10 June 2008
The thirty day average of the SOI as of 10 of June is 0.0. The SOI is simply a measure of the difference in barometric air pressure between Darwin and Tahiti. It typically ranges in value from plus 30 to minus 30.
Changes in sea surface temperature (SST) patterns in the central Pacific drive changes in the global circulation patterns and influence our local climate. Although the SOI is measured on a daily basis, it is the shift in value of the SOI from month to month that reflects SST patterns and the strength of the Walker Circulation.
By using a statistical analysis of SOI phases and historical climate data (rainfall, frost, hail, temperature, etc), a seasonal forecast can be developed. These forecasts indicate for example, whether the coming three months are likely to be wetter or drier than normal.
It is worth noting that the SOI influence on climate varies across Australia (greatest skill is for eastern Australia) and between seasons. Usually the greatest skill is for winter, spring and early summer. Skill is a measure of how closely the forecasts are to what actually happens.
Generally, periods of widespread pro-longed drought, especially across eastern Australia, are associated with Consistently Negative SOI phases (average monthly values of the SOI below minus 5.0) and with an El Niño SST pattern. This negative SOI value is a reflection of barometric air pressures over northern Australia being higher than those in the central Pacific. This may slow or stop the flow of the south-east trade winds as well as reduce the occurrence of rain depressions (and cyclones).
There is some evidence that Indian Ocean SST's affect our climate too. Currently the eastern Indian Ocean is cooler than normal, and the western basin is warmer (positive Indian Ocean Dipole). This suggests that there will be fewer north-west cloud bands which often bring rain, especially to southern Australia, during winter.
The MJO influences rain (and dry conditions) on a shorter timescale. Each of the 8 phases has some impact, and each phase lasts on average 6 to 12 days. For example the MJO is currently in Phase 3 indicating increased chances of rainfall for northern Queensland, but drier conditions for South Australia. Phase 4 generally has the best chances of being associated with rain in Queensland, during both winter and summer. So over the next week chances of rain are enhanced for most of Queensland. This should coincide with the passage of a trough from the west, further enhancing the chance of some associated rain.