About ENSO and IOD indices
An index is a measure (often a numerical value) that can be representative of a particular pattern or state of a system. Climatologists monitor several indices, some ocean-based and some atmospheric, to provide a quick indication of the state of certain climate variables and climate drivers.
El Niño–Southern Oscillation indices
El Niño and La Niña events (collectively referred to as the El Niño–Southern Oscillation or ENSO) 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 characterised by 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 use several sea surface temperature (SST) 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 NINO3.4 (which occupy the eastern and central equatorial Pacific respectively) and NINO4 (located in the western Pacific). NINO3.4 partially overlaps both the NINO3 and NINO4 regions.
The NINO regions (see map below) 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 ENSO events, the value of the NINO indices are often used in conjunction with other data, e.g., sub-surface ocean temperatures, cloudiness, winds, and the Southern Oscillation Index (SOI). Australian climatologists often cite sustained monthly NINO3 or NINO3.4 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 indices
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 (sometimes referred to as the Dipole Mode Index, or DMI) that is the difference between sea surface temperature (SST) anomalies in two regions of the tropical Indian Ocean (see map above):
- IOD west: 50°E to 70°E and 10°S to 10°N
- IOD east: 90°E to 110°E and 10°S to 0°S
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.
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.
ENSO and IOD outlooks
The Bureau produces long-range sea surface temperature outlooks for the NINO and IOD regions, to provide information on the potential development of El Niño, La Niña or IOD events in the coming months. The main regions focussed on are the NINO3.4 index and the IOD index as these are used to help identify ENSO and IOD events, respectively. A summary of the ENSO and IOD forecasts from international climate models is also produced.