About ENSO and IOD monitoring graphs
The El Niño–Southern Oscillation
El Niño and La Niña events are driven by changes 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 essentially the opposite, with cooler than average sea surface temperatures in the same regions.
In order to monitor the Pacific Ocean for signs of El Niño or La Niña, climatologists often use several "NINO" indices. These indices simply refer to the difference from the long-term (1961–1990) mean of the sea surface temperature in several regions located along the equatorial Pacific. These regions are called NINO1 and NINO2 (which lie on the South American coast), NINO3, and NINO4 (which occupy the eastern and central equatorial Pacific) and NINO3.4 (which partially overlaps both the NINO3 and NINO4 areas).
The NINO regions (shown in Figure 1) cover the following areas:
- NINO1: 5-10°S, 80-90°W
- NINO2: 0-5°S, 80-90°W
- NINO3: 5°N-5°S, 150-90°W
- NINO3.4: 5°N-5°S, 120-170°W
- NINO4: 5°N-5°S, 160°E-150°W
For monitoring of El Niño-Southern Oscillation (ENSO) events, the value of the NINO indices are often used in conjunction with other data (e.g., sub surface ocean temperatures, cloudiness, winds, the Southern Oscillation Index). Australian climatologists often cite monthly NINO values above +0.8 °C as typical of El Niño conditions, with values of below −0.8 °C as that of La Niña. These values are approximately one standard deviation from the long term mean (i.e., around 70% of monthly NINO3 values, for example, lie between −0.8 °C and +0.8 °C).
The Indian Ocean Dipole
Indian Ocean Dipole (IOD) events are driven by changes in the tropical Indian Ocean. Sustained changes in the difference between normal sea surface temperatures in the tropical western and eastern Indian Ocean are what characterise IOD events.
The IOD is commonly measured by an index that is the difference between sea surface temperature (SST) anomalies in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E and 10°S to 0°S) equatorial Indian Ocean. Figure 2 below shows the east and west poles of the IOD.
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. Learn about Australian rainfall patterns during IOD events.
For monitoring the IOD, Australian climatologists consider sustained values above +0.4 °C as typical of a positive IOD, and values below −0.4 °C as typical of a negative IOD.