Weak MJO event during late July Dave McRae, Qld Climate Change Centre of Excellence, 27 July 09.
The Madden-Julian Oscillation or MJO last crossed northern Australia as a weak event during the second half of July. It is timing remains around 40 days it would next be expected to cross northern Australia during very late August/early September.
The MJO is a band of low air pressure which originates off the east coast of central Africa. It travels eastward across the Indian Ocean and northern Australia roughly every 30 to 60 days. Because of the timing of the MJO the phenomenon is also known as the forty day wave. It can be used as an indicator for the timing of potential rainfall events.
The impact of the MJO on rainfall varies between the different seasons and location. For example the MJO has a greater influence on rainfall throughout northern Australia during summer and southern Australia during winter. For more information on the MJO go to www.bom.gov.au
When using a climate forecast you should remember that the probability or percent chance of something occurring is just that - a probability. For example if there is a 70% chance of recording more than 100 mm there is also a 30% chance of recording less than 100 mm i.e. 70-30; 30-70. It does not mean that you will get 70% more than 100 mm or 100 mm plus another 70%.
For example based on historical rainfall figures and a Near Zero SOI phase, Oakey currently has a 70% chance of getting at least 60 mm during July to September. Therefore Oakey has a 30% chance of NOT getting at least 60 mm during July to September.
Another way of looking at this is that 7 times out of 10 historically with the current SOI phase Oakey has recorded above 60 mm during July to September. Therefore 3 times out of 10 historically Oakey has recorded below 60 mm during July to September.
When looking at the seasonal outlook for your area it may make it easier to think of rainfall probabilities in these terms. Probabilities above 80% equal a high chance, probabilities above 60% equal an above average chance, probabilities below 40% equal a below average or low chance and probabilities below 20% equal a bugger all chance of that occurring.
For more on rainfall probabilities for your location refer to Rainman StreamFlow or www.longpaddock.qld.gov.au