Look towards winter rainfall and maybe an El Niño by Dave McRae and Roger Stone of Queensland Centre for Climate Application (QCCA) / Agency for Food and Fibre Science (AFFS)/DPI

Probability of exceeding Median Rainfall

The seasonal climatic outlook for the rest of 2001 will be strongly influenced by the pattern of ocean surface temperatures that develop during March to June. Historically, autumn is the greatest time of instability of the El Niño-La Niña climate cycle and changes between the two can often occur at this time.

An early indicator to the possibility of a developing El Niño was the presence of an intense low in the southern part of the Northern Hemisphere in mid February corresponding with the development of tropical cyclones Vincent and Wylva. These tropical cyclones, combined with this low pressure system would have helped create and enhance the westerly wind bursts which at this time of year can trigger an El Niño event.

Sea surface temperatures currently mimic a mild form of a 'La Niña'-like state. However, there is concern that ongoing changes in sub-surface sea temperatures and SOI trends may indicate a possible shift in the SOI phase during the critical April-June period similar to events in 1991.

There is the potential for an 'El Niño' to develop over the next 3-4 months. However, there remains some uncertainty about these ongoing SST changes. For forward planning, it is therefore important to keep track of these developments.

Watch for deep negative SOI by May

A key early indicator will be any deeply negative SOI values by the end of April or May. This would strongly reinforce that an 'El Niño' is developing.

Over the last few months the SOI value has continued to fluctuate. However, as Ground Cover went to press, the SOI continued to remain in a 'consistently positive phase'.

Rainfall probabilities

Based on the SOI, the rainfall probabilities across Queensland for the March, April and May period have not changed a great deal since the last forecast (February, March, April). The probabilities of getting more than the median rainfall range from 60 to 80 per cent for most of the south-east coast, 60-70 per cent across parts of the far north and central Queensland. There has been a reduction in rainfall probabilities (30-40 per cent) for parts of southern Queensland (refer to map). This is a good example of how a positive SOI phase does not equal high rainfall probabilities in all areas.

Frost risk?

Analysis by QCCA into frost-risk probabilities in Queensland and northern New South Wales has shown that there is an increased probability of late frosts in many cropping areas in years when there is a positive SOI phase at the end of February. The frost-risk forecasts at a number of selected locations for the coming winter are updated at the end of February and May. Producers affected by frost can check what phase the SOI was in at the end of February and what it means for their area.

Given this rather patchy map of probabilities, we strongly encourage producers to utilise the continually updated outlook in the climate and weather section of the DPI internet site at www.dpi.qld.gov.au/ — especially for the winter rain outlook.

Contact: Dave McRae 07 4688 1459 email mcraed@prose.dpi.qld.gov.au

Region National, North, South, West