Full fledged EL Nino was confirmed at the end of March 2015 when the five consecutive 3 monthly seasons had ONI index of +0.5 C or higher. February, March & April 2015 SST anomaly for Nino3.4 region is used to calculate the ONI Index for FMA as +0.6 .
As per NOAA Officially El Nino is declared as per conditions laid down and shown in ONI Index graph below:
El Niño: characterized by a positive ONI greater than or equal to +0.5ºC. La Niña: characterized by a negative ONI less than or equal to -0.5ºC. By historical standards, to be classified as a full-fledged El Niño or La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons.
The respective ONI Index are SON 2014= +0.5ºC and OND 2014 = +0.7ºC, NDJ 2015=0.7ºC, DJF 2015=0.6ºC, JFM 2015=0.5ºC & FMA 2015=0.6ºC.
CPC had issued an El Nino Advisory on 5th March 2015 in anticipation that JFM 2015 ONI would be 0.5ºC or more.
CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5°C along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months.
Southern Oscillation Index
The Southern Oscillation Index (SOI) had moved to negative zone from neutral zone during March 2015. However, the latest 30-day SOI value from BOM-Australian Government for period ending 26 April is −3.6 which denotes neutral zone.
SOI Monthly graph till end of April 2015 – The Long Paddock – Queensland Government. The Latest April 2015 30 days SOI was -3.1 .
This is the only El Nino to have been declared as full fledged El nino at the end of March of any year from 1950 on wards.
