The Full Story
1 October 2008.
A neutral ENSO pattern is now firmly established and is very likely to continue throughout summer 2008 - 2009. There is only a relatively minor possibility of an El Nino developing.
The spring-summer outlook for Queensland shows a marginal shift in the odds towards wetter conditions as a result of the SOI remaining positive, shifting from plus 8.0 in August to plus 13.7 during September. Based on the shift in monthly SOI values the SOI is in a "Consistently Positive" phase - Phase 2.
Based on this SOI Phase and historical rainfall records much of Queensland has a 50 to 70 % chance of exceeding median rainfall October through December. Areas in the north and north-east of the state have a higher 70 to 90% chance of exceeding median rainfall October through December.
Further analysis indicates that the October to December rainfall is likely to be above average (upper third) rather than average or below average (lower to middle thirds). This represents a shift in the odds towards wetter conditions compared with last months outlook.
For example Emerald has a 71% chance of getting above their October to December median rainfall of 175 mm; Roma has a 54% chance of getting above its October to December median rainfall of 166 mm, Gympie has a 71% chance of getting above its October to December median rainfall of 285mm, Longreach has a 61% chance of getting above its October to December median rainfall of 91 mm, while Charter Towers has a 85% chance of getting above its October to December median rainfall of 128 mm.
For more on rainfall probabilities for your location refer to "Rainman StreamFlow".
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 30% chance of recording more than 100 mm then there is also a 70% chance of recording less than 100 mm i.e. 30-70; 70-30. It does not mean that you will get 30% more than 100 mm or 100 mm plus another 30%.
For those who like to follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during October to December in the following years since 1950: 1950, 1954, 1955, 1956, 1960, 1962, 1964, 1967, 1971, 1973, 1974, 1975, 1981, 1988, 1996, 1998 and 2000. Find out your average rainfall and see how many times rainfall was well below, well above or close to average during October to December in the listed years.
The 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.
Changes in sea surface temperature (SST) patterns in the central Pacific drive changes in the global circulation patterns and influence our local climate. Although the SOI is measured on a daily basis, it is the shift in value of the SOI from month to month that reflects SST patterns and the strength of the Walker Circulation.
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 or drier than normal.
It is worth noting that the SOI influence on climate varies across Australia (greatest skill is for eastern Australia) and between seasons (usually greatest skill is for winter, spring and early summer).
Generally, periods of widespread pro-longed drought, especially across eastern Australia, are associated with Consistently Negative SOI phases (average monthly values of the SOI below minus 5.0 and often an El Niño SST pattern). This negative value is a reflection of barometric air pressures over northern Australia being higher than those in the central Pacific. This may slow or stop the flow of the south-east trade winds as well as reduce the occurrence of rain depressions (and cyclones).
For those interested, updates of 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. A summary of the seasonal outlook is available at (07) 4688 1623.
The MJO was in Phase 7 at end of September. This indicates that the MJO is located over the Pacific near the date line, about Kiribati. The MJO signal was incoherent and weak through July and up to mid-August, but strengthened and was organised for much of September. Provisional indications suggest the MJO will next reach Phase 4 mid to late October.
During spring (Sept- Oct-Nov) Queensland can expect enhanced chances of rainfall Phases 6 and 7 (NQ), and Phase 5 (S and SWQ). The MJO suppresses the chances of rain during spring in Phases 1 and 2. Check the Bureau of Meteorology's MJO page and go to 'seasonal composites' for more details.
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/ . Neutral SST conditions and ENSO pattern are firmly established, and the SOI for September was positive.
Central equatorial Pacific Ocean sea surface temperatures are near normal, and there are significant sub-surface cooler anomalies. Cloudiness near the date line is near normal and the trade winds are close to average in the eastern equatorial Pacific and stronger than average in the central and western Pacific. Dynamic Climate models forecast continuation of these neutral conditions over the coming summer months through to March 2009.
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 SINGLE 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/.