Monthly Climate Statement for September 2010

The Queensland Climate Change Centre of Excellence considers that there is an increased probability of above-median rainfall in Queensland for spring and the coming summer season. 

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).

As at September 1 2010, the Centre notes that:

A La Niña climate pattern is now well developed in the Pacific (see the latest Bureau of Meteorology ‘ENSO Wrap-Up (PDF)*’). This pattern is likely to persist through spring and summer.  In particular:

• The Southern Oscillation Index (SOI) remains quite positive when averaged over the last month (August: +17.0), two months (July-August: +17.9), and three months (June-August: +12.3).  The three-month value being the highest since that of November 2008 -January 2009 during the peak of the previous La Niña event.
• Observed sea surface temperatures in the key Niño 3.4 and Niño 4 regions (PDF)* are now much cooler than normal – typical of an established 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) remains quite positive (+1.69 ºC) according to the Centre’s experimental SPOTA-1 scheme), which is favourable for rainfall in Queensland.
• As reported last month, at this time of year, such anomalies in the SOI and sea surface temperatures tend to persist and thus ‘lock in’ to a La Niña phase.  It is now clear that this has occurred. The strong likelihood of a La Niña pattern persisting until the end of summer is also supported by global climate models.

The recent sea-surface temperature pattern in the North Pacific remains 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 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:

• 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 (September to November)

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 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. For example, an historical archive of SPOTA-1 reports is available on the Long Paddock website.  Users should also consider the wide range of information available each month describing the current state of the ocean/climate system.

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 QCCCE’s AussieGRASS model takes these factors into account in producing pasture growth seasonal probabilities.

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