Monthly Climate Statement for August 2010

The Queensland Climate Change Centre of Excellence has concluded, based on the current and projected state of the global climate system, that there is an increased probability of above median rainfall in Queensland for the coming spring and summer seasons.

The Centre’s understanding is based on the current and projected state of the El Niño-Southern Oscillation (ENSO) phenomenon and on factors which modulate the impact of ENSO on Queensland rainfall (for example the Pacific Decadal Oscillation, also known as the Inter-decadal Pacific Oscillation).

As at August 1 2010, the Centre notes that:

It is clear that a La Niña climate pattern is currently developing in the Pacific (see the latest Bureau of Meteorology 'ENSO Wrap-Up'). This pattern is likely to persist through spring and summer.

• The Southern Oscillation Index (SOI) has been quite positive when averaged over the last month (July: +18.7), two months (June-July: +10.0), and three months (May-July: +10.2). 
• Observed sea surface temperatures in the key Niño 3.4 and Niño 4 regions are cooler than normal – typical of a developing La Niña pattern.
• Associated with this La Niña pattern, the sea surface temperature gradient (west to east) across the South Pacific Convergence Zone (PDF, 622K, last updated 11:10AM, 8 December 2010)* (i.e. between eastern Australia and the central Pacific) is quite positive (+0.91 ºC according to the Centre’s experimental SPOTA-1 scheme) which is favourable for rainfall in Queensland.
• At this time of year, such anomalies in the SOI and sea surface temperatures have tended to persist and thus ‘lock in’ to a La Niña phase. This likelihood is also supported by global climate models.

The recent sea-surface temperature pattern in the North Pacific is consistent with a ‘cool phase’ of the Pacific Decadal Oscillation (PDO). The PDO modulates the impact of ENSO on Queensland rainfall particularly under La Niña conditions. A cool phase of the PDO coupled with La Niña conditions is particularly favourable for rainfall in Queensland.

There are various approaches to developing probabilistic rainfall outlooks based on the information considered above. These approaches tend to differ in terms of which components of the climate system are considered.  As such, each approach might convey a different outlook, particularly for specific locations. However, historically ENSO has had the strongest impact in north-eastern Queensland and the weakest impact in south-eastern Queensland.

The Centre produces two statistical climate risk assessment schemes:

• The Centre’s experimental SPOTA-1 scheme integrates the above sea-surface temperature information, including indices of ENSO and the PDO. SPOTA-1 currently indicates a high probability of exceeding median rainfall across the state over the coming summer (November to March).
• The Centre’s SOI Phase scheme, which relies on the SOI, currently indicates a high probability of exceeding median rainfall across the state over the coming three-month period (August to October).

It is important that users understand the nature of such seasonal outlooks and take a long-term risk management approach to such information. The above schemes indicate rainfall probabilities based on historical relationships. Users should appreciate that if, for example, an outlook is for a 70% probability of above-median rainfall, this also means there is a 30% probability of below-median rainfall. As such, users should also be aware that 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 (for example, see Australia’s Variable Rainfall poster (PDF, 1.0M, last updated 11:42AM, 24 June 2010)* or our archive of historical rainfall maps).

The QCCCE understands that each of the schemes may have its own particular following. Although such schemes cannot provide outlooks with absolute certainty each year, those who follow such schemes should benefit from doing so in the long-term.

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 that on rainfall alone. The impact of ENSO on pasture growth is also dependent on current pasture condition and soil water status. The QCCCE’s AussieGRASS model takes these factors into account in producing pasture growth seasonal probabilities.

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