Summer Outlook Optimistic
Recent storms and rain have been very welcome. The rains were still fairly patchy, however with the summer outlook for Queensland looking optimistic, hopefully those areas that have missed out to date will get some rain.
Currently (as at the 4th December) the 30 day average of the SOI is plus 8.8
The 30 day average of the SOI remained positive through November, and was plus 9.9 for the month. The SOI Phase for November was 'Positive' (Phase 2). The outlook for December to February shows a 50 to 70 % chance of above median rainfall for much of Queensland.
Some regions including the Atherton area and Fraser Coast have a 70 to 80% chance of exceeding median rainfall. North western regions and some isolated regions have a 40 to 50 % chance of exceeding their median rainfall. For more information try www.longpaddock.qld.gov.au http://www.longpaddock.qld.gov.au/> or contact me on 07 4688 1588.
Remember any probability forecast is just that - a probability. A 60 % chance of getting above median rainfall also means that there is still a 40 % chance of not receiving median rainfall.
Say your location now has a 60 % chance of exceeding the median rainfall. If your median for December is, for example, 150mm that means that in other years with an SOI similar to this, 6 times out of 10 there was more than the median. In 4 out of the 10 years less than 150mm fell.
For more information on probabilities, or to explore median rainfall figures try Rainman Streamflow available from the DPI&F on 13 23 25.
The Southern Oscillation Index (SOI) is simply a measure of the difference in barometric air pressure between Darwin and Tahiti. It typically ranges in value from plus 30 to minus 30.
There are five different phases of monthly SOI and they are used to categorise shifts in value of the SOI from the end of one month to the next.
The phases are:
Consistently Negative (SOI remains in negative values from one month to the next)
Consistently Positive (SOI remains in positive values from one month to the next)
Rapidly Falling (SOI significantly falls in value from one month to the next)
Rapidly Rising (SOI significantly rises in value from one month to the next)
Consistently Near Zero (SOI remains in a stable pattern near 0 from one month to the next)
By using a statistical analysis of SOI phases and historical climate data (rainfall, frost, hail, temperature, etc), a forecast can be developed to indicate for example, whether the coming three months are likely to be wetter (above the median) or drier (below the median) than normal (the median).
Lexie Donald 07 4688 1588
Lexie Donald, Qld Climate Change Centre of Excellence, 4th December 2007.
The seasonal outlook for Queensland is optimistic. Compared to this time for the past several years, there has been a moderate to strong shift of the probabilities towards a wetter outlook.
Based on a shift in monthly SOI values from plus 6.1 for October to plus 9.9 during November to the SOI is in a "Consistently Positive" phase. Further analysis indicates rainfall for much Queensland is more likely to be close to or above the long term average (or middle third to upper third) rather than below or well below average.
The chance of getting above median rainfall for the eastern two thirds of the state (say east of about Cloncurry) has a 50 to 70 % chance of exceeding their median rainfall. As Queensland receives much of its rain during the summer period there is a very reasonable chance of getting some useful rainfall.
Some coastal regions have a higher 70 to 80 % chance of getting more than the median rainfall. A region in the west of the state, adjacent the border with the Northern Territory and some smaller regions have a slightly lower 40 to 50 % chance of above median rainfall.
Probabilities will continue indicate median to above median rainfall if the SOI stays 'Consistently Positive' (say above plus 7.0) and remains there until autumn (March), when the existing ENSO pattern traditionally breaks down. The last time the SOI was in a "Consistently Positive" phase for more than one month was from September 2000 to March 2001.
The current outlook is an improvement on this time last year when much of the state had as low as a 10% to 30% chance of getting median rainfall.
Based on a "Consistently Positive" SOI phase at the end of November there is a 50 to 70% chance of getting median rainfall for December to February with higher rainfall probabilities (70 to 80%) along isolated coastal regions. The other exceptions are relatively scattered regions throughout the south west of the state where there is a lower 40 to 50% chance of getting median rainfall.
