Beginning November 2007, the Oceanic Niño Index (ONI) is calculated using Version 3 of the extended reconstructed sea surface temperature (ERSST) dataset. More information on this new dataset is provided here. This new monthly analysis replaces ERSST Version 2, which will no longer be updated. In the meantime, we are temporarily providing web links to the ONI based on ERSST.v3 and ERSST.v2.