WestVic Dairy News - July 2010 Edition
Does SAM make it rain?
By Rod Eldridge, DPI Colac
In last December’s issue we explained the main influences on the climate and key drivers of rainfall in south west Victoria. These included El Nino Southern Oscillation Index (ENSO), Indian Ocean Dipole (IOD), Southern Annular Mode (SAM) and Sub-Tropical Ridge (STR).
Now we are getting into winter and hopefully the rainy season, these key drivers start to have a stronger influence on the weather and can provide an indication of things to come later in winter and the spring. As winter progresses the degree of confidence in the predictions for ENSO and IOD increases considerably and their status is worth watching. Information on the status of these can be found on the Bureau of Meteorology website at http://www.bom.gov.au/climate/enso/
However SAM, and to a lesser extent the STR, also play a role in influencing winter rainfall, as they trigger the rain. SAM is an indication of the strength of the synoptic systems around Antarctica and influences movement of cold fronts towards Australia. When it is strongly negative (below -1) the polar systems are expanded and push the cold fronts towards southern Australia, while for a strongly positive SAM (above +1) the polar systems are contracted over the pole and the cold fronts track further south of the mainland.
Figure 1 below shows the daily fluctuations of SAM, and that it can fluctuate widely and change rapidly. Since mid May it was in fairly positive territory, and if you had watched the synoptic charts, you may have noticed the frontal systems travelling further southwards of the mainland.
Figure 1: Daily fluctuations of SAM from 17 Feb 2010 to 16 June 2010
Reference http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao_index_mrf.shtml
Figures 2 and 3 below show the correlation between SAM and winter rainfall for Timboon. SAM has been calculated since 1957. Figure 2 shows that for the 10 years when SAM remained strongly positive (above +1) during winter there was a 70% chance of below average winter rainfall, and 20% chance of average rainfall. However figure 3 shows for the 13 years when SAM remained strongly negative (below -1) during winter, there was a 38% chance of above average rainfall and 47% chance of average rainfall.
Figure 2: Chance of winter rainfall at Timboon in years of positive SAM (June to August)
Figure 3: Chance of winter rainfall at Timboon in years of negative SAM (June to August)
SAM is very difficult to predict and it can swing between positive and negative phases quite quickly, however once it enters a phase it can tend to stay there for sometime (ie a week to a fortnight). The SAM is currently the focus of a lot of research into how it interacts with other weather drivers such as the STR and how it influences the rainfall of southern Australia.
Remember the SAM is just one of a number of factors that influence rainfall. Presently, in the central equatorial Pacific Ocean, this summer's recent El Nino has cooled to neutrality. A number of models that take the IOD and ENSO into account currently suggest a somewhat promising outlook in regard to at least average winter and spring rainfall, but perhaps we will believe it when we see it.
By gaining a greater understanding of all four key weather drivers we can have greater confidence in using them to help make decisions for the farm and the business, as rainfall influences many of the key decisions made on the farm.
For more information, contact Rod Eldridge, DPI Colac on 52 335594.