While these probabilities may still not be as high as some would like, there is a reasonable chance of getting useful rainfall. For example during December to February Kingaroy has a 90% chance of getting at least 200 mm, Jondaryan has a 80% chance of getting at least 200 mm, Roma has a 75% chance of getting at least 150 mm, Emerald has a 80% chance of getting at least 200 mm and St George has a 80% chance of getting at least 80 mm.
Interestingly a Near Zero SOI phase around September (like this year) is associated with an increased chance of hail and thunderstorm activity throughout northern NSW and southern Queensland. This is due to the increased instability of the middle atmosphere (600mb to 500mb) combined with cool air at those levels.
When using any probability based forecast you should remember that the probability or percent chance of something occurring is just that - a probability. 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 more on rainfall probabilities for your location refer to "Rainman StreamFlow".
To follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during September to November in the following years: 1950, 1955, 1961, 1962, 1964, 1970, 1971, 1975, 1988, 1998, 1999 and 2000. Find out your average rainfall for December to February and how many times rainfall was well below, well above or close to average during September to November during those years.
For those interested daily updates on the SOI are available on (07) 4688 1439 or at www.longpaddock.qld.gov.au You can also receive a text message with the latest SOI values sent to your mobile - just contact me on 4688 1588.
The MJO is currently in Phase 8 (2nd December). It can be expected in Phase 4 sixteen to twenty-five days from now, around the 22nd December. In a La Nina year, I would expect the monsoon trough to develop in short order after the MJO reaches the Maritime Continent (Phase 4). Northern Australia can expect MJO enhanced rainfall probabilities Phases 4 through to 7 during summer.
At this time of year the MJO is typically associated with the Australian monsoon and increased rainfall over much of Northern Australian and also with features such as northwest cloud bands. As the summer progresses the MJO would typically be associated with Southern Hemisphere tropical storms, and cyclones in the Indian Ocean and western Pacific Ocean. We usually expect to see a greater impact on our rainfall (northern Australia) during our summer and autumn.
The MJO is a band of low air pressure originating off the east coast of central Africa travelling eastward across the Indian Ocean and northern Australia roughly every 30 to 60 days. Research has shown the MJO to be a useful indicator of the timing of potential rainfall events. For more information try www.apsru.gov.au/mjo/
According to the Bureau of Meteorology in their "ENSO Wrap-up" available at www.bom.gov.au/climate/enso/ the La Nina is now firmly established.
For example, eastern equatorial Pacific Ocean temperatures have continued to cool and the SOI has remained weakly positive, rather than being strongly positive as during many La Nina events. However, south east trade winds have weakened with the passage of the MJO over the past week.
This La Nina developed late and was not established before the end of winter. Hence, we did not see rainy southern Australian winter-spring often associated with LA Nina conditions. The climate models used by the Bureau suggest that the La Nina event will persist until about March, before breaking down into a more neutral pattern.
Another factor influencing this 'less rainy' La Nina was Australian coastal sea temperatures, which were cooler than normal for a LA Nina event. For the general summer seasonal outlook, the chance of rain would improve if ocean temperatures off the eastern Australian coastline became warmer than normal. Those to the west and north have warmed, and if the Coral Sea follows suit, that would promote cross-continental moisture flow.
Key points about how climate information is applied have been developed from client feedback. These include that management decisions should never be based entirely on one factor such as a climate or weather forecast. As always, everything that could impact of the outcome of a decision (soil moisture, pasture type/availability, crop and commodity prices, machinery, finance, costs etc) should be considered. For example, the level of soil moisture at planting is the major factor influencing crop yield or success.
A simple cost benefit analysis when making a major decision may also be useful. For example what will I gain if I get the desired outcome? What will I lose (sleep, money, family relationships) if I do not get the desired outcome and what other options (risk neutral) are there? A PART OF THIS PROCESS IS TO HELP MANAGERS TO BE CAREFUL NOT TO CHANGE FROM NORMAL RISK MANAGEMENT TO HIGH LEVEL RISK TAKING BASED ON A PIECE OF INFORMATION (SUCH AS A CLIMATE FORECAST).
The Climate Variability In Agriculture' (CVAP) program has an interesting site which highlights some case studies on how producers and businesses have used climate and weather information in their decision-making processes at http://www.managingclimate.gov.au/.