16 September 2008
SOI continues positive into spring
The MJO was in Phase 5 (16th September), and was fairly well organised with reasonable signal strength. The active centre of convection was located over the Maritime Continent region. Currently conditions are suppressed over equatorial Indian Ocean.
The MJO is expected to reach Phase 4 (Maritime Continent) again in about 40days, or mid to late October. In Northern Australia, the passage of the MJO is often associated with the beginning of the green season, as well as the start of the wet season proper. The timing of the next passage of the MJO could certainly trigger the green season in northern Australia.
During September to November the MJO is associated with increased chances of rain during Phases 5 (south and west) and 6 and 7 (north). Drier Queensland conditions connected with the passage of the MJO are during Phases 1 and 2 from September to November.
Most areas of Queensland received over welcome rain during early September. The exceptions were the far south west and north of the state. For example to date this September Nambour recorded 120 mm while inland centres such as Emerald received 41 mm, Roma 69mm, and Goondiwindi 70 mm.
This comes on the back of a close to average winter, where August were fairly dry, but June received rain in south west Queensland, and July was wet in the north and east of the state. As a result, around two thirds of the state recorded average to above average rainfall for winter.
In the mean time the monthly value of the SOI went from plus 3.3 for July to plus 8.0 for August. Based on a 'Rapidly Rising SOI phase and historical rainfall data there is a 40 to70 % chance of getting above median rainfall throughout most of the state for September to November.
This represents a shift in the odds towards wetter conditions for spring. The thirty day average of the SOI as of the 16th of September was plus 15.7.
Further analysis indicates rainfall for Queensland is more likely to be close or above to the long term average (or middle third to upper third) rather than well below (bottom third) average for September to November.
For those who like to follow the relationship between the SOI and rainfall patterns in more detail have a look at what happened in your area during September to November in the following years; 1954, 1962, 1964, 1966, 1967, 1970, 1971, 1983, 1985, 2000, and 2007. Look at your long term average rainfall for September to November and see how many times rainfall was well below, well above or close to average.