Likely La Niña developing, SOI values positive Climate Watch, Dave McRae, Qld Climate Change Centre of Excellence, 16/08/2010
Based on the shift in monthly value of the SOI from plus 1.3 for June to plus 18.7 for July the SOI is now in a "Rapidly Rising" phase.
For those interested in following historical rainfall patterns more closely, some of the years that have had the same SOI pattern at the end of July include 2003, 1999, 1995, 1990, 1988, 1985, 1984, 1974, 1963, 1960, 1954, 1949, 1948, 1947, 1943, 1939, 1936 and 1933.
It can be useful to find out what rainfall and farming conditions were like in your area. Find out your average rainfall for August to October and see how many times rainfall was well below, well above or close to average during August to October in the listed years. For more information on historical rainfall data for your region try Rainman Streamflow.
Based on the current phase of the SOI, rainfall probabilities across much of Queensland remain reasonable. At present there is a 50 to 80% chance of getting above the long term August to October median rainfall across most of the southern, central and northern Queensland. The exception is for the far west and north west of the state where there is a lower 40 to 50% chance of getting above median rainfall for August to October.
It is also worth remembering that throughout most of Queensland, August and September have the lowest historical average rainfall figures so high rainfall totals are not common at this time of year.
The 30day average of the SOI as of the 16th August is plus 17.5. 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, Dalby on the Darling Downs currently has a 70% chance of getting above 100mm for August to October. This also means that there is a 30% chance of not getting above 100mm.
Another way of looking at this is that in 7 years out of 10 (or slightly less than three quarters) with the current SOI pattern, Dalby has received more than 100mm for August to October. Therefore in 3 years out of 10 (slightly more than one quarter), Dalby has gotten less than 100mm for this period.
According to the latest ENSO Wrap-up from the Bureau of Meteorology, all climate indicators in the Pacific Ocean show that we are now in the early stages of a La Niña event.
This is reflected by continuing positive SOI values (30 day average as of the 9th August is plus 19.1), the continued cooling of surface and sub-surface ocean temperatures throughout the key ENSO regions of the Pacific (which have now reached or exceeded La Niña levels) and the stronger than normal south east trade winds.
As well, the surveyed global climate models are indicating some further strengthening of the La Niña event is likely. Also of interest is that SST’s around eastern and northern Australia are warmer than normal which has assisted the inflow of moisture across the Queensland coastline.
La Niña events are usually (but not always) associated with average to above average rainfall during spring and summer throughout much of eastern and northern Australia. This is a reflection of barometric air pressures over northern Australia during La Niña events being lower relative to those in the central Pacific. This helps to increase the intensity of the northern Australia monsoon trough and increase the occurrence of rain depressions (and cyclones).
La Niña events normally develop during late autumn/winter and decay the following late summer/autumn. Recent La Niña events include 2008/09, 2007/08, 1998/01 and 1988/89.
Based on soil moisture profiles and the seasonal rainfall outlook at the end of July, the 2010 winter wheat crop outlook for Queensland remains optimistic.
For example, throughout most of the southern Queensland there is greater than a 70% chance of getting above the long term median wheat crop yield. Similarly most of Central Queensland has a 60 to 80% chance of getting above the long term median wheat crop yield. The exceptions are for parts of the eastern Darling Downs and northern Central Highlands where there is a lower 30 to 50% chance of getting above the long term median wheat crop yields.
The current state wheat outlook shows a forecast median yield at the end of July of 1.73 t/ha (up from last months outlook of 1.53 t/ha). This remains close to the long-term state median yield of 1.41 t/ha. Useful rainfall totals especially during flowering will help maintain or improve the current crop yield expectation.
The regional wheat crop outlook is based on the assumption of cropping after a summer fallow and does not take into account effects of poor crop nutrition or damage due to pests, diseases, heat or frosts. For more information on the seasonal crop outlook contact Andries Potgieter, DEEDI on (07) 4688 1417.
The active convection and associated rainfall during early August appears to have been associated with an active MJO event. Based on its current timing the next MJO would be expected around early to mid 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.
It is worth noting that 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 I'm asked about how climate information can be used I refer to a couple of key points developed from client feedback. Key points 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).
Forecasts as well do not always give a strong signal as to likely conditions for your location. In assessing climate forecasts as a management tool consider the level of signal for the key decision times in your location. Rainman StreamFlow is a useful tool for this.