About POAMA Forecasts
The POAMA System
POAMA stands for Predictive Ocean Atmosphere Model for Australia. POAMA is the Bureau of Meteorology's dynamical (physics based) climate model used for multi-week to seasonal through to inter-annual climate outlooks. It is a state of the art long-range forecast system using ocean, atmosphere, ice and land data observations to initiate outlooks up to nine months ahead. POAMA was developed jointly by the Bureau of Meteorology and CSIRO Marine and Atmospheric Research. The Bureau utilises POAMA for several products including forecasts of the state of El Niño - Southern Oscillation (ENSO), Indian Ocean Dipole forecasts, and seasonal rainfall and temperature outlooks. Full details of the POAMA system and a range of experimental products can be found on the POAMA site.
POAMA forecasts
POAMA forecasts are run twice every week, and consist of 33 scenarios for the coming 9 months. The variability of the results among the 33 runs gives an indication of the uncertainty in the future evolution of the climate system. When many individual forecasts are considered together they are said to comprise an ensemble and the spread in the conditions they forecast can be used to gauge the likelihood of future conditions. For instance if 30 of the 33 ensemble members suggest dry ahead, we can say there is about a 90% chance of dry conditions in the next season.
El Niño - Southern Oscillation (ENSO) forecasts
A standard area in the tropical Pacific Ocean, called
NINO3.4
is used to monitor the state of ENSO. The sea surface temperatures in
this region are closely related to Australian climate.
The NINO3.4 index is defined as the average of sea surface temperature (SST)
anomalies over the region 5°N-5°S and 170°-120°W.
El Niño (a warm event)
is considered to occur when the NINO3.4 index persistently exceeds +0.8°C.
Similarly La Niña (a cold event)
occurs when NINO3.4 is persistently less than −0.8°C.
These temperatures should be sustained over a number of months to be
considered an ENSO event. POAMA forecasts of
NINO3.4 are given out to nine months ahead to monitor the possible
evolution of ENSO conditions. Similar forecasts of both NINO3 and
NINO4 are also shown to monitor the details of possible future ENSO
conditions.
Indian Ocean Dipole forecasts
Another region of sea surface temperature (SST) variability that impacts on Australian climate is located in the Indian Ocean. One mode of variability that affects Australian rainfall, particularly the southeast of the country, is the Indian Ocean Dipole (IOD)¹. The IOD index is the difference between SST anomalies in the western (50°-70°E, 10°S-10°N) and eastern (90°-110°E, 10°-0°S) parts of the tropical Indian Ocean. A positive IOD occurs when the western region is warmer than average and the eastern region is cool. Australian winter and spring rainfall generally increases in negative IOD years, and decreases in positive IOD years. More information is available on the impact of the IOD on Australian rainfall patterns.
A positive IOD is defined here if the index is greater than +0.4°C, a negative IOD if the index is less than −0.4°C. The standard deviation of the monthly-mean IOD index averages about 0.4°C. As with the NINO indices, POAMA forecasts of the IOD are given out to nine months.
Seasonal rainfall and temperature outlooks
The Bureau's seasonal outlook system for Australia is based on rainfall and temperature forecasts from the POAMA model.
Unlike the ENSO and IOD SST forecasts, the seasonal outlooks are based on the last three weeks of forecasts, i.e. five separate model runs combining to make a 165-member ensemble, as this was shown to give higher skill. Monthly forecasts use the most recent three runs, or a 99-member ensemble.
¹ Saji N.H., B.N. Goswami, P.N. Vinayachandran, T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401, 360-363.
