La Nina brings results
Rain and storms and flooding during January have bought much needed relief falls to most of Queensland. While storm rains are patchy, the reminder of the summer outlook for Queensland indicates that reasonable chances for at least median rain continue.
Hopefully those areas that have missed out to date will get some rain. Indeed, the dry southeast corner has received some moderate to heavy falls over the past 48 hours. For example Crow's Nest had 57 mm and Southwood had 213mm. A friend told me that rain has yet to interfere with his irrigation schedule - I certainly hope it does soon.
Currently (as at 5 February) the 30 day average of the SOI is plus 9.4.
The 30 day average of the SOI remained positive through January, and was plus 12.7 for the month. The SOI Phase for January was 'Consistently Positive' (Phase 2). The outlook for February to April shows a 50 to70 % chance of above median rainfall for most of Queensland.
Some scattered regions only have a 40 to 50% chance of exceeding median rainfall. For more information try www.longpaddock.qld.gov.au http://www.longpaddock.qld.gov.au/> or contact me on 07 4688 1588.
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.
In addition, the monsoon trough should be re-invigorated by the passage of the MJO this week. Cyclone development is also associated with the monsoon trough, and with the recent passage of the MJO. For more information on the MJO try http://www.apsru.gov.au/mjo/ .
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.
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).
The Full Story
Lexie Donald, Qld Climate Change Centre of Excellence, 6th February, 2008.
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.
This has been reflected over the past month or so with rain and flooding. Even the previously dry SE corner of Queensland managed enough rain for (some) runoff into storage in the past several days.
Based on a shift in monthly SOI values from plus 13.3 in December to plus 12.7 during January the SOI is in a "Consistently Positive" phase. Further analysis indicates rainfall for much Queensland is more likely to be above or close to 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 much Queensland is a 50 to 70 % chance of exceeding 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 small isolated regions have a 40 to 50 % chance of above median rainfall.
Probabilities will continue to 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 l.
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 February to April Kingaroy has a 92% chance of getting at least 100 mm, Jondaryan has a 75% chance of getting at least 80 mm, Roma has a 90% chance of getting at least 60 mm, Emerald has a 83% chance of getting at least 100 mm and St George has a 83% chance of getting at least 60 mm.
Most of these areas have already received these amounts, or close to them, reflecting the 3 month forecast of a 50-70% of above average rainfall, or, in other words, the impact of a La Nina.
A 'Near Zero' SOI phase around September (like last year) is associated with an increased chance of hail and thunderstorm activity throughout northern NSW and southern Queensland over all of summer. 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 January to March in the following years: 1950, 1951, 1956, 1957, 1962, 1974, 1976, 1989, 1997, 1999, 2000, and 2001.
Find out your average rainfall for February to April, and how many times rainfall was well below, well above or close to average during January to March 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 4 (6th Febraury). As expected in a Nina year, the monsoon trough developed with the last MJO and should re-develop in short order as the MJO crosses the Maritime Continent (Phases 4 -5). Northern Australia can expect MJO enhanced rainfall probabilities Phases 4 through to 7 during summer.
The past 2 MJO's have lingered in the western Pacific, after rapid progression eastward over the Indian Ocean basin. This is most likely due to the La Nina atmospheric signature in the western Pacific. The current MJO (Phase 4) might reasonably be expected to behave in a similar manner, and linger over the Maritime continent and/ or in the western Pacific for the next week to ten days. The next passage of the MJO (Phase 4) can now be expected in mid Mach.
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 tropical cyclones. Several tropical cyclones and tropical lows have/are developing with this passage of the MJO through the tropics north of Australia.
Over summer the passage of the MJO will typically be associated with Southern Hemisphere tropical storms, and also 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. There is now an increased chance of cyclones to the NNW of Australia post MJO passage.
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 continues.
For example, eastern equatorial Pacific Ocean temperatures have continued cool and the SOI has remained positive. However, south east trade winds anomalies intensified after the last passage of the MJO but may weakened with the passage of the MJO over the past week.
While the mid-Pacific basin remains cool, a patch of warmer water has developed in the western Pacific, and this will probably propagate to the east, encouraging the La Nina to gradually break down over (Southern Hemisphere) autumn.
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/.