EI Nino? Models say so, but limited evidence so far by Dave McRae DPI/AFFS Centre for Climate Applications

Probability of exceeding median rainfall-May/July based on consistently near zero phase during Match /April

WITH THE potential development of an EI Nino later in the year, the output of long-range forecast models is of particular interest.

Currently seven of eleven surveyed reputable ocean or coupled ocean/ atmosphere forecast models predict the possible development of EI Nino conditions between April and August 2002. A few of the models show a tendency for cooling in the second half of the year after the establishment of warm conditions a few months earlier. The Bureau of Meteorology has an interesting web site www.bom.gov.au/ climate/ahead/ENSO-summary that summarises the outputs from various reputable ocean or coupled ocean/atmosphere forecast models.

Our strong advice to growers remains to closely monitor SOI trends and sea surface temperature patterns.

The latest diagnostic discussion by the USA Climate Prediction Centre is also interesting. It states that warmer than normal sea temperatures were observed throughout most of the eastern equatorial Pacific Ocean. However, there was only limited warming of sea surface temperatures in the key regions of the central Pacific Ocean during this period.

Therefore, it appears a trigger event such as strong westerly wind bursts in the central Pacific or a Kelvin wave is still required for an EI Nino event to occur. It will be interesting to see if the last passage (early May) of the 30-50 day intra-seasonal oscillation helped create the needed strong westerly wind bursts and/or a Kelvin wave.

Given the large amount of media coverage recently, I urge readers not to place too much emphasis on the development of an EI Nino. It is well worth remembering that low-rainfall probabilities and drought conditions will often occur without the presence of an EI Nino.

The likelihood of an EI Nino occurring this year will reduce should either a sustained drop in the SOI (Southern Oscillation Index) value or a strengthening of the warming in the equatorial Pacific Ocean fail to occur.

With this in mind, the 30-day average of the SOI has continued its recent downward trend and as of 20 May was -10.1. If this fall in value is sustained through to June, it will further substantiate the potential for reduced rainfall across much of Australia for the following 6- 9 months.

For those readers interested, the daily value of the 30-day average of the SOI is available at the Long Paddock Internet site www.longpaddock.qld.gov.au/

Continuation of relatively low rainfall probabilities

Meanwhile, the key point is the over most of eastern Australia for May to July. Based on the recent pattern of the SOI, relatively low rainfall probabilities (20-50 per cent) continue to persist across most of the eastern half of Australia, including much of NSW and Queensland.

However, the probability of gelling or receiving median rainfall for May to July is 60-80 per cent for the northern half of SA, large sections of the Northern Territory and the north-west of WA.

For the rest of these states, based on the SOI, there is currently no strong signal either way as to whether the next three months will be excessively wet or dry.

For those who like to follow historical pallerns more closely, the last 15 years that have had a similar SOI pattern in March- April are 2001, 1996, 1988, 1986,1982,1979,1978, 1973,1972, 1970, 1968, 1964, 1962, 1958 and 1957.

What were the rainfall and/or farming conditions like for May- July in your area for those years? Details of the rainfall in those years can be found at the Long Paddock Internet site www.longpaddock.qld.gov.au or in the AUSTRALIAN RAINMAN CD.

If anyone requires more climate related information, try DPI's climate web site www.dpi.qld.gov.au/climate on which the 'Climate Note' can be found. The climate note contains more detailed information including rainfall probability maps, SST information etc.

Region North