Spring Outlook Reasonable, La Niña Developed
Dave McRae, Qld Climate Change Centre of Excellence, 13/09/10
Based on a Consistently Positive SOI phase at the end of August, rainfall probabilities for spring throughout Queensland are reasonable. Currently there is a 60 to 80% chance of getting above the long term September to November median rainfall throughout most of the state.
The last time there was a Consistently Positive SOI phase at the end of August was in 1998. Other years since 1950 that have had the same SOI phase at the end of August include: 1996, 1988, 1981, 1975, 1974, 1973, 1960, 1958, 1956, 1955 and 1950.
It may be useful to find out what rainfall and/or farming conditions were like in your area during September to November in those years. See how many times rainfall was well-below, well-above or close to average during September to November in the listed years. For more information on historical rainfall figures for your region try Rainman Streamflow.
It is worth remembering that we are still in Queensland’s traditional dry season of August and September and high rainfall totals are not common at this time of year.
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 100mm i.e. 70-30; 30-70. It does not mean that you will get 70% more than 100mm or 100mm plus another 70%.
For example, comparing the current SOI phase with the historical rainfall record, Miles currently has a 75% chance of getting above its long term September to November median rainfall of 140mm. This also means that there is a 25% chance of getting below 140mm.
Another way of looking at this is that, in slightly more than 7 years out of 10 with the current SOI pattern (or three quarters), Miles has received more than 140mm from September to November. Therefore in slightly less than 3 years out of 10 with the current SOI pattern (or 30 percent of the time), Miles has received less than 140mm for September to November.
According to the latest ENSO Wrap-up from the Bureau of Meteorology, a La Niña climate event is now well established.
This is reflected by continuing positive SOI values (30-day average as of the 6th September was plus 22.2), the continued cooling of surface and sub-surface ocean temperatures throughout the key ENSO regions of the Pacific (which now exceed La Niña temperature thresholds) and the stronger than normal south-east trade winds throughout the central and western Pacific.
As well, all the surveyed global climate models indicate Pacific Ocean sea surface temperatures will continue to exceed La Niña thresholds throughout spring with the majority indicating the event will persist into at least early 2011. Also of interest is that SSTs around eastern and northern Australia are warmer than normal which has contributed to 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. Other impacts of a La Niña event include warmer than average night time temperatures, increased intensity of the northern Australia monsoon trough, increased occurrence of rain depressions and increased tropical cyclone occurrence.
Given the recent flooding throughout southern Australia, it is interesting that a negative Indian Ocean Dipole (IOD) event appears to have commenced in the Indian Ocean. Negative IOD events are often (but not always) associated with above average rainfall over large areas of southern Australia during spring.
At this stage the MJO is next expected to impact on the Australasian region during very late September or early October. 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. 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 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
For more climate related information, updates on SOI values and the latest outlook map go to www.longpaddock.qld.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. One key point is that management decisions should never be based entirely on one factor such as a climate or weather forecast. As always, everything that could impact on the outcome of a decision (e.g. 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 JUST ONE PIECE OF INFORMATION (SUCH AS A CLIMATE FORECAST).
Furthermore, forecasts 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.