Climate monitoring graphs
Sea Surface Temperature (SST) indices and Southern Oscillation Index (SOI)

El Niño and La Niña (collectively referred to as the El Niño–Southern Oscillation or ENSO) are characterised by changes in the equatorial Pacific Ocean. During El Niño, sea surface temperatures (SST) in the central and eastern Pacific Ocean become warmer than average, while during La Niña these SSTs become cooler than average.

Niño indices regions

To monitor the Pacific Ocean for signs of El Niño or La Niña, climatologists use several SST indices. These indices measure the difference between the current sea surface temperature and its long-term (1991–2020) average in several regions located along the equatorial Pacific. The difference is referred to as an anomaly. These regions are labelled Niño1, Niño2, Niño3, Niño3.4 and Niño4 and are used by meteorological agencies around the world.

For monitoring of ENSO phases, the value of the Niño indices are often used in conjunction with other data, e.g., sub-surface ocean temperatures, cloudiness, winds, and the Southern Oscillation Index (SOI). The Bureau cites sustained monthly Niño3 or Niño3.4 values above +0.8 °C as being associated with El Niño, and values below −0.8 °C being associated with La Niña. These values are approximately one standard deviation from the long-term mean (i.e., around 70% of monthly Niño3 values in the historical record, for example, lie between −0.8 °C and +0.8 °C).

• Details about:
• El Niño
• La Niña
• ENSO
• SOI

The Southern Oscillation Index, or SOI, gives an indication of the state and intensity of ENSO, from an atmospheric perspective. The SOI is calculated using the pressure differences between Tahiti and Darwin.

Sustained negative values of the SOI below −7 often indicate El Niño is active while sustained positive values above +7 are typical of a La Niña.

Early monthly pressure readings from Darwin and Tahiti have been digitised for electronic use. Early daily pressure readings have not yet been digitised, so a shorter dataset is available.

Technical details

There are a few different methods for calculating the SOI. The method used by the Australian Bureau of Meteorology is the Troup SOI which is the standardised anomaly of the Mean Sea Level Pressure difference between Tahiti and Darwin. The base period used in the SOI calculation is 60 years (1933–1992).
Calculation

                        Pdiff − Pdiffav
            SOI = 10 x -------------------,
                            SD(Pdiff)
    

where:
Pdiff = (average Tahiti MSLP for the period) − (average Darwin MSLP for the period),
Pdiffav = long term average of Pdiff for the period in question, and
SD(Pdiff) = long term standard deviation of Pdiff for the period in question.

The multiplication by 10 is a convention to make the final value more readable. Using this convention, the SOI ranges from about –35 to about +35, and the value of the SOI can be quoted as a whole number. The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily values can also be averaged over a longer period to form a multi-day average. Single-day or weekly values of the SOI are not so useful for information on the current state of the climate, as these values are dominated by the effects of short-term weather variability, and accordingly the Bureau of Meteorology does not issue them. In particular, single-day values can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes.

• Details about:
• About ENSO, El Niño and La Niña
• SOI

Indian Ocean Dipole (IOD) phases 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 phases.

The IOD is commonly measured by an index (sometimes referred to as the Dipole Mode Index, or DMI) that is the difference between SST anomalies in two regions of the tropical Indian Ocean (see map):

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.

• Details about:
• IOD
• IOD phases

The Southern Annular Mode, or SAM, refers to the north-south movement of rain-bearing westerly winds and weather systems in the Southern Ocean, compared to the usual seasonal position. A positive SAM refers to a southward shift while a negative SAM refers to an northward shift. The typical impact on Australian rainfall from positive and negative phases of SAM depends on the time of year and interaction with other climate indicators such as El Niño or La Niña.

Sustained values of the SAM index above +1 indicate a positive SAM event, while sustained values below -1 indicate a negative SAM event.

• Details about:
• About SAM

Data periods

Daily datasets have a value for every day in their record. Similarly, weekly and monthly (30 day) data sets have values for every week or month (30 days), respectively, in their record.

Sea surface temperature data

The weekly and monthly datasets are formed from weekly or monthly averages of daily SST values, and are updated either weekly or monthly in near real-time. The daily values are obtained from interpolated (gap-free) analyses on a 0.25° latitude by 0.25° longitude grid of the temperature of the uppermost 10 metres of the ocean under well-mixed conditions, based on observations from both in-water instruments and satellites. As observations are not always available within the specified time interval for all areas covered, the daily analysis systems uses 'statistical interpolation' to fill in the gaps using a weighted combination of the previous daily SST analysis and previous weekly SST analysis.

The temperature estimate is generally considered to be at approximately 0.2 metres depth (the depth of drifting buoys). However, as the observations used for the analysis have been selected for only well-mixed conditions, these temperatures are similar to temperatures down to approximately 10 metres. The maps provide SST analysis values for each 0.25° of latitude and longitude (approximately 28 km).

The observations used to derive the global daily SST analyses are obtained from drifting buoys, moored buoys, ships, and infrared radiometers aboard Polar-Orbiting Environmental Satellites operated by the National Oceanographic and Atmospheric Administration (NOAA) and the European Space Agency (ESA). In order to fill in some of the data gaps due to satellite infrared sensors that cannot penetrate cloud, they also incorporate SST observations from microwave sensors on polar-orbiting satellites operated by the Japan Aerospace Exploration Agency (JAXA).

• BoM GAMSSA SST analysis: Beggs et al. (2011) and Beggs et al. (2020)
• Climatology: 1991–2020 European Space Agency (ESA) SST Climate Change Initiative (CCI) Climatology product version 3.0

Early SST data

Before the satellite era, the primary source of SST data was observations made by ships passing through the region. The frequency of these observations was too low to produce a useful weekly dataset, so it is shorter than the monthly dataset. IOD and ENSO event identification using early SST data has limited accuracy, particularly for the Indian Ocean.

SOI data

Data source: Bureau SOI data

The SOI data includes a long history of monthly pressure readings from Darwin and Tahiti that have been digitised for electronic use. Old daily pressure readings have not yet been digitised, so a shorter dataset is available.