Climate Model Summary
Negative IOD event likely to end by mid-spring; models suggest La Niña possible later in 2021
The negative Indian Ocean Dipole (IOD) event has weakened, with index values just shy of the negative IOD threshold in recent weeks. While three of five models indicate the event will restrengthen in October, two maintain neutral values throughout the spring and all models indicate a return to neutral by December. This is consistent with the typical IOD seasonal cycle, that sees events dissipating in early summer with the arrival of the Australian monsoon. A negative IOD event typically enhances spring rainfall in southern and eastern Australia. Additionally, daytime temperatures are typically cooler than average for southern Australia, but warmer than average in the north.
The El Niño–Southern Oscillation (ENSO) is currently neutral, i.e. neither El Niño nor La Niña. Most model outlooks indicate the central tropical Pacific will cool in the coming months. Three of seven models indicate this cooling will be sufficient, and sustained for long enough, to meet minimum La Niña event criteria from mid to late spring through to at least January 2022. Two additional models also briefly meet La Niña thresholds during the outlook period. La Niña typically enhances spring and summer rainfall in eastern and northern Australia.
The latest weekly NINO3.4 value to 12 September 2021 is −0.2 °C, within ENSO-neutral boundaries. Most surveyed models indicate central Pacific sea surface temperatures (NINO3.4) are likely to cool over the coming months, with three of the seven models suggesting the cooling will be sufficient, and sustained for long enough, to meet minimum La Niña event criteria. Two additional models briefly touch on La Niña thresholds during the outlook period, with the remaining two models predict a neutral ENSO to persist through to early 2022.
La Niña events increase the chances of above average spring and summer rainfall in northern and eastern Australia.
Persistent NINO3.4 values above +0.8 °C typically indicate El Niño, while values below −0.8 °C typically indicate La Niña.
Indian Ocean Dipole (IOD) outlook
The negative IOD event has weakened, with index values falling just shy of the negative IOD threshold in recent weeks. The latest IOD index value for the week ending 12 September is −0.3 °C, above the −0.4 °C threshold. While three of five model outlooks indicate the IOD is likely to return to negative IOD levels during October, two maintain neutral values throughout the remainder of spring, and all models indicate a return to a neutral IOD pattern by December. This is consistent with the typical IOD event cycle that sees events dissipating in early summer with the arrival of the Australian monsoon.
A negative IOD increases the chance of above average spring rainfall for parts of southern and eastern Australia. Additionally, daytime temperatures are typically cooler than average for southern Australia, but warmer than average in the north.
IOD events are unable to form (and therefore influence Australian climate) during December to April. This is because the monsoon trough shifts south over the tropical Indian Ocean, changing the wind patterns and preventing an IOD event from forming.
Australian Community Climate Earth-System Simulator–Seasonal (ACCESS–S)
The Bureau of Meteorology's climate model generates a six-month forecast for the NINO and IOD indices each fortnight.
The most recent model run (generated 11 September 2021) indicates sea surface temperatures in the central tropical Pacific (NINO3.4) are likely to cool, exceeding La Niña thresholds (−0.8 °C) during October to January.
The current model outlook for the Indian Ocean Dipole (IOD) indicates the active negative IOD event has likely weakened, with a neutral pattern expected for the remainder of 2021.
The forecast values, shown below in bold, are for the model's ensemble mean.
Product code: IDCKGL0000
Average of international model outlooks for NINO3.4
Average of international model outlooks for the Indian Ocean Dipole (IOD)
The arrows on the dials indicate the combined average of monthly outlooks from a survey of international global climate models. Note that the individual model runs vary around the average.
NINO3.4 covers the central Pacific region.
The graphs are based on the ensemble mean for the most recent model run.
These graphs show the average forecast value of NINO3.4 for each international model surveyed for the selected calendar month. If the bars on the graph are approaching or exceeding the blue dashed line, there is an increased risk of La Niña. Similarly, if the bars on the graph are approaching or exceeding the red dashed line, there is an increased chance of El Niño.
The graphs are based on the ensemble mean for the most recent model run.
Thse graphs show the average forecast value of the IOD index for each international model surveyed for the selected calendar month. If the majority of models are approaching or exceeding the blue dashed line, then there is an increased risk of a negative IOD event. If the majority of models are approaching or exceeding the red dashed line, then there is an increased risk of a positive IOD event.
Sea surface temperature graphs
NINO34 predictions for the next 5 months.
About these sea surface temperature outlooks
About the graphs
The plume graphs show outlook scenarios for sea surface temperatures (SSTs) averaged over particular regions of the Pacific and Indian oceans. The SSTs in these regions are related to different phases of the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD); climate drivers that can influence Australian rainfall and temperature patterns.
The graphs show 99 possible scenarios (grey lines), that are produced by the Bureau's climate outlook model, which represent the range of outcomes that may occur over the forecast period. For example, they may show the SSTs in the NINO3.4 region to be warming, cooling, or remaining mostly steady. At times the outlook might suggest a shift towards (or away from) values typically associated with El Niño or La Niña events. Each of the 99 scenarios is based on current conditions in the global oceans/atmosphere and how the model anticipates their likely development over the outlook period, with each given slightly different treatment to provide a range of likely possibilities. This technique allows us to see the range of what is possible, with a small spread in the range of scenarios meaning more confidence in the likely path, and a larger spread meaning less confidence.
The green line is the average of all these 99 scenarios, often known as the ensemble mean. The solid black line shows the observations (based on the Bureau's SST observation analysis for each region) for the previous months.
The graphs are updated fortnightly. As a result, the value given for the 'current month' can vary depending on at what point in the month the forecast is being issued. Forecasts made on the 1st to the 11th of the month show a forecast value for the current month. For forecasts made after the 11th of the month, a month-to-date observation (shown by an open circle and dashed line), based on weekly observational data, will be used for the current month as a preliminary value until the final monthly data is available.
About the maps
While the climate model runs a set of 99 possible scenarios, it can be useful to look at the ensemble mean (the average of these forecasts) to see the most likely scenario. The global SST maps show the most likely SST anomaly for the months and seasons ahead. This can be useful to see how ENSO and IOD look spatially. The SST anomalies show the difference from the 1990-2012 average (often referred to as the base period).
About the outlook model
The long-range SST outlooks are generated by the Bureau's climate model, ACCESS–S (Australian Community Climate Earth-System Simulator–Seasonal). ACCESS–S is the Bureau of Meteorology's dynamical (physics-based) weather and climate model used for monthly, seasonal and longer-lead climate outlooks. Prior to August 2018, climate outlooks (including these graphs) were produced by the Bureau's earlier model, POAMA.
Product code: IDCK000073
The models used within our survey are listed below with links to their agency homepages, model output and technical information about the model.
Model data are provided for Bureau of Meteorology use by the agencies detailed in the Models section. Respective agency copyright applies to these data.