SOI in positive values, Increased chance of La Niña Dave McRae, QCCCE, 15/07/10
Based on positive monthly SOI values for May and June the SOI has remained in ‘Consistently Positive’ phase. An analysis of historical rainfall records and a consistently positive SOI phase at the end of June indicate a 50 to 70% chance of getting above median rainfall for July through to the end of September throughout most of Queensland. This is in comparison to last year when there was a lower 30 to 40% chance of getting median rainfall throughout most of the state.
It is worth noting that for Queensland we have now entered our ‘dry season’ and that August and September have the lowest median monthly rainfall totals for most locations in Queensland. For example median rainfall for August and September at Blackall is 9 mm and 6 mm, at Emerald is 11 mm and 9 mm, at Goondiwindi is 26 mm and 34 mm, at Toowoomba is 33 mm and 41 mm and Mareeba is 2 mm and 2 mm. Therefore significant or ‘drought breaking’ rain is not usual during this period regardless of the seasonal outlook.
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 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 July to September in the following years since 1920 that have had a Consistently Positive SOI phase at the end of June: 1921, 1924, 1931, 1938, 1942, 1952, 1955, 1956, 1962, 1964, 1968, 1971, 1974, 1975, 1978, 1981 and1989.
Find out your average rainfall for July to September and see how many times rainfall was well below, well above or close to average during July to September in the listed years. For more information on historical rainfall data for your region try Rainman Streamflow.
According to the latest ENSO Wrap-up from the Bureau of Meteorology, the trend towards a developing a La Niña climate pattern has continued. This is reflected by continuing positive SOI values, the continued cooling of surface and sub-surface ocean temperatures throughout the key ENSO regions of the Pacific (in some regions around 3 degrees cooler than normal) and the stronger than normal south east trade winds. At this time of year this is consistent with a developing La Niña.
As well, the majority of the surveyed global climate models are indicating the potential development of a La Niña during the next couple of months. Also of interest is that SST’s around eastern and northern Australia are warmer than normal which may assist the inflow of moisture across the Queensland coastline.
Historically, about 40% of El Niño events are followed by a La Niña. For example the 98/99 La Niña developed onto the end of the 97/98 El Niño.
As autumn is the key transition time for the development and breakdown of El Niño and La Niña events we recommend monitoring SST and SOI values. Updates on the development of these climate patterns can be found at www.bom.gov.au/climate/enso or www.longpaddock.qld.gov.au or www.cpc.ncep.noaa.gov
As of the 12th July the 30day average of the SOI is plus 7.1. If the SOI remains in a consistently positive phase through to the end of July, rainfall probabilities for early spring would be expected to increase.