Monthly Climate Timeseries:
Oceanic Niño; Index (ONI) V2

This timeseries is a measure of ENSO.

Timeseries Plot:
Select this link to get a plot. Plot is taken from the Timesseries Plotting and Analysis webpage.
Calculation Method:
DESCRIPTION: 3 month running mean of ERSST.v5 SST anomalies in the Niño 3.4 region (5°N-5°S, 120°-170°W)], based on the 1971-2000 base period. For historical purposes cold and warm episodes are defined when the threshold is met for a minimum of 5 consecutive over-lapping seasons.
Time Interval: Monthly
Time Coverage: 1950 to present
Update Status: Monthly

Get Data:
ONI(SST): Standard PSL Format (What is standard format?)
Data is from NOAA CPC