Background
In a paper that addresses "the need for a reliable ENSO index that allows for the historical definition of ENSO events in the instrumental record back to 1871," the authors say that their Multivariate ENSO Index (MEI) was originally defined by them (Wolter and Timlin, 1993, 1998) as "the first seasonally varying principal component of six atmosphere-ocean variable fields in the tropical Pacific basin," noting that this index "provides for a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices."

What was done
To further improve on their original improvement, the two U.S. researchers describe their efforts "to boil the MEI concept down to its most essential components (based on sea level pressure and sea surface temperature) to enable historical analyses that more than double its period of record," noting that their efforts were "designed to help with the assessment of ENSO conditions through as long a record as possible to be able to differentiate between 'natural' ENSO behavior in all its rich facets, and the 'Brave New World' of this phenomenon under evolving greenhouse gas-related climate conditions."

What was learned
Wolter and Timlin report that "the new MEI.ext confirms that ENSO activity went through a lull in the early- to mid-20th century, but was just about as prevalent one century ago as in recent decades." In fact, they state that "so far, none of the behavior of recent ENSO events appears unprecedented, including duration, onset timing, and spacing in the last few decades compared to a full century before then," whereas prior computer model simulations have at various times suggested that: (1) global warming will increase the frequency of ENSO events, (2) global warming will increase the intensity of ENSO events, and (3) weather-related disasters will be exacerbated under El Niño conditions.

What it means
So far, there is no indication of the "fingerprint of man" in any aspect of ENSO behavior over the last few decades, when climate alarmists contend the planet warmed at a rate and to a level that were both unprecedented over the past millennium or two, throwing some welcome "cold water" on another "hot climate crisis."

References
Wolter, K. and Timlin, M.S. 1993. Monitoring ENSO in COADS with a seasonally adjusted principal component index. Proceedings of the 17th Climate Diagnostics Workshop, Norman, Oklahoma. NOAA/NMC/CAC, NSSL, Oklahoma Climate Survey, CIMMS and the School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA, pp. 52-57.

Wolter, K. and Timlin, M.S. 1998. Measuring the strength of ENSO events -- how does 1997/98 rank? Weather 53: 315-324.