About the Bureau's climate model: ACCESS–S

The model

The Bureau of Meteorology's climate forecast system for weekly to seasonal and longer-range climate outlooks is called the Australian Community Climate Earth-System Simulator – Seasonal (ACCESS–S). It is a state-of-the-art dynamical (physics-based) forecast modelling system, which uses ocean, atmosphere, ice and land observations to initiate outlooks for the season ahead. The ACCESS–S climate model is a collaboration between the Bureau of Meteorology and the UK Meteorological Office (UKMO).

The atmosphere and land model components of ACCESS–S operate at an approximate resolution of 60 km in the Australian region. At this resolution, the model can represent the markedly different climates of the Great Dividing Range and the eastern seaboard in Australia's east.

The ocean model component of ACCESS–S operates at an approximate resolution of 25 km in the Australian region. At this resolution, the model can resolve small-scale currents and eddies.

ACCESS-S outlooks are based on a 99-member ensemble. This is a common climate forecasting technique where the model is run 99 times with slightly different initial conditions to capture a range of likely future scenarios.

Being a dynamical model, ACCESS–S is undergoing continuous research and development. Advances in the science of seasonal prediction, improvements in the observations and how they are fed into the model, as well as increases in supercomputing power are just some of the ways the model's accuracy will increase over time.

ACCESS–S replaced POAMA in August 2018. POAMA, also a dynamical climate model, was used for official Bureau climate outlooks from May 2013 until ACCESS–S was brought into operation. Prior to 2013, the Bureau used a statistical method to generate climate outlooks.

For more technical details on ACCESS–S, see: Hudson, D. et al, 2017: ACESS-S1: The new Bureau of Meteorology multi-week to seasonal prediction system. Journal of Southern Hemisphere Earth Systems Science, 67:3 132-159

The outlooks

The Bureau uses output from ACCESS–S for several products including forecasts of the El NiƱo–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), as well as weekly to seasonal rainfall and temperature outlooks, and sea surface temperature (SST) anomaly forecasts.

Rainfall and temperature outlooks

ACCESS–S produces rainfall and temperature outlooks for the weeks to seasons ahead. They are based on an ensemble of 99 forecasts (or scenarios) for the future climate. The variability of the outlooks among the 99 ensemble members gives an indication of the range of possible future scenarios. For instance, if 80 of the 99 ensemble members suggest drier conditions are likely, we can say there is about an 80% chance of drier conditions in the upcoming month/season.

The rainfall and temperature outlooks are provided as the probability of getting above median rainfall or temperature (maximum and minimum) for the weeks to seasons ahead. For rainfall, this outlook information can also be displayed as the probability of getting a particular rainfall amount.

El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) outlooks

Sea surface temperatures in specific regions of the Pacific and Indian oceans are monitored to establish whether significant climate drivers, such as El Niño or La Niña, may be developing. These events can influence, or drive, Australian rainfall and temperature patterns. ACCESS–S produces temperature forecasts for these regions for the seasons ahead to monitor the possible development of ENSO or IOD events.

Ocean temperature outlooks around Australia | Coral bleaching risk

ACCESS–S produces seasonal sea surface temperature forecasts for regions around Australia. The forecasts provide information about likely ocean conditions. They can be used for various purposes, including monitoring of coral bleaching risk in the Great Barrier Reef and other reefs around Australia.

Northern rainfall onset outlooks

The northern rainfall onset date occurs when the rainfall total reaches 50 mm since the 1st of September. This is approximately the amount of rainfall required to stimulate plant growth after the dry season in northern Australia. ACCESS–S produces the northern rainfall onset outlooks in the winter months, with the outlooks based on an ensemble of 99 forecasts (or scenarios). The northern rainfall onset outlook is provided as the probability of getting an earlier than median onset.