The Bottom Line
Bureau of Meteorology Cyclone Forecast
Lexie Donald, Qld Climate Change Centre of Excellence, 14th November 2007.
The Bureau of Meteorology has launched their new website section dedicated to tropical cyclones: www.bom.gov.au/weather/cyclone
The Tropical Cyclones site integrates the Bureau's tropical cyclone warning services and supporting information. It also includes cyclone outlooks, reports and tracks of previous cyclones and information on activity, trends and links to climate change information as well as links to cyclone preparedness guides, State Emergency Services and frequently asked questions.
According to the Bureau, Queensland can expect more cyclone activity this season than last. Last season just two cyclones developed in the Coral Sea, neither of which was severe nor made landfall. However it's unlikely however that the coming season will be as active as 2005-06 when severe cyclones Larry and Monica struck the east coast.
While the number of cyclones is in doubt, it only takes a single severe cyclone to hit a coastal city or town to make the season a memorable one. Interestingly if the La Niña strengthens further, as may well happen, the likelihood of a major cyclone impact on the east coast would increase.
The 30 day average of the SOI as of the 14th November is plus 6.9.
Based on a "Consistently Near Zero" SOI phase at the end of October there is a 40 to 60% chance of getting median rainfall for November to January with higher rainfall probabilities (60 to 70%) along the tropical Queensland coast. For instance, Cairns has a good chance of getting 75-100mm this week.
For those interested daily updates on the SOI are available on (07) 46881439 or at www.longpaddock.qld.gov.au. You can also receive a text message with the latest SOI values sent to your mobile phone. To subscribe call (07)4688 1588.
The Full Story Climate Watch Lexie Donald, Qld Climate Change Centre of Excellence, 14th November 2007.
Compared to this time last year, there has been an improvement in the seasonal outlook for Queensland.
Based on a shift in monthly SOI values from plus 2.2 for September to plus 6.1 for October the SOI is in a "Near Zero" phase. Further analysis indicates rainfall for Queensland is more likely to be close to the long term average (or middle third) rather than well above or below average. So while the chance of getting above median rainfall is not high there remains a quite reasonable chance of getting some useful rainfall. The current outlook is also 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 Near Zero" SOI phase at the end of October there is a 40 to 60% chance of getting median rainfall for November to January with higher rainfall probabilities (60 to 70%) along the tropical Queensland coast. The other exceptions are relatively small, isolated regions scattered throughout the south west of the state where there is a lower 30 to 40% chance of getting median rainfall.
Again, probabilities would improve if the SOI went 'Consistently Positive' (say above plus 7.0) and remained there until autumn, 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.
While these probabilities may not be as high as some would like, there is a reasonable chance of getting some useful rainfall. For example during November to January Kingaroy has a 80% chance of getting at least 200 mm, Jondaryan has a 70% chance of getting at least 200 mm, Roma has a 70% chance of getting at least 160 mm, Emerald has a 75% chance of getting at least 150 mm and St George has a 90% chance of getting at least 60 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: 1949, 1954, 1958, 1959, 1960, 1961, 1966, 1967, 1968, 1978, 1979, 1980, 1984, 1985, 1990, 1995, 2001, 2003, and 2004. Find out your average rainfall for November to January 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 phone. To subscribe to this free service call (07) 4688 1588
There MJO is located north of Australia, over the Maritime continent, in the Phase 4-5 region. Following rapid eastward movement from the Indian Ocean in October the MJO now displays little eastward movement. Like the previous MJO passage eastward movement has deteriorated into convection over south east Asia, focused slightly north of the Equator. The weak MJO signal is mostly due to the lack of eastward movement. The past two MJO's have stalled to the north of Australia, and this is probably due to La Nina conditions dominating the MJO. The next MJO, due around mid-December, has the potential to favour monsoon onset conditions for northern Australia.
At this time of year the MJO is typically associated with increased rainfall over much of Northern Australian and 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 south east trade winds have been stronger than average. In contrast, the SOI has remained near zero, rather than being strongly positive as during many La Nina events.
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 seasonal (summer) outlook the outlook would improve if ocean temperatures off the eastern Australian coastline became warmer than normal. Those to the west and north have warmed, and fit east follows suit this would assist to cross-continental moisture flow.
Key points about how climate information 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/.