ENSO and IOD Indices

 El Niño–Southern Oscillation indices

El Niño and La Niña events (referred as El Niño–Southern Oscillation or ENSO) are driven by changes in the Sea surface temperature and atmosphere in the equatorial Pacific Ocean. During El Niño, sea surface temperatures in the central and eastern Pacific Ocean become warmer than average, while La Niña is characterised by cooler than average sea surface temperatures in the same regions.

To monitor the signs of El Niño or La Niña, several sea surface temperature (SST) indices are being used. These indices simply refer to the difference in sea surface temperature from the long-term mean (here 1991–2020) in several regions located along the equatorial Pacific. These regions are called NINO1 and NINO2, NINO3, and NINO3.4 and NINO4.

The Niño regions cover the following areas:

  • Nino1+2: 10°S-0, 90°W-80°W        (-10 to 0, 270 to 280)
  • Nino3    : 5°S-5°N, 150°W-90°W    (  -5 to 5, 210 to 270)
  • Nino3.4 : 5°N-5°S, 170°W-120°W  (  -5 to 5, 190 to 240)
  • Nino4    : 5°N-5°S, 150°W-160°E   (  -5 to 5, 160 to 210}
The Modoki regions cover the following areas:
  • Modoki:  RegA-(0.5*RegB)-(0.5*RegC)
  • Where:
  • RegA: 10°S–10°N, 165°E–140°W  (-10 to 10, 165 to 220)
  • RegB: 15°S–5°N, 110°W–70°W     (-15 to 5,    250 to 290)
  • RegC: 10°S–20°N, 125°E–145°E   (-10 to 20, 125 to 145)
El Nino Modoki is a coupled ocean-atmosphere phenomenon in the tropical Pacific Ocean and has been shown to be quite different from the canonical El Nino/Southern Oscillation (ENSO) in terms of its spatial and temporal characteristics as well as its teleconnection patterns (Ashok et al. 2007; Weng et al. 2007; Ashok and Yamagata 2009).


Figure 1: ENSO regions (Source: NOAA)



Figure 2: ENSO Modoki regions (Source: JAMSTEC)

Indian Ocean Dipole indices

Indian Ocean Dipole (IOD) events are driven by changes in the sea surface temperature in the tropical Indian Ocean. 

The IOD is commonly measured by an index (known as Dipole Mode Index or DMI) which is the difference between sea surface temperature (SST) anomalies in the western equatorial Indian Ocean (WEIO) and eastern equatorial Indian Ocean (EEIO).

  • WEIO: 50°E to 70°E and 10°S to 10°N
  • EEIO: 90°E to 110°E and 10°S to 0°
  • IOD: WEIO - EEIO

A positive IOD period is characterised by cooler than average water in the tropical eastern Indian Ocean and warmer than average water in the tropical western Indian Ocean. Conversely, a negative IOD period is characterised by warmer than average water in the tropical eastern Indian Ocean and cooler than average water in the tropical western Indian Ocean.



Figure 3: IOD regions (Source: BoM)


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