Weekly Update 25th October
The 30-day average of the SOI as of 25th October is -3.5.
An analysis of historical sea surface temperature (SST) maps provides a long term viewpoint from which to assist in determining the longer term seasonal outlook. For example, the current sea surface temperature map pattern closely resembles the pattern experienced in September 1992 especially in the central Pacific Ocean. This pattern also somewhat resembles the SST patterns experienced around August/September 1993. As many readers will remember most of Queensland also had below average rainfall in 1993 and 1994.
Sea surface temperatures in the far eastern Pacific Ocean are cooler than the long term average. However it is in the key central equatorial Pacific that SSTs are slightly warmer than normal and it is this region that appears to have most influence on Queenslands weather and climate. SSTs in this region have been warming steadily over the past nine months.
Most of the SST prediction models suggest that a warming trend in the central Pacific will continue but the question remains as to how much. Some models suggest the Pacific will remain in a "neutral" state but others still are suggesting the potential for this warming to reach El Niño proportions later this year or early into next.
As the SOI tends to follow sea temperature variability in the central Pacific we recommend that those readers looking to get an early indication of seasonal conditions for early next year keep track of any shifts in the SOI value and SST patterns.
An updated seasonal outlook covering the period November to January will be published on Thursday 1st November.
Slight Lift in Rainfall Probabilities across Qld
The 30day average of the SOI has risen over the last few weeks to +1.5 as of the 1st of October. Based on this shift in the value of the Southern Oscillation Index (SOI) over August/September there has been a corresponding slight lift in rainfall probabilities across much of Queensland for the October-December period (refer to map).
Regions of the state that have had a slight improvement in rainfall probabilities include parts of the Chinchilla, Wondai, Mundubbera, Gayndah, Perry, Monto, Murweh, Blackall, Jericho, Aramac, Longreach, Winton, Mt Isa, Bourke, Cloncurry, Nebo and Duaringa shires. These areas all have a 60-70% chance or probability of getting or exceeding the median rainfall over October-December.
However areas of lower rainfall probabilities still exist. These include the southern half of Dalrymple shire and a strip running from the western half of Quilpie shire to the southern half of Cloncurry shire where there is only a 30-50% chance of getting or exceeding the median rainfall over the October-December period.
The rest of the state has around a 50-60% probability of getting or exceeding the historical median rainfall over October-December.
As always when using probability based forecasts the opposite always applies. For example, if there is a 20% chance of getting above the median rainfall at a location over the next three months there is an 80% chance of getting below it.
During the past few months the 40day wave and its influence on Queenslands rainfall has been hard to identify. However it is next expected to effect us in early October. It is important to take into account when considering the influence of the 40day wave that it is an indicator as to the possible timing of rainfall rather than the amount of possible rainfall.
For those who like to follow historical patterns more closely years in the past that had a Rapidly Rising SOI Phase for August/September include 1989, 1983, 1979, 1970, 1949, 1936,1931, 1926, 1922 and 1921. What was the rainfall like for October-December in your area for those years? For example, at Crow's Nest for 8 of the last 10 times that a Rapidly Rising Phase existed at the end of September, above median rainfall was recorded over the October-December period. However at Boulia this only occurred in 4 out of the last 10 times that a rapidly rising SOI phase existed at the end of September.
Recent research has also shown that a "Consistently Rapidly Rising" SOI phase at the end of September increases the potential for severe storm activity over the coming months across southern Queensland and northern NSW.
Also of interest at present is the trend of Sea Surface Temperature (SST) changes in the central Pacific Ocean. This partially resembles sea surface temperature patterns and trends experienced throughout the 1992-94 period. It will be interesting to see if this warming trend in the central Pacific Ocean continues and if so by how much.
Further substantial warming in this region could be considered an early warning of the potential reduction in expected rainfall across northern Australia and PNG over late summer and well into next year. Even though the outlook for the medium term has improved slightly many climate research institutions around the world believe the necessary preconditions required for the onset of an El Niño in the longer term still exist.
Therefore the key point at present is to keep track of any shifts in the SOI value and changes in sea surface temperature patterns over the next couple of months, as these will provide the indicators as to rainfall probabilities for summer and into next year.
More climate related information is available on the DPI's climate web site www.dpi.qld.gov.au/climate or by contacting the DPI Call Centre on 13 25 23. A recorded message with the 30day average of the SOI is available on 46881439