A connection could be established between the phenomena of Southern Oscillation (SO), which is related to the atmosphere, and the El Niño / La Niña phenomena resulting from changes in ocean currents. The corresponding pattern has since been referred to as El Niño - Southern Oscillation (ENSO).
The SOI is calculated using the pressure differences between Tahiti and Darwin. A negative phase of the SOI is characterised by below-normal atmospheric pressure at Tahiti and above-normal atmospheric pressure at Darwin. Long periods of negative SOI are associated with warmer than normal ocean waters across the eastern tropical Pacific (El Niño). The opposite is true for long periods of positive SOI (La Niña). Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade wind, and a reduction in rainfall over eastern and northern Australia. Two of the strongest El Niño episodes of the century occurred on 1982/83 and 1997/98.
NINO3 is one of several climate indicators for El Niño - Southern Oscillation derived from sea surface temperature anomalies in the eastern tropical Pacific (5N-5S, 150W-90W). A long-term mean is usually subtracted from the NINO3 index, whereby an El Niño or La Niña event can be identified. In the specific example from Trenberth (1997), it was considered an event if the 5-month average of the index was exceeded by 0.4 ° C for at least 6 consecutive months.
An NINO3 index with monthly resolution covering the period from 1950 to the present day is provided by NOAA http://www.esrl.noaa.gov/psd/data/correlation/nina3.data
The SOI with monthly resolution covering the period from 1951 to the present day is provided by NOAA https://www.cpc.ncep.noaa.gov/data/indices/soi
Trenberth, K. E., 1997. The Definition of El Niño. Bull. Amer. Met. Soc., 78, 2771-2777.
Zhang, Y., J.M. Wallace and D.S. Battisti 1997: ENSO-like Interdecadal Variability: 1900-93. Journal of Climate, Vol. 10, 1004-1020.
Gershunov, A. and T. P. Barnett. Interdecadal modulation of ENSO teleconnections. Bull. Amer. Meteor. Soc., 79: 2715-2725.
ENSO at NOAA https://www.climate.gov/enso