Which El Niño Index is Best?
A Plethora of El Niño Indices
The November monthly Sea Surface Temperature Anomaly (SSTA) in Niño 3.4 region was a record +2.35, while the 3-month Oceanic Niño Index (ONI) for September-October-November is +2.04, second to a +2.18 for the same period in 1997. So, is this the strongest El Niño on record or not? How about the monthly SSTA from December 1877 of +2.51?
In reality they are all correct as there is really no “best” metric. One may have elements that best describe a particular season in a particular location, while another may have the best teleconnection for a different time and place.
A variety of different metrics are used to characterize El Niño; ranging from sea surface temperatures (SST) to sea-level pressure, surface winds, surface temperature and out-going longwave radiation. These are usually expressed in the form of an index; often in terms of the difference from long-term average (i.e., anomalies). Even indices based upon the same metric, like SST, may use different datasets, sampling periods (i.e., weekly, monthly, bi-monthly or tri-monthly) and/or different filtering techniques.
The Niño 3.4 region (map) has arguably the best teleconnections to seasonal weather in the United States and is widely used as a sampling area for SSTAs.
The Oceanic Niño Index (ONI), is the mostly widely used index in the United States to characterize El Niño events. The ONI is the 3-month SST Anomaly (SSTA) for Niño 3.4 and calculated from moving 30-year average values. See http://ggweather.com/enso/oni.htm and http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml.
The Niño 3.4 region SST is also sampled on both a monthly (http://www.cpc.ncep.noaa.gov/data/indices/ersst4.nino.mth.81-10.ascii) and weekly (http://www.cpc.ncep.noaa.gov/data/indices/wksst8110.for) using the ERSST4 dataset back to 1950. There is also monthly Niño 3.4 data going all the way back to 1870 using the HadISST1 dataset (http://icdc.zmaw.de/1/daten/ocean/hadisst1.html).
The Southern Oscillation, which was identified by British mathematician Sir Gilbert Walker in the early 1900s, is a periodic "see-saw" of pressure patterns in the tropical Pacific. This pattern was later linked to occurrences of El Niño and is the SO part of the combined acronym ENSO for El Niño Southern Oscillation. It is usually expressesed as the standardized Southern Oscillation Index (SOI). See https://www.ncdc.noaa.gov/teleconnections/enso/indicators/soi/.
The El Niño Modoki (translated from Japanese for "similar but different") Index quantifies ENSO events that are warmer in the central Pacific than the typical El Niño warming in the eastern Pacific. See http://www.jamstec.go.jp/frcgc/research/d1/iod/enmodoki_home_s.html.en.
And the Multivariate ENSO Index (MEI) brings together six different elements (sea-level pressure, zonal and meridional surface wind, sea surface temperature, surface air temperature and total cloudiness). See http://www.esrl.noaa.gov/psd/enso/mei/.
An excellent technical analysis of these various indices by Anthony Barnston of IRI can be found at https://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-enso-indexes-instead-just-one.
Jan Null, CCM
Golden Gate Weather Services