Long Paddock

 

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QCCCE monthly climate statement, August 2011

The Queensland Climate Change Centre of Excellence (the centre) considers that the probability of above-median rainfall for the next three-month period (August to October) is higher than normal (50 per cent) for most of Queensland. For the coming summer (November to March), the centre’s long range experimental Seasonal Pacific Ocean Temperature Analysis version one (SPOTA-1) scheme currently indicates a higher than normal probability of above-median rainfall across Queensland. Read more (PDF, 452K, last updated 01:52PM, 16 August 2011)*

The centre's understanding is based on the current and projected state of the El Niño-Southern Oscillation (ENSO) phenomenon and on factors which alter the impact of ENSO on Queensland rainfall (e.g. the Pacific Decadal Oscillation (PDO)). The centre's outlook for the August to October period is based on an historical analysis of Queensland rainfall and the SOI. The centre's long-lead outlook for summer is based on the current sea-surface temperature pattern in the extra-tropical Pacific.

As at 1 August 2011, the centre notes that the ENSO phenomenon is currently in a neutral state as measured by sea-surface temperatures in the equatorial Pacific (see the latest Bureau of Meteorology 'ENSO Wrap-Up' (PDF)*).  However, the SOI has risen from near-neutral values in May (+2.1) and June (+0.9) to a quite positive value (+9.1) in July.  The centre’s assessment of rainfall probabilities for August to October is based on this rapid rise in the SOI.

The centre also closely monitors sea-surface temperature patterns across the Pacific Ocean and, on this basis, makes a long-lead assessment of rainfall probabilities for the coming summer.

The 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. Historically, the ENSO and PDO signals have had the strongest impact on rainfall in north-eastern Queensland and the weakest impact in south-eastern Queensland.

Whilst sea-surface temperatures in the equatorial Pacific have returned to near-average values, there remains a strong sea-surface temperature gradient across the South West Pacific which is a remnant of the recent La Niña signal. Based on the Centre’s experimental SPOTA-1 scheme, this strong sea-surface temperature gradient in the South West Pacific, combined with a 'cool phase' of the PDO, increases the probability of above-median rainfall for the coming summer (November to March). The centre will continue to monitor sea-surface temperature patterns up until November and revise the rainfall outlook for summer on this basis each month until the start of summer.

Rainfall over the last three-month period (May to July) has been near or below-median for much of Queensland.

There are various approaches used to provide rainfall outlooks based on the above information. These approaches tend to differ in terms of the components of the climate system that are considered and, for this reason, each approach may convey a different outlook, particularly for specific locations.

The centre produces two statistical climate risk assessment schemes:

·      the experimental SPOTA-1 scheme integrates sea-surface temperature information, including indices of ENSO and the PDO

·      the SOI phase scheme relies solely on the SOI, an atmospheric measure of ENSO.

The centre's experimental SPOTA-1 scheme provides long-lead probabilities of summer (November to March) rainfall for Queensland from mid-April through to mid-November each year. The sea-surface temperature gradient (west to east) across the South Pacific Convergence Zone (i.e. between eastern Australia and the Central Pacific) remains quite positive (+1.32 ºC), a remnant of the recent La Niña pattern. According to the centre's experimental SPOTA-1 scheme, a positive sea-surface temperature gradient across this region is favourable for summer rainfall in Queensland. The SPOTA-1 scheme, which takes this sea-surface temperature gradient into account, as well as the state of the PDO, currently indicates a higher than normal probability of above-median rainfall throughout Queensland over the coming summer.

The centre's SOI phase scheme provides probabilities of rainfall for the coming three-month season based on SOI values over the previous two months. The SOI phase scheme currently indicates that the probability of exceeding median rainfall across most of Queensland is higher than normal (50 per cent) for the next three-month period (August to October). This is discussed further in the centre's commentary on rainfall based on phases of the SOI.

The above schemes indicate probabilities based on historical relationships. It is important that the probabilistic nature of seasonal outlooks is 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 all of 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 the historical archive of SPOTA-1 reports and past commentaries on the SOI phase scheme. 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.

For further information, please visit www.LongPaddock.qld.gov.au or contact QCCCE@climatechange.qld.gov.au 

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Last updated 23 November 2011

Seasonal climate outlook