive" link).

NOAA CPC/IRI declared El Nino in March 2015. It is our opinion that this is the start of El Nino irrespective of the ERSST dataset. This is because we do not declare ENSO events -- from month-to-month -- on the basis of a single index (or dataset) that is SST only (covered here: http://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-e… and here: http://www.climate.gov/news-features/blogs/enso/decembers-enso-update-c…)

The Oceanic Nino Index (based on ERSST) is only one index to monitor ENSO and it is an simple way of classifying historical events and drawing comparisons. However, please always keep in mind that no index (or single dataset) is foolproof and ENSO is a *coupled* (meaning ocean + atmosphere) phenomenon, so for the weaker/borderline events in particular, we think it is important to consider other ENSO indices in addition to the SST based ones. In late 2014, we did not see a clear atmospheric response, so we did not declare an El Nino (http://www.climate.gov/news-features/blogs/enso/do-recent-global-precip…).

Since March 2015, we have seen both the tropical ocean and atmospheric indices aligned and more clearly exhibiting coupled behavior, which is why we have been calling it an El Nino. Also, our forecast probabilities for El Nino were fairly elevated (40-50%) prior to March (Feb forecast: http://iri.columbia.edu/our-expertise/climate/forecasts/enso/2015-Febru…), so we don't think it's accurate to say we never saw it coming.

A final (more minor point), recent values of the ONI will continue to change when the climatology changes. ONI values for 2014-15 will change slightly until the climatology becomes fixed in the year ~2030. More details here: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI…