Monthly Climate Statement for September 2014

The Science Division of the Department of Science, Information Technology, Innovation and the Arts (DSITIA) considers that the probability of exceeding median rainfall during spring (September to November) is slightly below average for most of Queensland, whereas the probability of average to well-below average summer (November to March) rainfall remains higher than normal for much of the state. Read more (PDF, 284K, last updated 08:59AM, 16 September 2014)*

DSITIA’s rainfall outlooks for Queensland are 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 (i.e. the more slowly changing extra-tropical sea surface temperature (SST) pattern in the Pacific Ocean). 

At this time of year, and in the coming months, the prevailing ENSO pattern (as measured by indices such as the Southern Oscillation Index (SOI) or central equatorial Pacific Ocean SST anomalies) offers a useful basis for providing seasonal outlooks for spring and summer.

Although sea surface temperatures in the central equatorial Pacific remain below El Niño thresholds, the SOI, the key atmospheric measure of ENSO, fell to an extremely negative monthly value (-10.1) in August.

Currently:

  • Observed SST anomalies in the Niño 3.4 region of the central equatorial Pacific Ocean remain warmer than normal (July +0.18 ºC, August +0.20 ºC).
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  • The majority of international global climate models indicate warming of central equatorial Pacific Ocean SSTs in coming months with the risk of a weak El Niño event developing.
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  • As at 14 September, the 30-day mean SOI value remains extremely negative (-9.8).
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  • The most recent three-month (June to August) SOI value was -5.0 and the most recent (as at 14 September) 90-day mean value is -7.9.
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  • Recent negative SOI values highlight the continuing risk of the ocean and atmosphere 'coupling' to form an El Niño event during spring and/or summer.
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  • If an El Niño event does develop, it would likely persist until the end of summer 2014/15.

What if an El Niño event develops this year?

More than 75 per cent of Queensland remains drought declared under state government processes, including most inland regions and all of south-eastern Queensland. The possibility of an El Niño event developing, and with it the threat of another dry summer for some regions, poses a risk of current drought conditions becoming more protracted. This risk should be factored into decision- making and allocation of resources. In this context, DSITIA’s long-lead outlook for summer rainfall (next page) should be taken into consideration.

Outlook for summer rainfall

DSITIA scientists have shown that extra-tropical SST anomalies, when measured in specific regions of the Pacific Ocean in March, provide a useful basis for long-lead forecasting of summer rainfall in Queensland. This outlook can be modified, with increasing accuracy, as the monthly ENSO-related SST pattern is also taken into account from June to November.

This understanding has been incorporated in an experimental system known as SPOTA-1 (Seasonal Pacific Ocean Temperature Analysis version 1), which has been operationally evaluated by DSITIA scientists for over a decade. 

Currently, DSITIA’s long-lead outlook for summer rainfall indicates a higher than normal probability of below-median rainfall for most of Queensland over the coming summer (November to March 2014/15) and, conversely, a low probability of widespread drought-breaking rainfall. This outlook will be updated monthly until November, with accuracy increasing each month.

In summary, it should be noted that:

  • The current long-lead outlook for summer is based on both extra-tropical and central equatorial Pacific Ocean SST anomalies. 
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  • Central equatorial Pacific Ocean SST anomalies over spring and summer will define the state of ENSO (‘El Niño’, ‘La Niña’ or ‘ENSO-neutral’) for 2014/15.
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  • Should an El Niño event occur, it is increasingly likely to be a weak event. However, both ‘weak’ and ‘strong’ El Niño events have an equal tendency to produce dry conditions in Queensland.
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  • For Queensland, the state of the extra-tropical Pacific Ocean determines whether an El Niño event (weak or strong) is most likely to result in a dry summer.
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  • The extra-tropical SST pattern related to the Inter-decadal Pacific Oscillation is the main contributor to DSITIA’s current long-lead outlook for summer rainfall. 
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  • DSITIA’s long-lead outlook for summer rainfall will be updated each month until November, by factoring in the developing ENSO-related SST pattern.
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  • 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 may be stronger than on rainfall alone. The impact of ENSO on pasture growth, for example, is also dependent upon current pasture condition and soil water status. DSITIA’s AussieGRASS model takes these factors into account in producing seasonal pasture growth probabilities.

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Last updated: 30 March 2018
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