Grains Research and Development

Date: 01.09.1999

La Niña starting back up?

Probability of exceeding Median Rainfall

Oceanographers are telling us that the La Nina pattern in the Pacific Ocean may just be starting to regenerate. While we have witnessed a La Nina pattern during winter, it has been in a very subdued form. The upper-air circulation pattern associated with La Nina — The Walker Circulation — has recently shown some signs of increasing in intensity with slightly stronger easterly winds evident across the tropical Pacific Ocean.

What does this mean for our rainfall patterns?

The rainfall probability analyses (in such programs as Rainman) issued by the QCCA and also by the Bureau of Meteorology tend to indicate some increase in the chances of getting any given amount of rain for some regions through to November. As the mechanisms responsible for our climate patterns tend to persist from autumn to autumn, it is unlikely that the current outlook will alter much in general terms before January 2000.

The accompanying map shows most areas of Australia have fairly even chances of getting or exceeding their climatological median for the period up until the end of November. This type of result also suggests low probabilities of being either excessively 'wet' or excessively 'dry'.

Rainfall probabilities beyond November are unlikely to change much from the above pattern, although there is some suggestion from the oceanographers that our rainfall probabilities may increase a little as we near the end of 1999.

Key things to monitor over the next few months

Please keep a watch on the SOI and any news you may hear on the state of the La Nina pattern in the Pacific. Indicators to watch for are signs of warming sea surface temperatures around northern Australia and SOI indicators climbing above 10 (strongly positive).

Any continued improvement in the strength of the La Nina pattern would have important implications for rainfall in mid- to late summer in almost all parts of Australia. A 'strong' La Nina with especially high SOI values, if they persist, increases our summer rainfall probabilities quite substantially, especially in northern Australia. We will update this information as we near the end of 1999.