Grains Research and Development

Date: 01.09.2001

Dry likely to continue by Dave McRae, DPI Centre for Climate Applications

Probability of exceeding Median Rainfall

Pacific climate and ocean patterns are remaining in a rather volatile state unusually late into the year. Normally the climate patterns have settled the notoriously unstable autumn period by mid- to late June. However, this year, the Southern Oscillation Index (SOI) continues to shift around rather erratically.

The monthly average SOI for April was +1.4, -9.9 for May and +2.4 for June. Since then the 30-day average of the SOI has started to drop again and as of 16 July was -4.1. An analysis of historical data has shown that, in years when the SOI was in a rapidly falling phase at the end of May (as in this year), there is a 46 per cent chance the SOI will in a consistently negative phase by the end of September.

By comparison, there is only a 20 per cent chance the SOI will be in a consistently positive phase at the end of September.

The positive effect of the rise in the average SOI value from May to June is to slightly increase rainfall probabilities across much of the state. This slight rise in rainfall probabilities provides the opportunity for relief rain.

However, the overall long-term climate picture suggests that this is only a brief respite. The current pattern of ocean temperatures indicates that in the longer term the probabilities of high rainfall amounts are low.

With any three-month-based forecast system it is essential to take into account the previous two seasonal outlooks. Readers may remember the previous two seasonal outlooks had relatively low rainfall probabilities, especially over the draughted areas.

El. Nino winds?

Strong westerly wind bursts have again been recorded in the western equatorial Pacific Ocean. These westerly wind bursts continue to create the necessary preconditions for the possible, but by no means definite, onset of an El Nino. It is important to remember that there can be low probabilities of highrainfall events without the presence of an El Nino.

Also note the latest information from the USA research body NASA, which reports a pulse of warm water (Kelvin Wave) travelling towards South America. At the time of writing, this warm water was due to arrive at the west coast of South America in late July.

The arrival of the Kelvin Wave is expected to cause a warming of the eastern Pacific Ocean similar to an El Nino-like sea-surface temperature pattern. Kelvin Waves are triggered by westerly wind bursts (winds blowing in the opposite direction from the normal south-east trade winds).

Ocean warming

The latest Sea Surface Temperature (SST) anomaly map (www.dnr.qld.gov.au/longpdk) indicates sea temperatures in the key regions of the eastern equatorial Pacific Ocean are mainly close to average or slightly warmer than average. However, at depth the ocean has warmed significantly over the past four months.

It will be interesting to see if this warming of the eastern Pacific Ocean is strengthened by the presence of the Kelvin Waves.

For those who like to follow the historical patterns more closely, years with a rapidly rising phase in June also include 1901, 1929, 1934, 1947, 1953, 1957, 1958, 1966, 1967, 1970, 1973, 1986 and 1996. Maps of the rainfall in those years can be found on the www.dnr.qld.gov.au/longpdk Internet site.

More climate-related information is available on the Department of Primary Industries’ climate web site www.dpi.qld.gov.au/climate, the Longpaddock Internet site www.dnr.qld.gov.au/longpdk or by contacting the DPI Call Centre on 13 25 23. A recorded message with the 30-day average of the SOI is available on 07 4688 1439.