Milking the Weather - September 2010 Edition
Rainfall Outlooks—Why They Sometimes Differ
Dr Andrew Watkins (Bureau of Meteorology) and Rod Eldridge (DPI Colac)
Sometimes you will read that the SOI is positive and you get all excited about some good rains coming, but then someone says ‘but I saw on the BOM website, below average rainfall is predicted’. Well, we now know that our climate is driven by many other factors apart from just the SOI. It will also depend upon which type of model you look at. For instance, the Bureau of Meteorology (BOM) runs two models – one official and one experimental.
The official BOM outlook uses a statistical model. It takes the historical patterns of sea surface temperatures in both the Pacific and Indian oceans and compares these to the past patterns of rainfall over Australia. It then uses these relationships to create a forecast based upon the current status of the two oceans. This outlook can be found either in map form at Bureau Home > Water and the Land > Rainfall Outlook or as a map with a report at Bureau Home > Climate > Seasonal Outlooks > National rainfall outlook.
The second type of model is a dynamical model. This is based upon the physics of the atmosphere and ocean, and although generally used by researchers, these models are increasingly being used for climate forecasting. BOM’s experimental dynamical model is the Predictive Ocean Atmosphere Model for Australia (POAMA). This model takes atmospheric and ocean data collected from around the globe, and forecasts forward only using the equations that govern how the oceans and atmosphere interact and move.
As POAMA makes no assumptions about what happened in the past, it may be more responsive to changes to a range of key drivers of the weather that affect our rainfall. In contrast, the statistical model, although fairly reliable in the past, assumes that the key drivers and influences on the weather will be the same now as they were in the past, which is becoming an increasingly difficult assumption to make.
Both types of models have their place and both are used by climatologists to gauge what may happen in the months ahead. Dynamical models are approaching the point where they will have enough skill to replace statistical ones for climate forecasting. Until they do, it’s worth considering both. If there is a big gap between their forecasts, check out BOM’s ENSO Wrap Up, which may help give you the bigger picture. It is found at Bureau Home > Climate > ENSO Wrap-Up.
