winter 2000 - mixed messages

Probability of exceeding median rainfall

Most climate forecast indicators now suggest a return to a near 'normal' state in key climatic indicators. For example, the Southern Oscillation Index (SOI) appears to be falling slowly from its high positive values maintained since last spring.

Sea-surface temperatures in key regions of the Pacific and Indian Oceans also appear to be returning to a near 'normal' state. Interestingly, sea-surface temperatures have remained quite cool in the central equatorial Pacific until very recently, suggesting the La Nina pattern of the past nine months or so has shown little willingness to fade away. It may only now be showing signs of doing this.

In the Indian Ocean, the overly warm sea-surface temperature pattern is now also showing signs of returning to near 'normal' values, at least for the immediate future.

What does all this mean for Australia?

The climate forecast systems in use in Australia simply employ known conditions at the time of compilation of the climate outlook.

For example, at the end of April and in May, the SOI was in a 'consistently positive' phase. This means the rainfall probability values for most of Australia remain somewhat neutral.

There is little sign of many regions showing either very high or low probabilities of exceeding the long-term median rainfall in the country.

Exceptions appear to include parts of central inland NSW, some very patchy areas of Victoria (where they desperately need the rain), northern inland Queensland and areas where the probability of exceeding median rainfall reaches 80 per cent for June-August this year. These are the areas shaded blue on the accompanying map.

By contrast, parts of southeast Queensland still have only a 30 per cent probability of exceeding median rainfall during winter.

Longer-term outlook

Most climate models that attempt to predict the sea-surface temperature pattern a year or so in advance are suggesting a fairly 'wishy-washy' pattern for the rest of the year.

Although we may even witness occasional weak warming for parts of the central Pacific, it does not appear at this stage that it will be a lengthy or extreme event.

I strongly suggest readers of this column monitor climate forecast indicators such as the SOI and sea-surface temperatures until the end of June to watch for any sudden changes.

Normally, climate indicators established at the beginning of winter tend to stay Tocked-in' for about a year.

Useful sites include the Long Paddock www.dnr.qld.gov.au/longpdk and the Bureau of Meteorology site http://www.bom.gov.au/climate/ahead/rain_ahead.shtml.

The work has been funded by the Climate Variability in Agriculture Program (CVAP), GRDC, IWRRDC and RIRDC

Region North