The Queensland Climate Change Centre of Excellence (the Centre) considers that, at this time of year, the Southern Oscillation Index (SOI) or other measures of the El Niño-Southern Oscillation (ENSO) are less reliable as indicators of rainfall for the autumn season ahead. However, while the current La Niña pattern persists, there remains an increased probability of above-median rainfall for Queensland. Read more (PDF, 236K, last updated 12:33PM, 29 March 2011)*
The Centre’s understanding is based on the current and projected state of the ENSO phenomenon and on factors which alter the impact of ENSO on Queensland rainfall (e.g. the Pacific Decadal Oscillation, PDO). This time of year is known as the ‘autumn predictability gap’ when there tends to be less persistence in the ENSO signal and El Niño or La Niña events tend to break down.
As at 1 March 2011, the Centre notes that a La Niña pattern currently remains in the Pacific Ocean. However, there are signs of this pattern weakening (see the latest Bureau of Meteorology ‘ENSO Wrap-Up (PDF)*’). This pattern is likely to continue to weaken in coming months. Currently:
The recent sea-surface temperature pattern in the North Pacific remains consistent with a ‘Cool Phase’ of the PDO. The PDO modulates the impact of ENSO on summer rainfall in Queensland, particularly under La Niña conditions.
A cool phase of the PDO, coupled with La Niña conditions, is particularly favourable for November to March rainfall in Queensland (see the Centre’s experimental SPOTA-1 scheme which incorporates a measure of both ENSO and the PDO). These conditions are also usually associated with enhanced tropical cyclone activity in the Coral Sea. Further information is available in the Bureau of Meteorology’s media release. So far this summer, three tropical cyclones have made landfall in Queensland. These were Tasha (25 December 2010), Anthony (30 January 2011) and, by far the largest, Yasi (2 February 2011).
So far this summer, almost all of Queensland has received rainfall totals exceeding the long-term median for the entire summer (PDF, 457K, last updated 04:12PM, 14 March 2011)* (November to March) with most regions having received rainfall totals exceeding the 70th percentile level. Over half of the state has received rainfall exceeding the 90th percentile level for the current summer.
There are various approaches for developing probabilistic rainfall outlooks based on the above information. These approaches tend to differ in terms of which components of the climate system are considered and, for this reason, may convey a different outlook, particularly for specific locations.
The ENSO and PDO signals have historically had the strongest impact on rainfall in north-eastern Queensland and the weakest impact in south-eastern Queensland.
The Centre produces two statistical climate risk assessment schemes:
As the above schemes indicate rainfall probabilities based on historical relationships, it is important that the nature of seasonal outlooks are understood and long-term risk management is undertaken. For example, if an outlook indicates a 70 per cent probability of above-median rainfall, this also means there is a 30 per cent probability of below-median rainfall. Additionally, an increased risk of above or below-median rainfall in Queensland due to ENSO will not necessarily result in above or below-median rainfall occurring throughout the state (see Australia’s Variable Rainfall poster (PDF, 1.0M, last updated 11:42AM, 24 June 2010)* or the Centre’s archive of historical rainfall maps).
The Centre understands that each of the schemes may have its own particular following. Although such schemes cannot provide outlooks with absolute certainty each year, users of the information who follow a skilful scheme should benefit from doing so in the long-term. Users should consider the historical track record of any scheme and such information is becoming increasingly available. The Centre’s Long Paddock website provides an historical archive of SPOTA-1 reports. Users should also consider the wide range of information available each month describing the current state of the ocean/climate system, for example the ‘ENSO Wrap-Up (PDF)*’.
ENSO influences other climate variables apart from rainfall (e.g. temperature, pan evaporation and vapour pressure). This means that the impact of ENSO on crop or pasture growth can be stronger than on rainfall alone. The impact of ENSO on pasture growth is also dependent on current pasture condition and soil water status. The Centre’s AussieGRASS model takes these factors into account in producing pasture growth seasonal probabilities.
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